Vintage boss T- wah debug

[HeavyFog]

A friend of mine (same one who gave me his muff fuzz to fix that i still haven't managed to get working) recently gave me his vintage 80s Boss T-wah to see if i could get it running again. The pedal turns on, i get bypassed and effect signal, but the effect signal sounds more like a fixed wah toe down. I replaced the peak pot with a new one (original pot was busted) and the pot does lower the peak like it should but i still get a fixed wah sound. The sense pot does nothing at all and i don't think its busted but il check resistance readings and continuity to be sure. To me it sounds like it might be a problem with the envelope filter since its stuck at a high point but i really don't know. The circuit board is in great shape and i couldn't spot any sign of shorts, broken wires, damaged parts or repair work. Perhaps one or more electrolytic dried out?

Here's the only schematic i could find:

[anotherjim]

If the sense pot was bust it would act fixed  - feed to envelope detector comes off the wiper of that pot.
Old fx often suffer broken pots that have been stomped on too many times by oversized feet.
Definitely suspect that pot.

[xaxxop]

A friend of mine (same one who gave me his muff fuzz to fix that i still haven't managed to get working) recently gave me his vintage 80s Boss T-wah to see if i could get it running again. The pedal turns on, i get bypassed and effect signal, but the effect signal sounds more like a fixed wah toe down. I replaced the peak pot with a new one (original pot was busted) and the pot does lower the peak like it should but i still get a fixed wah sound. The sense pot does nothing at all and i don't think its busted but il check resistance readings and continuity to be sure. To me it sounds like it might be a problem with the envelope filter since its stuck at a high point but i really don't know. The circuit board is in great shape and i couldn't spot any sign of shorts, broken wires, damaged parts or repair work. Perhaps one or more electrolytic dried out?

Here's the only schematic i could find:

Hello, I got the same issue on my aw-2. Any joy?

[Mark Hammer]

Have you played with the trimmer at all?  It adjusts how the LED responds to the input signal.

[Kipper4]

May i suggest you start a new thread.
It seems your aw maybe different schematic to the T wah.
Post up a schematic please. Take some voltages.
Go to the debugging page and give us as much info as possible. So helping you to help yourself.
I'll gladly help as much as I can.
Rich

[duck_arse]

anotherjim, if you're watching - how's the led drive work in the tw-1, is that a current mirror, or is the opamp turning on robbing the led-connected transistor's current? [and does the led action reverse with npn substitutes?]

[edit :] dumb money on me. it looks like two transistors differencing the single emitter resistor's current. I wonder what that config would be called ....

[anotherjim]

I glanced, I suppose that counts as watching, what with today's short attention spans....
I too recognise a current mirror. The op-amp has that invert/not invert switch thing.

If you want to use npn, I guess you must turn the mirror pair upside down, with the common "tail" resistor to ground and the LED up at +supply. The emitters still have to be coupled I think. Oh, and that little trimmer into the control base would be wired up to +supply instead of ground. Off top of head that, so take with pinch of saline.

[HeavyFog]

Sorry everyone i had had university exams and i couldn't do many  pedal related things, but now that that's out of the way i went back and took another look at the old twah. I measured continuity and resistance on the sense pot at various positions and both checked out just fine, so that may not be the issue. Il see if i can get some good pictures of the inside and of the pcb. On a side note i did notice that the on off switch isn't working so great. Its only working maybe a third of the time. Not sure if that could have anything to do with it but Il take a look at it. Like i said before the pcb and wiring appear to be in great shape and don't look like they've been modified or worked on at all. Apparently this pedal has been broken since the 90s so this might be a bigger challenge than i thought. I can post voltages if i need to

[Tony Forestiere]

Have you played with the trimmer at all?  It adjusts how the LED responds to the input signal.

What he said.

[HeavyFog]

Have you played with the trimmer at all?  It adjusts how the LED responds to the input signal.

What he said.

Ya i adjusted it a for a bit and it didn't seem to make a difference

[maiko]

I have a twah as well and it is even older (i assume) than that of the two dual op amps it uses a single 3403.   No effect at all
ive tried swapping out the quad for another but when i insert the 3404 (from the wah) to another project via a socket it works
so i know the quad is good.   also tried swapping the transistors its the same.   checked all the caps everything ok.  hmmm very
frustrating

[anotherjim]

Time to do some testing with a dmm.
I'm assuming bypass works and when the effect is active, the filter sounds as though it's doing something, but it don't wha or quack.

I'd check if it trying to sweep first. This means clipping test leads on and playing something. In the diagram above (2 chip variant)
IC2 pin 7 is the sweep signal. Depending on the switch up or down selected, the voltage there should kick up or down when you whack a chord. The Sens' pot should control how far the voltage sweeps, so try it at both extremes of that controls range.
Also check for sweep on the collectors of Q4 and Q5. This will depend on the setting of the trimmer control Vr3 - so you can check that.

If sweep action exists, my suspicion falls on 2 special parts, the coil/inductor and the vactrol.

The coil could fail open-circuit - it is very fine wire and may be seen to be soldered at 2 of the 4 pins of the coil former. You might remove it from the board if you're careful, so you can confirm the coil by measuring its resistance. You might measure with it in circuit (and power off), but the peak control pot + 4.7k is across it and will confuse the reading. If you disconnect one wire of the peak control pot, the resistance reading should pretty much be that of the coil.

For the Vactrol, if you take one leg of its output LDR out of the board (the one connected to IC1 pin1 will do), then connect the meter (Resistance setting) to that leg and the adjacent one (connected to IC1 pin2). When playing, you should see the resistance changing. Probably going high around 1M ohm and maybe down to a few K ohm - depends on the particular type of vactrol.

[HeavyFog]

Finally getting back around to fixing this thing and i'm still lost. The version i have has a single chip that i cant find any info on labelled "rayc7945" and "rc340sa DB". Not sure what the ic is but i'm assuming its a dual op amp. Like i mentioned before nothing looks wrong and no one seems to have attempted to fix this thing before me. I can get voltage readings if needed.