Need some guidance with DSC 40C

[chikov]

Hello. I was asked to diagnose 3 year old Marshall DSC 40C. Problem is that after a few seconds going off standby, the sound becomes crackly and distorted, and you can smell like something is getting hot (no smoke though). I checked all the tubes on my tube tester and they all checked ok. Next, I pulled the chassis and checked the voltage at the BIAS terminals (one black, two red ones). At first the voltage showed about .85mV dc, then it dropped down to .50 ish and I began to hear some hum and popping noises. After I began to feel heat, I turned the power off and discovered that one of the 5W resistors (see photo, circled in red) was burning hot.
I tried to find the schematic for this model, to check voltages, but could not find it. Can anyone help, which direction i should move next, or does anyone has the schematic? it has two EL34 output tubes.

[bepone]

dont even try to diagnostic this cheap pcb type of amp, throw all out, ,keep the transformers and build any decent amp

[rangdipkin]

Your screen be pulling mad current, I'd look at replacing tubes. If only one of the two 1K 5 watt resistors is getting hot, switch tube positions and see if the issue stays in the same socket or follows the tube, and then dig deeper if needed.

[chikov]

dont even try to diagnostic this cheap pcb type of amp, throw all out, ,keep the transformers and build any decent amp

I hear you bro! I build my own amp all from scratch (minus the transformers) and I love the tone of it. But this amp - it seems that the problem is one of the tubes.

[chikov]

Your screen be pulling mad current, I'd look at replacing tubes. If only one of the two 1K 5 watt resistors is getting hot, switch tube positions and see if the issue stays in the same socket or follows the tube, and then dig deeper if needed.

Thank you for your help. Yes, I also think the problem is one of the tubes. Tomorrow I will swap them around. the funny thing is that they pass the test on my vintage tube tester, but the tested does not hit them with the higher voltage like the amp would.

[rangdipkin]

I rarely use my old tube tester for that very reason... newfangled rock and roll machines are damn abusive to the poor screens! Another thing they cant always test for is intermittent shorting and mechanical failure in vibration prone setups... what could measure rock solid on a tester could become a shorting, arcing mess in a high powered combo amp, once the tube starts receiving the beatdown from the speakers.... a little mechanical damping goes a long way! If the problem remains with the socket, you could have something like a bad grid leak resistor or a leaky coupling capacitor from the PI, but IIRC you mentioned the bias current actually dropped... id be more suspicious of an intermittent screen short pulling a heap of current to god knows where!