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I should add that you also needn't permanently disconnect the ground connection to chassis, or install a ground lift switch. You can simply use a ground-lift plug adapter aka "cheater plug"

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In any case, I still think it worth replacing the stock power inlet with an IEC unit. The original power cables are often in bad shape, and finding a replacement that fits can be a pain (same with modifying a cable that doesn't fit, such that it does fit and remains safe to use after doing so)

Having an IEC inlet means you never have to search around for special 2-prong extension cord for this instrument. If you forget your power cable at a gig, there's almost certainly another one nearby you can borrow.

It would be simple to add a ground lift switch so you can have the best of both worlds. All you need is an SPST toggle and put it inline with the ground conductor between the IEC inlet and where it connects to the chassis. I've done this on a couple 140B overhauls, so that it can be grounded for normal use where a little 60Hz hum is not a problem, like a live band setting, but it can be lifted and quieter for recording scenarios.

Alternatively, just cut the ground wire off the IEC inlet and leave the instrument ungrounded, as it was originally. This does remove a safety feature, of course, but you can assume that risk if you so desire.

If you scroll back to my first reply on this thread you can see in the first photo how I've implemented a ground lift mod. The ground lift toggle is to the right of the fuse holder. The ground wire goes through this switch before connecting to the chassis.

Well, may as well copy that photo into this post since it's easy and clearer, for those following along:

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Here's a photo of how it looked from the outside:

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It's been a few years now, but I'd say the Warneck amp I installed wasn't necessarily brighter than any given Wurlitzer sound I've heard. It was also going into a piano that arrived at my bench with no electronics, so I had no basis for comparison to the stock amp in that particular Wurly.

It definitely has a way lower noise floor than an unrestored original amp. As for overdriven/fat vs clean, you can adjust the gain to taste, much like the original amp.

If you already have the amp in hand, I say give it a try and hear it with your own ears! If you don't like it, you can always put the original amp back in and sell the Warneck, or save it for something else.

Glad you got it sorted!

Yep those transistor housings are isolated from chassis ground, good this did not cause you any further grief and that it was an easy fix once you discovered this issue.

Hey, also want to say it sounds really nice. For a 200A, on this recording it really has a nice throaty tone that's almost akin to that of a 140 or 120 series.

You'll love the Warneck amp. I installed one of those for a client and was very impressed with all aspects of its design and sound, and I also had a good experience communicating with the product engineering and sales folks there. They were really helpful in sorting out some questions I had for my particular application.

Personally, I don't rely on google ai search results, especially for relatively obscure questions. I've seen it return a good many inaccurate or misleading results.

If I was working on that amp, first I'd ask myself why that fuse is blown. Is something else in the power amp/output section shorted out and pulling too much current?

Looking through a scanned copy of a 200A service manual, the parts list does not indicate fast or slow for those two fuses. However, the line/mains fuse is explicitly indicated as slow-blow in both the parts list and the schematic drawing. So my *assumption* is that if F1 and F2 are supposed to be slow-blow fuses, then they would be labeled as such.

Did you pull the old fuse out to examine it? Not that we can be sure it is original or correct, but I'd at least have a look to see if it was even of the correct amperage rating.

Personally I wouldn't even bother, to me they are part of the vintage character of these instruments, and unique to each individual instrument. Kind of like finish checking of a very old guitar or bass. Or as you suggest, wrinkles (aka the accumulated markings of earned wisdom...)

It is amazing to me that this ~60 year old plastic holds up as well as it does. "Erasing" these cracks seems quite obsessive unless you are attempting to restore an instrument to factory mint, in which case you may be better off replacing all of the key tops and fronts with new material.

I have a 720 that suffered a badly chipped front edge of a key top due to an unfortunate incident, and was fortunately able to swap out the keycap for a good one I nicked off a 206 I keep around for parts. It's not an exact color match nor edge profile match, but it's not terrible, though I wish it had never happened at all. Oh well! It's not like my 720 is aesthetically minty, anyway. Character, and part of the story of this particular instrument.

Back on the technical side of the question, I'm not certain of a reliable method to suggest for all cases. Relatively shallow scratches or cracks can potentially be buffed out, which I have done - although at potential cost of color mismatch since you might buff through age-discolored surface to fresh material. Best to look at the back edge of the key cap where it sits underneath the lid/felt and assess how discolored it has become over time.

You could attempt to fill a deeper crack with say, baking soda and superglue (this is a luthier's trick for building up a too-deep slot in the nut of a guitar) but this method would probably look far worse than just leaving it alone. Deeper scratches and cracks are likely filled with dirt, and again there are color-matching concerns for more aggressive sanding/buffing.

If you've ever seen touch-up paint on an old car or bicycle, you might agree it looks worse than had it been left alone, and there are no significant functional issues to consider as with a vehicle, such as inhibiting rust formation.

If this is a stage instrument, nobody in the audience will likely ever notice it.

¯\_(ツ)_/¯ I personally gave up on caring about the new model when the info came out that it was basically just a digital piano inside a fancy (aka overpriced) shell

And I had signed up for the email list and followed that company on socials and all I ended up getting was a firehose of nostalgic jukebox propaganda and absolutely no news about the piano. Meh. What a waste of time it was to be interested in this at all.

As such I had forgotten all about it until now. Did a little search, and apparently as of at least a couple months ago, it has been "shelved"

The quote below is from: https://www.facebook.com/groups/wurly/posts/2523211341408683/

Received an official email from Wurlitzer. The 250D project has been shelved. This could be good or bad news:

"Hi there,

We want to share a clear update with those of you who joined us for news about the 250D digital piano.

Reintroducing a new Wurlitzer instrument is something we take seriously. Rather than releasing something that does not fully meet our standards – musically, mechanically, and creatively – we have chosen to pause and reassess the right path forward. After considerable effort and evaluation, the 250D project will therefore not be moving forward in its original form.

Our ambition to bring a new Wurlitzer piano into the world, however, remains very much alive. We are committed to finding the right way to realize it – in a form that truly honors the legacy of the instrument while meeting the expectations of today's players.

At the same time, our broader mission remains unchanged: to preserve and evolve the Wurlitzer sound for today's musicians.

WURLITZER by Rhodes is part of that commitment – bringing the character and feel of the original 200A into modern studios. Whether through software or future physical instruments, our focus remains authenticity, playability, and musical integrity.

If you'd like to experience our virtual instrument, you can explore WURLITZER by Rhodes at
www.wurlitzerpianos.com

Thank you for your patience, your belief in the instrument, and for being part of this journey.

Warm regards,
The WURLITZER Family"

So, "shelved" aka they probably got a ton of negative feedback from the potential customer/influencer base, which taken alongside what I assume would be a high-end pricepoint made it a completely non-viable from a business perspective. That's just my take - don't take it as the plain truth.

Hard for me to say, but maybe others have more insight to offer. You'll do the recap anyway so if the excessive thump on power-off is still there after, that's something else to look into

Good on ya, yes the harp assembly can sometimes seem black magic between taming resonance, rattles, and dead notes

Whatever makes it work and will hold adjustment

Those are sheet metal screws, not machine screws, so the threads are not exactly standardized nor would I imagine the manufacturing tolerances are very tight.

They also thread into clip nuts. If the clip nuts are damaged or worn, then you may need to replace those bits if repairing is not possible (for example if the metal where the screw threads in is deformed and can be re-formed to a more functional condition)

I would suggest just taking one of those screws to a hardware store and trying to find one or more good matches, then doing a test with one only to see how it threads in, before committing to doing the rest like that.

These harp screws need to be somewhat fine-adjustable and they need to hold position, since any of them being too-loose or too-tight can affect the sound significantly. I"ve never needed to shim a harp mount, but I suppose it is possible, for example if the wooden case warps and changes the relative positions of the harp mounts.

Maybe sacrilege, but I'm not sure how much the choice of electrolytics brand/series matters much for this application, as long as the specs are appropriate. The Wurlizter EP amplifier is not a hi-fi or pro audio device, so IMO it's not worth sweating over audiophile type considerations, if that's part of the question at all. Forgive me for assuming so if it isn't.

In terms of quality, any of the brands you name should be totally fine. I've even used some NTE stuff when I needed to, but I tend to prefer Nichi or Vishay. Where I work we have a boatload of assorted new and NOS film caps, it's unclear even what brand some are but they've all worked for me.

Might find it difficult to source some specific cap values in the optimal lead configuration - for example you might only be able to easily source a radial cap when the PCB is meant for axial leads. In my experience it doesn't matter, since caps are now less than half the size they were when these were first made. There's plenty of room to operate. I just add a little adhesive to help support any radial cap installed in this manner.

You may also need to replace the resistors R12,13,14 and .12mfd film caps C16,17,18 in the twin-t vibrato oscillator, if you've got thumping or other issues with the function of that circuit. After replacing those caps, it may require tuning the resistor values to produce a stable and thump-free signal at the desired frequency. Would leave R13/14 at stock value and adjust R12 to taste.

Sure, all three action centers might play a factor in what you are observing but I'd just start with the one at the top since the hammer butt directly pivots around that point. Then you could move on to the pivots on the whip and fly if you think those are causing any sluggishness. I generally attempt to be methodical in this sort of investigation so I can pinpoint the cause of an issue.

One other thing I forgot to ask is if you're sure the hammer or butt isn't getting hung up on anything along the sides of those parts. Sometimes these can get jammed against part of the harp casting, hardware, or even action parts if they are very close together. A little bit of finesse work with the mounting screws or even some careful, light sanding of the wood parts is sometimes necessary to create enough clearance and resolve this.

Also - you said two hammers were affected by this issue - are they right next to one another or in different areas of the keyboard?

Yeah, since the photo seems to show you have clear and direct access to the hammer pivot in question, you can easily lubricate it in situ. I will sometimes shrink/lube action centers without disassembling the components it's more of a touch-up regulation job as opposed to a deeper inspection and overhaul.

I'm not experienced with using protek clp on action centers, rather I use a naptha/silicone fluid mix as described in the service manual. This mix both shrinks the felt (which expand over time from moisture absorption, I believe) to reduce mechanical interference friction while the silicone provides lubrication. I only mention this since I can't speak directly as to how CLP performs or if the use considerations are any different from what I use. YMMV!

Assuming you have a needle applicator, just put the tip right onto the felt bushing that surrounds the metal pin. You shouldn't need much product at all. Gently work the pivot back and forth a bit to ensure the product gets in there and does its thing.

Check to see if there is any noticeable difference before/after both in manually lifting the hammer and then dropping it, and also with normal use of the associated key.

Have you shrunk and lubricated all the action centers? The one that is visible here for the hammer pivot looks discolored, perhaps rusty or almost like someone has put used motor oil on there as a lubricant. If the action center is sluggish, that could prevent the hammers from falling away completely when releasing the key.

I find it's generally folly to go chasing after action regulation issues if you aren't certain the action is moving freely, all parts are structurally sound, and all screws that hold the flanges to their respective rails are tight.

Of course there are plenty of exceptions, and the quick-n-dirty must be done in the middle of a gig or while on the road. But when I have something on the bench I don't even spend my time on regulating until the mechanical assembly is in a proper state.

Other folks' eye may see other things going on here - or not - just sharing how I'd approach this issue.

Oh... well I suppose there is one other question to ask:

It's unclear from your post - what part of the piano are you listening to/recording that you are finding a sharp roll-of above 2-3kHz? Is it some sort of line out/aux out jack mod to the original amp; an aftermarket amp; or are you mic'ing the speaker?

If the latter, is the speaker original? In that case, I would compare the freq response of the speaker output to that of a line/aux out (if present) to see if there is any difference. Weber makes a very nice alnico that is a drop-in replacement for the 140, if you decide the speaker is the culprit.

Late to the party but I do not think of the Wurltizer EP as any sort of "spanky" or "bright" sounding instrument. I've always felt it was a very mid-focused instrument, even considering the differences in tone across the various model ranges.

If you can, always use your ears - the way other people describe sounds is the epitome of subjectivity and is chock full of dialect/slang. For example, when I hear "spanky/bright" I think telecaster. Which the Wurly 140b is not, IMHO.

I have worked with a client who felt their 140b was too mid-rangey/muffled/muddy for the studio sounds they were after. So we tried an experiment replacing the original amplifier circuit with a minimalist "bias-supply" type preamp, which bypasses the entirety of the stock electronics and any tone-shaping that happens therein.

The minimalist board includes a HPF to block DC from the pickup output, and we found the cut-off frequency of this filter was fairly low. In studying various schematics I got the impression this is typical of a Wurlitzer EP, with the cutoff frequency often falling in the subsonic range. All it really needs to do is block DC (which is essentially equivalent to 0Hz)

We then altered the HPF circuit, raising the cutoff frequency fairly far into the midrange in an attempt to re-balance the frequency response of the instrument (cut lows, boost everything, see what happens...)

I could detect an admittedly small and somewhat pleasing difference between the stock filter and the modified one, but the client decided to sell the piano and get a completely different instrument that was going to provide the brightness they were after.

This is anecdotal, mind you. But it was a good exploration to have undertaken, since I learned a bit more about the "raw" sound of these instruments.

I mistakenly wrote there were two wires on that terminal for the positive DC rail. This is probably because I had been using the VV power supply rebuild kit instructions as a reference, but that apparently contains an error. It is ringing a bell for me now - I did install one of those kits on a 140B and remember getting momentarily tripped up by the same confusion.

Looking at photos of my own work more closely and studying the schematic, yes it makes sense there are more than two wires connected to that can cap terminal. One goes to the bridge rectifier, one goes to a lug on the level set potentiometer on the amp chassis (assuming your amp was made after that revision was implemented) and two go to the PCB.

Move all four wires from the tab on the old cap over to the new cap.

It is good not to assume things - because production examples do not always follow the available schematics, someone else may have changed what was originally there before you arrived, and also because people can make inaccurate statements in their product literature and in online forums... 

OK, I think I understand now what the actual cause of your excessive hum was. Started writing this up earlier but had to head off to work.

On a typical guitar amp 1/4" input jack, for example, the tip contact is shunted to ground when no cable is plugged in. Think about how a guitar amp often sounds like when you plug in a cable but no instrument attached at the other end...

I assume with the cable harness you have, that 1/4" input is not a shunted jack. So that would explain why the sound is excessively noisy when you have the harness connected to the 930 amp but no piano plugged into it.

On the 930 schematic, there is no indication that the phone plug input is a shunted device. In practice, it might be, but we'd have to get a closer look at the one Vance has to know how one is assembled - or at least the one example we have ready at hand. (If it has a 1/4" input at all... read on)

The mystery control plate on Vance's 930 does look to have a jack with a double-shunted tip contact, but I can't tell exactly how it is wired, or what all those caps and resistors are doing there. I'm not interested in studying it that closely at this hour.

But now I'm starting to wonder if maybe that mystery jack is actually the input for the signal from the piano, and that the "control box" lacks an input jack? It might only be functioning as a power inlet for the aux amp and a volume knob. There doesn't seem to be a pilot lamp visible in the photos, and it has a metal plug filling what I'd expect to be the input jack or lamp location.

That mystery chassis has an awful lot of parts on it, and then what's the use of having two volume pots? Is the "volume knob" on the control box only acting as a power on-off switch?

Starting to think this is either a variation from the factory not captured by the schematic, or it has been modified after it left the factory. Or perhaps, none of the 930s ever made match the schematic! It will need to be proven 😂

You'd have to get a response from OP to really know what they meant with their words, but I assume if the post was about broadly adjusting strike line, "high and low" means treble and bass ends of each harp. If the strike line for a whole bunch of reeds is off, as opposed to a single reed, then that may indicate the harp is not optimally aligned to the action.

There is a little bit of left-right/front-back adjustment of the harps available when loosening the harp mounting screws and nudging things around in the horizontal plane. This is a necessary check on reassembly if you've removed the harps for any reason during other service.

You can't really fully check the results of this type of adjustment without properly tightening the harp screws back down - it's like tuning a reed in that way. So it can be a trial-and-error process.

If you get into this, be careful to pay attention when tightening those screws. I find it helps to play the piano as you are proceeding with that task so you can listen to how the sound may change. If you under- or overtighten the harp screws it can make reeds sound dull, choked or uneven. This could be user error or you might need a shim. I have never needed to shim a harp mount, but here is from the 200-series manual on that task:

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Another wrinkle - my 200 isn't at the house right now, but I seem to recall all the harp bushings on a 200 series are nylon. On my 140B and my 720, all bushings are nylon except the four at the left side of the treble harp are rubber. I am unsure if this was intended as a means of damping out unwanted resonances or affording twist adjustment without shims, but those would be my guesses. These are squishy so it's easy to accidentally overtighten them. And when the get old, they probably either deform, harden, or both. But I haven't had any motive thus far to change those on my pianos or others I have serviced. I don't think replacements are available other than the nylon ones anyhow.

(Adding, there is no adjustment for the pickup mounting. I don't recommend touching that assembly at all unless you are adding reed bar shields, or there is some other issue that requires it.)

You've misunderstood - please don't just connect random points on the amplifier PCB to the input jack, or start modifying the power and signal connections without a fairly clear understanding of what you are actually doing. This is not the way to go about it, you could damage the device, create a potential electrical hazard, or both. Being patient and methodical is the way to go. Even with experience, sometimes I find I need a night's sleep to help me solve a challenging puzzle.

For a typical TS phone plug, the sleeve connection is ground. The bare wire on a shielded instrument cable is the ground conductor. Does that help clarify?

In your case, the entire metal shell of the jack should likely exhibit continuity with ground. I add that note because not all phone jacks have their frames grounded to sleeve. Some isolate signal ground from chassis ground. I doubt that is the case here, but I could be wrong.

I still suggest the following diagnostic procedure: WITH THE AMPLIFIER POWERED OFF AND UNPLUGGED, use a multimeter to check continuity of ground from the 1/4" input jack - the sleeve - to Pin 1 of the connector plug on the input/power wire harness.

Then check continuity from Pin 1 of socket on the amp to a known chassis ground point on the amp - don't just guess without knowing what you are doing and randomly add connecting wires. This is how you could cause a short or zap parts of the amplifier or yourself with the voltage of a charged up filter cap.

So, if both of these two tests above show continuity, then put the connector together and check continuity of ground all the way from the 1/4" jack to the amp itself. This is to rule out a bad connection at the plug and socket. For me, this would be the first step in attempting to determine if your audio problem is related to a missing or bad input ground connection, or if the cause is likely to be something else.

Incredible

This is a 930 I presume. The control box sits on the carry handle then. Kooky, I have to say. And all very cool and seemingly rare of course.

No worries posting so many photos. In fact if you spread out the attachments across several posts, you can put up some larger sizes. Feel free to share more if you others. I don't think that'll run afoul of any forum rules here, and this is great photo documentation. Having higher-res images available would be a good resource.

Yeah, good reading of the schematic to notice the differences for your input/power cable harness. It could be a professionally-made custom job, or it might just be a factory variation of what's on the schematic. I honestly have no clue. The main feature it seems to lack is a volume control that is separate from the piano. I'd think you'd want that feature so you could control the aux cabinet volume separately, but Wurlitzer apparently eliminated it in the 930B revision so maybe they thought it was simpler to just have one volume control at the piano itself.

If the original "control box" was made of metal, the 1/4" jack would have been grounded there. Following the "white" lead from that ground point and/or sleeve terminal of the input jack, it also goes to Pin 1 of the connector. On the amplifier side, the corresponding wire coming from Pin 1 should be grounded somewhere on the amplifier. It might not look like a ground lug right on the chassis - it might be soldered to ground on the PCB itself, which isn't as visually obvious.

It would be good to rule out a bad or missing input ground as the source of noise and hum before worrying about other possibilities. I suggest you use a meter to check continuity of ground from the 1/4" input jack to Pin 1 of the connector on the harness. Then check continuity from Pin 1 on the amp to a known chassis ground point on the amp. If both of these have continuity, then put the connector together and check continuity from the 1/4" jack to the amp itself.

Here's a tip: keep a short length of a 1/4" instrument cable around for this type of test. Standard wiring to the plug on one end, bare wires on the other end to clip test leads to them. This allows you to test that the jack contacts are mechanically and electrically functional, as opposed to just checking the solder points on the back end of it.

As to the differences from what we see in the schematic, aside the lack of switch and volume control, it's unclear if there is the .22ufd cap in there or not. Have you opened up that pin connector housing or the jack housing to see if anything is wired inside? The cap in the schem is most likely for blocking DC from entering the amp input. It's interesting that it is spec'd for 400V...

Anyway, interstage caps for DC blocking is fairly common practice in audio devices. You'll also notice in schematic for the actual amp, there is also a cap on the input. I don't have a recommendation as to whether you should add one to the harness or not, if it is indeed missing.

Hi, not vague at all - good questions here. Was a busy week for me, sorry it took a few days to respond.

Yes I replaced the old 2-prong inlet with a standard IEC 320 inlet. You are correct, neutral wired to the transformer and hot first to the fuse, then the switch, then the transformer. I believe I put a piece of black tape or heat shrink on the transformer lead that was originally connected to hot. I'm honestly not sure if transformer polarity matters at all in this instance or not, since the AC gets converted to DC and filtered anyway. But I have noticed mains polarity affecting noise floor on my 720 (tube version, not yet converted to three-prong power) so I figured why not try to preserve the original polarity configuration.

You'll also notice that I installed a ground lift switch between the IEC inlet and the chassis ground lug. I did this to provide some convenient flexibility if there was excessive ground loop hum for recording situations. Not a necessary option. I don't typically install this mod because in a studio setting, it's trivial to use a ground lift plug adapter if desired. I was just testing some ideas with this particular overhaul/mod and the client was enthusiastic about all of it.

Directionality for the fuse socket wiring does not matter - otherwise fuses would be directional! It's a very good question to ask.

I also typically delete the phono input. Good eye! Completely removing the wire that goes from the tip/hot terminal on the phono input and amp PCB is good practice. This is basically an open signal input and can be a source of noise.

I do leave the RCA jack in place unless I want to repurpose the hole that's already there for some other mod, because it's less work, doesn't hurt anything to leave it there, and already in use as a grounding point for other components. If you already removed it and relocated the wires to another ground point, that's fine.

Well you are lucky - I've never actually seen any of the 920, 930, or 930B auxiliary amps in person, and there is hardly any info or photographic evidence on the web that I've ever seen past or present. Somewhat recently a 930 was listed on Reverb, but the seller had taken the amp out of it to use it as a passive speaker cabinet and didn't post any photos of the amp itself:

https://reverb.com/item/82951859-wurlitzer-930

However your query did spark some memory that I might have original copies of the schematics for at least one of the aux amps. Turns out I have schems for both the 930 and 930B. There is a 930 schem already floating around on the web but I am unsure if anyone has previously shared a 930B schem, so here are some quick photos of both. These are just reduced-size JPGs so I can fit into the attachment limit here on the forum. I'll have to do a high-res scan of this stuff and put PDF copies on archive.org.

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Not being directly familiar with these, hard for me to say precisely what your problem might be, but they look to be fairly simple power amps, especially the 930B. Excessive noise or ground hum could indicate that a signal ground is disconnected somewhere. Have you even played a guitar amp that had no ground on the input jack? It's noisy as heck. A fairly common problem and extremely easy and quick to fix.

On either a 930 or 930B I'd start at the input jack and on the 930 I'd work towards both sides of the 6-pin connector checking for continuity to ground. Pin 1 is ground on the 6-pin connector. You might find the problem right away if it's at the input jack!

Noise/ground hum can also indicate the power supply is needing an overhaul. Which you might want to do in any case since all of these amps are 60+ years old at this point. It's not that many components to replace, and they are cheap, and you are gonna buy some for your 140B anyway, right?

I agree that it can be challenging to translate schematic layout to the actual layout/topology of a device on the bench, especially older stuff which can be pretty messy. I also encourage you to continue getting familiar and comfortable reading schematics with the device right in front of you. I am by no means an EE expert, and even for me it can take some time to confidently map a relatively complicated schem onto an actual device. Old schematics may contain unfamiliar variations of components symbols as well. But with more time and experience, it gets easier. The knowledge gained and the "aha" moments are always satisfying, and nearly always worth chasing, if you ask me.

Just doing the can caps is fairly straightforward. The two can caps are slightly different - you can see from the outside one is taller than the other. The smaller can that is closer to the power tx contains just a single cap; that's for the -32vdc rail. The other, larger can contains two caps; one for the +32vdc rail and the other is connected to the vibrato oscillator circuit.

2200uF/35v electrolytics are OK to substitute for the 2000uF/35v can caps called out on the schematic.
470uF/6.3v electrolytic is fine to sub for the 500uF/6v.

You can safely substitute caps with higher voltage ratings - for example 50v instead of 35v, 10v instead of 6.3v.

Basically, you need to cut the wires going to the terminals on the can caps and solder them to the appropriate legs of the replacement caps, and solder the other legs of those caps to ground. I have observed that the particular wires at hand are not the same color in different 140B amps that have been on my bench, so I'm not going to use wire insulation color as a reference.

The -32v can cap (label #62 on the June 1966 schematic) has two wires on its sole terminal. Cut those two wires off the tab and solder them to the negative leg of one of the new 2200uF caps. Solder the positive leg of that cap to ground.

The +32v can cap has two wires that go to the terminal associated with the old 2000uF cap inside (#54 on the schematic. The terminal is marked with a half-circle symbol. Cut those two wires off and solder them to the positive leg of the other new 2200uF cap. Solder the negative leg of that cap to ground.

The other terminal on the larger can cap should have only one wire soldered to it (#53 on the schematic). The terminal is marked with a triangle symbol. Cut that wire and connect it to the positive leg of the 470uF cap. Connect the negative leg of that cap to ground.

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The example above was done a long time ago, one of my earliest 140B overhauls. There are a number of customizations going on with that amp rebuild, but the replacement caps you asked about are labeled in the image so you can easily identify them in the rats nest.

Alternatively, for C53 you can follow the wire from the triangle terminal on the old can cap all the way back to where it is soldered onto the PCB. Remove the wire from the PCB and solder the positive leg of the 470uF cap there, and the negative leg to a terminal in the PCB ground plane. Remove the old wire completely. Below you can see a more recent example of me doing it this way - it's quite a bit cleaner IMO. I used a 50v cap which is way overkill but I already had one on hand.

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I tend to leave the old cans in place but disconnected so that I don't have to fill or cover the holes that would be left behind. But you can remove them if you want. You may also choose to snip off the old solder tabs of the can caps (not the mounting/ground tabs, though!)

If you are lucky enough to still have a brick-and-mortar electronics supply shop in your area, shop there. I wish I still had one down the street but those days are over. Any online supplier should be able to fulfill your need. Mouser, Digikey and Newark are reputable, bigtime suppliers and have pretty much everything, but shipping can cost more than the components themselves. Shop around... These caps aren't in the signal path, so you don't have to get anything fancy, at least in my opinion.

No problem, glad to offer my thoughts.

Have you checked that the damper flange screws are all tight? If any of those are loose, they could rattle against the damper rail, and it would explain how you can damp the noise by pressing down on the dampers. Make sure the tongue-and-groove features on the mating surfaces are aligned and use care when tightening these screws (and other wood screws that hold the action parts together) as the flanges can split if handled roughly. Fixing them is fairly easy - wood glue and a small clamp - but best to avoid, of course.

If it continues to be a bother, some audio and/or video would be helpful. It's hard to say, but it is notable perhaps an encouraging sign that you can make the sound go away by pressing on the damper levers.

Welcome to the ol' forum!

Hard to say what the problem is exactly without sound/video clips or in-person inspection but it should be possible to hunt this down, considering the conditional aspects you've already identified.

• Does the noise happen even if the sustain pedal is not connected to the piano at all?

• Is there any slack in your sustain mechanism when the pedal is connect but it is not engaged/under tension? I wonder if there could be rattle in the mechanical linkage between the upper end of the pedal cable and the damper rail lift arm.

• Check the tightness of the bolt/nut that attaches the sustain connecting rod to the damper rail lift arm. There should be felt washers on either side of the lift arm that act as thrust bearings for the bolted connection, as well as metal washers outboard of the felt washers.
• Check the metal cap that bears against the big return spring - ensure it is not loose/rattling and that the spring is seated inside the lip of the metal cap.
• Also, check that the wooden block underneath the spring is screwed down tight against the floor of the case.

Don't touch the sustain adjusting nut or cable adjustment inside the sustain pedal right away! Try to find the problem and positively identify it before making any changes to the sustain mechanism.

If the rattle does not seem to be coming from the sustain linkage, the fact that removing the "damper return springs" is another clue - by which I think you must mean the long-neck screws sticking up out of the damper connecting rods? The ones with the rubber grommets that push into metal clips at the rear of the damper levers.

It's possible some of the damper levers themselves are rattling around, perhaps because the damper flange screws are not tight, or it could be something else in the action. For example, those rubber grommets can become quite hard over time, which could increase the noise level of any screws rattling around inside the grommets.

Troubleshooting like this can require patience, and sometimes a few attempts at hunting down the problem. But it is very satisfying to figure it out, even more so when the solution presents itself as being quite simple and quick.

(Yes, Wurlitzers do make mechanical noises and can have odd resonances, but in my experience these aren't always an issue for the player depending on the use context. If it is distracting or audible to players/listeners/recording devices then for sure attempts should be made to mitigate it. Recently I put some reed bar shields into a 720 and found it created a high-pitched resonance. I had to fuss with the tightness of the screws holding down the shields a bit to get it to go away, among other poking and prodding. But hey, I'd rather hunt down weird mechanical anomalies than rework a SMD PCB)

I've also come to understand that excessive let-off can create this sort of double strike. It seems counter-intuitive no doubt. Something to do with the effect it has on aftertouch?

I'd also suggest that it's important to check the entire regulation of the keys in question to be sure there aren't other factors making it more difficult to find a let-off adjustment that is acceptable. It's been a while since I touched a 700, but on the later pianos I work on, if something is messed up with the earlier steps of regulation, attempting to fix an issue only through let-off can become a fool's errand.

Caveat that I'm almost assuredly less experienced with mechanical aspects of piano tech work than Steve. I've been doing this type of work for well over a decade, but primarily as a hobby and occasional paid side gig for friends and acquaintances. I'm sure there's a lot more that I can learn about regulating a Wurlitzer. Part of why I still check in here regularly!

If you're in Canada but close enough to the border, could you have it shipped general delivery to a post office, or to a familiar store on the US side? I've sent small, relatively cheap parts to folks in Canada this way before, when it was convenient enough for them at least. Shipping is cheaper, faster, less hassle, and the items are more likely to actually make it to the destination....

Doc Wurly's excellent website states that some of the early production 200A models had the speakers mounted to the chassis rail and not the lid. If I'm reading correctly, your piano which appears to have the updated amp+preamp would have been produced between October 1974 and April 1975, approximately.
Following the longtime policy of using up parts from prior models, this is a version of the 200A that has speakers on the amp rail, like a 200, instead of mounted on cover. Most known examples have atypical, non-chronological (higher) serial numbers in known range of 91xxx-93xxx, then 99xxx (USA) and 98xxx (Europe, 220Volt).

(86001L-86666L numbers seem to bookend the 9xxxxL run of rail-mount 200A's.)

All evidence suggests the earliest instruments were assembled from parts dating no later than August 1974 (European instruments in 98XXX) range), and from "latest parts" of October 1974 for USA instruments. This run continued until sometime in early 1975 (known March, possible April instruments). Instruments sometimes use up "pre-A" 200 series parts dated as early as January 1974, mixed with parts from 9 months later; so it is important to check as many date stamps as possible to assess the most likely approximate date of a given instrument.

As with some early non-hybrid 200A's, the hum shields are cut for the prior amp's heat sinks, and may fit poorly. Some do not have aluminum-tubed transformer wires (as with 200 series).

Around May 1975, production switched to instruments with speakers in lids, and numbering picked up in the prior 86XXX range.
I have a 200, and having worked on a number of 200As, I far prefer having the speakers mounted to the chassis rather than the lid. It's an additional annoyance to contend with from a service perspective, in terms of the speaker cable connector, as well as not having the speakers right there while working on it with the lid removed. Plus the lid is heavier and when it is removed from the piano, and the center of mass is lopsided so it's easier to knock it over.

If you want to have the speakers in the lid, I'd suggest drilling new holes to match the proper/factory mounting pattern. Original-style mounting screws with the decorative flower-shaped heads are available from online retailers.

Whether you bother to fill the old, wrong-pattern holes is up to you. Plastic plugs in various sizes are widely available online for low cost. Personally I would go that route rather than epoxy simply because it's faster, no-mess, and easily reversible if you don't end up liking it. A dab of adhesive could address rattling or the potential of falling out.

My 200 has had four extra holes drilled in the left and right sides of the lid since forever, probably to attach some sort of "upper deck" platform for another keyboard. It doesn't bother me, but I guess I don't have to look at it all the time while playing, as it is in your situation.

Lastly, if you really wanted to make it factory or just dislike having the speakers attached to the lid, you could source an original 200 amp rail and new production 200-style speakers. I don't know if it's OK to name retailers here, but both of them have pinned posts at the top of this forum.

You might eventually find an old 200 amp rail for cheaper on the auction site if you keep an eye peeled for it. I used to have one, but chopped the lengths to install the 200 amp in a 140B that was missing all the original electronics. This was back in the days before aftermarket, turnkey 140B amps hit the scene.

That's great that reflowing solder joints helped resolve some of your problems. Some were never done properly at the factory, and on top of that, gig wear and thermal cycling over many decades can only add to the potential for issues in this regard.

The reed bar shields do make a noticeable improvement when added to a model 200, in my experience. Well worth it, IMO, if for nothing else than that they significantly reduce hum when you have the main hum shield off to adjust the tuning or action.

Re: volume being low. Could be something with the amp but it could also be that the trimmer pot for the pre-amp gain is adjusted too low or has some dirt/corrosion affecting signal continuity. That is the part that looks like a plastic wheel, soldered to the amp PCB to the right of the reed bar input RCA jack. Mark the current position somehow and then try adjusting it to your taste. Too much gain can introduce distortion, but you might like it a little crispy sounding.

(Edit to add: if the signal crackles when you adjust this trim pot, it could benefit from some contact cleaner. you might spray some in there anyway, even if it doesn't seem to be a problem)

The main trem parts are as follows:
TR-4 - driver transistor
C27,C38,C44 - nonpolarized poly/film caps, all .12mfd/75V
R32,R33,R43 - resistors, various values - R32 is the primary one for adjusting the tremolo frequency

Check the function and connections/continuity of the trem potentiometer first. If the pot isn't working or isn't properly connected to the amp, the trem won't have any effect on the output.

The resistors and caps might be fine but they are cheap and widely available. I wouldn't bother replacing the transistor before doing the other components, unless you happen to have spares on hand already, or are placing an order with a specialty supplier that has these transistors.

Old-style carbon comp/carbon film resistors can drift in value with use and age. If they drift far enough from spec, it can detune the oscillator network causing it to become unstable and cease functioning. In some cases, replacing the resistors might be all that is required to get the trem working.

Adding that another reason for massive hum could be bad or missing ground connection of the pickup output. If you disconnect the reed bar from the amp, you can remove the pickup noise from the equation as part of troubleshooting.

But you definitely need the hum shield, and I recommend adding the reed bar shields to any 200. Those make a big improvement in noise floor, IMO

+1 To Steve's questions - where is the hum shield?

From the factory, a 200 would lack the reed bar shields that are located between the dampers and the pickups, as found on a 200A, but either model should definitely have the larger L-shaped hum shield that sits over top of the dampers. If your piano has neither shield, you are going to have a massive amount of hum like this - and no amount of money spent on a replacement amp will solve such a problem.

I've attached images of the hum shield and reed bar shield to better identify what I'm referring to.

As to the tremolo circuit, there are a few typical reasons why it might be malfunctioning on a 200-style amp:

*Caps and/or resistors in the trem circuit that have drifted in value to the point that the oscillator is longer stable. these passive components are cheap and easy to replace. You may need to try different resistor values to dial in the trem speed once you get it working. It can also make a "thumping" sound if it's not quite tuned correctly.
*Tremolo driver transistor is kaput
*Could be a bad potentiometer or else an issue of continuity in the wiring to the pot
*Some combination of the above
*Possibly none of the above

[new]

Maybe there is a generous soul still lurking here with a bin full of these cables in good condition. But if otherwise: just buy a new cable. There is one vendor on [the big music gear listing site] and one on [the big auction site] selling them for around $50 before shipping - don't pay $130 or whatever some of these internet bozos want for rotting vintage junk.

I guarantee you won't cry about the price a year from now. It's like having a sports car in the driveway but you don't wanna pay for the ignition keys, meanwhile your investment is reduced to an enormous paperweight. Not to put too fine a point on it, but if you are worried about the cost of the power cable, you're not gonna like the price of replacement reeds either.

IMO, folks should not be using a mains power cable that's ~50 years old anyway - get a new production item for the sake of safety if you are not going to mod the piano with a standard IEC320 inlet.

From what I've observed over the years, these cables used to be expensive because they were hard to find, and now they are expensive because Wurlitzer prices are through the roof. Can't win.

FWIW, this is also known as a PH163 connector. I have some old Hewlett-Packard test equipment from the '60s and those use the exact same type for the mains inlet.

I can confirm the wrench I have is 3/16" as well. If you are in an area where it's easier to find metric tools, 5mm would probably work fine. In my experience there usually isn't enough torque required to where I'd worry about stripping the wrench flats with a slightly too-big wrench.

On that note, my wrench is actually slightly oversized and also tapers out a little bit at the opening. To be honest, I can't remember if I modified it myself to be like this! It does seem to help get it more easily seated on the capstan flats, given the obstructed visibility and limited room to work.

Absolutely! I hope I didn't come across as too harsh, or judgmental of your capabilities. I just feel it's better to strongly advise caution on this subject, rather than encouraging folks to jump in head-first when the consequences are potentially lethal.

With some basic instruction and guidance from an experienced mentor, you can absolutely gain the knowledge, techniques and skills necessary to safely work with hazardous voltages.

This thread has been dormant for nearly 8 years, and the member who first posted hasn't logged in for over 3 years! I'm not a forum mod, but I believe you were correct to start a new thread with your question about wiring for IEC conversion.

Anyway, don't assume that *any* existing wiring is correct. "Assumption is the mother of all..." See my reply on the other thread, please don't reply here.

First, if you don't know what you're doing with electrical wiring, don't mess with it. Especially when it comes to mains voltages. You could injure or kill yourself or someone else, or you could start a fire, or all three.

If you have to ask if green is ground, then you have some homework to do, and you should ask for assistance from someone with experience.

But for the sake of providing some useful reference info, which readers would do well to independently confirm instead of relying on a forum post:

Black (US standard) or Brown (int'l standard) is hot or "line"
White (US) or Blue (int'l) is neutral
Green (US) or Green w/ Yellow Stripe (int'l) is earth ground

Wurlitzer EPs with what appears to be original wiring most likely adhere to US standard, but you can't be certain, since they are all old at this point and may have been altered since originally manufactured.

IEC 320 C14 receptacles (aka "power inlet") are often labeled with L, N, and ⏚, which correspond to the conductors noted above, but may not always be labeled.

Attached is a C14 pinout diagram as seen from the normally-visible side. Of course, the terminals on the other side will be mirrored from this view.

Note also that the original oval-type power inlet is lower-profile than a typical C14 inlet, and the wooden case may need to be modified to fit the substitute component, either in the original location or somewhere else.

I've installed a Borish 200B for client - they wanted it put into a 140B tho, which, aside the need to fab a custom hardware solution with zero support from the vendor, we both determined we didn't care for the sound of the stock lowpass/DC blocking filter circuit. As I've not auditioned one with a 200-series piano, I can't be sure if that's just how the amp sounds generally speaking, or if the fact that it was a 140-series model made any difference.

Anyway, I changed out the wima film cap for a different value to raise the filter cutoff frequency and I think it made a significant improvement to the sound. Fortunately it is fairly easy to tweak this component value, as it's a through-hole part and not SMD like pretty much every other part on the Borish preamp board.

In considering the options for this client, before choosing the Borish we also looked at that viva-analog post, but the info shared was a bit unclear to me and the long string of ALL-CAPS discussion in the comments was hurting my brain to try to follow.

For my part, I can't call this approach a "passive" conversion - a bias voltage is necessarily applied to the pickup, else it does not produce any electrical signal on the output whatsoever. There's no such thing as a passive Wurlitzer EP, because it is unlike a guitar or passive Rhodes that produces signal via electromagnetic induction. My preference is to refer to these types of amp alternatives as a "minimalist bias supply/preamp"

If more ventilation holes are added near the bottom, wouldn't that help with the air flow? I don't mean just randomly adding holes in the back, but towards the amp on the bottom.

There's already a hole by the amp at the bottom. Adding more holes near the bottom/inlet won't add to the overall ventilation capacity unless the exhaust/upper open area is increased proportionally, or if it was already larger to begin with. Putting a hole on the other side where the amp is not located would be less effective or even counterproductive in terms of establishing convective airflow over the amp/power supply.

For passive convective ventilation, if the inlet and exhaust are not equal in open area, then the ventilation capacity is roughly determined by the smaller of the two.

The rule of thumb I use is 1 in² of open inlet AND exhaust area each, per watt. We can use nameplate power consumption to be totally sure, but the affordances can be shrunk significantly if we are able to confidently calculate or estimate the overall efficiency.

For example if there's a 100w device  that operates at 85% efficiency, there's 15w of waste heat to vent, so you need at least 15in² of inlet and 15in² of exhaust, with the inlet low and the exhaust high and on the opposite side of the components that require cooling. The positional arrangement generally optimizes convective flow.

Ground loop hum is common with solid state Wurlitzers, in my experience. The intensity can vary depending on the particular scenario - what it's plugged into (and what else is plugged into that as well that shares a ground plane) single vs. multiple mains supply circuits for various gear involved, etc.

Doing some troubleshooting as discussed in thread here is good to sort out, for example if there are particular scenarios that are worse than others. Recently I was getting a lot of noise on a new recording setup, and it turns out the source of noise was a mixer I was using to submix the drums mics going to another mixer feeding a tape machine. I had to put a ground lift on that piece of gear to quiet things down, but it required making changes one-at-a-time to narrow in on the noise source and solution.

With Wurlitzers, I typically just deal with it in live settings, and if I need a lower noise floor for a recording, I use a ground lift plug. From a standpoint purely accounting for electrical safety, this is not recommended. However, my personal assessment is that there aren't really that many grounded/conductive bits on a Wurlitzer 200/200A that are exposed to the player, other than the sustain pedal. It's not quite the same risk, IMO, as a microphone or guitar strings that come into direct contact with skin.

Caveat: I am not an electrical engineer nor licensed electrician, but I do work with electrical safety design and assessment as part of my job. Do not follow my example without first carrying out your own assessment and decision-making process.

[A]

No updates on the panel design/tests yet but I did want to correct what I wrote at the top of this thread. It turns out I do have a 720A - I was mistaken because the nameplate has "720" printed upon it in the typical fashion, and then the letter "A" stamped into the metal. I didn't notice the "A" before crawling around down there with a flashlight at some point recently.

For reference, the serial is 40690, so that seems to put it on the earlier end of "Series 1" 720A production, according to Doc's model rundown. That would jive with a possibility that they were using up old "720" nameplates and just stamping the "A" into them, unless that's how all those were done across the entire period of production prior to the debut of the 720B.

I can look at the transformer and speaker date codes at some point to add some data. Sorry my thread is drifting slightly, topic-wise, but I figure it's an OK place to add some model data.

Adding bigger/more holes near the amp is probably not optimal with this design. The warm air rising off the amplifier then exits the enclosure through the two upper slots due to convection. This establishes an airflow current which draws fresh air in through the bottom vent by the amplifier. It's a passive ventilation scheme that behaves somewhat like a fan blowing air over the amp.

If the lower hole is enlarged without a corresponding increase in the open area of the upper vents, then it probably won't make much of a difference in the amount of cooling, and could potentially reduce the cooling factor if for example it creates turbulence that disturbs the convective flow.

Think about how a wood stove/fireplace works - you need enough chimney height so that it "draws" else if it's too short then you might end up with a room full of smoke/hot gases.

@wordsandsigns apologies if I missed this being addressed previously, but have you noticed if fussing with the volume knob has any affect on the "fuzzy" sounds? My first hunch with fuzzy or crackly sounds from any piece of older or heavily-used gear with a volume knob is that the potentiometer has some crud inside of it, or some other factor that would mess up the continuity of the signal.

With a Wurlitzer EP, "fuzzy" sounds I hear have often traceable to dirt/crud/moisture in the pickup and reed/ground interface, but it sounds like you're not getting signal from your pickup.

Another place I've noticed issues, particularly in my 720 (non-A, non-B, with tube amp), is at the RCA jack for the pickup. Fussing with that connection sometimes makes all kinds of horrible sounds come out. I've tried contact cleaner but I think the next step for me is replacing the entire wiring harness and connector hardware, which some pro techs recommend anyway as a means of reducing noise floor in this model (along with relocating the power switch away from the volume pot and down to the rear panel of the amp)

So if your RCA cable is original you might consider replacing it with a new one. But before I would bother with that, I'd double check all the solder joints in this part of the circuit to make sure none of them are iffy.

I was thinking about this some more and it makes sense there are reports 145 amps can fail in the portable models if left on for extended periods of time - there's barely any ventilation affordance in that cabinet! There's the one array of slots on the bottom face (below the speaker, on the other side of the cabinet brace/bulkhead from the amp) and then the cutouts for the power cable, fuse holder, extension speaker and phono jacks.

There's no upper vent to encourage passive convective cooling, and I doubt the painted steel lid is efficient enough at radiant cooling to do much.

In the 720, at least the hand-hold cutouts are above the amp, encouraging convection flow. I'm thinking to test the potential difference in sonics, I could just shove a piece of plywood up against the back face of the console. Anyhow, that didn't stop me from whipping up a quick model, screenshots attached.

I can lasercut this out of butcher paper or cardboard to check the fit before bothering to cut some masonite (even though that stuff is cheap, we don't generally keep a lot of it around the shop space that I have access to)

If it fits well enough, I'll be happy to post a drawing and a dxf so folks can more easily make their own if they like. This layout hews close to the original examples I've seen photos of, in that the amplifier is mostly covered up except for the rear panel connections. I thought about making the cutout taller so more air could flow over the tubes etc but then I'd also want to enlarge the upper slots to match the area of the lower opening.

I've read a couple of Doc's posts here about the 145 tube amp being susceptible to transformer failure, especially (according to them) if left powered on for long periods of time:

https://ep-forum.com/smf/index.php?topic=10145.msg56515#msg56515
https://ep-forum.com/smf/index.php?topic=9541.msg52622#msg52622

My 145/720 amplifier has a replacement output transformer but the power tx appears original. I suspect it's just as likely the output was blown due to operating the amp without a speaker load as it is to have failed due to overheating under ordinary use. The transformer mortality rate is a bit of a tangential topic but worth citing when thinking about this. From your story it strikes me as possible and likely that a number of back panels were removed (and then lost) due to a perceived need for more ventilation. It is also likely that many back panels were removed and lost for less intentional reasons.

In pursuing this idea I'd apply the same principles I use when designing equipment enclosures at my day job. Since the amp is in the bottom of the cabinet and there are already vents/hand-holds at the top, passive convection is the existing and preferred solution here (else it needs fans, etc). I use a formula that relates open area of the ventilation affordances to the amount of power that needs dissipating, in relation to ambient temps and a target operating max temperature. I am considering the surface area of the wood enclosure to be a non-factor in terms of radiant cooling.

So it may well be a good idea to increase the ventilation affordances, and I'd have to do some measurements and calculations to assess the design needs.

FWIW, adding more/enlarging the outlet vents doesn't matter unless the inlet open area is correspondingly large enough to avoid restricting the convective air flow. As an analogy, if you're inhaling air through a straw it doesn't matter if you exhale it normally, you're going to notice something feels a bit unusual...

Had some time this morning to hunt around for images and found a few in places I hadn't seen before. None of these are of a 720 (or 720A) with the tube amp, and I'm not sure whether or not Wurlitzer would have designed the panel any differently for the solid state variants.

The 726B that is pictured was shown on the Tropical Fish Vintage site and later sold on Reverb which included a lot more photos, which is where I ultimately found that image. Note the extra cutouts for the student model multi-pin plugs:

https://reverb.com/item/5692439-wurlitzer-720-electronic-piano-with-vibrato-in-walnut

I also found a 720B listing on Reverb that shows the back panel in full:

https://reverb.com/item/12335086-wurlitzer-720b-vintage-electric-piano

Also found back panel images from a couple examples of the model 700, which seems to indicate Wurlitzer may not have added any extra ventilation for the tube amp variant, but I can't say with certainty:

https://reverb.com/item/80893036-wurlitzer-700-64-key-electric-piano
https://reverb.com/item/84021807-excellent-wurlitzer-700-serviced-and-tuned-includes-unobtanium-bench-and-music-stand-extremely-rare (image is actually from a CL ad for the same item)

This is probably enough for me to go on, though if anyone has an actual 720 or 720A with tube amp and the original back panel, it would be nice to confirm whether there was any design variation such as additional ventilation etc.

Thanks for the input!

I'll add that if I'm able to get enough intel to reproduce the original design, I'll gladly produce and share out a technical drawing and CAD assets for others to utilize.

Greetings all, for a long time I've been thinking about fabricating a back panel for my 720 (non-A/B - it has a 145/720 tube amp and the nameplate says only "720")

However I have never been able to find any image of any model 720 variant with a back panel in place.

Of course it's an assumption that it was even provided with a rear panel to begin with, but there are screw holes in places that make me suspect it was the case. As well, in my ancient digital copy of a xerox of the 700 service manual, there are a couple of grainy B&W photos which show there is indeed a back panel on that one. Those can be found on pages 700-4 and 700-5, where the procedure for removing the top cover is explained.

In all the digital copies of the 140/720 service manual I have seen, there is no mention of this component nor any photos.

My interest in this is both for the sake of keeping dust out of the action and reedbar area, possible enhancement of EMI shielding, as well as the possible changes in frequency response that come from having a more fully-enclosed speaker cabinet. My 720 has a 12" speaker, FWIW. Of course, port size and geometry can have an effect, and I'm not sure whether or not Wurlitzer would have concerned themselves with this or not.

I am guessing any full-size rear panel has to have some cutouts for hand-holds underneath each end of the dividing panel between the speaker/amp area and the action/reedbar area. As well as a cutaway for access to the amplifier. These together would provide some airflow for cooling of the amplifier, and I'm wondering if there were other features such as additional ventilation or access cutouts.

I could design something from scratch using these common sense notions but I am interested in seeing what the original design actually looked like. I also assume it was probably made from masonite paneling, 3-6mm thick, in keeping with the typical conventions of the era for back panels of audio electronics cabinetry, but being able to confirm any of this would be great.