DSL100H Blows mains fuse with I/P signal

[paddy]

Hi guys,
I have a fault which is causing me some serious headaches and I'm wondering if anyone has some suggestions.
A customer brought me a Marshall DSL100H (New Vietnamese Model) which he said keeps blowing the mains fuse. After replacing the fuse I was treated to a light show inside one of the power tubes and the fuse blew. Suspecting that I had an internal short, I replaced the output quad. After biasing everything went well until I strummed a few chords at a decent volume. Once again there was a flash inside one of the tubes and, before I could hit the off switch, the 100 Ohm heater balance resistors started to smoke.
I swapped out the tubes with a known good quad from my own amp and the same thing happened again.
I am considering that the power tranny may be shorting hv to the heater windings or that possibly the O/T is somehow causing this. Importantly, the blown fuse/tube arcing only happens when strumming the guitar.
Given that both transformers are expensive, I'd like to know if anyone here has any ideas before ordering any. Unfortunately I don't have a schematic but one is on the way.

Will appreciate any help or suggestions.

Cheers,
Paddy

[Stevem]

That resistor is heating due to the outputs shorting every time heater to plate, or heater to screen.
This will only happen due to arcing, either due to a bad OT, arced over carbon traced output tube sockets or both!.

take a good long look at the tubes sockets and with a very sharp thin nail or scribe scrape off any black arc traces until you see good new material, also look for signs that something got spilled into the output tube socket area.
This crud can be swabbed out most times with a few Q tips wet with windex.
If the amp has a HT fuse and not just a mains fuse I would replace the fuse with one of half the amperage and this should blow open fast enough to save you from killing another output tube in the process of confirming that the amp is now ok.
If this fuse pops I would have to say that your OT is done, I would also at this point replace those 2 100 ohm balance resistors and if this amp has plug in connectors off the PT to feed the heater voltage into the main board, make sure that the slip on spade connector and the male counterpart on the board show no sign of getting hot.

If they do be sure to resolder that board connector and solder the heater wires right to that connector, or the board directly.
.
Good luck and let us know how you make out!

[martin manning]

I'm with Steve. A likely scenario is that the amp was played without a load connected, and extreme plate voltage resulting from that caused arcing between pins 3 (plate) and 2 (filament). With high voltage on the filament, the balance resistors are a path to ground, and/or the heater insulation can break down causing an arc to form from the filament to the cathode inside the tube. A conductive carbon trace is left on the socket, so there will now be HV leaking into the filament circuit and the failure will recur more easily.

You know exactly where to look- on the socket that keeps blowing an output tube, betweens pins 2 and 3. The suggestion to scrape away carbon traces is good, but the socket may have to be replaced. The OT could have been damaged by breakdown of its primary insulation, but you will have to fix the socket problem first.

[paddy]

Hi Guys,

Thanks for your suggestions, they are greatly appreciated. I tend to agree that the OT may now be breaking down. There are no signs of carbon on the socket but I will replace it anyway. One of the faulty tubes has a measurable resistance between Plate and Heater which obviously explains the burnt heater resistors.
I will re-post when new parts are fitted.

Thanks again,
Paddy