Re: Marshall 2203 eats fuses

Mike Schway wrote:
In article <mschway-434DAD.18580129022008@xxxxxxxxxxxxxxxxxxxxxxxx>,
Mike Schway <mschway@xxxxxxx> wrote:

In article <d1dgs3pinsaiht1ijp2a921ang6t8l8rvr@xxxxxxx>,
Ernie Snakewater <nospam@xxxxxxxxx> wrote:

My Marshall 2203 blows the mains fuse as soon as I take him out of
standby.

Even with pulled power tubes the fuse blows.

[snip]

shorted rectifier diodes? (somewhat common)
Shorted preamp tube (less likely)
Shorted power transformer (unfortunately somewhat common)
Shorted output tranny (but it'd take out the HT fuse first).

First I'd do a diode check using your DMM's diode checker (in circuit usually works). If you see a shorted diode, (bridge rectifier?) replace it.

Lift the PT primaries. If it's still over-currenting, you have a ground fault BEFORE the transformer.

Reconnect the PT primaries and lift ALL of the PT secondaries from the rest of the downstream circuit and see if you're drawing too much current. If so, it's likely to be the PT. Bummer, but not completely fatal.

Other than that, it's just a matter of isolating parts of the DC load and seeing if it over-currents. A line current monitor and variac is EXTREMELY useful...dare I say necessary? If you don't have one, at the very least, do the light bulb AC line cord thingie. (if you're unsure, google this).

--Mike



OOPS.

I should have read the original post more carefully.

The PT is probably OK since the mains fuse only blows when the standby is switched, (but presumably the HT fuse is OK ?)

This leaves the rectifier diodes and everything else downstream (filters, preamp tubes, random wiring fault, etc.)

I'm betting it's a shorted diode. IIRC, this amp used four discrete diodes, not a monolithic bridge. Easy to check in-circuit with the DMM.

-Mike

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Mike Schway | [Picture your favorite quote here] mschway@xxxxxxx |
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Did it recently have a shorted tube? If so check the plate resistors.
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