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Post subject: Fender Princeton Chorus Solid State Amp Hum???
Posted: Sun Oct 21, 2012 11:53 am
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Any help on this one is greatly appreciated. I have owned this amp since 1994 and bought it new at Guitar Center in Hollywood. This amp has been through a lot, but I have taken good care of it. Several years back one of the can-type capacitors blew. A replacement cap was installed at a local shop and that one blew a year or two later. In 2008 I replaced both capacitors with some JJs and they work great. The speakers were bad and all of the switches sounded dirty. When turned on, the amp generates a constant, hum; even when nothing is plugged in. The hum occurs at all volumes. I can play and the tone comes through clean, but the hum remains. Additionally, when I activate the Chorus effect it no longer works and essentially turns the amp into a mono where sound only comes out of one speaker.

I decided to make a project out of this amp and cleaned all of the pots and check all connections. It cleaned up very well. Vintage Weber 10s were installed (10F150 50w speakers). The tone achieved with these speakers is exceptional. However, the hum remains. Should I replace the transformer and is this the most likely culprit?

Specs: Fender Princeton Chorus Solid State amp black face
Input power: 120W ~ 60Hz (125w)
Made in the USA
Ser# LO-655953


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Post subject: Re: Fender Princeton Chorus Solid State Amp Hum???
Posted: Sun Oct 21, 2012 2:39 pm
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Hi Jborup,

It's not the transformer. You might have a poor connection somewhere, maybe a loose ground or solder joints, and I would have a close look at the effects loop jacks. Try a stereo patch cable in the loop to see if any difference. Possibly a component failure in the chorus circuit is causing the hum. Of course the caps are suspect also, considering the history. Did you have the hum after replacing the caps last time, or did the hum happen suddenly or gradually after?

Is this your amp?
http://support.fender.com/schematics/guitar_amplifiers/Princeton_Stereo_Chorus_DSP_schematic.pdf

If not, you can contact Fender to get a copy of the other SS Princeton Chorus Schematics.
http://www.fender.com/support/articles/fender-guitar-and-bass-amplifiers-owners-manuals-and-schematics-hard-copy-archives

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Post subject: Re: Fender Princeton Chorus Solid State Amp Hum???
Posted: Sun Oct 21, 2012 5:14 pm
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shimmilou,

Thank you for your reply. It's hard to say when the hum came on. The hum and the speaker distortion began when the first cap went bad. When it was fixed there was zero hum, but when you would power down the amp it would give off a weird high-pitched sound. I think this was the sound of the new cap discharging. When I replaced both caps the hum was still there, but quieter. During that time I also double checked all solder points and re-soldered anything suspect.

I tried the patch cable in the stereo and mono loops and the hum remains, but it is quieter. When I plug a cable in the headphones jack the sound is gone. The schematic does not look like mine. It looks like it is for the reverb?


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Post subject: Re: Fender Princeton Chorus Solid State Amp Hum???
Posted: Sun Oct 21, 2012 5:50 pm
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DSP version. Get your schematics and have a look inside your amp.

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Fender Princeton Chorus Solid State Amp Hum???
Posted: Tue Oct 23, 2012 9:25 pm
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shimmilou wrote:
Possibly a component failure in the chorus circuit is causing the hum.


If this be the case then muster the roadie crew on the fantail, have the ship's bugler blow "Taps", then deep-six the amp. The chorus module has been OOP for about eight years now.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Princeton Chorus Solid State Amp Hum???
Posted: Sat Oct 27, 2012 3:49 pm
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shimmilou wrote:
DSP version. Get your schematics and have a look inside your amp.


Thanks, email sent to Fender for the correct schematic.

Not gonna float-test this amp as it still sounds really good even with the hum...


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Post subject: Re: Fender Princeton Chorus Solid State Amp Hum???
Posted: Sat Oct 27, 2012 3:51 pm
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jborup check for a bad solder orprinted circuit broken in the filter cap circuit


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