Grief trying to fix a vintage Blubber Maxon Wah

[Iplayloud]

Hi guys, it's been years since I posted. I'm an amateur trying to figure why this wah loses 80% of volume when engaged. I attached the schematic. The inductor measures 30 ohms and I replaced the only electrolytic cap (10uf).

Q1
B +0.69v
C +4.44v
E +0.10v

Q2
B +2.74v
C +8.91v
E +3.27v

[Iplayloud]

I tried all the obvious stuff like bypassing the switch, doubling all ground points with alligator clips.. It's like an inverted volume pedal now. From what I read on other threads is it must be the inductor or the 10uf cap. If the inductor was dead I would read 33k. They say the inductor should read under 100 ohms. Mine reads 30 ohms.

[The Dude]

Are you sure the readings you posted for Q2 are correct? The transistor is biased off. Base voltage is lower than emitter.

[Iplayloud]

Thanks, yes the readings are correct. From what I read Q2-b should be about 1.1v lower than Q1-c (3.4-3.6v). Also the 470k resistor just before the Q2 base is the one that bias Q2. What do you recommend, putting a 1M pot and twist until I get the correct voltage?

[52 Bill]

If the pedal acts like a volume control, then my guess is that the second stage is working. The control pot sends an out of phase signal back to the input, so if there is a volume change the second stage must be working.

The problem would be in the tone altering filter network with the coil, the 33K resistor, the 10 uF cap and the 100K resistor. I know that you said you checked all of the ground by bypassing them, but did you check for continuity between components?

[Iplayloud]

I'm sorry the problem isn't exactly what I said. When I hit the switch, there is a huge volume drop, I have to crank the amp way up. Then there is a wah effect AND an inverted volume pedal.

Now I bypassed the 470k resistor just before the base of Q2 with a pot, I'm down to 200k total to get the base of Q2 to reach 3.6v and pass above the emitter voltage, which also crept up as the base voltage went up. In that state the wah still has pretty much the same problem except the volume doesn't shut all the way down in upwart position. I'll unsolder the caps now and measure individually.

[Iplayloud]

Okay!!! Bingo, I decided to pull each component off and test them outside the circuit, then reinstall them. Now it's finally working, even though I had tested for continuity between each parts.

One thing now is the voltages for Q2 are still far from textbook. Should I leave it just like that? I would really like to bring it to spot-on values. Should I play with the 10k emitter resistor to keep that emitter voltage down to 3.3v While I bring the base voltage up?

[Iplayloud]

Can somebody give me a quick hint about biasing the Q2? Should I play with the emitter 10k or collector 1k resistors? Thanks !!

[52 Bill]

If it's working, stop fixing!