140B electronic repairs

[Kevin Odhner]

I need to replace the capacitors in the two cans. I can read what's written on the outside of the cans.. and the Wurlitzer schematic. Vintage Vibe and Tropical fish sell kits, which I've bought in the past, but in this case, I just want specific components. Can anyone recommend a good source?

Also, any experiences you can share te doing this upgrade would be very much appreciated. I know the new caps are usually added below the can rather than putting them inside.

Pictures of the underside of the amp would be great too, as I am not great at reading schematics and the actual circuitry isn't always consistent with the original schematics.

Thank you!!

[velo-hobo]

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.

[Kevin Odhner]

Thank you so very much - thus is exactly the tutorial I needed. The photos and verbal explanations will get me through this, since I get lost reading schematics.

I see you installed an EIC panel plug on that chassis - I did that too, but I had a few questions you don't mind:

1) I put the live to the line switch, and the neutral to the transformer - is that right or should it be the other way around?

2) Does it matter which direction the fuse is wired? I've seen it both ways.

3) I removed my phono jack and cut a hole for my IEC there. One of the phono jack's posts seemed to served as a bridge for several wires and also seemed to be grounded to the chassis, so I left those wires connected and ran a ground ti the chassis. Does that sound right? There was also a long wire that went from that same post all the way to the other end of the circuit board where the reed pickup connects - I could figure out no purpose for this with the phone jack gone, so I removed it. I see that wire is missing from your board too, which gives me hope that I did the right thing.

Sorry of that is too vague, and thanks  for your patience and any guidance you can give me.

[velo-hobo]

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.

[Kevin Odhner]

I finally got my components from Mouser after and installed one of the the 2200uF caps under the -32v can as instructed.

I also had no trouble installing the 470uF cap under the +32v can..

Then I hit a snag with the other 2200uF cap when I found there were four wires attached to the terminal marked with a half circle, not 2.

It stands to reason that I should cut off all four and solder them to the positive leg of the new 2200uF, but rather than assume, I figured I'd ask and wait for clarification.

Thanks again for all your help!

[velo-hobo]

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... 

[Kevin Odhner]

Well, I replaced the can caps, and I'm fairly confident I did everything right based on your very thorough instructions..

Unfortunately, the caps were not what was causing the malfunction I have been trying to hunt down.

What is frustrating is that I had the piano working several weeks ago and was just trying to eliminate some hum, sone radio interference and sone scratchy pots. 

Somewhere along the line something went very wrong and all I get is a loud noise - like when you plug a bad cord into an amplifier - the noise is not affected by playing the keys, adjusting the volume or vibrato - if I put headphones into the headphones jack, I get the same sound through the headphones. In fact, unplugging the pickup cable from the amplifier makes no difference at all.

I don't know if it's something I did or if some old components failing now either by coincidence or because I disturbed them.  Here is a little history of what I've done, how the malfunction developed and some of the things I've tried - in hopes it will ring a bell and someone will recognize these symptoms..and perhaps a cure:

I replaced the vibrato lamp - vastly improved the strength of the tremolo even the audio signal in general.

I replaced the on/off volume pot - got rid of some scratchiness.

I replaced the neon pilot bulb which I had accidentally destroyed while working on the wiring harness.

I installed components from a Vintage Vibe amp rebuild kit - resistors, transistors, capacitors - not including the can caps. This really didn't make as much difference as I had hoped, but the piano worked, though still a lot of hum.

I then replaced the two prong plug with 3 prong IEC and this is where something began to go wrong. When I first tried it out, the piano worked but I got some loud pops and noises and the signal was cutting out - there were certain notes that seemed to trigger it, so I figured I had gotten something on the reed bar that was shorting it out.

I used compressed air and a vacuum to clean everything out but got no improvement.

Someone suggested maybe it was bad can caps which had never been replaced - I couldn't see why they would suddenly fail now after 60 years, but when I touched the cans there were loud crackles and pops - so I replaced those - now the crackle pop sounds are no longer happen when I touch the cans, but now the malfunction is constant - I can't get any tones from the pickups at all - just a loud noise like something is shorted.

Is it possible I damaged the transistors or the transformer when I cut the hole for the IEC with a Dremel? There was aluminum dust - which I clean off - at least everything visible - but what if some found its way inside the transformer - or could the vibrations from the dremel have disturbed something?

I am totally stumped. Is there anything about what I described that could narrow it down, or binary tests I can do to rule out various culprits, or narrow down my list of suspects?

For example, can I plug something other than the reed bar pickups into the rca input jack on the amp to prove whether it's the amp or the reed bar that's malfunctioning? 

Or can anyone tell be what it's supposed to sound like with the reed bar unplugged?

What would be the symptoms of a bad transformer or bad output transistors?

Any advice you can give me to track this down would be much appreciated!

Thanks,

Kevin

[Kevin Odhner]

I think my last question was too open/ended. 

I've decided to try testing my diodes since it can apparently be done with them in place. 

Are there any other "low-hanging-fruit" things I can check?

I'm getting no signal from the reed bar - in fact unplugging it has no effect..  just a loud 60hz buzz from the speaker (or headphones) that does not change when I change the volume or vibrato.

I assume I'm getting the wrong voltage..

Thanks!