JC-120 in need of a lot of love

[laurie]

Just connected an external speaker to the one remaining channel and Its Alive! Plus the vibrato works, so the MN3002 is good. Chorus doesn't work so there is an issue with switching or LFO or something.

Bottom line - because vibrato works, the MN3002 (unobtanium) is working. That was the biggest question mark.

[Pepe]

That's a fantastic price for the pair! If the speakers turn out to be great sounding with the Jazz Chorus, then you have a real bargain (if we disregard the fact that it needs hours of maintenance).

[The_Doc]

Great news that it’s not a complete write off. I reckon those speakers will do the job just fine.

[bigtone23]

Very cool! Those speakers look like they will work well, kinda flat-ish, maybe sort of EVM-ish..?

[laurie]

Very cool! Those speakers look like they will work well, kinda flat-ish, maybe sort of EVM-ish..?

Yeah. May...be. I'm hopeful!

[laurie]

The slow job of restoration has started.

This is the correct schematic - based on serial number, and the fact it is the only schematic with an MN3002 BBD:
https://music-electronics-forum.com/fil ... ?id=842270

However. The output transistors are not the ones shown in the schematic (2SD425). Of course. They are actually 2SC1079.

The output transistors in the blown (and removed) channel are short-circuit. So there's a place to start.

There are many 2SC1079 transistors on eBay, however, the consensus from the tech boards I checked is that they are fake. Based on specification, a modern equivalent is the MJ15001G which Mouser has in stock.

I'll do some more fault-finding before placing the order - I'm sure there will be more things to buy.

[laurie]

While I'm doing the repair I'll replace every electrolytic capacitor. On the blown board they have been subjected to a lot of heat as the nearby components burned. And in the power supply the big electros are 40 years old. Time to be replaced.

I'll replace the 1uF electro capacitors in the signal path with modern metallized polyester. I've always been an advocate of getting old electrolytic caps out of the signal path.

[Pepe]

I've always been an advocate of getting old electrolytic caps out of the signal path.

But ... but ... the MOJO!

No, just joking. I think in the very same way.

Only recently I had to repair the one remaining KORG KDD-501 Digital Delay module that I have had left*. There was annoying lo-fi sounding glitchy noise in the delay signal that was increasing at high feedback rates. The circuit is based around an '80s NEC delay chip and it has only one single trimpot (for feedback amount, I think). There were no visible bad solder joints. So the first thing that I did was getting rid of all the electrolytic capacitors. Shoo! Shoo! Out with them! Once there were new capacitors inside, the delay worked like a charm again. Which one was the culprit? I don't know, and I don't care. It works and sounds perfectly again with the new capacitor set and that's the main thing. And I suppose that these components are of better and sturdier quality in comparison to those of the mid-'80s. So I can expect my delay to work for at least another 40 years.

*two units went to the Switzerland in a great deal and I sent the guy the two ones that sounded exactly the same and without noise issues.

[laurie]

I've always been an advocate of getting old electrolytic caps out of the signal path.

But ... but ... the MOJO!

[laurie]

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