noisy reverb and moderate hum on princeton chorus

[NA_NC_MX5_DRVR]

Hello all

  I have a couple of issues with my princeton chorus.

1. a noisy spring reverb circuit.  The noise is acceptable until ~ 5 on the reverb control and gets really bad from there.  I have replaced the cable and cleaned the connections but still noisy even with the reverb disconnected. when I had the amp apart last I noticed that when i place my hand over the area where the reverb cables are soldered into the amp, I get more noise.  I took a look at the reverb unit and used deoxit to clean all of the internal connections, the unit seems good. Is there something else I should look at?

2. Moderate hum all of the time. I have replaced all of the electrolytic caps in the amp and still get some hum. I noticed that with the chassis out of the cabinet the transformer vibrates a little. I have used cardboard shims where the transformer is attached to the chassis and it has helped a little but the hum is still there. Are these amps ever completely hum free?

Any suggestions would be helpful. Thank you.

[DrGonz78]

Well to be honest the reverb connections will hum when not shielded and out of the cabinet. Try turning on and off the chorus with that amp out of the cabinet, it will make a strange noise too. Strange noises occur when the chassis is out of the cabinet as the cab provides a shield plate.

As far as the next item to look at is noisy op amps. The last Princeton chorus I did some repairs had no reverb and one of the opamp was bad. I did some scoping of the signal and voltages tests. In your case it could be ground issues and also op amp noise. When I fixed the reverb circuit on that last amp it was not noisy like you describe your amp to be...

[Roly]

Welcome @NA_NC_MX5_DRVR.


Check the continuity of the screening on the two cables that run to the reverb tank.

Generally speaking reverb tanks are screened and earthed with reference to their output end, the input of the gain recovery stage following, and the eatrhing at the tank end is sometimes only through a compression rivet which can work loose and cause noise pickup problems.

By shorting Main In you should be left with the power stage noise contribution on its own; this tells you where to look for the injection, is it early or late in the signal chain?  Different basic causes call for different corrective measures.

HTH

[awander]

I am working on a Princeton Chorus that was very noisy when the reverb control was turned up. hum, and an occasional crackling noise.

I treated the jacks on the tank with some deoxit D5, and it made no difference.

I removed the reverb tank from the cabinet, and found that someone had been n ther already-the wires from the jacks were soldered directly to the transducers, and the connections were gooped up with silicone sealer.

While I was in there, I discovered 2 things:

-Whoever had the amp before me had the reverb tank plugged in backwards.

-the Output jack on the tank, which is supposed to have it's ground("shell") connected to the metal case of the reverb tank, was severely oxidized and no longer making contact. Shorting the "shell" of the jack to the tank case got rid of the hum, so I scraped a clean spot on the case, and one on the "shell" of the jack, and connected them with a bit of solder(the stock install relied on the "shell" of the jack touching the metal case).

I put everything back together, and now the amp is almost silent with the reverb all the way up.