Fender Princeton Chorus - chorus circuit problem

[Emil]

Hi all,
I’ve recently acquired a Fender Princeton Chorus (Black Knobs) and there seems to be problem with the chorus circuit. With chorus switched on, there is an intermittent cracking noise or distortion of some sort when playing, especially with somewhat heavy picking. Stronger input signal seems to activate it. And this is audible only through the speakers and there is nothing wrong with the headphones listening. There is no problem whatsoever with the clean or the OD channel. I checked and resoldered the PC board and sprayed the jacks but didn’t do any good. I may have missed a point, though. I have the schematics and detailed photos of the board and am able to do basic tests and parts replacement if someone could point me to a specific section of the circuit. I’d appricate any thoughts and suggestions to help me out.
Thanks in advance
Emil

[nashvillebill]

The headphone out is driven by the same amps that drive the speakers, so it does not make sense that the speakers would be crackling with chorus on but the headphones wouldn't... Can you plug the effects out into another stereo amp, plug in the headphones on the Princeton Chorus, and with the chorus engaged, do you hear distortion/crackling through the second amp? We need to isolate whether the problem is in the chorus section, or the amp section, or the speakers themselves. The circuit design doesn't show any obvious way that distortion/crackling could be present only through themain speakers and only when the chorus is on, they are in two separate parts of the signal chain.

One note of clarification: this is a "regular" Princeton Chorus (red or black knobs doesn't matter, same schematic AFAIK) and not the Princeton Chorus DSP?

[Enzo]

Unless the headphones jack was causing the issue. Dirty contacts within it.

[Emil]

Hi again,
Upon Nashvillebill’s suggestion I looped out the chorus circuit to another stereo amp. It turns out that it is not the chorus circuit responsible for the noise, as it works flawlessly with the other amp. As I mentioned before it also sounds alright with the headphones. Another finding is that the noise is on one channel only. I listened carefully and swapped the speakers to confirm that. So it looks like there is nothing wrong with the speakers, either. To describe the noise in a bit more detail, it sounds like a delayed distortion or clipping, especially when playing certain notes. I need to mention again that there is no problem with the clean and OD inputs and the reverb. So the problem is likely to be in the amp section I guess… but where?? Smacking the cabinet seems to help a little. Could it be the chorus pots? They seem to be sensitive and cause a bit of noise when tapping on the rods. Of course this might also be something nearby on the PCB. Perhaps the cabin vibration will cause that… I can pass on the schematics and photos if any one cares to take a crack at narrowing down the possibilities.
Best wishes
Emil

[Jazz P Bass]

Have you had the circuit board out?
My suspicion is solder joints that are flaky.
You will need an O scope & a signal generator to literally find the problem.

[Enzo]

Oh geez, I'm not thinking clearly these days.

Doesn;t do it through the phones, but does through the speakers.

It is a combo amp, so the speaker vibration may be at fault. With the amp running, ball up your fist and whack the top of the amp. Does that make it do the noise? That exposes loose connections inside. Some part needing solder or whatever.

[Emil]

Hi there,
I got back in and checked the PCB more thoroughly this time and spotted two loose cap connections (C 74 and C 35). Resoldered them and some other not-too-good-looking joints and the problem was solved. It seems one of the weak points of Fender Princeton Chorus amps is the poor solder quality, cos’ it’s really seldom, at least for me, one could witness joints giving way like this. So like you suggested one would be better off checking the PCB for loose solder joints in the first place. Thank you all for your contributions.
Emil

[Jazz P Bass]

Bad soldering is not limited to Fender Princeton Chorus amplifiers.
Or Fender amps in particular.
The wave soldering process leaves many avenues for short cuts to be applied.
Think "piece rate".
Unfortunately, this is the way it is.
And the issue is certainly not limited to musical equipment.

[Enzo]

Exactly, there is nothig different about the Princeton CHorus. All the Fender amps are made with the same parts on the same equipment.

[schottk]

The same style Princeton Chorus that I picked up used started having the same problem - a crackling distortion which was more sensitive to certain frequencies. Thanks to the information supplied by this forum, I was able to trace the problem to a fractured solder connection on C59 of the power supply. The amp is now working perfectly. Many Thanks !