Author Topic: Rhodes 80W Peterson Amp Issues - help!?!  (Read 292 times)

Offline jonesy013

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Rhodes 80W Peterson Amp Issues - help!?!
« on: August 07, 2017, 12:50:09 AM »
Hello all,
Long time reader, but first time poster!

I've got a 75 suitcase rhodes with the 80w peterson amp, which I've been trying to get working forever without success. When I initially turned on the unit, it gave a loud hum but with no signal. Neither of the external amp outputs give a signal when I plug an external instrument into them and listen through the headphones. 

My PSU is putting out +39/+25/-39VDC on both molex connectors, and the 4pin connection (to the harp output) has the +24VDC between pins 1 and 4.

From the above, I figured it was the power amp modules that were blown and not the PSU. I bought the PSU refurbish kit from vintage vibe, including the silicon transistor replacements. I tested all the components for shorts, and tested all of the transistors (out of circuit), but I didn't find any shorts, and the resistors seemed to be putting out the resistance they were supposed to. I still swapped out the components, but didn't do the silicon swap, then hooked it up to the PSU to see whether anything had changed.

Unfortunately, nothing had changed, and in addition, as soon as I fired the PSU up (with the amp connected by the molex connector and the RCA jack), the circuit breaker tripped right away. When I reset it, it immediately did the same thing. I noticed that the .5 Ohm resistors would get really hot, but no other components.

I tried replacing the Germanium transistors with the silicon ones (with the corresponding resistor swap which comes with the kit), but there was no change, except that the 0.5 ohm resistors would no longer get hot.

At this point, I'm at a loss as to what is wrong. I don't think I made an error in doing the re-build (though I'll certainly allow for that possibility :)), and the components seem to test fine and I can't find any shorts.

If anybody is able to assist, I would be most grateful! 

Offline Ben Bove

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Re: Rhodes 80W Peterson Amp Issues - help!?!
« Reply #1 on: August 14, 2017, 01:15:31 PM »
The Peterson amp repairs can be quite intensive to track down what's happening.  If you'd rather tackle it yourself than sending it to an amp tech, check to see if Vintage Vibe is still selling its DVDs or MP4s (or youtube videos if they go deeper) on troubleshooting the amp system.  It's less common to see both amp modules blown at the same time, and with the circuit breaker tripping, it's only a guess that the Power Supply is the culprit (without testing the system against the schematics).  Does the power supply trip the circuit breaker with the amp modules unplugged?  (molex connectors removed and powered on)
« Last Edit: August 14, 2017, 06:15:27 PM by Ben Bove »
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Offline jonesy013

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Re: Rhodes 80W Peterson Amp Issues - help!?!
« Reply #2 on: August 26, 2017, 06:20:22 PM »
Hi Ben,
Thanks for the reply! And yes, I'd definitely prefer to send it to an amp tech, but I've had a hard time finding anyone who will or can fix it...

The circuit breaker on the PSU does trip right away when the molex connectors are removed as well. I'm guessing connecting the amp took something out when I hooked it up.