Princeton 112 Plus - Intermittent "Popping"

[Jayz]

Background:
I bought this amp as a "project" last year. I replaced the reverb pot, reverb tank, input jack and some knobs. The amp has been working fine for several months. I've been using in in weekly rehearsals. It's always been noisier that I think it should be and the reverb goes from zero to full on setting 1 but other than that I've had no issues. I've been considering making some of the mods I've seen suggested on this forum but haven't made any yet.

Problem:
Last evening during band rehearsal it started making intermittent "popping" sounds. I unplugged the guitar at the amp input jack and the popping continued (about 1 "pop" per second). I turned the amp off and back on again and the popping continued. I only know enough electronics to be dangerous and so am hoping you can advise me where to begin looking for the problem. This was on the clean setting, no reverb, bass--mid-treble set to 5 - I haven't tried the dirty setting.

http://support.fender.com/schematics..._schematic.pdf

Any suggestions would be greatly appreciated.
Thanks

[Mark Black]

The schematic link didn't seem to work - let me try: http://support.fender.com/schematics..._schematic.pdf

One way to start would be to plug a guitar cord (or better yet a 1/4" plug with no wiring) into the effects loop Return jack. That should eliminate all the preamp circuitry so if the popping stops you have narrowed the source to the preamp. Or if it keeps popping you know it's in the power amp. Divide & conquer as they say.

Another possibility is environmental noise - does the amp pop when in a different location (different house/wiring/etc.)?

[Enzo]

That^^^


Also, in general, do ANY of the controls affect the noise in ANY way? If all controls are at zero, does it still pop?

[Jayz]

Thanks Mark & Enzo,
When I turned it on again tonight it was initially quiet but started crackling-hissing within a minute or so and popped then went quiet for a few seconds then crackle hiss pop. I "smacked" the top of it and the crackle -hiss-pop stopped. I was able to repeat this a couple of times. I assume that means there is a bad solder joint so I turned it off and I will pull the PCB out and look for cracks, bad joints, etc. unless you think the sounds I am hearing could be a component degrading on its way to complete failure. Mark - there are no "effects send-return" jacks on this amp. Only "pre-amp out" and "power amp in". Here is the link to the correct schematic: http://support.fender.com/schematics..._schematic.pdf
Enzo - none of the pots seem to have any influence on the "crackle-hiss-pop".

Thanks again guys.

[Enzo]

preamp/poweramp are same thing as FX loop. The schematic labels them as FX loop. Plug into the power amp in jack then, does the noise stop or continue? Schematic shows voltages on the power amp IC, how are they?

[Jayz]

Thanks for clarifying. So this morning I turned it on to plug into the power amp in. Before I did that I plugged a guitar cord into the input. Its been quiet for 10 minutes with all pots set to 3! I can't believe that "smacking" it a couple of times last eve cured the problem and of course I know that one more smack or jar could bring the crackle back again.
I plugged another cord in the power amp in jack - the amp becomes extremely quiet (no "normal" amp hiss whatsoever).
Should I assume there is a bad connection somewhere? If so, where should I look first?
PS - I won't be able to check the power amp IC voltages until I take the amp apart later this evening.

[Enzo]

So smack it again, see if the issue comes back. I doubt you cured it, you just caused the symptom to stop.

The power amp jack disconects the preamp, so all the preamp noises will be gone.

So we still want to know if plugging into the PA jack stops the noise while it is making that noise. If the amp is not noising in the first place, we cannot tell if the jack stops it, now can we?


There is no one place to look for loose connections. Narrowing it down to the power amp or not is one step. getting inside while it runs and poking everything with a wooden chopstick is one way to find parts that react when moved.

[Jayz]

Thanks for your patience Enzo! So it's been on for 20 minutes. I smacked it, rocked it, banged it on the floor a couple of times and still can't get it to crackle. I guess I'll just have to wait until it starts crackling again and then plug a cord into the power amp in jack and see what happens to hopefully narrow it down. I'm only using for rehearsals so don't have to worry about it in a live gig.
Thanks again, I'm much better prepared next time it happens.

[Amp Kat]

Just to verify that when you plugged a cord into the return you basically bypassed the preamp altogether which is why it got extremely quiet. It's important that you realize that the preamp out and the preamp in are a bridge point that seperates each one from each other. If you were to use the preamp out to another amp and the crackles return the problem would be in the preamp section from the preamp out back to the input section.

[Jayz]

Great advice Amp Kat! Thanks. If, (probably when!) the crackle returns I'll take the preamp out to another amp and see if the crackle shows up in the secondary amp.