Marshall VS65R hum on both channels

[Osiris81]

Hello,

I've recently tested my old amp and it had an awful hum, specially on the clean channel. I've opened it up and replaced the filter caps with no success. I've then realized, that the tube is microphonic, so I've replaced it with a new one.

Now the extreme hum on the clean channel is gone, but the amp has still an annoying hum even without the guitar cable inserted. When I insert the cable, the hum intensifies and quite some hiss is audible as well on both channels (volume turned all the way off). This gets much worse when I turn up the reverb, even if volume is still off. The same cable/guitar on my small cube amp is almost dead silent.

When playing, the amp sounds quite harsh (clean channel) and is extremely loud, it is quite hard to dial in a normal room level with the volume knob.

I'm not sure if this is the normal behavior of this amp or if there is still something off?

I've inspected both PCBs of the amp, all the capacitors seem fine (no leakage) and I could not find a cracked solder joint.

The big 5W resistors next to the power button get very hot, about 110 degrees Celsius (230F), is this normal?

Thanks for any help!
Cheers,
Rico

[The Dude]

Here's the schematic (see attached). Do you have a scope? marshall_vs65r_65w.pdf

[Osiris81]

Yes, I have a scope. I've just tested the +-15V rails and they are 15.9V and -15.4V. The resistors R18 and R17 are the ones that get so hot as well as the two 15V zener diodes. Other than that, only the tube gets about 50 degrees Celsius, everything else seems normal.

[The Dude]

Those dropping resistors do typically run hot. It's not something I'd worry about.

[Osiris81]

Ok.

The problem is, my skills with analog circuits are very limited, even with the schematic I have no clue where to begin...

[x-pro]

Ok.

even with the schematic I have no clue where to begin...

If there is no problem in the input connectors, then you are on your way to either service or a master for this unit.
Otherwise, a good-looking device will become both sightless and soundless.

[Osiris81]

I want to fix the amp by myself or I will sell it as broken. I'm sure the cost for letting it repair here is more than buying a used one.

I thought it'd be fun to fix it and just need some hints where to look...

Anyway, I've disconnected the preamp and shorted the input of the power stage to ground and the hum is still there (the same as when I don't connect the guitar cable). I guess I will now try to power +LT and -LT with my lab power supply (+-30VDC) to see if the hum is coming from the power supply or not.

[Osiris81]

I've now tried the output stage with my lab power supply and it is dead silent. I've fed some music to the input (R1) and it plays normally but very quiet, I guess the levels of the preamp are higher than a headphone output...

So the hum seems to come from the power supply, right? I've already replaced C11 and C12, so what else could it be there?

[x-pro]

so what else could it be there?

BR1, D14, D15, R43 fault

[g1]

When you replaced the filter caps, was that just the 2 big ones or C17 and C18 as well?

[Osiris81]

I've replaced C17/C18 now as well, no change.

BR1, D14, D15, R43 fault

These are fine, just tested it.

Maybe the hum of the power stage / supply is just normal for this amp, it is not loud. The hiss and hum of the preamp with the power stage together is much worse.

I also noticed that it now makes a plop some seconds after power-up. I've recorded it here:

Hmm, just realized that the old tube does not make that plop...

[g1]

With speaker disconnected, do you measure any DC on the output?

[The Dude]

Since you have a scope, I would scope all of your power supplies and see if any have excessive AC. If the amp is quiet with your bench supply, you almost certainly have a power supply problem.

[Osiris81]

With speaker disconnected, do you measure any DC on the output?

about 5mV, so I'd say no.

[Osiris81]

If the amp is quiet with your bench supply, you almost certainly have a power supply problem.

I've only tested the output stage with my power supply so far. To test the whole amp, I'd need both, the internal transformer (for the tube) and my power supply.

As far as I can see it, it can simply disconnect the two red wires and the black wire from the transformer and connect it with my power supply, then plug in the mains as well and power everything up and test it this way, right?