JC-120 in need of a lot of love

[laurie]

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

I just did both sides, but only because the component side had a film of grease.

Usually I do the solder side. It will be a little sticky for a few hours, then it dries out.

[laurie]

FX board cleaned and recapped.
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Board cleanup6.jpg (219.38 KiB) Viewed 8989 times

[laurie]

All the caps and other items removed so far...
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JC-120 recap.jpg (212.63 KiB) Viewed 8989 times

[laurie]

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

Well yeah... there was less damage than I thought, I was able to find equivalent components, and got super lucky with the speakers.

Still, I think it will take about 100 hours over many weeks.

[Pepe]

[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.

Unknown1.jpg (138.77 KiB) Viewed 8976 times

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I've traced the circuit and the potentiometer that doesn't appear on the schematic is definitely chorus depth. When in chorus mode, the chorus LFO drive coming out of the 4558 passes through the potentiometer into the BBD drive oscillator (the four transistors to the left) and the potentiometer allows adjustment of the magnitude of the LFO signal.

For future reference if anyone finds this thread...
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JC-120 External potentiometer not on schematic SML.jpg (236.17 KiB) Viewed 6816 times

[laurie]

Cleaned up the reverb tank. Removed the surface rust and clear-coated it to stop any future deterioration.

The wiring has old-school construction. The connectors are RCA, but instead of using plugs, the shielded cable is run into the tank through the center hole in the RCA jack, then soldered. MUCH more reliable than plugs, and twice as long to build.
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JC-120 reverb tank1.jpg (141.61 KiB) Viewed 8810 times

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JC-120 reverb tank2.jpg (275.34 KiB) Viewed 8810 times

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JC-120 reverb tank3.jpg (186.66 KiB) Viewed 8810 times

[Pepe]

"MANUFACTURED BY BEAUTIFUL GIRLS IN MILTON, WIS. UNDER CONTROLLED ATMOSPHERE CONDITIONS."

[laurie]

"MANUFACTURED BY BEAUTIFUL GIRLS IN MILTON, WIS. UNDER CONTROLLED ATMOSPHERE CONDITIONS."

Definitely from the 1970's, eh?

[laurie]

I ordered some furniture tacks for when I get time to do the cabinet.

I am now sure that the amp was stored in a shed or somehow in the weather for a few years. There were leaves and organic debris in the amp chassis, and many of the furniture tacks that hold the binding on the cabinet are rusty.

I'm not sure what else I'll find structurally when I dismantle the cabinet, and I may end up having to manufacture a new cabinet. We shall see.
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Furniture_tacks.jpg (186.13 KiB) Viewed 8070 times