I have a Roland Jazz Chorus 120 that is stone cold dead. Not even an indicator light or AC hum. The fuse appears good. What should I look for? Are old, dead solid state amps worth anything as fix-up projects?

JC120s aren't worth a whole lot, but they're sought after enough that they're worth more than most solid state amps (they have some of the best cleans you can get, period).

As far as the fuses go, I would replace them anyway, because a bad fuse can look ok, but still be fried. Replace the mains fuse first. If that doesn't work, there are other fuses inside.

***The next part comes with a word of caution, so pay attention***

Guitar amps can have lethal voltages in them, due to large capacitors holding a charge. So be very very careful NOT to touch anything inside the amp with your hands. If you have to replace a component, leave the amp unplugged over night to let the caps discharge, and test in various places to verify that there isn't any remaining voltage in the caps/circuit.

That said. You can go get a cheap digital multimeter from Walmart for like $20. Use it to test wether or not power is getting around the circuit. You should be able to isolate the bad internal fuse and/or component. It's possible that there's a bad internal fuse, capacitor, resistor or diode.

But all of that said, I would guess that it's just a bad mains fuse. And they're cheap to replace.

There are other fuses?

I had to pull the chassis to get to the main fuse. I gather there are different versions over time, and this one does not have an externally accessible fuse. I didn't see other fuses.

I was assuming that there were other fuses. Many guitar amps (bigger ones like that, anyway) have more than one fuse. But if you only found one, that's it.

I think that is the only solid state amp with any value.  I am sure you could flip it on ebay or reverb in it's current state.

This might help you, although it sounds like there were many different revisions over the years.

Looks like there might be two test points marked "120" or "100"  that you can read to chassis ground that would be a good place to start. If these are dead, your power switch may be a gonner

Then check the two points marked "AC" after the transformer. That might tell you something. (all of the above on AC on your meter)

If all of those are hot, check +15, and -15 to G on DC.

ETA:

Found some more schematics

http://music-electronics-forum.com/a...d-jc120-82.pdf
http://music-electronics-forum.com/a...d-jc120-84.pdf
http://music-electronics-forum.com/a...120-160-79.pdf

JC120
Great Amp,
Get it Fixed.

Check that it's not something stupid like a loose wire, broken wire inside the power cord, bad plug, etc.

That... one of the best clean amps ever, and likely the only solid state amp I'll ever lust after.

I'd find a good local amp tech and get it looked at.

the ONLY good SS amp ever made.

Thanks for the replies, y'all, especially the schematics. I just haven't got a meter handy right now. Hoping to poke around next week.

ETA - I found my meter! (we've been moving )

xmission, thanks for the schematics, but they don't match mine. First, there's no power outlet. My SN starts with ZN, not numbers. And, the circuits don't look like that. Of course, I barely know enough to be dangerous. I got the chassis open, and checked the fuse (good), but I don't know where to go from there. The power cord checks good, too. (continuity checker is my favorite function. ) Here's what I'm looking at. There are seven terminals on the little board where the power comes in, where the switch is, before it goes to the transformer. I suspected the switch, but I'm getting continuity between the appropriate (I think) terminals when power switch is on. Could the transformer be dead? How would I check that?   Attached File


Thanks again.

Found some other schematics that look like a match. JC 120 UT?

Link to schematic or a pic of the power supply section schematic?

From your pic, I would put you meter on the two terminals labelled PRI

on the upper portion of the circuit board. Set your meter to AC volts

and turn the unit on. You should have line voltage (120VAC) there when it's on.

If you do, then connect probes to the red wires on the other end of the transformer.

That is the secondary winding. It should also be putting out AC voltage but different than

the primary side, normally less in a SS amp.

As mentioned before, use caution when probing around.

ETA -- Semi-Educated guess..
Attached File

Thanks for the reply. Found schematics for my version here.

The first two terminals on that power input board are labeled gnd - should that have continuity to the chassis?

Can you pop a pic of that whole board to the left of the transformer?

Look close on those big cap. Look for bulges, or white shit like you'd see on a battery terminal on your car.. Good chance those electrolytic caps are your problem ( C31, and C32). It's not to hard to recap something like this.

ETA: KB7DX has you on the right track. Looks like the secondary wires from the transformer go to the small board to the left of the transformer.

You should be able to read  AC from the secondary on the solder terminals KD7DX has marked "Connected" by putting the black lead carefully on the center solder connection, and read one at a time to the ones on either side of it. you should see voltage each way.  Be really careful not to slip and short the terminals together, or you will see sparks, and may cause yourself more trouble.

If you look at the schematic here: file:///C:/Users/druss/AppData/Local/Temp/JC120UT_JT-PMBoard-pg6-7-1.pdf

You'll see that little board in the upper left side of the board layouts. There is a rectifier on there. It feed the terminal block called W3. These are the three solder terminals you will see from the top to the left of the terminals KB7DX has marked "Connected". If you carefully put your black lead on the on furthest to the inside, and red lead on the one closest to the outside of the board, you should read some sort of DC voltage.

That's a good start.

CAUTION: Don't touch anything with your hands, even with the thing unplugged, because those caps can store some voltage.

Looks like the green wire to the chassis on the right side of the transformer that is marked ground does come from the power input board, so most likely, yes.

You can probably keep your black lead on that nut on the chassis when reading all of the above stuff, it might make it a little safer and easier.

Just looked at your first post again.

The power LED comes right off one of the transformer secondary feeds. It's probably the brown wire that snake out under the righ hand side of the little rectifier board, and heads over to the main amp board.

Your problem is more than likely right in that area. There is a cap marked c127 on that little board, that if blown would kill the LED, and would keep voltage from the rectifier, giving you no power.

Agree with caps as likely culprit if fuses? good.

There is a small cap on the small board next to the transformer coming off the secondaries. Pulling a cap out of a board is outside my technical ability. Even if I found my soldering iron (in the middle of moving again), it doesn't have a fine enough tip for a PCB.

The two terminals on the power input board labeled gnd do not show continuity to the chassis. But I don't see how they could, anyway. The ground wire comes off the power cord to the chassis, and there's no lead from chassis to the power board I can see.

View included from farther down the line. Lead for the main power light comes from the main amp board. I see no visual sign of swollen or burned components, although there are a ton of resistors & capacitors.

Attached File

Thanks for the great leads. Sadly, there is no voltage at those primary leads on the power input board (labeled T5 & T6). I double-checked, and there is continuity between those two leads, but when I turn power switch on, no voltage comes across there.

And I just blew my meter!

If it were me, I would disconnect the wires from the T5 and T6 terminals and apply line voltage

directly to the wires with a fuse of course. This will bypass the AC input board and switch completely.

It sounds like the problem lies on the AC input board. There isn't much on that board to troubleshoot.

A switch, fuse and an MOV for protection. Maybe there's a bad/cracked solder joint on the

underside of that board.

ETA - Is the amp's AC input cord a standard plug in 3 pin "computer" cord?

Sometimes the socket it plugs into has a fuse integrated into the socket.

ETA 2 - It looks like there's two terminals to the right of the T5 and T6 terminals

labelled "AC". I assume this is where the line voltage comes into the board. There

should be line voltage there when plugged in. Power goes through the switch and fuse and applied

to the T5 and T6 terminals to the primary side of the transformer.

Good point about the power socket. Possible it might have a fuse built in.

If there is nothing across the primary side of that transformer, there isn't much left, and it should be fairly easy to isolate.

Sorry about the meter. I remember the old days using Simpson meters, and blowing fuses if you forgot to take it off on continuity, and hit power with it :)  There is probably a fuse in your meter somewhere though.

Yeah, just blew the fuse on the meter. Now I have to find one of those fuses with the right amperage to replace, probably after the holiday.

Power cord is hard-wired, no IEC cable.

Thanks, guys!

I still haven't got the fuse back in my meter, but I did contact a vendor, and was told that both the power input board and the power switch are discontinued items and not available from Roland.  I might try a second opinion somewhere else. But a power switch couldn't be that hard to substitute, right?

Yeah, should be easy. Just call mouser.com at 800-346-6873 or DigiKey.com at 800-344-4539 . I've dealt with Digi Key on the phone when I was ordering parts to mod my JVM and they we're more than helpful. The guy I talked to knew right off the bat I was modding an amp from the list I had, and they have a few guitar amp nerds working there.

Got back into this finally. In the diagram quoted, the upper right of the power input board two terminals (labeled T1 and T2) go to ground. Is it wrong to be getting voltage on there?

ETA - closeups of the suspect area, power switch from top & bottom.
Attached File

Attached File

The two solder pads I circled are what's being connected/disconnected with the power switch.

If you bridge these two pads together it will make the switch "ON" all the time bypassing the switch itself.

The T1 and T2 terminals appear to be ground from the looks of the copper trace side of the board.


Edit -- Let me look a little deeper at the schematic for the T1 and T2 terminals. The printing side says GND as you know

so there shouldn't be any significant voltage on them if they are functioning like normal ground connections.

Attached File


Edit2 -- The yellow oval on this pic are the same terminals I circled on the pic above.

If you bridge those two together, you will bypass the switch and AC voltage should appear at

T5 and T6.  C1 is the MOV (metal oxide varistor)(blue disc). It's purpose is to protect the rest of the unit

from voltage spikes. Of course F1 is the fuse.

Attached File