Tsl 602 pops fuse when turned off

[madkatb]

I'm trying to troubleshoot a problem in a Marshall Tsl 602. The fuse pops when the amp is turned off.
One of the power tubes broke during a recent move so both power tubes have been replaced and the amp has been rebiased. The fuse that pops is F1, part of the standby circuit. But it pops even when the standby switch isn't used. And it doesn't pop every time. It doesn't seem to be affected by how long the amp is on.
I'm guessing that something must be arcing in the power switch but I'm not sure how best to test it. Replacing the switch seems to be the logical option but I'm curious whether someone else has had this problem and what the cure was.
http://www.drtube.com/schematics/mar...03)%20iss7.pdf

[g1]

F1 is the HT fuse. It is before the standby switch, but it's load is connected by the standby switch.
It must be a slow-blow type, is that what you are using?
How about turning the amp on and off without ever going out of standby mode, does the fuse ever blow?

[R.G.]

Marshall was semi-infamous for putting their bias supply connection on the switched side of the standby in older models, so I looked there.

That wasn't the issue with this amp, but I did notice that the bias supply is powered directly from the transformer. If the bias supply somehow goes to zero quicker than the main filter caps run down, that turns on both output tubes and could well pop the fuse. It's a plausible theory, at least.

What you do about this theory depends on your approach to a fix. I personally would just remanufacture the bias section, with new caps and diodes and measure the resistors as well as checking the solder joints, etc.. The theory might not be the problem, but renewing the bias components would at least not harm the amp, and might fix it. A more analytical approach would be to get out the scope and watch the B+ and bias voltages at turn off and see if bias drops to zero faster than B+.

[madkatb]

It is a slo-blo. Seems to blow regardless of main or standby switch setting.
The amp actually belongs to a friend. He has tried all of my suggestions. Guess I'll have to bring it home and crack it open. I was hoping someone had seen this before.

[madkatb]

I'll check out the bias supply components. As I said, this started after the amp was moved, breaking a tube. (This is what happens when the bands drummer is allowed to touch guitar equipment ) So maybe turning on the amp with a broken tube could have damaged a component.

[Enzo]

Does the fuse really blow when you turn it off? I mean can you see it go? Or is it that your turn a working amp off, then back on and the fuse is blown? That usually blows the instant the power returns rather than leaves, though anything is possible.

[madkatb]

You can hear it pop. We tried it several times. The amp plays fine. When you turn it off and then check the fuse it's blown. That's why it's confusing.

[Enzo]

Just a weird hunch: Can you disconnect the standby switch? Doesn't it have two wires down to push-ons on the board? If so, pull those wires and see if it still does this. The you can use a couple clip wires to serve as standby switch during service.

[madkatb]

Just to let you know how this ended up:
My friend got some new fuses from Mouser. Solved the problem. But the original fuses he had were labelled as slo-blo so I'm not sure what was going on.
And they were blowing when the amp was turned off not when it was turned on. We blew the rest of them confirming this (after the Mouser ones showed up).