Something fried in my Marshall "Studio 15"

[Amplitude]

Was practicing a few days ago with my '85 Marshall Studio 15 (mod. 4001), and when I went to power down with the standby switch, there was a serious electrical noise, and then the fuse blew before I could hit the power switch. Replaced the fuse (T1 250v) and it immediately blew. BTW, all the tubes test excellent (Eico 667). Even with tubes out, it will blow a fuse. I was advised elsewhere to disconnect the 2 red PT wires at the standby switch, and test the unit with a Dim Bulb Tester. The DBT doesn't dim, so I'm guessing the PT is shot (??). Until this happened, the amp was sounding excellent, no humming or anything and great tone. Anyone have a clue what the problem might be, or how to further test the power trans?

[52 Bill]

Removing the two red high voltage wires tests that winding, but there is still the filament circuit with a bridge rectifier to test.

Remove the red wires as well as the red and black filament wires to test the transformer.

[Amplitude]

Bill, I disconnected the red & black secondary wires that lead to the power tube socket (V4 I believe). There's no change with the light bulb, it stays bright (tried 40 & 60 watters). The 4 wires/terminals on the primary side have continuity. On the secondary side, the red/black filament wires have continuity (these I disconnected at V4 socket). The two red wires that lead to the standby switch have continuity, but not with the black ground wire between them. Hope I'm making sense here (I'm kinda new to this stuff, at least with testing). Would any of this determine a shot PT?

[Jazz P Bass]

If all of the secondary wires for the power transformer are 'flying' (ie: not connected to anything) & the lamp is still glowing bright, then your power transformer is shorted.
The power transformer by itself should not consume any real power when unloaded.

[Amplitude]

I'll probably just get a new trans (or a rewind if it can be done?). While I'm at it, I may as well put in a new filter cap/s, as the amp is approaching 30 years. Thanks for the advice folks!

[Fragger]

I was curious about these three circled areas near the standby switch.

[Amplitude]

I was curious about these three circled areas near the standby switch. [ATTACH=CONFIG]22112[/ATTACH]

I don't know Fragger, I suppose there could have been some arc'ing going on inside. There's a brown spot on the blue wire coming from the standby switch as well (near the filter cap terminal). I rubbed it off after that pic, wire looked ok though.

[Amplitude]

Does this capacitor look like it has shorted on the negative end (is unusually blackened)? If I'm reading the schematic correctly, that cap is associated with the PT secondary, and standby switch as well. Cap is C19 on schematic, and value is 350v 10uF. Could this cap have led to the shorting of my PT?

[Enzo]

That is a weld mark from when they welded the wire lead to the aluminum can.

[Amplitude]

Thanks Enzo.