Wurlitzer 930 auxiliary amp

[Kevin Odhner]

I have one of these 930s that works great but it's noisy.. a pretty loud 120hz hum/buzz. I saw a post on here from about 10 years back regarding one of these rare birds and am looking for someone that has one who is willing to share a photo of the actual circuitry under the amp.

I've had great luck working on these amps by comparing mine to pictures of the actual circuitry.. I've spotted cases where a prior owner wired something wrong.. but when I look at the schematics I just don't know how to interpret them.. 

Like they say, a picture is worth 1000 words! 

Thank you for any help you can provide.

[velo-hobo]

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.

[Kevin Odhner]

Thanks for this very helpful information.

The first difference I notice is that the 930 schematic has a separate control box, complete with an on/off switch, a pilot light, a two prong ac cord and a grounded 1/4" jack - but grounded to what?

Instead of a box, my 930 has a floating jack and a two prong ac cord combined in a 6 pin Amphenol socket (only 4 pins used). It sounds like something a prior owner rigged, but actually it looks kind of professional - I always assumed it was factory, but now that I see the schematic shows a control box I'm thinking maybe this is a hack job after all, and maybe explains my hum/buzz.

In the amp itself, the green and black lines going from the Amphenol plug go directly to the circuit board with no grounding that I can see.

So do you think if I just install a jack and ground it to the chassis that might solve my hum?

Thanks again!!

[velo-hobo]

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.

[vanceinatlance]

Wow! I have looked for info on these in the past, but never found much until I ran across this post. Really neat to see schematics! Here are some pics from mine if it helps you out. Sorry if I attached too many.

[velo-hobo]

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.

[Kevin Odhner]

A treasure trove!! Thank you.

My much simpler floating jack only had two conductors, so I'm not sure how to ground it, (see picture). Also, I'm not sure what good it would do, since a 1/4" audio plug (TS) only has two conductors as well (inner multi-strand wire soldered to the Sleeve, and outer braided multi-strand wire soldered to the Tip). 

I'll experiment with a test wire from the amp chassis to various points on the floating jack and see if this solves my buzz/hum problem - if so, I'll mount a proper grounded jack in the cabinet, and perhaps a grounded AC plug as well, and do away with the duel purpose Amphenol connection altogether.

Please let me know if either of you think any of this sounds like a bad plan, as I don't really understand grounding..

Thanks again for all your help

[velo-hobo]

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.

[vanceinatlance]

Glad the pics are of use. I am not sure if this is a 930, there is no identification plate to be found. I have the control box on the speaker handle just as a convenient place to put it until it gets properly attached to a piano. I am guessing there are two input possibilities on mine? There is another metal box i found loose in the case that has a 1/4" jack with what I assume is a volume potentiometer that is wired to the side of the amp case. There is also the round "amphenol?" plug with cable I assume is a direct connection for the piano. I have not reviewed the schematics to confirm anything yet. I am not an image expert, but I will try and add some better quality photos.

[Kevin Odhner]

Yes, sorry to give you a scare with my dumb comment about trying various grounds - I'm less reckless in my actions than with my comments. 

What I did - with some success, is connect the 930's 1/4" jack with a Wurlitzer 270, the 200a in a walnut butterfly cabinet. The amp is identical to the 200a, but happens to be factory fitted with a 3 prong grounded AC cord.

With this combination, the hum got a LOT quieter.. so just being connected to a grounded instrument did the trick.

I think I'll call that a win - thanks guys!

[velo-hobo]

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 😂

[pianotuner steveo]

I don't know if this will help or not, but I once had a 720 with a loud hum/buzz, and after much trial and error, it turned out to be the volume control was worn out. Hope that helps.