JC-120 in need of a lot of love

[laurie]

Checking things with the scope - the chorus LFO isn't "O"ing.

Not sure why yet. Might be a failed 4558 (it is used as the LFO oscillator).

Will continue tracking it down...

[Pepe]

Sounds like that should be fixable.

[laurie]

So it turns out everything is OK. Chorus is working fine.

Here is the technical description.
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JC-120 chorus LFO_1.jpg (396.71 KiB) Viewed 16410 times

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In the pic above you can see a level detect circuit. What that does is take the speaker signal and when there is "enough" volume sent to the speaker Q46 turns on, taking the collector to ground level.

Until that happens, pin #6 of the 4558 is sitting at -1.8V which turns the LFO off.

When the level detect circuit turns on Q46, pin #6 of the 4558 goes to ground, and that turns on the LFO. So the Chorus LFO doesn't run until you actually play something. Hence my original thinking that it wasn't oscillating - I was taking the measurements when there was no signal.

Once the LFO is turned on, it works just fine. Very subtle, but beautiful.

[laurie]

Here is a really special part of the circuit.

The old MN3002 chips are REALLY noisy. Hence the NE570 compander used in early analogue delays (eg. the DM-2).

In this old model JC-120 they use the level detect circuit to turn an opto coupler on and off. When there is no signal at the speaker, the opto is turned on and the resistor in the opto coupler is near short circuit, muting the output of the MN3002.

When the level detect circuit detects a signal, not only does it turn on the LFO, but it also turns off the optocoupler, unmuting the MN3002. Genius.
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JC-120 chorus LFO_2.jpg (366.57 KiB) Viewed 16410 times

[laurie]

I'm calling it done. I'll recheck the wiring, then button it up.

Time to start work restoring the cabinet...

Oh, and the Samick drivers sound really nice:
viewtopic.php?f=14&t=954#p10052

[Pepe]

Awesome!

[The_Doc]

You beat me to it Laurie because I dug out my CE-1 yesterday to find out how that sounded, as it should be a good approximation of your amp’s chorus. It is, as you have found, a subtle and beautiful sound. I had it running into my JC-40 (with the chorus off!).

[laurie]

In case anyone needs it in the future... right click the image below and "open in new tab".
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JC-120 board connections.jpg (380.57 KiB) Viewed 16396 times

[laurie]

Ready for reinstallation in the cabinet when it is done. Before then after shots.
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JC-120 before.jpg (260.58 KiB) Viewed 16391 times

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JC-120 finished2.jpg (223.05 KiB) Viewed 16391 times

[laurie]

JC-120 before2.jpg (298.85 KiB) Viewed 16391 times

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JC-120 finished1.jpg (204.27 KiB) Viewed 16391 times