JC-120 in need of a lot of love

[laurie]

Thanks!

A little more progress.
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[laurie]

Getting close...

Tested with the speakers in the cabinet and when chorus or vib are engaged the right speaker volume drops 6-9dB. Obviously something amiss with the wiring. Will fault-find it tomorrow.
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[Pepe]

Awesome! Are you sure that something is wrong with the effect? My vintage KORG CX-3 stage organ has a Chorus/Vibrato effect inside and the volume does noticeable drop when I activate it (especially in Vibrato mode).

The Lineout signal is mono, correct? Might it be worth checking first how the chorus sounds through this output?

[The_Doc]

There is certainly a perceived volume drop in the chorus mode on my JC-40. Not sure if it’s a real decrease in volume but it is noticeable.

It is looking so good now. Amazing job Laurie !

[laurie]

I dug out the scope and checked the circuit.

When the BBD is engaged (either Chorus or Vibrato) the right channel level was dropping by about 3dB which was annoying, but more importantly it was greatly reducing the stereo chorus effect.

I don't know why the volume was lower - I suspect because the MN3002 is VERY noisy it made the amp easier to sell back in the day to have the chorus channel backed off and thus less noisy.

Between the vintage hum, hiss, and the uncorrected BBD noise this is not a quiet amp. Definitely vintage, and definitely not high fidelity. But then, it is a veritable old guitar amp and when using at stage level you aint going to hear any of the hum and noise

The fix was to adjust the effected level. There is no adjustment in the original circuit, but it is easy to add one. I put a 20k trim pot across R159 (the input resistor for the "discrete opamp" used for preamplification).

I then used the scope and adjusted the original trimpot (VR11) to balance the two outputs when in mono ("OFF") mode, then rebalanced in effected mode using the 20k trimpot across R159.

Although a little noisier, the chorus is much (much) better.
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[laurie]

What's left to do:

• The casters. The original ones are worn out - the axle holes in the rubber wheels flogged out and the swivel bearings very sloppy. Can't find a direct replacement, of course, so I'll have to figure out a work around.

• Install new amp corners. All but one of the original corners was lost or damaged beyond repair. I've ordered some that "kinda look right", but again originals are impossible to get.

• Repair and reinstall the back panel. The brace on the panel had been cut(?) and the glue wasn't holding what was left very well. I'll make a new brace and glue it in on the weekend.

• Replace the cup-washers on the chassis screws. The amp only came with three washers and again they are impossible to get. So I'll paint some modern chrome ones black and use them. Plus clean and repaint the heads of the chassis screws - they have had a hard life with people trying to repair this amp for years.

I'll also keep an eye out for some darker speakers. The Sammi ones are good, but they lack bottom end because I think they are meant to be in a ported enclosure, not the back of a guitar amp.

Other than that it is good to go. One fully restored bit of vintage goodness.

[Pepe]

Great to hear that you found a solution for the volume drop!

[laurie]

Managed to find some casters:
https://www.amazon.ca/dp/B079JJ71LV

They are almost correct - I had to ease the holes in the swivel base a little with a rat-tail file to allow the bolts to line up, but they are close enough.
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[laurie]

I made a dodgy video of the beast... many thanks to pepe for hosting it!

http://www.pepemusic.de/downloads/1978_JC-120_SML.mp4

[fernieite]

Great restoration job, Laurie! It sounds quite good.
I hadn't realized that so many famous players have used a JC120 before.
https://www.thegearpage.net/board/index ... r.2214561/