Reed bar makes only a buzz

[Jezza]

Hello all,

I have a 112 that I'm working on little by little.  New tubes and power supply caps got my amp working again, although it's still a little buzzy.  However, the reed bar doesn't seem to be working.  I'm listening to music through the amp right now, but when I plug in the reed bar all I get is a buzz.

I'm pretty sure I'm plugging it in correctly.  There is only 1 ohm of resistance between the bar and the chassis ground.  The pickup portion has about 130V, which is probably close enough to the 140V the schematic says should be there.  Installing or removing the shield over the pickups makes no difference.  I'm getting no sound from it at all, but it looks so simple that I can't figure out what's wrong.

It must be something simple and stupid, but I can't see it!

Thanks in advance.

Jezza

[pianotuner steveo]

It sounds like a reed is shorted. Look for a loose piece of solder or something else that is conductive,( dust,dirt,pet hair, lint, a paper clip, etc)  or a reed is off center and is shorting. If the amp works fine when you unplug the reeds, then you know it is not the amp.

This also happens in cold weather from condensation, but that shouldnt be the case here.

Be careful of the high DC voltage.

Check impedance of the reed bar with it disconnected from the amp and the power OFF. It should not be shorted, it should be open between the + and -   or very high resistance.... I have not worked on a 112 specifically.

[Jezza]

The impedance of the reed bar is about 20M.  I needed to use a handful of 120 reeds because 112 reeds are impossible to find.  They're just a tiny bit wider so a short is possible, or likely.  I thought I took care of the reed alignment already, but I'll triple check.

Thanks.

[Jezza]

I pulled out a few of the slightly oversized reeds and now I'm getting a mixture of sound and buzz.  I never saw any shorts though, and it's still not fixed, so it's still a little mystery to me.   

[pianotuner steveo]

It sounds like you are on the right track, but something is still shorting between the reeds somewhere. Use a higher power flashlight under the reeds and look down between them, this makes it easier to look for shorts.

Do you see any little sparks in the reeds when you play?

[Jezza]

Sparks when I play... I'm a long way away from that!  This poor Wurli is in many, many pieces and won't be playable for a while.  It's just a dismembered amp and reed bar on a bench right now.   

[Jezza]

I unsuccessfully spent some more time trouble shooting my Wurli 112 today, so I would greatly appreciate the advice of the sages here on the EP forum.  Here is my dillemma:

There is some self-noise in the amp that's too loud for me right now, but it's within the realm of acceptable for something this old.  (I'll fix it later.)

Plugging in a quiet headphone feed does not change the noise floor at all, so I can listen to music through it if I want to.  (The 57 year old 6x9 sounds like 1" laptop speakers.  )

I have two reed bars, one from my 112 and one from a 120 that I'm stealing reeds from.  Both of them appear to be free of shorts.  When plugged in both of them make sound, but the self-noise is exponentially louder and becomes an overwhelming buzz that nearly drowns out the sound.  The 112 bar now reads 70Mohms between ground and pickup, so it's not quite fully open as it should be.  The 120 bar is fully open, but it makes the same noise.

Unplugging them stops the buzz, the amp returning to its own self-noise. 

If a different sound source doesn't create the same buzz then it HAS to be the reed bars, right?? 

I don't know where else to look.  There are no electronics and no moving parts.  I've tried connecting different wires, so it's not that. 

Is it possible that the dielectric is no longer a perfect insulator? 

I really don't get it.  I looks like there's nothing to break or go wrong.   

[velo-hobo]

If I'm accurately interpreting what you've written, you have the amp and reed bar out of the piano on a bench for testing, yes?

If the reed bar is just sitting out on the bench with no shielding whatsoever, there's going to be a significant amount of hum when you connect the pickup to the amp, although you should also be able to hear a signal from the reeds mixed in with that.  Is that the case, or is there no signal from the reeds when you strike or pluck them (just hum)?

If there's no signal, check C3 - it looks to be the coupling cap between the pickup and everything downstream of there.  I would imagine a misbehaving cap or connection there would give you trouble. 

Assuming you have no shorts between the pickup and the reed bar/ground, something tells me the issue lies not with the reed bar but in the signal path between it and the amp, since you say you get a decent signal using the record player input, and especially since two different reed bars produce similar or the same results.

[pianotuner steveo]

I didn't realize this was all apart either.  Shielding and proper grounding will help a lot. Remember, the case is lined with shielding.  Check that cap, and be sure the reed bar ground is connected to a proper amp ground.
70 Meg is not like a dead short, so I don't think that is part of the problem. Did you try swapping reed bars BEFORE you started taking reeds out? If so, was it any different then?

[Jezza]

New development!  Velo-Hobo, you're right that there's something else going on.

The grounding/shielding question is easy to test so I put the amp, speaker, and reed bar back in the Wurli case where they belong.  When I powered it up there was a new and scary buzz coming from the main power transformer much like you hear from old power line transformers on a foggy day.  It was not transformer hum, but rather an electrical buzzing sound.

Back on the bench it's now doing the same thing, so I'm afraid to leave it on for more than a second or two.  Somehow just moving it 4 feet changed something, but I don't know what.     It makes me nervous when the electronics themselves make sound.

To answer the other questions:  Both reed bars were passing both signal and buzz on the bench.  The pickup coupling cap C3 and every other ceramic cap I've pulled out for testing is still within spec.  The electrolytics and tubes are new.  I was running music through the pickup input, not the record input.

I'm afraid I only know enough to get myself into trouble and not quite enough to get myself out of it.   

[velo-hobo]

Hmm, is the transformer noise like electric arcing, accompanied by any smell?  That could be bad - but you'd know pretty fast if it really crapped out because the fuse would probably blow.

If it is just a mechanical noise, check the transformer mounting.  The transformer on my 200 rattled like a small buzzer before I played around with the mounting, now it runs quietly (can't remember if I tightened it up or just nudged it around to keep it from acoustically coupling with the amp rail).  Every once in a while if I bump the transformer while doing other work on the piano, I can hear the same sound come back momentarily.  So it's worth checking for mechanical noise.

As far as measuring cap specs, sometimes a cap meter will tell you a cap is fine, but since the meter is testing capacitance with a very small voltage, it may not be telling you the truth about the cap's functionality (or lack thereof) at normal operating voltages.  Just sayin'.  But the fact that you have successfully run audio signal through the reed bar input is odd.

Have you measured DC voltages around the amp to compare them to the schematic?

Also, with the reed bar disconnected from the amp, what's the resistance between the tip and the sleeve of the pickup's RCA connector?  It should be completely open, as I'm pretty sure the pickup is supposed to be fully insulated from the harp.

[Jezza]

Thank you guys so much for the prompt replies.  I REALLY appreciate it.  I'll check all of this out over the next couple of days and get back to you.

[Jezza]

I couldn't wait and it turns out that the transformer was a false alarm - just mechanical noise that made me nervous.  Many thanks for that. 

The reed bar seems to behave a little better in the case with the shield on, so I'm guessing it's not my main problem.

I checked the DC voltages last week, and they're high near the little 12AU7 tubes.  Farther away from that part of the circuit they're pretty good.

In the main volume knob there is one little sweet spot in the middle where the buzz is cut almost in half.  It's still annoying but much better.  The same is not true of the record player input level, which has almost no effect on the buzz level.

I think that now I'm just chasing a noisy amp problem that's most likely a grounding issue.  I need to go out now, but to be continued...

[Jezza]

PS:  It's not the power indicator light.  I saw that VV sells an LED upgrade for it and so I removed it to see if that was the problem.  It isn't, and it has no effect on the noise level.

[velo-hobo]

Ah, sometimes the volume pot can cause trouble - have you used a contact cleaner like De-Ox-Id or the like on the pots and jacks?  Is the volume knob very crackly when you adjust it?

For example, my roommate has a Fender 75 amp that recently started misbehaving.  The reverb stopped working and the amp generally sounded dull and noisy.  We thought it might be a bad tube or reverb tank, or something else deeper in the circuit even.

Before we decided to really start digging into it, I cleaned all the pots and the reverb and footswitch connections - it immediately sounded 100% better, with minimal effort.  Remember, the pots are part of the signal path so if the contacts are corroded or dirty then you're going to be inviting all sorts of problems.

I've also read here and there that volume pots on wurlies can be a source of hum, though there's never really a thorough explanation of why in any of the accounts I've seen - just that sometimes folks have replaced the pot and hum magically goes away.  Could be that the resistive element is worn out and leaking stray signal to the wiper??  Who knows...

One other question are you using a 2-prong or properly grounded 3-prong plug?  If 2-prong, does flipping the polarity help?  It's inconsistent, but sometimes I get more noise with a wurly with the ground in place and sometimes I get more noise with the ground lifted.  I think it also depends on the quality of the power coming out of the wall in your location.

[pianotuner steveo]

I agree completely- I once had a 720 that drove me nuts with noise and it turned out to be a bad control.
Cleaning that one did not help, only replacement did.

I think the answer is that they are just so old, and worn out on the tube models, and they may have been cheaply made to begin with.

[Jezza]

I cleaned the pots some time ago and they all operate completely silently.  I haven't tested them though, and I will. 

There WERE some very hard to see shorts from the 120 reeds that I put into my 112 reed bar, so I'll have to spend some time with a metal file to make those reeds work for me.  I think the remaining hum and buzz is coming from somewhere else.

I'm still using the original 2 prong power, but it shouldn't matter.  The original design worked well, so it can work well again.  This Wurli is cosmetically in very good condition, so I want to restore it and keep it as close to original as possible.  (Once it's working and in tune I may add a vibrato upgrade though.)

[Jezza]

I have a question regarding the pots.  All three of them read their proper spec values and move smoothly from that value to zero.  (The rec input had to be removed from the circuit for a proper reading, the others read correctly in place.)  Could they still be bad somehow?  They're cheap enough and easy enough to replace, I just wonder if it's worth doing since the meter says they're good.

[pianotuner steveo]

I would simply based on their age.

[Jezza]

They seem to behave like linear pots.  Is that correct?  My schematic doesn't specify.  They seem to be working properly and within spec, though, so I'm not too worried about them.

[Jezza]

I am more suspicious of the old caps.  I wired up some leads to a speaker so that I could put the caps in series with the audio signal, effectively using the caps as part of a crossover network.  All of the new electrolytic caps in the power supply behave as expected, cutting varying amounts of bass depending on their size.

None of the older caps pass signal at all in that test.  The others are all ceramic disc caps.  Could they all be bad caps, or is it not a good quick-check test for ceramics?  Despite the noise it makes, the amp turns on and amplifies audio from both inputs.  I find it hard to believe that all the ceramic caps are bad and yet the amp still nominally functions.

[pianotuner steveo]

 It is always a good idea to replace any cap in a very old amp,as long as you can find suitable values and know what you are doing as far as polarity w/ electrolytics, etc.

This amp is likely older than I am and I have had parts replaced in me! 

[Jezza]

That certainly applies to the electrolytics, but ceramic caps are very stable and fail less frequently.  I think there is no need to replace them unless there is evidence of failure.

[velo-hobo]

Electrolytic caps do tend to fail with age, yes.  Ceramic, not so much, although they are cheap so cost is not a factor in replacing them, just labor which may or may not make a difference.

One thing though, film type caps can go bad, even if they seem fine with a cap meter.  I think this is one of the reasons why vibrato circuits get weak or stop working (the others being resistor drift and failure or poor performance on behalf of whatever is driving it, be it a tube or transistor).

Film caps can also go microphonic as they age.  I recently overhauled a Pianet N preamp, the film caps would make noise if you tapped on them with a pencil.  New ones fixed that.

[Jezza]

Circuits are not my forte and I'm at the extremes of my knowledge and experience with this project.  Any help in locating this buzz is much appreciated.  I have three new clues:

1.  One of my ceramic caps goes from the tip contact of the aux output to one of the little 12AU7 tubes.  That cap, and only that cap, is very susceptible to what is around it.  If I bring a tool or my hand close the hum increases substantially.  I know the last stage is a sensitive part of the circuit, so I don't know if the proximity effect is because of the circuit design or because that one cap is behaving oddly.

2.  I tested the pots a little bit more closely tonight and they seem to be logarithmic, changing quickly on one side and gradually on the other.  Since they're all for volume and not for biasing it makes sense.

There are three pots in this amp, and all three have one end grounded.  The grounds all check out, and the body of the pots are well grounded also.  It appears that the pots are in good shape and not misbehaving.  They all test within spec, but could they also be leaking something to ground?  A couple of them look to be $20 on DigiKey so I don't want to replace them all unnecessarily.

3.  Another piece of the puzzle:  Whenever I give firm pressure with my hands to anything that's grounded the hum dips noticeably in volume.  That happens whether or not my reed bar is plugged in.  Adding a second ground from the chassis to earth has no effect, presumably because my body absorbs the ground leakage and the wire does not. 


On the bright side I have a much easier time with the mechanical things:

The key pins are all aligned and vertical again, key bushings eased.
The action is reinstalled, regulated, and feels much better.  Some lube is on the way.
The old damper felts are removed and new ones are on the way.
The fragmented pedal is glued back together, repainted, cable lubed, and working.
The reed bar is IN TUNE!  Many of the reeds were slightly oversized 120 reeds and not the correct pitch so it involved a LOT of soldering and filing, but it's done.
The odd size aux output jack has been replaced with a standard 1/4"
A new speaker is on the way.

Once I can cure this 60hz hum, better shield my reed bar, and adjust the strike points I'll have a fully overhauled and practically mint condition 1955 Wurlitzer 112 that's ready for another half century of music.

Damn this buzz!   

[velo-hobo]

I you haven't tried a 3-prong cord that may help (really it should be done anyway for safety reasons).

If you want to try seeing what effect it would have without modifying your amp with a new socket, take a 3 prong cord, and at the end opposite the plug cap the hot and neutral leads with wirenuts, and install a ring terminal on the green (ground wire).  You can attach this ground wire to the amp chassis (at one of the mounting screws would be the easiest way to do it).

Then plug the 3-prong cord in to a properly grounded outlet (ideally the same outlet as the other, original power cord).  Because you've got the mains covered up and isolated, it should be safe to do this.  Just make sure the amp is off and everything is unplugged when installing the temporary ground connection on the amp, if there is stray voltage on the chassis it could arc when offered a path to ground.

If using a 3-prong seems to help with the hum, I'd recommend replacing the 2-prong cord with a 3-prong.  It would be wise to do so anyway for safety since there are high voltages in the amp, but in the past I have noticed at least of couple old tube models that produced less hum with the ground lifted (only through the internal speaker - the headphone/aux out was quieter with the ground in place).  They were operating in a more dangerous state but I knew to be very careful and only ever used a cheater plug to temporarily lift the ground, rather than permanently altering the cord or amp.

[Jezza]

Thanks velo-hobo.  Unfortunately, as I just mentioned in clue #3, I have tried exactly that and it does nothing to the hum.

[Jezza]

Correction to #1 above:

There are two caps that are responsive to the surrounding electrical field.  One comes from the reed bar pickup (not the ground) and the other comes from the aux output tip (not the ground).  Both go to different pins on the same little 12AU7 tube.  Putting my hand or a metal tool in proximity of both increases the hum.  They are not microphonic - tapping them with plastic or wood has no effect.

Could they be bad caps or is it just a sensitive preamp stage of the circuit?

[Jezza]

Clue #4:  All of my output jacks pass the buzz to an amp as expected.  However, if I plug in a pair of headphones to the Aux Out then the buzz almost completely goes away.  However, they work like a microphone.  I can tap on the headphones and I hear it in the Wurli speaker.  The Wurli speaker is about half volume with this setup too.

Very weird.  This unfortunately does not cure the buzz in the other outputs.

[velo-hobo]

here's a project 112 amp on the well-known auction website in case you have any interest in monkeying around with trying a different amp:

http://www.ebay.com/itm/Vintage-WURLITZER-Electronic-Piano-Tube-Amp-Amplifier-For-Restore-Model-112-/370688048116

(not my auction)

[Jezza]

(accidental duplicate deleted)

[Jezza]

Thanks, but I have enough problems of my own!

I'm going to change out the ceramic caps for orange drops and replace the pots.  Hopefully that helps.  It's a bit of a shot in the dark for me because I can't actually locate the problem, but the most I have to loose is a little cash and a bit of time.  If that doesn't help then I'll probably send the amp to Ken Rich.  http://kenrichsounds.com/

How many watts should the main volume pot be?  It's meant to be a 500k switched pot, presumably logarithmic.  The only one I can find that looks like the right specs and shaft is this one, which is only .1W.  It's from a guitar series from Bourns.  Since the main volume pot is between the inputs and the first tube it's probably okay, but I want to double check.
http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=121843345&uq=634882855492318619

Thanks guys.

[Jezza]

I noticed that the schematic inside the amp has two differences from the schematic in the manual.  One of those differences is the presence of a capacitor.  It's not in my amp but it's in the manual's schematic, where it's on the 12AU7A that's in the very first input stage.  I'm suspicious of that part of my circuit, so I'll try adding the cap.  It goes from one of the cathodes to ground, parallel to a resistor.

New caps are in the mail.  Fingers crossed...

In the mean time, I have new dampers and the action is lubed up.     This buzz can't stop me completely.