Voltage before resistor (between resistor and ground) measures 43V, after resistor with ground 0V. Voltage measured parallel to the resistor is 43V.
Tomorrow I will disconnect the PCB to measure the big caps there.

Hi Captain,
Thanks a lot for your elaborate reply; it really helped me understand the circuit better.

Unfortunately my multimeter can't measure AC mV, so measuring the mV values that are on the schematic won't work (for now), I fear.
As the pcb is still mounted I could not measure C144 and 145, but I did measure R195 (2,7K 0,5W), that should supply C145 with -15V (right?) It measures +45V on one side and 0 on the other side that should pass the -15V to C145. I guess that means that it's fried? The + and - 43V supplies work fine.

throughout the chorus circuit I do measure +15V correctly (although it reads little over 14V, which shound be fine?) but at R109  don't measure -15V. Could that be the problem? As I don't see that many -15V points in the parts of the circuit that are connected with the clean signal path, would it be logical that the clean signal path is fine but the fx signals don't work because -15V is missing?

Hi all,

I hope it's ok I ask this here because of all the other Jazz Chorus questions I found on this and other forums already. However, none of them describe the issues I'm facing with my newly acquired JC-120. It's probably a late 70s/early 80s amp, serial 176578. Toggle switch for the effects and bright switches. The BBD IC is a MN3004.

I was told the effects did not work, and they were right. Chorus, reverb and tremolo are all not working. Channel 2 works fine though, clean sound and even (a tiny bit of) the distortion. When I engage the chorus, a very soft 'pop' can be heard, indicating that something is switching. However, no chorus sound. Vibrato switch does not make a sound. Both speakers and power amps work fine.

A solder joint in the reverb tank was loose so I fixed that. Still no reverb. I suspect some connection in a part of the circuit that all effects share is faulty. Does anyone know a certain region or connection in the schematic that could be the source of this problem? Schematic (I think it's the right one) is attached.

I will start tapping solder joints soon, but I would like to ask first if there is something obvious I should check. On first sight, I don't see components that might be burnt or broken. I don't own a scope but I have a multimeter and I can solder. However, I don't understand the schematic well enough to see where a possible fault could be. Hope someone can help!

Cheers,
Rens

Edit/solution:
- disconnected legs 3 and 4 of IC5 (op-amp) that were connected during a previous faulty fix by someone; this restored the -15V supply.
- Replaced R195 in the power supply section for good measure, as its resistance was too low and might have blown something.
- Replaced transistor 2SC1624Y with a 2SD726 as the former turned out to be broken
- Replaced Zener diode D6 to restore +15V supply

Everything worked after this.