Fender M-80 Loud Pop on Switch on/off

[argenta]

Hi everyone. I have a Fender M-80 which came in making a lot of noise as well as loud switch on/off pop noise. On opening it, I found two zener diodes and  filter caps O/C. Replaced these, and the noise has gone but the pops remain. I have tried turning the volume down but this has no effect. I would like to put a mains filter cap across the switch, however, this is a DPDT switch with active and neutral on one pair and two connections running to a thermal breaker on the other pair. I checked the breaker and its in working order. Can I still use a filter cap and will it be effective?
Thanks,
Rhea deLascy-Wells.

[Kaz Kylheku]

Check for any DC offset at the speaker output.

On/off popping in an otherwise working power amp could indicate that there is a significant DC offset on the output, so that's something to check for.

Audio power amps usually have single-digit millivolt DC offsets. Sometimes higher, but rarely above 0.1V by design.

See the attached zip file: is that the schematics matching your amp?

I don't see a muting circuit in the power amp, other than that the limiting circuit in the power stage has some capacitors in it which looks like should mute the signal while they are charging. Two zener diodes are found in that circuit (CR42 and CR44: CR stands for crystal rectifier, same as D for diode); I wonder whether those are the ones you changed.

[Jazz P Bass]

To try and prove whether or not the power switch is the issue, turn the power switch to On & then plug the power cord in to the outlet.

[argenta]

@kaz kylheku: The schematics you sent are for the M-80 Chorus amp; the M-80 I'm working on is just that - M-80. Thanks anyway, that said, I will check the dc offset on the speaker.

@jazz p bass I did verify the switch, thanks for your advice.

Will keep you informed.
Cheers.

[g1]

the M-80 I'm working on is just that - M-80.

I believe the plain M-80 is the HM series amp.  Schematic attached.

[Kaz Kylheku]

I believe the plain M-80 is the HM series amp.  Schematic attached.

I'm not looking at them side by side or anything but the power section looks identical to me based on the details I remember from studying the chorus model's power section yesterday. Of course the problem might not be the power amp; you never know.

[argenta]

Thanks for the schematic. I changed out CR25 and CR26 - these were burning out. Apparently these zeners have a tendency to run hot anyway.

[argenta]

Ok. So DC offset was 0.003mV, which is negligible. I notice it will buzz when switch ed on, even with volume turned down, and pop loudly on switchoff. The buzz is a quick sound. When I plugged in to the headphone input it was quiet. Not sure I can eliminate this problem, as there isn't any way I can put a snubber circuit across the mains. Unless you have thought of something I haven't?

[J M Fahey]

I believe the plain M-80 is the HM series amp.  Schematic attached.

I'm not looking at them side by side or anything but the power section looks identical to me based on the details I remember from studying the chorus model's power section yesterday. Of course the problem might not be the power amp; you never know.

Both are "basically" same, only the stereo version has *one* pair TIP142/147 per channel, each driving some 60W per single 8 ohm speaker.

The non chorus one is Mono, drives (roughly) same 60W into 8 ohms but almost 100W into 4 ohms, so it needs *two* pairs to meet current needs.

Troubleshooting is same for both. 

Ok. So DC offset was 0.003mV, which is negligible. I notice it will buzz when switch ed on, even with volume turned down, and pop loudly on switchoff. The buzz is a quick sound. When I plugged in to the headphone input it was quiet. Not sure I can eliminate this problem, as there isn't any way I can put a snubber circuit across the mains. Unless you have thought of something I haven't?

Different problems:

Turn on *thump* comes from empty electrolytic caps charging up, so for a fraction of a second amp "wakes up stupid" (just like me ), it´s almost inevitable.
Buzz comes from same.

BIG amps (PA, rack type. (powered mixers) suffer the same but hide it by using a time delay relay.
Too expensive for small amps.
Some incorporate some kind of mute.

You *can* add a snubber.

If double power switch (hot + neutral) add one snubber per switch section .

Or a single snubber across PT primary terminals.

[Loudthud]

To try and prove whether or not the power switch is the issue, turn the power switch to On & then plug the power cord in to the outlet.

Did you do this ? What were the results ?

[argenta]

@Loudthud, yes I did. Same result. As the mains power cable is connected directly to the switch which then goes to primary side transformer via thermal breaker, I believe the snubber is the only answer. The amp works perfectly otherwise, just needs something to lower the sound of the pop.

[g1]

If you got the same pop at turn on by plugging it in with the switch on, that eliminates the switch as a cause for the pop.
Look elsewhere for a solution.

[argenta]

If you got the same pop at turn on by plugging it in with the switch on, that eliminates the switch as a cause for the pop.
Look elsewhere for a solution.

When you plug headphone jack in, that quietens it...thanks anyway.

[joecool85]

My Dean Markley does this.  I added a switch next to the power switch.  It connects/disconnects the speaker output.  I turn the amp on, then turn on the speaker connection - no noise.  Turning off is the reverse (although it's fairly quiet on turn off anyway).

I had thought about putting in a time delay relay type circuit, but it was more complex than I wanted to do and a second switch was cheap, easy, and reliable.