Marshall Valvestate 8080 buzzing

[mathieuloutre]

Hello! I've just got a Valvestate 8080 for a decent price (a good deal, but not cheap) and unfortunately found out a bit too late that when turned on, with nothing plugged in and all knobs to 0, it has a constant, relatively loud buzzing sound (my fault for buying without testing!). The buzz goes away when the loudspeaker is disconnected. I've tried a few outlets too but no luck.

I took the head out and cleaned all the pots with contact cleaner but that didn't do very much (though it's probably good maintenance anyway). Visually nothing seems to be amiss, but I know very little about electronics and amp repairs. One thing to note maybe is that the transformer doesn't appear to be original.

Is there something obvious I should try? This happens on all channels, with a guitar plugged in or not. On similar thread (here and across the internet) people suggest to replace the valve but I'm not sure it's the issue since it happens even on the clean channel which as I understand doesn't use the valve. Could it be the power supply/transformer? Is there something I can look at that involves minimal soldering? (I can do it, just not very confident with an iron that's all!) — I guess what's puzzling is that it disappear when the loudspeaker is disconnected...

When turned off, it also gives a sharp pop sound. I unplugged the reverb and that seems to fix it. Any way I can get the reverb not to do that?

Here are some pictures before the clean

[g1]

1st thing would be to leave the speaker disconnected and check for any DC voltage between the wires that normally connect to the speaker (one meter probe to each wire).

[Jon Snell]

Main smoothing capacitor has dry joints.

[Mick Bailey]

Pretty much every one of these amps I've worked on has at least one failed solder joint. It's worth flipping the board over and inspecting it closely. Bad joints look granular, have a circular crack around the component leg and sometimes the joint is raised like a small volcano. If in any doubt, resolder them - I use lead-based solder for this. The larger vertical can capacitors to the bottom left of your picture are a common failure point, (as per Jon's suggestion) as well as the smaller vertically mounted ones, the larger ceramic power resistors and the legs of the output transistors. Be aware the HT smoothing capacitors for the valve hold a charge long after the amp is switched off.

[mathieuloutre]

That's fantastic insights all, thank you!