boogie simul-class 2:ninety question

[pontiacpete]

ONe of these came in with both HT fuses blown. Owner tells me he was using one channel, as per the manual, he turn the volume down on the channel not being used and turned up the presence.
For what its worth I put the 8 6L6s on the tester, B&K 700, and no shorts or leaks and the meter stays steady for the mutual conductance test.
On each side of the push pull there is a cool amount of current being pulled, 42mA. 21mA per tube and both sides and both channels are really close. I'm using the ohms law method of measuring current V/R, (probe is fried and too many 400ma fuses blown to bother doing it any other way)
Did a lot of tapping on the pcb and tubes.
For the past several days I turned it on and off many times and have run it for hours, played through it and still no fuses blowing.
I don't like jobs like this when I can't find a problem, so what to do.
Is this and issue of a voltage spike or an underrated fuse, 1amp fast blow? BTW I'm using slo blow in there now.
Is there some other things I should be checking?

Also I can't seem to figure out how to upload the schematic
Thanks

[g1]

zip the schematic and you will be able to upload it.

[pontiacpete]

Thanks g-one.

[g1]

So, they are not HT fuses but cathode fuses, one per side. They are shown as fast, not slo-blo.
Did they both blow at the same time?
You said the one side was not being used, so for that side, the fault must have occured when idling
The only thing common to both sides amps I can think of that might cause it is the bias supply. Or maybe some oscillation.

[pontiacpete]

The owner said the amp just died on him during a show. He wasn't specific, that is, if it was while playing or at turn on.
I'm assuming the fuses went at the same time. Maybe I shouldn't.

Do you think the bias caps should be changed?

[g1]

If it is related to the bias, it is some kind of intermittent, as it is currently working fine for you. Normally caps will not cause this kind of problem, but I suppose it is possible. I was thinking more of some kind of intermittent connection.

[pontiacpete]

That makes sense.
I resoldered the bias circuit from the brown wires off the transformer to the diode and on to 'D' and checked the resistors and caps.
It looks like they have their caps drawn in backwards. C24 and 25 have the negatives ends going to ground.
I went ahead and cleaned the connector that carries the brown wires.
Anything else?

[tedmich]

a guy on my Clist is selling one of these for $185 (broken)...worth it?

[The Dude]

I'd say hell yes. Look what working ones are going for. I have no use for one, but if one came up that cheap in my area, I'd buy it just to repair and resell.

[soundguruman]

Very often a bad speaker cable or a bad speaker will blow the fuse.

Inspect the plugs on the speaker cables. Look for loose or intermittent connections.

MAKE SURE that the speaker impedance is correct!
If you use a 16 ohm amp output into a 4 ohm speaker.....you KNOW why the fuse blows.