Marshall VS65R hum on both channels

[x-pro]

I hear a buzz from the electronics itself coming somewhere from where the filter caps of the tube are but I'm not sure if its coming directly from the caps, it does not come from the transformer however.
Grounding the output of IC7 (thats the input of the power stage) gets rid of most of the hum/buzz.

Electrolytic capacitors operating in the hard voltage mode change their characteristics slightly over time. The rated capacitance may stay the same or become larger. However, if you shake it in your hand, you will hear it jiggle inside. It's a dead body.

("Grounding the output of IC7") So the output amplifier is good.

[x-pro]

It's a dead body.

[Osiris81]

These are multi-segment and switching op-amps. Please be specific about which exact pins you are grounding.

IC5: I've grounded pins 2+3. I've also replaced IC5 with IC4, which is not used when the tube is not inserted. Still no change, so IC5 seems fine.
IC7: I've grounded the negative input of IC7c (should be pin 6). I've now also replaced IC7 with a new chip, still no change...

[Osiris81]

Hmm, I've now grounded the connection between VR10/R59/C4 and also grounded the connection between VR11/R61 and I still have the hum at the output, remember, when I ground the output of IC7c I don't have the hum anymore...
I still can't upload anything because of the upload limitation unfortunately...

[x-pro]

I suspect this is natural background introduced into the line output by neighboring conductors in the connector. Try shunting the line output with a 50-100 ohm resistor (hot output to the connector body). The connector connection to the board is probably not shielded.

[Osiris81]

I've tried that with no luck, still the same

[Osiris81]

I've also removed the FET that grounds the input of the output stage (T16 on the power module sheet) and grounded the input myself during startup, but that did not help either, as soon as I removed the ground from R1, the hum was back.

[Osiris81]

I just saw that on the back of the preamp PCB, there are four ceramic capacitors soldered across the 4 rectifier diodes (D1-D4) for the tube supply voltage. These capacitors are not mentioned in schematic, maybe one of these is bad, I have to check them tomorrow.

[The Dude]

Won't hurt to check, but those caps almost never go bad. If one were bad, you'd be blowing fuses and/or supplies would be low.

[Osiris81]

You were right, they are all fine. While I had them off, I also replaced the four diodes just for fun, of course no change.

[Osiris81]

I did another test with IC7c: I soldered together Pin 5 and Pin 6 directly on the backside of the PCB (so the positive and the negative input are ground). When I power the amp up this way, I have almost no hum/hiss when no guitar cable is plugged in, but I have an extremely loud hiss when the guitar cable is plugged in (much louder than in "normal" configuration, I cannot even hear if there is a hum as well or not)... I don't get it, it should make no difference if I plug the guitar cable in or not, right?

[x-pro]

The quality of the cable, the position of the controls, the shielding, the quality of the soldering in the guitar, the quality of the connector all contribute to the background in the amp. The amplifier itself, without cable and with a bypassed input, should have no background or noise within the specified limits. But this does not mean complete silence.

[x-pro]

Try shunting the line output with a 50-100 ohm resistor (hot output to the connector body). The connector connection to the board is probably not shielded.

If you tried that and there was noise, how come you didn't have it when you connected the output of IC7 to ground?

[Osiris81]

x-pro

See here:
https://freeimage.host/i/JAQWfwB

Since I'm still not allowed to upload images here I need to use one of the **** free image hoster...

[Osiris81]

I don't get it, it should make no difference if I plug the guitar cable in or not, right?

Now I found out why it is that way: The PCB of my preamp does not match the schematic!

The schematic shows a connection from the guitar plug JS1 to the input of the tube (connection R9/R10/tube socket). This connection is ground when no guitar is plugged in. My preamp has no such connection, instead, the plug is connected to the output of IC7c (connection between C10/R16). So when no guitar cable is connected, the input of the power amp is grounded and that's why the amp is silent.

Now I wonder what else is different on my amp than on the schematic... I guess the preamp of my amp is older, it has written VS65-60-00 on it, the schematic mentions VS65-60-02. Does anyone know what differences there are between these two preamps? I could not find another schematic than the one I already have...