JC-120UT Right POWER AMP channel not working,

[Mr_bibbles]

Hey y'all,

I have a JC-120UT (the new one) that is not putting out signal from one speaker, but the symptoms are very strange.

1. Both speakers OK, checked, pass sound no problem.
2. Low volume on both Preamp Channels
2. Line out OK, both sides, both preamp channels.
3. SEND only works with Channel 2, Channel 1 does not pass through. ???
4. Amp sounds for like the appropriate volume for half a second on startup, then is quiet. Also it occasionally pulls a ton of juice through the light bulb limiter on startup until I cycle it off / on again....
5. DC offset of 50mv on the working channel.


So, the weird part is that, SEND doesn't work with Channel 1, but the preamp channels are not linked to L / R power amps. This means we are dealing with 2 seperate issues at least, not counting the DC offset? Hopefully someone with some better SS amp experience can point me in the right direction! Thanks guys and gals.

[Enzo]

Unless this JC120 is radically different, the preamp channels are one thing, the power amp channels are another. Usually on chorusing amps, the preamp - either channel - is a mono signal, and is fed to both amps for both speakers. Most chorusers use stereo only while chorus is on. Typically they send the dry signal to one amp and the chorused signal to the other. A chorus pedal mixes a chorus effect with the dry and sends it to the amp, but a chorusing amp like this has two speakers with different signals, so it makes a real swirling sound as the wet and dry signals beat against each other.

So in some cases one amp fails and the mono comes out just the other speaker and sounds more or less normal. But turn on the chorus, and we then either get only the dry still or only the effect side, depending which amp is faulty.

In your case, I am not sure what I need the FX board drawing for yet. Power amps would be good to have.

I would see that both power amps work, or does one have a problem. Do the FX returns go right to the power amps? Are there power amp in jacks? Hmm I see there are not. SO do both returns function or not?

I see the send jack is clearly labelled as CH2. Many amps have FX on one channel and none on the other. But the returns are labelled L and R.

[g1]

it occasionally pulls a ton of juice through the light bulb limiter on startup until I cycle it off / on again....
5. DC offset of 50mv on the working channel.

I don't think you have any need to have it on a bulb limiter, and it may just cause confusion troubleshooting. (unless you have much more DC offset on non-working side)

[Enzo]

Agree, the bulb is probably why it sometimes wakes up goofy. 50mv offset is nothing.

[Mr_bibbles]

Whoopsie daisy, here's the rest of the Schematic. I'll also remove the light bulb limiter as there is 0mvdc output on the second channel.

1. RETURN R-, Extremely low (less than a whisper) volume in both +4 and -20 positions
RETURN L- Same extremely low volume in +4, but much louder signal through -20????? Definitely not JC120 loud, but it is audible and seems to at least pass to through to the speaker. Chorus makes no difference in any position.

Note- The +4 and -20 do not work correctly with the FX loop bypassed with a patch cable. It seems a couple DB louder in the -20 position.

That makes a lot of sense for the "fx loop" only being used for Channel 2. I see that IC1a/IC1b are in the return section next to the limiter, here are the measurements:

1. 8.9v
2. 8.9v
3. 8.9v
4. .7v
5. 8.9v
6. 8.9v
7. 8.9v
8. 17.8v

I wasn't able to find VB+ on the schematic, so I measured q38, q39, q47, q48, and found they also have 17.8v where VB+ should be. Since the line out works, we can rule out IC 1?

Am I right in thinking that the effects loop is the right place to start due to the complex switching and the malfunctioning DB limiter?

Thanks y'all.