Fender Princeton DSP lower freq feedback?

[HaxorMachine]

I'm new and registered to post a question on an issue I've had with my 1999 Fender Princeton Chorus DSP for several years now.  I'm getting back to playing electric and this amp has what seems like some sort of low-frequency (but undesirable) honk/buzz/fart/resonance/feedback when playing low E/G area with mid volume and gain around 5 (Electra/Westone strat copy with JBJr bridge bucker).  It seems like it's also trying to do it in the clean channel with a good bit of volume, but it doesn't go into a resonant/feedback mode like it does with some gain. Hard to describe the noise, but it goes from a nice rich ringing E or G to a braaaaaaaaaaaaaaaaaap. If I don't change to another note, it will continue to self-sustain.

I read here that SS Princetons had filter cap issues like solder and vibration (glue) issues and I opened mine up to compare.  Caps seem solidly mounted with a large bugger of glue between them - should I squeeze some additional hot-glue between them or is this likely a different problem? Speakers (2x10 stock Fenders) look fine and when this happens both speakers seem to be doing it.

An interesting observation -  As there was some glue in there spacing the caps, I put a strong rubber band around the caps to see if it would make a difference (perhaps some vibration issue and resonance might change).  The same noise was at same freqs still, but I also heard another similar effect around B blues/power chords and the amp darkened/lost some gain. Took the rubber band off and it went back to brighter without the B-ish resonance. Must be an electrolytic cap reaction to slight pressure?

I like the amp, but this noise must go away!  Any thoughts?

[joecool85]

Definetely sounds like old caps, I'd replace all the electrolytic caps.  At minimum the power supply caps.

[HaxorMachine]

OK - I've never actually heard what seems to be a common Fender 10" amp/speaker fart-out, so is that possibly what my uneducated ear is hearing or does bass fart-out just produce a rattly/ragged cone sound? I'd hate to invest in new speakers that handle bass better and still have this noise (a pair of Cajuns would be around $130)......This guitar is not "bassy" (no tone controls and no volume - pickups direct from switch to output/ground) and I'm running amp bass setting around 6.

If I do the caps, are there recommended 'upgrade' caps (ala the orange drops folks put in guitars) or just go with basic RS replacement electrolytics?

[nashvillebill]

Several possibilities come to mind.  The issue with the Priceton caps that I mentioned some time back, regarded the large caps on the power supply side.   If they came unsoldered, the result would be a constant 120 hz hum regardless of whether you're playing or not.  If I understand your description, you get the noise only after you play the note, then it stops when you mute the string.  I doubt it's a loose power supply cap if it stops when you mute the string.

Have you tried another guitar with this amp?  Could be the pickup on that guitar, does it do it with all pickups?  Does this guitar have a preamp; if so, is the battery dead/dying?

Do you have any effects pedals plugged in?  If so, take them out of the equation and try it.

I think this amp has a headphone jack.  Can you plug in a set of headphones and try it?  If you don't hear the noise in the headphones, then the issue is in the speakers or power amp--or could be mechanical vibration of the amp making something short or ground.  If you still hear the noise through the headphones, it's in the preamp stage of the amp.  To me, your description sounds like the digital stage might be clipping.  Try turning the gain down, compensating by turning the volume up.

If the noise only happens through the speakers, not through the phones, and reducing the gain did nothing to help, try a different speaker to see if it's the speaker (I think you have a "speaker out")  Disconnect the built-in speakers and move the alternate speaker a ways away.  If the noise doesn't come back, it's in the original speakers--or the vibration is making something short.  The vibration-induced short can be investigated by playing a note that doesn't cause the noise and then hitting the side of the amp hard several times to see if you can induce a noise.

I would not randomly just start replacing components--that's the "shotgun approach" and rarely works IMHO.  Look very carefully, with a good magnifying glass, at all of the solder joints at various components, starting with large components like caps, if it's possibly a vibration-induced noise.  Look at each joint, and *slightly* wiggle the component, if the  solder joint is bad, you'll see the lead or broken solder joint move.  (A few weeks ago I fixed a Tascam 488 Portastudio 4-track that had 3 broken solder joints on the main faders, go figure!!)

I would NOT recommend Radio Shack for several reasons: 1) their components are generally low-quality, 2) their prices are outrageous on things like caps, and 3) they have no selection at all.  Try online, through Mouser www.mouser.com or Digikey www.digikey.com or even Parts Express  www.partsexpress.com

I am not a believer in "upgrade caps", but that's my personal opinion.  Some audio folks or even guitar folks swear by high-dollar exotic components, IMHO *most* of that is the "placebo effect".  I use whatever Mouser or Parts Express has for a decent price.

[teemuk]

In addition to what has already been said...

Some amplifiers go to instant "fart mode" when you hit the speaker's resonant frequency at reasonable sound pressure levels.  This only happens at a certain note, though, usually somewhere around 80 – 100 Hz. I don't know if this is the case (judging by your description it doesn't really sound like it) but you can easily rule out this theory: Just advance through the fretboard playing one note at a time (have the amp set quite loud when you do this). If the amp instantly farts out (the way you described) at a certain note then this is the problem. The only cure I know is to change the speaker (the resonant frequency must change to something proper) or roll off the bass.

Since the rubber band around caps had such a notable effect I would renew the solder joints and the hot glue that holds the caps in place. I doubt that you could have "strangled" the caps enough to make any audible effect, likely you just weakened their electrical contact with mechanical stress. There might be a contact that has not yet failed totally but is about to.

If I figured out correctly the amp is less than 10 years old so likely aged caps aren't an issue just yet. But who knows, some may have decided to die prematurely due to some reason. Anyway, I would start changing caps only if I found out that they indeed were the problem – and if resoldering them didn't help. The exception is of course if the cap is definitely looking faulty (e.g. bulged or leaking). I have never bothered with brands either; I use what I can get from component stores. Well, of course if there's a possibility to choose I try to avoid the cheapest option. If you have to buy some replacements you should rather worry about the physical parameters (in other words, does the new part fit the place of the part you're replacing). Capacitor lead lengths and pitches (distance of leads), along with the size of the can, vary so much that these issues might be the ones that end up dictating what brand you need to buy.