JC-120 in need of a lot of love

[laurie]

The 2SC1628 high voltage driver transistor is blown. Very hard to find an original. Am on the hunt for an equivalent.

[Pepe]

Meh. That's really dumb that this iconic amp has been used for taking out spare parts.

[bigtone23]

Starting the board clean-up. Quite a few components missing - looks like it had been harvested for spares.
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Board cleanup1.jpg

Dang it. That's always the catch, something was used as an organ donor.
Hopefully something pops up that will work. If anyone can figure out a work around, it's you!

[laurie]

Even more interesting... someone had jumpered the high voltage rails to the low voltage rails.

On the schematic ... B1 jumpered to B3 and B2 jumpered to B4.

It looks like someone who had no clue was "trying things" to make it work.

So far there isn't too much wrong with the channel 2 board I'm working on, just a couple of blown resistors and transistors. But I bet that's why the chorus doesn't work - the JRC4558 opamp is likely dead on the FX board.

[Pepe]

Even more interesting... someone had jumpered the high voltage rails to the low voltage rails.

On the schematic ... B1 jumpered to B3 and B2 jumpered to B4.

It looks like someone who had no clue was "trying things" to make it work.

[laurie]

OK, I'll admit I was wrong.

The board hasn't been harvested. I found the service manual finally and the board is multi-purpose (can be used for either channel) and for the board I'm working on, those components are actually not needed.

Burned areas on the board are cleaned up and sealed with superglue (keeps the moisture out of the PCB).

There were a few resistors gone, but they are now replaced and I'm in the process of swapping all the electrolytics.

Only one transistor blown that I can find. Will order tomorrow.

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Board cleanup2.jpg (148.09 KiB) Viewed 1431 times

[laurie]

I just powered it up (with only the main board in place) and checked the power supply voltages - all good. Plus checked the reverb - all good.

There is hum, so the filter capacitors are tired. Was going to replace those anyway.

Checked every active component on the board above (transistors and diodes) and apart from the one known dead one on the board, plus the dead output transistors (on the chassis), everything else tests good.

It is coming along.

[Pepe]

Awesome! Could be a quicker repair than intially thought, eh? I hadn't thought that Roland used multi-purpose boards back in the day. But the power rail thing was not stock, I suppose?

[laurie]

The power rail jumpers are definitely from someone trying a repair. I think maybe they tried to power the whole board with the 14-0-14 low voltage supply, to help faultfinding.

[laurie]

Something interesting... here is a control that is clearly factory installed, but doesn't appear on any schematic, and is not labeled on the chassis.

I appears to be part of the bias circuit for the BBD drive for the chorus. I won't know until i pull the boards to do the capacitor swap and can trace the circuit.
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Unknown1.jpg (138.77 KiB) Viewed 1399 times

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Unknown2.jpg (158.79 KiB) Viewed 1399 times