Fender '94 Twin Amp Noise...

acorkos · 11-09-2009, 05:34 PM

When I flip the standby switch, there's a static noise coming from the speakers. All front panel control pots are set at zero. And noisy with/without a guitar cord plugged in.

The noise persists when preamp tubes V1-V6 are removed. The noise is gone when V7 (effects loop) or V8 (PI) are removed.

It's not the tubes. It's not the plate resistors. It's not the power supply filter caps. I know because I replaced all those things. I've cleaned and retensioned all the tube sockets, too.

I replaced the .022uF PI/power tube coupling caps C32/C33. I also replaced coupling caps C29, C23 and C26 which run between V6 and the PI via the effects loop tube V7. Noise persists.

I also replaced power supply resistors 5W/1K R91/R154 and 2W/4.7K R103...noise persists.

Any ideas as to what to check next to determine where the noise is coming from? Thanks for your help!

The schematic is on page 2 of the link....

http://www.fender.com/support/amp_sc..._Schematic.pdf

oc disorder · 11-09-2009, 08:55 PM

Hi acorkos
I would have thought it would be a plate resister R146 even R 145 but you say you have replaced these.
If you insert a jack plug in the "power amp in" dissconecting the first half of V7 is the noise still there ?
That may help to narrow it down further.
Maybe explore where the negative feed back comes in and components around the presence control.

acorkos · 11-09-2009, 11:15 PM

Quote:

Originally Posted by oc disorder

If you insert a jack plug in the "power amp in" dissconecting the first half of V7 is the noise still there ?

when i insert a plug into the power amp in jack, the noise (and other normal sounds) disappears

melvin · 11-10-2009, 06:34 AM

It seems you've narrowed it down to V7b.

any resistor connected to it is suspect. could one of the cathode resistors inject hiss in a follower?

bad solder joint?

acorkos · 11-10-2009, 12:09 PM

R140 off V7B (100K) was the problem

11-09-2009, 05:34 PM	#1
acorkos Senior Member Join Date: Jul 2006 Posts: 204	Fender '94 Twin Amp Noise... When I flip the standby switch, there's a static noise coming from the speakers. All front panel control pots are set at zero. And noisy with/without a guitar cord plugged in. The noise persists when preamp tubes V1-V6 are removed. The noise is gone when V7 (effects loop) or V8 (PI) are removed. It's not the tubes. It's not the plate resistors. It's not the power supply filter caps. I know because I replaced all those things. I've cleaned and retensioned all the tube sockets, too. I replaced the .022uF PI/power tube coupling caps C32/C33. I also replaced coupling caps C29, C23 and C26 which run between V6 and the PI via the effects loop tube V7. Noise persists. I also replaced power supply resistors 5W/1K R91/R154 and 2W/4.7K R103...noise persists. Any ideas as to what to check next to determine where the noise is coming from? Thanks for your help! The schematic is on page 2 of the link.... http://www.fender.com/support/amp_sc..._Schematic.pdf Last edited by acorkos; 11-09-2009 at 05:59 PM.

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11-09-2009, 08:55 PM	#2
oc disorder Senior Member Join Date: Jul 2006 Posts: 434	Hi acorkos I would have thought it would be a plate resister R146 even R 145 but you say you have replaced these. If you insert a jack plug in the "power amp in" dissconecting the first half of V7 is the noise still there ? That may help to narrow it down further. Maybe explore where the negative feed back comes in and components around the presence control.

11-10-2009, 06:34 AM	#4
melvin Member Join Date: Aug 2009 Location: Seattle, Washington Posts: 86	It seems you've narrowed it down to V7b. any resistor connected to it is suspect. could one of the cathode resistors inject hiss in a follower? bad solder joint?

11-10-2009, 12:09 PM	#5
acorkos Senior Member Join Date: Jul 2006 Posts: 204	R140 off V7B (100K) was the problem

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