JC-120 Low Volume

[g1]

No problem running it without the mute Fet's. You will find out if the other one is ok or not, or if the control line is ok. Matching should not be an issue.

[AtomicMassUnit]

Pulling that fet did make one side play with full power.

[52 Bill]

Pulling that fet did make one side play with full power.

So that proves that the mutes are not being turned off.

I don't know the pinouts for those fets, but the gates connect to the control voltage through diodes. The other two pins are the source and drain. There should be a low resistance between the source and drain like the 15 ohms that you have. When the fets are turned on the resistance will increase. What voltage is on the gates?

[AtomicMassUnit]

So that proves that the mutes are not being turned off.

I don't know the pinouts for those fets, but the gates connect to the control voltage through diodes. The other two pins are the source and drain. There should be a low resistance between the source and drain like the 15 ohms that you have. When the fets are turned on the resistance will increase. What voltage is on the gates?

If I understand you correctly, the one I removed caused that side of the power amp to work, while the other side's mute is still on, making that side quiet.

What's the purpose of those mutes?

I will measure the voltage at the gates of both in the morning. Thanks again for all the help, everyone.

[52 Bill]

If I understand you correctly, the one I removed caused that side of the power amp to work, while the other side's mute is still on, making that side quiet.

What's the purpose of those mutes?

I will measure the voltage at the gates of both in the morning. Thanks again for all the help, everyone.

Yes, the fet grounds out the signal to the input of the power amp. It's like having an automatic master volume control that turns down the volume until the noises and glitches that are created at power on from the preamp are done making noise.

[g1]

It is also possible that the mute Fet still in the amp is actually the bad one and is pulling down the control line. You could try swapping it for the one you have removed.

[AtomicMassUnit]

Yes, the fet grounds out the signal to the input of the power amp. It's like having an automatic master volume control that turns down the volume until the noises and glitches that are created at power on from the preamp are done making noise.

I've got no voltage at either gate. Also, I'm having trouble finding VB on the power supply schematic to check the source, but Q47 on the JK Board is also attached to VB, and is getting 19vdc.

I have -46vdc at R54 (-VE) and one side of R96/D13. At + side of C61, I have 19vdc. At + side of C60 I have 9vdc. At the -side of C60 I have nothing.

D13 and R96 both test good.

Also, my pilot lamp is not working. I don't have any voltage at the connector. Checking into that...

[g1]

You may need to have both the mute Fet's removed to measure the gate voltage. Any leakage in the Fet still in the amp will pull down the control voltage real quick as it is a high impedance circuit.

[AtomicMassUnit]

g-one, thanks. I have both fets pulled, those measurements are without either of them in circuit.

[52 Bill]

What about D14 and D15? With the amp off try reading the resistance to ground at the junction of R96 and C60. Or for that matter the resistance from each gate pad to ground.

I would expect to see a negative voltage applied to the gates, you need to figure out why it isn't there.

[AtomicMassUnit]

R96/C60 junction is giving me 287k to ground.

Gate pads are 1.9m to ground, and rise from there as I leave the meter on them. When I use my meter's diode check on D14, D15 I get .58v one way, and nothing the other way.

[AtomicMassUnit]

I undid D14, and now I'm getting -25v at R96/C60.

Soldered it back, now I'm getting open resistance to ground at the gates and -35 v on D14, D15. When I take the two fets side by side with the diode tester, they don't measure out the same, I think it's just one of them is bad, and perhaps a hair of solder around one of those pads. I have one on order, so I will reinstall and see if it's all good at that point.

So, on to the chorus troubleshooting. On the Vibrato setting, Rate and Depth work, and on the Chorus setting they don't. Any ideas? I have no clue how the chorus circuit works or how to troubleshoot it. I started by checking voltages VF in a few spots and they're there.

[AtomicMassUnit]

Another thing I'm noticing is, the vibrato works perfectly, and the chorus works, but the rate and depth for the chorus don't. Any ideas there? This thing is a mess.

[g1]

Seems to me the rate and depth for the chorus is fixed in those amps, and the controls are only for the vibrato. Consult owners manual before spending any time on this .
One more thing with the mute circuit, did you check the R96 value out of circuit?

[AtomicMassUnit]

Seems to me the rate and depth for the chorus is fixed in those amps, and the controls are only for the vibrato. Consult owners manual before spending any time on this .
One more thing with the mute circuit, did you check the R96 value out of circuit?

Wow... I spent hours messing with it today, and started going through more threads here, and your post confirms what I read. That is a really dumb feature... holy crap, why not have it adjustable too?

I did pull R96 out and measure it, measured fine so I put it back. I notice with the jfets I pulled, if I use the diode checker on my meter, one of them reads different from the others, and there was some terrible soldering going on here too, which I cleaned up.

While I'm thrilled to be playing through it finally, I will be getting this mute thing figured out, because I want it to be 100%.

This amp was given to me as a project, and has been sitting for a long time. A friend of mine sent it to a local tech who resoldered and hot glued a bunch of stuff, and then he sent it to the state Roland service center and they wanted to just replace the whole main amp board for $380, and he gave up on it.

I've been scrounging up as much broken solid state gear as I can to try and learn how to troubleshoot it, and it's been a big eye opener. I learned a lot from you guys here in this thread and others. So very much appreciated, g-one, teemuk, 52 Bill, soundguruman, JM Fahey, jamesmafyew, melvin.