Blown PT

[fopoman]

Whats the safest way to test for a blown power transformer? I accidentally ran the bias tap to ground. The amp blows the main fuse as soon as it powers up. It's a 5e3 type amp.

thanks,
Nate

[sportster4eva]

The quick and dirty way would be to meter all the pairs of leads. If it is blown you'll have to disconnect it anyway, so just make sure you don't have anything connected that will alter your readings, like diodes if it's using SS rectification. Measure across the primary (120v) side, the secondary side(s) both between each other and from each to the center tap, and the bias tap to center tap, and from each lead to ground. By ground I mean the steel part of the transformer. You may be able to use one of the bolts, or scatch off a little of the varnish on an end lamination. What you need to watch for is open or shorted windings or shorts to ground. The resistance between each half of the secondary is usually not exactly the same as the other half, so if it's a few ohms different don't be surprised. The bias tap will be a fraction of one half of a secondary to the center tap. You probably won't see anything that shows resistance to the transformer body. If it shorted that bad you probably would have seen the magic smoke leaking out. Maybe someone in here who has it fresh in their head or could give you some ballpark figures for resistance for a 5e3 type tranny.
Good Luck!
Paul

[mhuss]

Disconnect all the secondary leads -- if it still blows a fuse, your PT has 'joined the choir invisible.'

--mark h

[fopoman]

I disconnected the secondaries and it still blew.

Nate

[drz400]

I disconnected the secondaries and it still blew.

Nate

If you disconnected all the secondaries and nothing was connected and a fuse blew you could have a switch shorted to chassis ground but more likely your tranny is dead

[unklmickey]

...you could have a switch shorted to chassis ground but more likely your tranny is dead

hey, good point. i would have said it's definitelly dead, but there still is that one last remote possibility.

so, i guess disconnect both leads of the primary and try it one last time.

the good news is, if your tranny IS dead, you won't waste another fuse.

the bad news is, you probably won't waste another fuse.