Fender 125W Princeton Chorus w/Loud Buzz

[PowerPro]

I have a Fender 125W Princeton Chorus amp that give out a fairly loud (you'd have to raise your voice to talk over it) buzz from both speakers when I power it up, even with no input and the knobs all turned down to their stops. Turning volume up or down doesn't affect it. Normally with any kind of amp I'd inject a signal from a frequency generator and trace it out with an o'scope, but if I leave this amp on for more than a couple of seconds, I get a nice "toasted component" smell, so that route is out.

Can someone tell me where to get a schematic and / or troubleshooting chart? Or, if there is a tech out there who has done this one, what was it?

[Jam]

Princeton Chorus

Try Fenders website or go to Mr gearhead.com to get a schematic but Iam sure if they have this one

[52 Bill]

I have a Fender 125W Princeton Chorus amp that give out a fairly loud (you'd have to raise your voice to talk over it) buzz from both speakers when I power it up, even with no input and the knobs all turned down to their stops. Turning volume up or down doesn't affect it. Normally with any kind of amp I'd inject a signal from a frequency generator and trace it out with an o'scope, but if I leave this amp on for more than a couple of seconds, I get a nice "toasted component" smell, so that route is out.

Can someone tell me where to get a schematic and / or troubleshooting chart? Or, if there is a tech out there who has done this one, what was it?

Did you try the Fender site for the schematic? It may be there in pdf form.

The buzz is usually cause by one of two things, a bad or loose filter cap, or a shorted output device. The first test would be to see if there is any dc voltage on the output. Set your meter to read dc volts attach the leads across the output (speaker) wires and turn the amp on for a few seconds, while watching the meter. If you see anything other than a few millivolts, then you will probably need to replace an output device.

If there is no dc on the output, then pull the chassis and see if one of the filter caps has come unsoldered from the board.

[Rick Erickson]

There is no 125W Princeton Chorus. That is the nominal AC power consumption. The output is 25.5W per channel, stereo for 51W RMS total.

This is from the F.P.R.C. owners book.

POWER OUTPUT: 25.5 Watts R.M.S. per channel = 51 Watts R.M.S.

POWER REQUIREMENTS: 120 Volts AC 60 Hz., 1.2 Amps Max., 145 Watts Max.

SPEAKER COMPLEMENT: Two 8 Ohm Fender Special Design 10 inch (P.N. 029753) speakers.

RE

[PowerPro]

Jam, thanks for the lead, 52, thanks for the lesson, and Rick, thanks for the corrections. After I opened it up, I noticed that both of the large (4500 microfarad?) caps in the power supply area rattled! Geez, is that an extra feature? After I pulled the board, I inspected the solder joints for the caps, and the solder had broken in a ring around all four joints. They didn't look like cold joints gone bad, the original solder was shiny and slick looking, but they had cracked completely around each cap terminal. Weird. I am usually fixing things in a power plant, or in generators, and I haven't seen a fault like this before. Is it common in amps, and if so, why?

{re-posted under original thread}

[WholeToneMusic]

The high production amp companies use wave flow soldering ( among other things to keep the cost of the amps low) and that is not the most reliable solder technique. It is known to crack and cause problems. Best of luck to you!

[Enzo]

Oh, I don't blame it on the wave solder, otherwise ALL the solder joints would crack.

Moreso than all the other parts on the board, those two caps stick up from the board and have a relatively high center of gravity. They also are relatively heavy as parts go. So they vibrate and shake from the speakers right below them. Large parts like that are usually glued in place as well as soldered. Was there a ring of hot melt around their bases? The glue helps hold them steady, but eventually vibration takes its toll.

This is a very common problem on Fender solid state amps of the era, especially the ones with the sloping chassis - the one tall in the rear and short up front at the panel.

It would be a very good idea, now that it is resoldered, for you to run a bead of hot melt or silicone sealer between the two caps - glue them together - and globbed around their bases - glue them to the board. Clamps would be better, but that isn't going to happen.

[T-bone]

princeton amp

hi guys, lee turner from blue logic studio QLD> ive got a princeton chorus amp that really vibrates the speakers when the bass is turned up past 4. is there a special way to test speakers for ineffectiveness? would really appreciate the experianced responses. cheers T-bone .

[Enzo]

Hi Lee, welcome to the forum.

I'd suggest you start a new thread for your amp, you'll get better response than at the end of another amp's thread.

The easiest thing to do is connect the speakers to a different amp and see how they sound.

Also, determine if it is one or the other speaker - or both. Note that each speaker has its own power amp within the chassis.

ANd make sure all the hardware is tight - speaker bolts, chassis bolts, etc.

[T-bone]

thanks

hi thank you so much for responding so quick, resolderd the main caps, cheaked for dry solders, etc, there is also a prob with the chorus cuircitry, if i activate the chorus channel there is no change in the sound from clean? very strange.. ???? any thoughts?/ Regards T-bone

[T-bone]

sorry

sorry to mention, ive changed speakers, and amps, standard fender speakers seem kind of flabby at high vol, so could be them, tightend all screws, etc... obviosly trying to eliminate the obvious, before i wash up for operations lol. cheers any thoughts?/

[Enzo]

Are both speakers being driven?

[52 Bill]

Are both speakers being driven?

And are the speakers wired with the correct polarity?

[T-bone]

bill & enzo

gday guys, yea, both speakers are driving, with correct polarity, dc reads
20 mv @ each speaker, chorus led pulsates as it should, after reading some of the other threads,(which i dont know how to start) ill clean resolder the input jacks. any other things i should look out for? many thanks, T-bone

[T-bone]

i dont suppose anyone has correct scematics for this princeton chorus or a link, fender has a princton chorus dsp, whatever that is, but the confiquration dosent look right. i :dont think this is the true stereo model, any help guys. T-bone...