Marshall TSL 122 hum and V8 cap ID help

[beedoola]

I picked up this amp and it had some bad motor boating sounds going on. I replaced V4 and that helped. Amp seems to work but there is some bad hum.

The 1k 5W screen resistor on V8 was cracked. I replaced it but I see there is a small ceramic cap on V8 between pins 6 and 7. It looks a little burned; anyone know the value? I might replace it just to rule it out.

[Jon Snell]

Where? Photo?

[beedoola]

This isn't might amp but a pic from the same board. See the cab in question in the lower left of the photo.

[Pedro Vecino]

If it is shorted it will inevitably generate hum. You can remove it without problem and it will work without it.
But, if your board is identical (light green color) it is susceptible to problems (bias drifting and other sources of hum).

[beedoola]

Thanks, I'll clip it and see if that helps.

I ordered one of those Bias adjust kits from a place in the UK:

https://valvetubeguitaramps.com/prod...it-tsl-series/

[Enzo]

Schematic clearly states it is a 22pf 1000v cap, visual tell us a ceramic disc.

As Pedro says, in almost all cases it can be removed and not replaced.

[g1]

I see there is a small ceramic cap on V8 between pins 6 and 7..

The pinout is a bit confusing as pin6 is missing from the socket. The cap is actually between pins 3 and 4 as shown on schematic in post #2.

[beedoola]

Got it. Clipped the cap and the hum went away.

I'm still getting some random popping that seems to be coming from the power tubes. If I hit them with a chopstick it pops.

I cleaned all the sockets with Deoxit previously so I'm not sure if that is the issue.

[beedoola]

https://www.dropbox.com/s/rik1p9tnhd...%20PM.mov?dl=0Here is a short video of the crackling.

I resoldered all the power tube sockets and that didn’t help.

The amp can be shut down, started up and turned off standby and it will idle fine but if I tape the tubes, chassis, transformers, it will pop and then go silent again.

More tapping and it will start again. Eventually I might start going on its own and going continuously.

[Pedro Vecino]

Mark the tubes for easy identification and test them in pairs until the faulty is isolated.

[beedoola]

Can the amp be run half power with the two inner tubes?

[g1]

Yes you can run only 1 pair for test purposes. But before that, have you isolated the issue to the power amp section? Does the fault still occur when you run your signal into the FX return?

[beedoola]

Fault still occurs with fx loop.

I’m not getting signal with two inner tubes.

[The Dude]

When the amp goes silent, measure voltages at the output tube sockets and see if anything is missing- plate voltage, screen voltage, filament voltage, etc.

[beedoola]

So I started to get the output noise issue again and the output transformer was making a kind of crackling sound. Not sure if it's new or if it was happening before and I just didn't hear it as now the amp is on my bench. Like it literally sounded like crackling in the OT.

When I got the amp - got it at a garage sale for next to nothing - and got home, the speaker out was connected to the 8 ohm jack. I can't remember if the switch was on 4 or 8 but the combo speaker load is 16 ohm so it could have been used with improper impedance for some time, I don't know.