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.
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.