Randomly Blowing Fuse...Bad PT?

[FunkyE9th]

Hello,

I'm working on this amp that randomly blows a fuse. In trying to debug it, I'm at point where the whole 2ndary is disconnected (un-soldered all 2ndary connections) and I'm still blowing fuses. I wanna say 1 out 20 times of switching the power on it would blow a fuse, and this is with no load (2ndary not connected at all other than a voltmeter on the heater winding). Any ideas?


Forgot to mention... When the amp was fully assembled, if the amp makes it to the power on without blowing a fuse the amp worked fine. The amp is using a 2AMP slow blow.

Thanks,

-FunkyE9th

[Phil_S]

With apologies for stating the obvious, the PT is drawing more than 2A if the fuse blows with secondaries disconnected. I wonder, is the some sort of intermittent short on the primary? Try simple stuff first, like removing and reworking the primary connections. Maybe it's the power switch? Don't know.

[FunkyE9th]

I've already resoldered all the primary wiring. I doubt its the switch, because I can get it to blow a fuse by switching the multiple outlet on and off instead. That's why I'm thinking it'may be PT problem. Just wondering what specfically could possibly wrong with the PT.

Maybe it's an intermitent short, but then why does it only occur at power on and not during normal operation. Maybe something transient related at power up...don't know...

[Structo]

That is strange.
If the seondaries were connected I would suspect a shorted filter cap or something in the PS that was drawing too much.

At this point I would suspect the PT is bad.

[FunkyE9th]

That is strange.

That's what I've been telling myself for 2 days...

[Phil_S]

Pull the end bell and look at the primary wires as far up as you can. Maybe you'll get lucky and find a problem that you can solve with some heat shrink.

I'm thinking there is an internal short that will require unwinding it until you find it. I don't expect you'll want to go that far.

Look carefully for something that is grounded in the primary path, that shouldn't be grounded.

I assume you've tried this on more than one outlet and that means on a different branch.

Can you get a consistent ohm reading on the primary? Wiggle the wires and see if it gets unstable.

[Andy Le Blanc]

It might just need a 3A, or you might want to look at some sort of current in
rush limiter. As its been stated, usually if its popping with no load its le mort.

[FunkyE9th]

Phil,

I did try different outlets and measured constant DC resistance on the primary and 2ndary.

Andy,

I did try a 3A fast blow and it still blew. Maybe I'll try a 3A slow blow. But this is an Express PT and the Express uses 2A.

"Le Mort" = "dead"??

[Phil_S]

"Le Mort" = "dead"??

Literally, "the dead."
I think so, too. With no load on the secondaries, the amp really shouldn't be drawing any meaningful amount of Amps. A 2A Slo should not blow. I think the only way your are going to find the problem is the unwind it. Too bad it is intermittent, as this is frustrating. Your best bet is to get another PT.

BTW, you can probably get $10-20 for it on eBay. List it as shorted. There are people who buy them for the iron and rewind them. And this way, it doesn't go to waste.

There is one last ditch thing to try, which is to dismount it from the chassis and see if it still blows. You might also see if any of the windings have continuity with the frame -- clip one meter lead onto a bolt hole and make the rounds of all the primary and secondary leads. See if one gets a reading. If it does, then you've confirmed there is a problem.

[billyz]

If you want to have some fun. Take the covers off the transformer, turn the light off and plug it in . I 'll bet you'll see a flash when the fuse blows, probably arcing internally. A bigger fuse is only going to cause the short to do more damage. With no load on the secondaries you should be able to run a 500ma fast blow fuse .

[FunkyE9th]

When I have some time, I'll probably take the PT apart and see how it is. Thanks all for chiming in.

[butwhatif]

Make sure it is a 125-or 250 volt fuse, and not an automotive 32 volt fuse.
Disconnect the secondaries and measure the primary winding to ground/core resistance-make sure it's infinity. Does it have any 'burnt' smell ? Otherwise if
you follow billyz -the dark test will reveal--