Tuesday, 4 October 2011

A bit of a fire in a Marshall DSL50 guitar amp

Here's an interesting little job just done.

It's a Marshall DSL50 valve guitar amp with the report "keeps blowing fuses and there's a funny smell".

Let's open it up. The output tubes checkout at very low emission using my Mullard MkIV valve tester. So whatever else I find, they need replacing anyway.

One of the valve bases is a bit black and sooty, and so is the socket that it came out of.  Better take a closer look.  Let's get to the PCB.

 Valves out and top plate removed. That'll be the trouble  then!

 A closer view at that valve socket.

 Get the socket out of the way so we can see what we're really dealing with.

 That's major carbonisation between plate (pin 3) and heater (pin 2).

All that conductive area needs to be milled out and generally cleaned up.
The hole in the board is fine, the high voltage won't track across that!  The pad at the anode is burnt away, but there's no connection on this side of the board.  The burnt pad at the heater is a bit more of an issue, we'll need to make a repair there - de-solder braid is still useful after it's been used for de-soldering the socket.  Don't forget to clean up the socket as well!

Here's the topside view, note preventative medicine at the second valve position.
We've lost the pad at the heater pin, but again it's not used on this side of the board - we don't worry about it.

A decent repair at the PCB heater track using a bit of used de-solder braid.

Now reassemble with a new pair of EL34 outputs, refit the original pre-amp tubes, power-up and set the bias.  Working great, no more fire or smoke and no funny smells.  Soak test for a couple of hours and sign it off.

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