1979 Janus preamp problem at big clients venue

Started by Seanfir, January 16, 2025, 04:53:50 AM

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Seanfir

I started working for a major venue in St Louis this week, maintaining their stable of vintage gear...
And I thought this would be easy, fixing the vibrato on this Rhodes.
Power supply is fine. Ground and cable are good. I recapped it straight away, taking care to use NP caps for the 1uf... and when I turned it back on, it had vibrato. And it stayed on for an hour until I was satisfied.
So then I put it back together.
No vibrato. I checked to make sure nothing was grounding out on the cover, and no. The circuit is oscillating on one side of the 10uf, but not the other. Another thing... this one does NOT match the late era schematic for this pre... that one lists a pair of 22/25 caps by the LDR units... which aren't present. The numbers and everything are off... it's way off from the older schemo... so I don't really know what to do. I told them that we probably should get a pre from Avion, since they're local to our area, but I HATE that this one stumped me. I changed out the transistor,  the op amps on the far right, pulled a bunch of resistors, all tested good, tried putting a ground jumper on the switch,  but realized that in THIS ONE, the ground appears to be floating, and I wasn't aware that LED'S could work on negative voltage. Do they??? Because the power led was running on -15v...
And the 20uf cap had polarity markings on the board, but is there a chance it could be BP?? The transistor had negative voltage on one leg,  and positive on the other... I was really hoping to find a schemo with voltages, after a while!
I did everything... I pulled the whole board out, flexed it, re ran solder traces... was there another preamp before the transition?
This was the 3 knob, 2 slider one, but the late era schematic was not accurate. It kills me that it was working until I put it back together. I reread all the posts in here, tried for two days. Then just to prove myself, fixed three leslie cabs in the last hour. Lol. I hate this thing! Lol. Any pointers??

Seanfir

Well, I don't think anyone would have been able to help with this thing. While not too terribly complicated it was different enough from the other schematics to where I couldn't figure out what the voltages should be or if I was getting correct ones or not. I think I've talked them into getting one of the Avion units which are made kind of locally here. And I have a lot of respect for those guys anyway.

sean

#2

There are three different Janus preamps:
Schematic for Janus preamp with sliders: https://www.fenderrhodes.com/org/manual/fig11-1.jpg
Schematic for Janus preamp with sliders, version 2: http://www.fenderrhodes.com/pdf/late-mark1-suitcase-janus1.pdf
Schematic for Janus preamp with five knobs: http://www.fenderrhodes.com/pdf/late-mark2-suitcase.pdf


(Fixed broken link.)