Happy New Year all. I did indeed replace the power transistors and the driver ones as well. With the power transistors, I made sure they are insulated from the chassis, however, when testing for shorts between the transistors and the heat sink I observe the following:
The meter reads short circuit initially, then over a few seconds resistance increases to open. Why is that?

So here's an update. On replacing all 4 power transistors, I eliminated the loud switch on pop. Now I get a loud hum, no sound. Noticed the TIP142 were the smaller version, so changed them out. Turned on, blew fuses F2, F3. Tested the diodes CR21-24; these were out of spec so replaced them. Replaced the washers on the power transistors as they were causing a short between collector and ground. powered on, no pop, just an incredibly loud hum, which I thought would be eliminated when I changed out the washers. Nothing doing. Where do I go from here?

Quote from: g1 on November 26, 2024, 06:56:06 PMIf 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.

@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.

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?

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

@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.

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.

Hi there, I am interested in the Honey Amp kit. Where can I purchase please?
Cheers
Rhea

Wish I could post pictures. It would so help answer your questions. I have the original circuit diagram. I can't tell what the wattage is. I would hazard that the voltage rail is 12-15V.

I plan to keep the transformer from power supply,and the reverb. Everything else to be rebuilt.

I'd post pictures but is more trouble than it's worth. Anyway - the amp. Two channels,each with two inputs, volume control and tone. Then reverb, intensity and speed. Small reverb tank, and two eight inch speakers. power supply, pre amp and power amp all on  one board. Tried to get the board reprinted, but came back nothing like the original. So just looking to put something in there.

Hi all. Customer brought in an old Dynamic solid state which looks like its had one mod too many. Don't want to give up on it, so wondering if I can lay hands on a solid state amp kit? Ideas? Cheers
Rhea.

Hey all. Doing up a little Fender Harvard Reverb Solid State Amp. The input jacks were rusty, so decided to replace them. So I rewired them, like the original (or so I thought) and input 1 is silent and input 2 buzzes quite loudly.
I have wired as follows:
Input 1 Lug 1 +ve (red wire)
Lug 2 Ground (Shield) also to Lug 3.
Lug 3 to Lug 3 input 2
Lug 2 to Lug 1 input 1
Lug 1 Black wire.

I am certain in copying the original I have gone awfully wrong.
Appreciate ideas and help.
Cheers

For some reason I can't post the photos. It keeps telling me there's an error - file of the same name uploaded. Most off-pissing.