Hybrid Amp Diodes

[adam75dfw]

My Marshall Valvestate blew its fuse with no warning and for no apparent reason the other day. The tube and the transformer are ok, but I discovered 4 bad diodes in the power module (D12, D13, D14, D15).

Question #1: D14 and D15 are parallell to a resistor and a capacitor in their own little isolated circuit that connects to nothing but the chassis. What is the purpose of this little circuit?

Question #2(&3): What are the most probable causes of these 4 fried diodes, and will replacing the diodes likely fix the amp or just fry 4 more diodes?

Any help appreciated!

The 1 page schematics are available at: http://www.drtube.com/schematics/marshall/v100-62-02.pdf

[Enzo]

Are you sure D14, 15 are shorted? That is a 10 ohm resistor across them.

That little curcuit bonds the common or ground of the amp circuit to the chassis or earth ground. I would be very surprised to see them shorted, flip your meter back to ohms and see if it says 10 ohms across them - the resistor value. I bet they are OK.

But I also bet D12, 13 are also OK. They are right across the output transistors if you look at the schematic. It is very rare for those diodes to short, but it is common for someone to blow up their output stage. I think you will find the power transistors on their heat sinks aer shorted, at least one per polarity. ANd once the bad transistors are removed, those shorted diodes will have healed themselves.

Those are darlington power transistors, and I generally replace them with TIP142 and TIP147, noting polarity and PNP vs NPN.