Marshall 50 Watt Keeps Blowing Fuses

[BIG Dave]

I was playing my Marshall 50 watt Metal Panel (model 1987) last night when the sound stopped but the Power light stayed on. I checked the HT fuse (1/2 amp SLO BLO) and it was blown. I replaced it and it blew again. I opened her up and found the white wire from the bias section of the PCB had become detached from the Standby switch. I resoldered it (rear lug, closest to the Pilot lamp) put in a new HT fuse and the fuse blew again.

I have an old Fluke 83. All five diodes on the PCB measure 0.57 volts one way and 'Overload' the other way. The two resistors in line with the Bias Pot measure 15k and 56K. The resistor in line with the fifth diode measures 220k. The two 10uF bias supply caps do not give a reading (I'm thinking 10uF is out of range for my VOM). Finally, The resistors at pin 5 of the output tube sockets measure 5.5k each, and the ceramics across pins 4 and 6 measure 1K each.

Any ideas what I should check next? Thank you.


[IMG:800:532]http://www.davidgrover.co/pics/1987as.jpg[/img]

[IMG:800:532]http://www.davidgrover.co/pics/1987bs.jpg[/img]

[IMG:800:532]http://www.davidgrover.co/pics/1987cs.jpg[/img]

[ToneMerc]

Power tubes did not like that no bias condition. I have you pulled the power tubes yet?

TM

[Reeltarded]

Yeah the bias ran off the farm.

[Structo]

Ditto

[ampgeek]

Dave,

As suggested, pull the power tubes, load up a new HT fuse and hit it with power again.

If the fuse holds, make sure that you can sustain a negative DC voltage out of the bias system. Measure from pin 5 on either power tube to ground. Spin the bias adjust pot from end to end and record the range that you find. Just guessing...but it ought to be in the -30'ish VDC to -50'ish VDC.

Also measure the plate voltage on both power tubes and record for reference. Pin 3 to ground should be high 400'ish VDC to low ~500 VDC'ish.

Obviously, be wicked careful when poking around an energized amp.

Let us know what you find!

Good luck,
Dave O.