Fender SF Twin Reverb Hum

[JAelec]

I'm trying to fix a 1976 Silverface Twin Reverb that has some hum...

My first thought was to change the filter caps... but looking inside the amp, it seem they have already been replaced with F&T ones.

I'm not sure what too look for next... any suggestions are welcome!


Schematic is here:
https://www.bustedgear.com/images/sc...er_100W_SF.pdf

[Jon Snell]

Please state the frequency of the hum, Mains hum 50/60HZ or Smoothing hum 100/120HZ.
Level shifts with control adjustment or not.
That woulld be helpful.

[Helmholtz]

Does the amp work otherwise?
Output tubes good and balanced?
Preamp tubes good?

Scope availalable?

[JAelec]

Volume controls do not change the loudness of the hum... I think it sounds like 50hz hum. So perhaps it is filament hum from a tube? What is the correct procedure for checking this... would I remove one tube at a time in a certain sequence?

I tried to catch the hum with my scope, but found it hard to get a stable reading that I could take a picture of. But I found that my scope has a hold function which made the signal freeze on the display. I'll post a pic of how that looks.

The time and voltage scales are not where they should be because I had just discovered the hold function after fiddling around trying to get the trigger to catch a stable reading.

Perhaps it is not possible to catch hum with a stable reading? Is it correct to use the hold function to "catch" the hum?

I am a little unexperienced at looking at noise on the scope... When looking at sines, which I am more familiar with, they are quite still on the display as long as the trigger is set correctly....so I hope you will excuse me for the setting on the scope I can do a better pic tomorrow... perhaps you can get something out of it anyway....but If you have any suggestions on how to make my scope catch the hum, I would welcome them!

Volts/div is set to 10mV
time/div is set to 5ms

[Helmholtz]

First of all set the trigger source to "~" (meaning line frequency).
Make sure the probe's ground clip is connected to amp ground.
Then try to increase signal magnitude by setting input sensitivity to a lower mV/div value.

Make sure the inner knobs with the arrow are in calibrated (arrested) position.

Where did you connect the probe?

[g1]

Output tubes good and balanced?

This is critical and there should be a user adjustment on the rear panel for this ("output tubes matching"), as well as a "hum balance" pot.
JAelec , have both those trim pots on the rear panel been adjusted for minimum hum?
(and if the hum balance pot does nothing, chances are it is burnt out)

[JAelec]

I listen again referencing a 50Hz and a 100Hz tone from an sig generator. It is definitely 100Hz not 50Hz as I said before. I also have a Bassman 70 that has the same problem; 100hz noise independent of any volume pot. The Bassman hum is is a lot louder then the Twin Reverb hum.

Today I tried measuring with the scope again, but his time I used the speaker instead of a dummy load. Now I finally get a reading.

Here are the results on video first for the Bassman 70 and then the Twin.


Bassman 70

Voltage/Div = 20 mV
Time/Div = 5 ms

Comment: I'm pushing the hold button here to get a better look, which is makeing the reading go down in amplitude for some reason. Why does this happen?

Twin Reverb
Voltage/Div = 10 mV
Time/Div = 5 ms

Comment: In this video I also push the hold button, but I'm not sure if the value being displayed is for the Twin or the Bassman... could be that the scope stores the value until the reset button is pushed therefore displaying the stored value from the Bassman... will have to check the manual for the scope.

I have no idea why the dummy load did not work???


I also found the problem on the Twin... one of the new filter caps have gone bad... I paralleled a 100uF 500V capacitance to each cap in the filter section one at a time... When parallelling the cap circled in red below (100uF 350V)... the hum almost disappears and is now at an acceptable level.





So this is the faulty cap... strange as the caps look brand new. F&T is supposed to be a reputable brand high quality German made part. But I see the rating for this cap is only 350V perhaps this is too low for startup voltages?

[Helmholtz]

Please do not mix different amps in one thread.
Scope pics clearly show 100Hz power supply ripple.
Scoping should work with dummy load.
I don't expect a difference between speaker and dummy resistor.

As the Twin uses 2 stacked 100µ caps, a 350V rating is sufficient.
Make sure the two 220k balancing resistors are good.
Also check cap polarity. Whenever an ecap was eposed to reverse voltage it's likely damaged.

[JAelec]

Please do not mix different amps in one thread.

Sorry, I'll keep that in mind.

Scope pics clearly show 100Hz power supply ripple.

Would you be able to pick out the 100Hz in the twin reverb video without the hold button?

As the Twin uses 2 stacked 100µ caps, a 350V rating is sufficient.
Make sure the two 220k balancing resistors are good.

So are these caps equivalent to one with 50uF and 700V rating then?

What is the purpose of the balancing resistors... is it to divide the voltage equally between the two caps?

Also check cap polarity. Whenever an ecap was eposed to reverse voltage it's likely damaged.

They seem to be in the right positions.

[JAelec]

This is critical and there should be a user adjustment on the rear panel for this ("output tubes matching"), as well as a "hum balance" pot.
JAelec , have both those trim pots on the rear panel been adjusted for minimum hum?
(and if the hum balance pot does nothing, chances are it is burnt out)

Thanks for your comment... I'm not sure about the trimpots on this amp... could you please guide me?

There are two adjustment pots on this amp... One is inside the amp:




Is this the bias balance?


The other is at the back of the amp:



Is this the hum balance or output tubes matching?

[Helmholtz]

Would you be able to pick out the 100Hz in the twin reverb video without the hold button?

Distance between peaks is 2 divisions corresponding to a period of 10ms.
Frequency is 1 over period, i.e.1/10ms = 100Hz.
If you can't get a steady trace without the hold function, there's a problem with the scope.

So are these caps equivalent to one with 50uF and 700V rating then?

Yes.

What is the purpose of the balancing resistors... is it to divide the voltage equally between the two caps?

Yes, they make sure each cap gets the same (50%) voltage.

[g1]

There are two adjustment pots on this amp... One is inside the amp:
Is this the bias balance?

Yes. And the one on the back is the hum balance.
You should hear some difference in hum level while adjusting either one. Set for minimum hum after you replace the bad cap.

[JAelec]

Make sure the two 220k balancing resistors are good..

They are good... they measure 227k and 228k.

Is there anything else that you can think of that could have made the cap go bad?
If not then perhaps it is just a lemon.

[mozz]

Be sure it is connected correctly. Maybe the previous tech hooked it up wrong then corrected it, could have damaged it and caused it to fail early in it's life.

[JAelec]

Yes, they make sure each cap gets the same (50%) voltage.

But if those resistors were removed... wouldn't the caps still get the same 50% voltage?