Finally fixing the vintage boss t-wah from hell

[HeavyFog]

Back in April i posted a thread about a vintage boss T-wah a friend gave to me to fix. Lucky for me he's patient and doesn't mind at all that i still haven't got the thing working, but i don't want to bury the nail and fix this thing for good. I didn't want to revive a dead thread so here's the original http://www.diystompboxes.com/smfforum/index.php?topic=117397.0

The pedal is a vintage T-wah from the mid 70s (i think) and according to my friend it hasn't worked since the 80s. He doesn't remember how it broke, all he remembers is that it just stopped working one day, without being dropped or having beer spilled on it.



The problem is that it doesn't sweep the filter at all. No matter how the sens pot or the up/down switch is set the wah stays stuck at a full toe position with no sweep at all. The peak pot was broken when i got it but i replaced it and the pot worked to control the peak level of the wah, but the problem stayed the same. I did notice a few things when probing it with a DMM (with the sense pot 3/4 open):

-The voltage straight from the power jack drops from 9.6vDC (plug measurement) to 7.6vDC (from the jack lugs). Not sure if this is normal or not.
-The ground lug from the power jack doesn't test for continuity with ground with or without the power plug in. (Added a wire to test it and it didn't make a difference)
- The voltage on the - leg of the led side of the vactrol remains fixed at 1.56v even when playing. The trimpot lowers that voltage to 1.47v or a bit lower but it always stays the same.
-I measured the voltages on Q4 and Q5 and none of them changed as i played anything.

I wouldn't suspect damage by a bad power supply either since he always used batteries. When i opened it there was no visible damage or repairs done to it, so i know it hasn't been messed with before i got it. Judging from the reading i got and the sound it makes i'm guessing that the envelope generator isn't working but i'm not sure why i doesn't work considering the circumstances. If anyone has any insight or ideas i'd rally appreciate it because i'm stuck.

[Transmogrifox]

The first thing you can do is replace all the electrolytic capacitors.

--The ground side of the power adapter input has the reverse-connection blocking diode in series with 470 ohm resistor.  This explains why you don't measure continuity to ground.  The 470 ohm resistor accounts for the ~2V drop from the mains, assuming the load is about 4 mA (and this seems reasonable).

-- The - leg of the LED side of the vactrol should be 0V with respect to ground, but based on your report of 7.6V on the power rails, then I would expect about 2V there...why the difference between 2V expectation and 1.56V, I can't easily guess what you're using for ground reference in each measurement since the ground connections to the power adapter in this circuit are funny.  It will be different depending whether you're measuring to the power jack or audio jacks.

Does anything change if you switch the up/down switch?

Can you get a change by alternately shorting C15 + side to ground and to power?

Back to my first statement, failure of electrolytic capacitors could definitely cause these symptoms.  A lot of other things could too, but with older circuits like this sometimes the electrolyte has dried up and/or the capacitor has failed short.

If all is working correctly, you will see the direct change to your playing at the output of IC2 at R32. 

It seems unlikely to me, but it is entirely possible the Vactrol is bad.  I would expect that changing the trim pot will make an audible change in the sound, so that could be a lead for you to follow.  Measure across R35 (2.7k) while you adjust the trimmer and see if the voltage changes on it.  This will give you a clue there is current going to the Vactrol LED.

Also measure across the Vactrol LED + to - while you adjust the trimmer and see if you can observe any change while doing that.

If you have a guitar hooked up I expect you will hear a change when you adjust the trimmer if the Vactrol is ok, but who knows?  Maybe the trimmer itself is bad.  Maybe you should also be looking at the voltage at the base of Q4 while adjusting the trimmer to make sure it's changing there as you turn the trimmer.

Anyway there are several ideas that might help the light bulb come on for you.

[Slowpoke101]

Your other thread mentioned that your unit is using a single chip and not two chips as shown on the service sheet. The single chip is a standard quad op-amp (If I recall correctly - hopefully). A LM324 could be a suitable replacement or TL074, MC3403..almost any of them. It may be helpful to measure the voltages on the op-amp's pins and post them here.

And as Transmogrifox suggested - the electrolytic caps are to be doubted (I have replaced many of them in Boss pedals and others over the years). Also make sure that the inductor is not open circuit. Another thing to check is to make sure that the sweep (drive) switch actually works and hasn't corroded its contacts.

[HeavyFog]

Thanks for the replies, il try and get some voltages next time i get the chance to work on it. The switch doesn't make a difference in the overall sound or to the measured voltages. I'm beginning to think it might be one or more dried up electrolytic at fault. If its just one or 2 caps in the envelope generator at fault then that would explain the problem and why i have a strong dry signal. Either way IL be replacing them all as soon as i can get some. At this point i refuse to let this one go unfixed

[HeavyFog]

The first thing you can do is replace all the electrolytic capacitors.

--The ground side of the power adapter input has the reverse-connection blocking diode in series with 470 ohm resistor.  This explains why you don't measure continuity to ground.  The 470 ohm resistor accounts for the ~2V drop from the mains, assuming the load is about 4 mA (and this seems reasonable).

-- The - leg of the LED side of the vactrol should be 0V with respect to ground, but based on your report of 7.6V on the power rails, then I would expect about 2V there...why the difference between 2V expectation and 1.56V, I can't easily guess what you're using for ground reference in each measurement since the ground connections to the power adapter in this circuit are funny.  It will be different depending whether you're measuring to the power jack or audio jacks.

Does anything change if you switch the up/down switch?

Can you get a change by alternately shorting C15 + side to ground and to power?

Back to my first statement, failure of electrolytic capacitors could definitely cause these symptoms.  A lot of other things could too, but with older circuits like this sometimes the electrolyte has dried up and/or the capacitor has failed short.

If all is working correctly, you will see the direct change to your playing at the output of IC2 at R32. 

It seems unlikely to me, but it is entirely possible the Vactrol is bad.  I would expect that changing the trim pot will make an audible change in the sound, so that could be a lead for you to follow.  Measure across R35 (2.7k) while you adjust the trimmer and see if the voltage changes on it.  This will give you a clue there is current going to the Vactrol LED.

Also measure across the Vactrol LED + to - while you adjust the trimmer and see if you can observe any change while doing that.

If you have a guitar hooked up I expect you will hear a change when you adjust the trimmer if the Vactrol is ok, but who knows?  Maybe the trimmer itself is bad.  Maybe you should also be looking at the voltage at the base of Q4 while adjusting the trimmer to make sure it's changing there as you turn the trimmer.

Anyway there are several ideas that might help the light bulb come on for you.

Did some more testing today. Shorting c15 gave no results other than bypassing the pedal when shorted to the power. Also measured voltage at r32 with no results, just a fixed voltage. On r35 its the same deal except on one side the voltage jumps by about 0.1v consistently if the trimmer is adjusted right. Its really starting o look like the envelope generator isn't firing. Il try and get some chip voltages as soon as i can.

[HeavyFog]

UPDATE: Iv'e done a few more tests on it. I noticed the pedal is marked on the rear plate that it needs an ASA power supply, not a modern psu style power supply. I corrected this by bypassing the diode+resistor from the negative lug on the power jack but with no change to the sound or voltages. I also tried 12vDC with and without the diode+resistor and still no difference. I also replaced a few of the electrolytics with new ones but still nothing. I didn't replace c2 and c24 (0.47uf electro), c12 (1uf but shouldn't be a problem since i'm getting through signal) and c15 (polarized tantalum might be 35uf but i'm not sure). c2 and c15 are both highly suspect as they both look to have a direct roll in the envelope genereator so il make sure i replace them both. I can't seem to find any electrolytic 0.47uf caps, so might i be able to use a regular unpolarized cap in place of c2?

I also tried shorting the legs of the opto-resistor side of the vactrol together and surprisingly i got a bit of drop in filter frequency, so i now that the wah part of the circuit is working somewhat, really enhancing my suspicion that the envelope generator is at fault here.

I swear this pedal is testing my sanity.

[HeavyFog]

Still no luck with this thing. While working on taking out and socketing electro caps c14 and c2 i found that soundwise there was no difference with or without the caps, which i guess could mean that the caps were dead to begin with or something else. I also measured the opto resistor side of the vactrol and i found that the resistance is changing slightly by about 10k-15k when i apply a high signal from my moog. It hovers mostly around 320k and doesn't change at all without an input signal applied. The overall measured resistance changes a bit depending on how the trimpot is set and i found with the trim set around halfway the change in resistance was greatest by a few k. The datasheet says the dark resistance is about 1M so it looks like the led might be stuck on. I should also mention that the sound was hardly different at all regardless of this resistance so there's a chance that the filter isn't sweeping at all. No even sure if a change in resistance that small would even trigger a noticeable change if the filter was working noticeably. I haven't swapped the c14 and c2 yet but i did try a few other random value caps out of desperation with no success. If c24 (right above Q2 were dried up would the filter even work?

[Rob Strand]

I would start at the Vactrol and work back.

Start provoking each part of the circuit until it stops.

1) Apply current manually to the LED.  If you hear no sweep at all then maybe it's the Vactrol.  I'd then put a pot across the Vactrol and see if you can sweep it manually.

2) Connect a 10k resistor to the base of Q4.  Connect the other end to 0V and then to +V.  You should hear a change in the filter.  If not the problem is around Q4, Q5.

3) Connect a 100k resistor to pin 5 of IC2.  Connect the other end to 0V and then to +V.  You should hear a change in the filter.  If not the problem is around that part of IC2.

4) Connect a 22k resistor to pin 2 of IC2.  Connect the other end to 0V and then to +V.  You should hear a change in the filter.  If not the problem is around that part of IC2.

5) If you get this far there' something wrong between C2, the sense pot and C14.