Vintage EH Doctor Q doesn't quack

[yeeshkul]

Guys i am trying to repair an old Doctor Q unit. The sound comes through but no quacking at all.
I have another working unit so i compared the voltages and they seem to be ok - see the picture

blue voltages - working unit
red voltages - this broken unit.

The IC has been changed and socketed before (by someone else - eBay). So the IC is not original.
Do i need any special IC? I've tried RC4558, LM1458 and some other chips so far, but no luck.

I 've checked both with my audio sonde and my working unit seems to have more boost in the input of the
IC (input part) and quacks right after. This one keeps clean sound, just modulated by the trimpot.

[oskar]

My guess is the sensitivity potentiometer. The voltages looks good and you have sound but no envelope.
I'ld start tracing the envelope path from the input...

[yeeshkul]

I measured the 100k pot right on the PCB (with the rest of the circuit in parallel) on both units.
MY working unit measures 100k, the broken one 82k at max.

EDIT: Actually the sound is very distorted and spluttering at the output of the input OA (diodes).
The complete output sound then is clean but it splutters when i play a hard not - i guess you must have been right.
I am just not sure where the problem is ... it seems be like that with all ICs i have tried ... and i changed the input cap (2u2 originally -> 47n)

Do you think the that it can be the 10u elyte between the diodes?

[Mark Hammer]

The first place to look is almost always the rectifier path.  So measure the AC voltage at the junction of the 2M2 resistor and 1N914, then measure the AC voltage at the junction of the second diode and the 25k trimpot, while plugged in and strumming.  You should have some indication of an AC voltage in respopnse to your playing.  If not, then you have a problem with the input signal getting to the op-amp, or something miswired in the rectifier path.

The second place to look is going to be the transistor.  Is there any remote possibility that the pinout is different than you think, or than is assumed by any layout you might be using?

Finally, the Dr Q wants, nay, insists on a 1458.

[yeeshkul]

Mark thank you! I had to stick in a different transistor (2N3904) already as the previous owner messed up the pinout as he was probably trying to repair it. Too bad he sold it as a working unit . Emitter is positively on the ground terminal and base in the middle now. I'll check the ACs now.

[yeeshkul]

I used a Strat and i played one string (A on 5th fret).
AC voltages were like:
Input: 40mV
2M2/diode junction: 120-180mV
diode/trimpot junction: 120-150mV

Audioprobe:
input up to the opamp:  clean
both junctions on the output of the opamp: distorted, gated, spluttering ...

[yeeshkul]

The working unit behaves differently:

AC voltages were like:
Input: 40mV
2M2/diode junction: 200-500mV
diode/trimpot junction: 150-400mV

Audioprobe:
2M2/diode: strong signal, fairly clean with some distortion
diode/trimpot: same as the broken unit - awful, spluttering, gated ..

[Gus]

Use the 1458 like Mark posted, check or change the 10uf cap(between the 1n914s) in the sidechain section.  The 10uf cap is the "storage" part of the sidechain.

When I repair something that was worked on before by someone unknown I check every part and connections first.

[yeeshkul]

I am using 1458, ill change the cap and report here. Thank you.

[yeeshkul]

It was the 10u cap. It dried down to 2nF. Everything back to normal now