Loud hum when turning the standby off on my 1990 Fender the twin "red knob" for 20sec

[jalexquijano]

Im experiencing a loud hum whenever i turn my 1990 Fender red knob twin standby switch off. This only happens when the amp is cold and has not been used for last couple of hours? Just to mention all of the filter caps were replaced in both the power section and the preamp section including the bias filter cap. Should i call my tech to solve this or can i fix it myself? The loud hum just lasts 20 secs. After that the noise starts to decrease and goes back to normal operation.

Any clues???

[Jazz P Bass]

It may be that the main power supply capacitors are at the end of there life.

If one of them is really bad, the amp will hum at turn off.
When the caps are bad you will see an abnormal amount of Volts ac ripple on them, which can be measured with a decent volt meter set to read Vac.
If you have anything with a close value (voltage value must be equal or greater) you could run them parallel to the old ones to see if that helps.

[potatofarmer]

Have you tried turning the amp on in standby mode, letting it sit for maybe five minutes, and then flipping the standby switch into "play" mode?

My guess is one of the power tubes (or a pair on one side) takes a little longer to warm up, but I guess that shouldn't matter much on this model.

[jalexquijano]

Have you tried turning the amp on in standby mode, letting it sit for maybe five minutes, and then flipping the standby switch into "play" mode?

My guess is one of the power tubes (or a pair on one side) takes a little longer to warm up, but I guess that shouldn't matter much on this model.

I guess not. I am just letting them warm for 30 secs and then fire it up. It wasnt producing this loud hum last week. the 4 power tubes are Brand new (Groove Tubes 6L6 GE matched quad) made in china and bought at guitar center in Orlando. Paid US$155 for the set. Only V7 is Brand new (Groove tuve 12ax7-C).

[alexradium]

sounds like very slow charging filter caps,if the amp is original i guess its time to replace them,you can try parallel a good 22/47uF one at a time on the preamp/PI ones and hear if it gets better.

[jalexquijano]

Its weird since i replaced the three preamp 22uf filter caps with brand new ones.

[J M Fahey]

Unplug the PI tube and repeat the test: results?

As a side note on:

Im experiencing a loud hum whenever i turn my 1990 Fender red knob twin standby switch off. This only happens when the amp is cold and has not been used for last couple of hours?

If an amp is cold and unused, so presumably OFF, how can turning Standby switch OFF create any sound?
It would be switching zero Volts in any case.
By the same token, how can any amp cold an unused produce any sound at all, no matter what you do with the Standby switch, which would be "switching" zero Volts?
What element would be producing an acoustic/audible hum sound?

EDEIT: please post schematic.

[eschertron]

What?

I presume turning standby 'off' is the same as putting into 'play' mode. Power on, standby off.

I am just letting them warm for 30 secs and then fire it up.

If 30 sec isn't long enough, but 20 sec more is, try letting it warm up with standby 'on' for a whole minute. If it still hums after that, there's something screwy with what ever follows the standby switch.

please post schematic.

+1 so we can see what's before and what's after the standby switch.

[Enzo]

STandby SWITCH must be on. Both power and standby have to be on to operate.

Standby switch ON is PLAY mode.

Turning that switch ON brings the amp out of standby.

Turn the switch OFF to put it in standby mode.

[J M Fahey]

Thanks but we so far are having a communications problem with our Friend jalexquijano, we must first sync with him for a successful solution to his problem.

In any case and to save some time, I already suggested a test to be performed

[Enzo]

I was responding to eschertron.

[eschertron]

STandby SWITCH must be on. Both power and standby have to be on to operate.

Standby switch ON is PLAY mode.

Turning that switch ON brings the amp out of standby.

Turn the switch OFF to put it in standby mode.

Why of course, you are right
My personal confusion results from odd labeling. While some amps label their switches as "standby/on", others do not. Case in point: the standby rocker switch on my Fender Super 60 has an "off" position and an "on" position. So of course for the amp to be in standby, the switch has to be in the "off" position. Clever? Confusing enough that I never look at the labels, I read them with my ears.

[jalexquijano]

Ok. Once again. I turn turn the amp on, wait for 20 secs and flip the stanby switch on. I start hearing a loud hum for 20 secs which suddenly dissapear. I already swapped some 12ax7 tubes and the proble continues. Should i consider changing the plastic input jacks on the effects loop or else. I reas somewhere that these jacks must be replaced to avoid noises.

[Jazz P Bass]

Should i consider changing the plastic input jacks on the effects loop or else. I reas somewhere that these jacks must be replaced to avoid noises.

If you want to 'rule out' the Fx jacks, simply run a jumper cable from one to the other.

[pdf64]

...Should i consider changing the plastic input jacks on the effects loop or else. I reas somewhere that these jacks must be replaced to avoid noises.

Clever, experienced people have taken the time to respond to your specific query.
How about reading, and acting on, the 1st lines of post #3 and #7?