New Member / Hello! / Roland JC-55 Question

[Taylorplayer]

Hello to all -

Very happy to have found this forum! I was recently given an 80's Roland JC-55 amp. A way cool little amp! It was DOA when I got it. On-half of the main fuse holder was missing. A quick trip to my local Radio Shack resolved that. So, then I had AC power but it was still dead. I then checked and found two small fuses blown on the PC board. I replaced those and it came back to life. I cleaned all the pots and got rid of lots of dust. The amp works pretty good now, with one exception. There is a 3 way switch to the far right side of the top front control panel that controls the "chorus" function. One way it's "fixed" - the other it's "manual" - and the center position should switch it off completely, leaving a very clean sound. The problem, however, is that in the center position the volume drops dramatically - almost to the point of being non-existent. Any ideas on how to proceed with troubleshooting this problem would be most appreciated!

Thanks in advance,
Will

[Jazz P Bass]

Roland Jazz Chorus 55-77 schematic

The first item on your agenda should be to locate why the fuses were blown.
As this is a 'stereo' amplifier (two output sections), I would bet the one amplifier is shorted.
The output transistors on the non chorus amplifier should be checked for a shorted condition.
That would be the 'Speaker Out/ Left' on the schematic.
Here is the schematic.

[52 Bill]

As this is a 'stereo' amplifier (two output sections)

The first thing I'd check would be speaker polarity/phasing. It's a stereo chorus, when it's turned off if the speakers are out of phase there will be drop in volume.

Try reversing the wiring to one of the speakers.

[Taylorplayer]

The first item on your agenda should be to locate why the fuses were blown.
As this is a 'stereo' amplifier (two output sections), I would bet the one amplifier is shorted.
The output transistors on the non chorus amplifier should be checked for a shorted condition.
That would be the 'Speaker Out/ Left' on the schematic.
Here is the schematic.

Thank you very much.

[Taylorplayer]

Thank you.

[Taylorplayer]

This is getting interesting, as this amp only has one transformer... (?)

[52 Bill]

This is getting interesting, as this amp only has one transformer... (?)

Why do you think it should have more than one?

Solid state amps usually do not use transformers in the audio circuits like tube amps do, so the only transformer is in the power supply.

[Taylorplayer]

Why do you think it should have more than one?

Solid state amps usually do not use transformers in the audio circuits like tube amps do, so the only transformer is in the power supply.

Because I misread one of the above posts. My mistake on that issue.

Could someone please explain to me the best way to accomplish this task? "The output transistors on the non chorus amplifier should be checked for a shorted condition.
That would be the 'Speaker Out/ Left' on the schematic." I do know how to read a meter, and I'm OK with reading schematics.

Thanks very much,
Will

[52 Bill]

Could someone please explain to me the best way to accomplish this task? "The output transistors on the non chorus amplifier should be checked for a shorted condition.

It's possible that you have a dead power amp, but I'd think that if there were shorted outputs, the fuses would continue to blow. Is there sound coming out of both speakers?

The output transistors are off of the main board and are mounted to the chassis near the back edge. They sometimes have push on connectors, but I think on your amp they are soldered to small breakaway pc boards. The schematic show that the wires connected to terminals 13-18 connect then to the main pc board.

This test is done with the amp unplugged and left off for a few minutes to let any voltages drain down.

If your meter has a diode test function setting use that. If it doesn't use a low ohms setting. The transistor has three terminals the Base, the Collector and the Emitter. On transistor Q9 these are connected to terminals 13, 14 and 15 respectively.

Find the terminal number 13 and follow the wire to the transistor. This is connected to the transistor's Base. Attach or hold one of the meter leads to this pin. Now touch the second meter lead to the other two transistor pins connected to terminals 14 and 15 and note the readings. Now reverse the two meter leads and note the readings. If the transistor is working you should get a low reading with the leads connected one way and then get a high reading when the leads are reversed.

The final test is to test from the Collector to the Emitter pins and then reverse the meter leads. Note the readings if any.

Now repeat the process with the transistor Q11 that connects to the terminal 16, 17 and 18.

[mozwell]

Does your JC55 have one speaker, or two.
Here is the JC50 schematics for both the 1979 & 1983 versions
Check the power amp rails, are they close to +/- 36VDC

[Taylorplayer]

Does your JC55 have one speaker, or two.
Here is the JC50 schematics for both the 1979 & 1983 versions
Check the power amp rails, are they close to +/- 36VDC

Mine has two speakers. Thank you for the information!

[Taylorplayer]

It's possible that you have a dead power amp, but I'd think that if there were shorted outputs, the fuses would continue to blow. Is there sound coming out of both speakers?

The output transistors are off of the main board and are mounted to the chassis near the back edge. They sometimes have push on connectors, but I think on your amp they are soldered to small breakaway pc boards. The schematic show that the wires connected to terminals 13-18 connect then to the main pc board.

This test is done with the amp unplugged and left off for a few minutes to let any voltages drain down.

If your meter has a diode test function setting use that. If it doesn't use a low ohms setting. The transistor has three terminals the Base, the Collector and the Emitter. On transistor Q9 these are connected to terminals 13, 14 and 15 respectively.

Find the terminal number 13 and follow the wire to the transistor. This is connected to the transistor's Base. Attach or hold one of the meter leads to this pin. Now touch the second meter lead to the other two transistor pins connected to terminals 14 and 15 and note the readings. Now reverse the two meter leads and note the readings. If the transistor is working you should get a low reading with the leads connected one way and then get a high reading when the leads are reversed.

The final test is to test from the Collector to the Emitter pins and then reverse the meter leads. Note the readings if any.

Now repeat the process with the transistor Q11 that connects to the terminal 16, 17 and 18.

Yes, there is sound coming out. Thank you for the detailed reply.

[g1]

If you are positive that there is sound coming out of both speakers, then have you tried reversing the wires on one of the speakers as 52Bill suggested back in post #3 ?
And I am only asking if you are positive about sound coming out of both speakers because it can be difficult to tell when they are close together in a cabinet. One speaker can cause a "dead" speaker to move along with it, giving the illusion that it is also receiving power. This is the idea behind "passive radiator" speakers.

[Taylorplayer]

If you are positive that there is sound coming out of both speakers, then have you tried reversing the wires on one of the speakers as 52Bill suggested back in post #3 ?
And I am only asking if you are positive about sound coming out of both speakers because it can be difficult to tell when they are close together in a cabinet. One speaker can cause a "dead" speaker to move along with it, giving the illusion that it is also receiving power. This is the idea behind "passive radiator" speakers.

If I disconnect one set of speaker wires, should the other speaker still work? Or do all (4) leads have to be connected? I have disconnected one set, and one speaker still works. When I disconnect the other set, however, I get no sound at all. Could I safely reverse the leads - taking the set from the one speaker that does work, and then using them for the other one (that may not be working)?

Once again, thanks!
Will

[52 Bill]

If I disconnect one set of speaker wires, should the other speaker still work?

Yes the other speaker should still make sound.

Could I safely reverse the leads - taking the set from the one speaker that does work, and then using them for the other one (that may not be working)?

Yes, that would test whether the problem was in the amp or a dead speaker. Just be careful not to have two sets of wires hooked to one speaker at the same time. Also be careful not to short the two wires together.

Make a simple diagram to be certain that you can reconnect all of the wires back to their original places. Most speaker terminals are marked with either a dot or a plus and minus sign to indicate polarity.