Jet City 20 combo Troubleshooting

[flatrockmobile]

I need some help with troubleshooting a JCA2112rc.

I've had this since they first came out. About 2 years ago it started cutting out as in volume would suddenly drop but would return only after turning it off and letting it cool off. Sometimes it would go for months at a time without a problem but then it would happen again. I finally pulled the circuit board about a year ago thinking maybe a cold solder joint and re flowed all joints. While doing this I noticed where they trim off the excess leads with some type of grinder, one piece of a lead was cut but only leaning over and extremely close to one of the preamp tube pins under the edge of the socket. Thinking this might be making contact after heating up, I removed it, buttoned up everything and played away until a few days ago, it started again. This time, every time. Sometimes after a few minutes, sometimes after a few seconds.
I changed out the phase inverter and the power tubes with new and set the bias, but no luck in fixing the problem. One by one, I replaced each preamp tube with known good ones, also no change.

When this volume drop happens, I still have all the distortion characteristics in the sound which led me to think it was the power section.
Also when it happens, the presence control still affects the sound as does gain, master, reverb and treble, however, the bass and middle controls have absolutely no affect. I've also checked each resister and all are within spec. I'm at a loss at this point. Any help would be appreciated.

[Mick Bailey]

Hi and welcome to the forum.

Maybe the bass and treble controls are the clue. With a stock FMV tone stack (I'm making a big assumption here as I don't have the schematic) if the anti-clockwise side of the treble pot (looking from the front) becomes disconnected, the bass/middle controls stop working and all the signal is then routed through the treble cap (220pf or 470pf). The treble control works, though not as well as it used to. The volume drops drastically, but controls except bass/mid still work. My thought is that either the treble pot is faulty, or one of the legs has a bad solder joint.

Can you post the schematic just to confirm what tone stack is fitted?

[1ampman]

Does the pwr transformer have thermal breaker? Get the chassis on the bench and monitor the supply voltages till you find the weak one....

[flatrockmobile]

Here are the schematics
JCA2112RC PRE.pdf
JCA2112RC POWER.pdf
Thanks for the replys

[Mick Bailey]

Ok, FMV tonestack, so check the treble pot VR3. I'm thinking your signal is only passing through C8 when the fault occurs.

[flatrockmobile]

I removed the treble pot but it ohmed out good and seemed to be sweeping normally. I had another so I installed it. While I was in there, I removed the bias trim pot. I had read that others were having problems with it on their models of Jet City amps. I measured the resistance and across the outside terminals, it was open. It would sweep between the 2 grounded terminals but not the 2 that mattered. I soldered in a normal B50k pot, fired it up, set the bias, then played for about 20 minutes without any issues. I'm keeping my fingers crossed. Thanks for all the help.

[1ampman]

Are these surface mount circuit boards?

[Dave H]

I hope the bias pot has fixed it.

Are D2 and D4 missing from the power amp? if they were fitted they would short out the transformer B+ winding.

[52 Bill]

This is the worst example of schematic Chinglish that I've seen so far. The tubes use the wrong elements and in the first preamp tube the two halves of the tube are intermixed. If it wasn't for the pin numbers, you wouldn't be able to figure the circuit out.

[flatrockmobile]

ampman - no, they are regular components mounted on the pcb.

Dave - All 5 diodes are in the circuit and I have a strong B+ of 379v. What I don't understand about the bias trimmer is that I was able to set the bias with it. It must have been breathing its last breath. I do hope this has fixed the problem. I played it for almost another hour last night without a problem.

Thanks again for all the help.

[Dave H]

This is the worst example of schematic Chinglish that I've seen so far. The tubes use the wrong elements and in the first preamp tube the two halves of the tube are intermixed. If it wasn't for the pin numbers, you wouldn't be able to figure the circuit out.

Check out the EL84s on the power amp schematic!

[Justin Thomas]

The first new tube design in 20 years: the ECC84! Watch these new "character display tubes" light up in fireworks as you put 300+VDC on the grids, oh so close to the cathode, which is grounded at one end and has 300VDC on the other end! Hooray!

Justin

PS throw that schematic out; it's useless.

[Mick Bailey]

I'm still suspicious of the original treble pot, or the solder joint. If the bias pot fails and there's still some sound coming through, this wouldn't account for all controls except the bass and mid operating. Maybe you fixed two problems?

[J M Fahey]

Is this a joke?

Who drew that piece of crap?

My 8 months old granddaughter can do a better job.

Who gave you that "schematic" ?

[flatrockmobile]

Mick - Unless the volume dropped so low that all I could HEAR was the treble pot affecting it, but you're right, I might have fixed 2 issues.

Justin - Where does the ECC84 come from? It clearly says EL84.

JM - I can't remember where I got them, but here's the head version of the same amp, just no reverb circuit. They look pretty similar to me.

JCA20H service schematic.pdf