Marshall DSl 401 power tubes not lighting up

[holt2010]

Hey guys my 2002 DSL 401 died about a month ago and I am struggling to find out what's wrong with it. Before I go any further, it has had the bridge rectifier changed and heat sink added. When it died I was playing and the sound just cut out so I removed the chassis and found fuse F4 blown. I replaced F4 and it was good for about an hour and then blew again. I took the board out and checked all the solder joints and touched up some of them. I couldn't find any components that looked damaged or burnt. I changed the tubes out for another set I had and fired it up and it ran for about 20 minutes and the bias held at 90mv and didn't runaway. However after about 20 minutes I lost the tubes again. Could the power transformer be at fault. Any help would be much appreciated guys.

[Jon Snell]

The transformer is the last thing I would suspect.
F4 is a weak point as are the connectiont to and from it.
Check the heater voltage is on the cables coming from the transformer and follow the test through up to the pins 2 & 7 on the octal bases.
There will be an issue with a connector in and around there.

[g1]

Did you replace it with a 'slow-blow' type 6.3A fuse?

schematic attached.

[holt2010]

Yes I did. Also the fuse doesn’t blow now but the tubes still don’t light up.

[Jon Snell]

Check you have 6.3 ish volts between R34 & R35 then keeping one meter probe on the fuse follow the traces to the valve and back through to see where you open circuit is.
Check you have heater voltage on BR1.
Keep a logical mind and follow the Fil circuit through.

Watch out for charged up smoothing capacitors!!!

[holt2010]

Thanks for the help everyone, I didn't get 6.3 volts at R34, R35 the BR or the fuse. Also didn't get it at W!9 or W20 either.

[The Dude]

To be sure and for clarification measure AC volts between W19 and W20- not to ground.

[Jon Snell]

Due to poor crimping/soldering the spade connectors can go high resistance on the crimp where the transformer wires enter. Check the transformer connections on the connectors and pay attention to the wire entering the connectors.

[holt2010]

I checked voltage between W19 and W20 and got 0.

[The Dude]

Unplug the transformer wires from W19 and W20 and measure the resistance of the winding. You may have an open filament winding.

[holt2010]

Unplugged W19 and W20, the winding was not open however it was reading about half an ohm

[Helmholtz]

Unplugged W19 and W20, the winding was not open however it was reading about half an ohm

The heater winding uses few turns of heavy gauge wire, so 0.5R is certainly not too low. Real resistance should be more like 0.2R or even lower, but such measurements are tricky.

[Helmholtz]

I checked voltage between W19 and W20 and got 0.

Sure you used the AC voltage setting of your meter?

[DrGonz78]

Unplugged W19 and W20, the winding was not open however it was reading about half an ohm

Well that would make me think that the open connection is located in the circuit. A low reading across the filament winding seems about right.

[g1]

With W19 and W20 disconnected, check AC volts between the 2 wires from the transformer.