Very Strange Problem Fender Princeton 112 Plus

[jwhmca]

Hello All,

I have been beating my brain against this amp for a month now. It makes a "sizzling/crackling" sound intermittently. It IS shock/vibration sensitive, but only certain frequencies. It very much sounds like "light clipping" in that it happens all the time, when it's happens if that makes sense... as opposed to just a random "POP" or "CRACK" The only way to get the amp to act up is to play it kinda loud and play certain notes on the guitar. If I take the amp section out of the cabinet it will play fine. That is what led me to say it's vibration sensitive.

But, no matter how much a push and prod around the amp I can't get it to act up... again, only playing a note through it.

It doesn't really seem to be thermal. It will act up right away if the right note is hit. Although a few times it really did seem to get worse the longer a Played it.

I can see the "distortion" with a scope. It appears after R50 (see schematic) but not before.

I have purchased, but not installed all the diodes after R50... I was thinking that it might be a bad internal connection on a semi-conductor.

Anyone have any thoughts or ideas???

[bkahuna]

The output of the amp runs through the headphone jack to get to the speaker. The switches in those jacks get dirty and might cause an issue like this. Have you checked that?

What does the distortion look like?

You might pull the board and inspect all the solder in that area, especially the bigger components like emitter resistors, and the outputs themselves.

If you can get to the pcb while the unit is hooked up and failing, you could try the opposite approach. Set it up to make the distortion, then see if you can poke and prod it into *stopping* the distortion.

[jwhmca]

Well, the distortion appears to be after R50, which is before the headphone jack... I did think of that. Could it be "bouncing back" somehow?

It looks like very fast, high level spikes.

[bkahuna]

Well, the distortion appears to be after R50, which is before the headphone jack... I did think of that. Could it be "bouncing back" somehow?

Yep. Through the feedback loop.
Also, if the problem is causing noise on the power rails, then the noise could show up almost anywhere in the circuit.

[jwhmca]

Yep. Through the feedback loop.
Also, if the problem is causing noise on the power rails, then the noise could show up almost anywhere in the circuit.

yeah, I see that! "Hudson, run a bypass..."

[olddawg]

Have you tried a heat gun and freeze mist? Sometimes the shock will encourage a latent intermittency.

[jwhmca]

I bypassed the headphone jack, same problem.

** Also, not sure if it matters, but I can not turn the amp all the way off on the clean channel... it gets really close, but you can still hear the guitar**

[jwhmca]

Found C39 47u/50v causing the problem... I used a "vibrator tool" to isolate the part.

[Jayz]

I haven't made any mods yet but 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.
Any suggestions would be greatly appreciated.
Thanks.

[nashvillebill]

I haven't made any mods yet but 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.
Any suggestions would be greatly appreciated.
Thanks.

Are you the original poster? Is this the very same amp (same serial number) continuing to show problems, or is this another Princeton 112? If it's another amp--even though it's the same model--a new thread starting with all the symptoms, history, etc would be less confusing.

[Jayz]

Thanks Bill. I'm not the original poster of the "very strange problem...." thread. Other than replacing the reverb tank I haven't had any problems prior to this "popping". My reference to the "mods" is just something I am considering doing based on others experience and advice. Of course making mods is a mute point if I can't fix this new popping problem.
Thanks again.

[nashvillebill]

I would suggest starting a new thread, describing how long you've had this amp, what you have done to attempt to diagnose the problem, etc. It just gets confusing when one thread has more than one amp being referenced, that's all. We'll try to help.

[Jayz]

OK. Thanks. I'll start a new thread.