JC-120 in need of a lot of love

[laurie]

Parts came from Mouser today. Will slowly do the repair and cap replacement over the next few weeks.

[laurie]

The board that was out of the amp is now repaired (but not yet tested). All burned components replaced, re-capped. Signal path caps now metallized polyester. Electrolytics all replaced with higher voltage, lower leakage.

Burned out driver transistor (center of board) is replaced. Had to do a dance with the device legs - different pin-outs to the original.
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Board cleanup3.jpg (261.09 KiB) Viewed 10431 times

[Pepe]

Burned out driver transistor (center of board) is replaced. Had to do a dance with the device legs - different pin-outs to the original.

I really don't like that (or understand why) there is no global rule for the position of B, C and E.

Good job!

[laurie]

I'm working through the amp slowly. Decided to check out the input jacks this morning.

One was completely bent out of alignment. I've tried to fix it but wasn't successful (no way to get it completely flat/level again). The pic shows it after I had done some unsuccessful work on it.

Luckily it is the same type with internal switching as the jacks in some pedals, so I had a spare.
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Jack12.jpg (93.2 KiB) Viewed 10402 times

[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.

So this gets really interesting. The amp boards definitely have input for the low voltage supply, but the schematic shows the entire amp board is powered by the high voltage supply.

I pulled the working amp board to do the recap, and it also has the "dodgy jumpers" between high and low voltage supplies.

The "dodgy jumpers" between the high voltage rails and low voltage rails on the amp board are actually factory installed. The only place the low voltage rails actually go is the effect board (correctly).

Damn. Add to that the fact the amp boards are are identical, but with different component sets, and this must be some sort of transition model.
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Board cleanup4.jpg (366.69 KiB) Viewed 10387 times

[laurie]

Power supply recapped. Check out the difference in size between the old main caps and the modern ones (and the modern ones are higher voltage).
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PSU_before.jpg (114.71 KiB) Viewed 10391 times

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PSU_after.jpg (221.29 KiB) Viewed 10391 times

[laurie]

Amp board 2 (the one that was working) recap and check complete.

Next step is to clean the boards with isopropyl alcohol and lube the pots.

After the amp boards are done I'll pull the effects board and do the same, and once all the boards and wiring are out of the chassis I'll clean the chassis, clearcoat the reverb tank to stop the surface rust, and replace the power transistors.

Slowly coming along.
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Board cleanup5.jpg (169.45 KiB) Viewed 10371 times

[Pepe]

Slowly? Haha! It is getting repaired in a much faster way than we all expected after your first postings.

When I repaired my BOSS PH-1 I also used pure isopropyl alcohol to clean the board. The solder side became very sticky with it. Do you dilute it for this purpose, or do you avoid the solder side getting in touch with it?

[laurie]

Slowly? Haha! It is getting repaired in a much faster way than we all expected after your first postings.

When I repaired my BOSS PH-1 I also used pure isopropyl alcohol to clean the board. The solder side became very sticky with it. Do you dilute it for this purpose, or do you avoid the solder side getting in touch with it?

The trick is to use enough of it... a small bath of isopropyl, and a paintbrush.

[Pepe]

But only on the component side? Or both sides? I use a toothbrush.