Marshall DSL100H cutting out... What's causing this?

[cluster]

Hello all,

I have a Marshall DSL100H that cuts out after about 20 minutes of playing. The sound just drops. No crackle, boom or buzz. Just silence. The weird thing is the power light (the switch light) stays on. The fuse doesn't blow or anything. Just silence. The only way to get the sound back on is to switch power and standby off and then back on again. This is one very weird bug. Play for 15-20 minutes -> sound cuts out suddenly -> power amp off -> power amp on again -> sound comes back for another 15-20 minutes -> rinse and repeat. What gives?? Has this happened to anyone here before. Thanks

[Helmholtz]

Well, it's something intermittent, probably triggered by temperature (or vibration).

Many possible reasons.
Start troubleshooting by measuring essential voltages while in failure mode.

And please post schematic.

[cluster]

Well, it's something intermittent, probably triggered by temperature (or vibration).

Many possible reasons.
Start troubleshooting by measuring essential voltages while in failure mode.

And please post schematic.

Thanks. Will do.

[The Dude]

It could be as simple as a tube that dies after warmup. I might try swapping tubes to see if that takes care of the problem. You could start by seeing if the preamp out will still drive another amp when the amp dies. That would at least tell you if the problem is preamp or power amp. Do the filaments still light when there is no sound?

[Jazz P Bass]

Please do not forget the FX loop.
If you are not using the jacks they may have become dirty.
Run a jumper cord from Send to Return.

[Mick Bailey]

The power switch is an illuminated type, so even with nothing connected it will light up when in the on position. You can plug your guitar into the FX return to see if the power amp is working. After trying the previous suggestions (which are the main cause of this type of problem), when it cuts out, try thumping the cabinet to see if it comes back on. These amps have plenty of connectors that link the different sections of the boards. They can also cut out due to the push-on connectors on the transformers being loose. Whenever I get these amps in for work I always check these. You could even have a bad fuseholder - especially the HT fuse.

[cluster]

Ok.. so I played around with the amp and found if I use a patch cable on the effects loop I could get it to NOT cut out. Problem seems to be in the effects loop area. The weird thing is the bias adjustment. One side reads 65mV (sounds about right) but the other side reads 26 volts!! How is this even possible. I checked to make sure my voltmeter was set right. Used the 3 prong lug on the board to check each side. Any idea as to why the bias is so off on one side?? Thx

PS: Just checked. The side read 26 volts is not connected to ground. So there is a short somewhere.

[Helmholtz]

Clean send and return jack switching contacts.

Sure you mean 26V not 26mV?

Still not seeing a schematic.
I don't consider it my "job" to find the appropriate schematic.

[g1]

Probably one of the 1R resistors in the power tube cathode circuits is open.
Schematic attached, but may not exactly match your revision of DSL100.

[cluster]

Here is a picture of the board. The ground on the right is shorted. Therefore I get a weird 26volt reading on the bias. Both plates are at 475DCV. Bias at grids is about -36.

[cluster]

found this schematic...

[cluster]

Probably one of the 1R resistors in the power tube cathode circuits is open.
Schematic attached, but may not exactly match your revision of DSL100.

thx. i'll check

[cluster]

Probably one of the 1R resistors in the power tube cathode circuits is open.
Schematic attached, but may not exactly match your revision of DSL100.

You were right. Found the bad 1ohm resistor. Thx