59 Champ popping fuse

[madkatb]

So this lights up a current limiting bulb with no tubes in it. I ran it off a variac and the bulb starts to glow at about 35 volts. Everything points to the transformer but all the resistance checks I did seem normal. Primaries to ground read open. Primaries to secondaries and filament supplies read open. Secondaries read about 200 ohms to ground (center-tapped). I isolated the power supply from the on-off switch and tacked in a different fuse holder. I can't get a secondary voltage, even the primary voltage drops to about one volt when I turn it on (118 volts at the wall). Something's sucking a lot of current and, I would guess, dumping it to ground. What am I missing?

[nosaj]

So this lights up a current limiting bulb with no tubes in it. I ran it off a variac and the bulb starts to glow at about 35 volts. Everything points to the transformer but all the resistance checks I did seem normal. Primaries to ground read open. Primaries to secondaries and filament supplies read open. Secondaries read about 200 ohms to ground (center-tapped). I isolated the power supply from the on-off switch and tacked in a different fuse holder. I can't get a secondary voltage, even the primary voltage drops to about one volt when I turn it on (118 volts at the wall). Something's sucking a lot of current and, I would guess, dumping it to ground. What am I missing?

With no tubes in it and it lighting up the limiter From what you've written it seems the primary side is shorted. It doesn't have to always short to ground to be bad, the insulation could've compromised and shorted against another turn. Anyone correct me if I'm wrong.

nosaj

[g1]

Secondaries read about 200 ohms to ground (center-tapped).

Not sure what you mean "to ground"? Do you have the secondaries and center tap disconnected?

[Chuck H]

Pull the fuse. Does the current limiter still light up bright? Primary short to some other winding or case likely = bad PT, but not necessarily. If no then put the fuse back in and... With all the secondaries disconnected from the circuits they feed, literally dangling in the wind, does the light bulb go bright? Secondary short = bad PT. If no then you need to look into the rectifier and/or filter caps and/or their wiring.

[R.G.]

What am I missing?

You're missing that any wire or connection from the power cord entry to the PT, the rectifier, filter, inductor (if there's one in a 59 Champ) and bleeder resistor wiring could cause a short that would pop a fuse.

You have to keep eliminating possibilities. You've only eliminated a few. The suggestions so far are good - isolate one thing, then move to the next. But you're not following through.

I would:
- unsolder both PT primaries; now see if it lights the limiter bulb. If not, the problem is in or past the PT primaries. If yes, it's in the wiring before the primary.
- resolder the PT primaries, and unsolder all of the PT secondaries. Insulate the secondary leads so they cannot short to things while flopping around. Try the limiter again. If the bulb lights, the problem is inside the PT. If not, it's past the PT.
- resolder the PT heater windings ONLY; try the bulb. If it lights, it's in the heater wiring. If not, it's somewhere else.
- resolder the PT B+ windings; disconnect the rectifiers from the first filter cap. Try the limiter. Lighting means it's a problem in the rectifier or first filter cap, otherwise, continue down the power chain

In general, you head from the AC power side down the power chain, reconnecting ONE section at a time until you find the first section that reliably causes the limter to light. The trouble is likely to be in there.

The sneaky problems are (a) internal shorts in the PT; find these with the geofex internal short finder; and (b) an internal short to frame in an inductor.

[nosaj]

You're missing that any wire or connection from the power cord entry to the PT, the rectifier, filter, inductor (if there's one in a 59 Champ) and bleeder resistor wiring could cause a short that would pop a fuse.

You have to keep eliminating possibilities. You've only eliminated a few. The suggestions so far are good - isolate one thing, then move to the next. But you're not following through.

I would:
- unsolder both PT primaries; now see if it lights the limiter bulb. If not, the problem is in or past the PT primaries. If yes, it's in the wiring before the primary.
- resolder the PT primaries, and unsolder all of the PT secondaries. Insulate the secondary leads so they cannot short to things while flopping around. Try the limiter again. If the bulb lights, the problem is inside the PT. If not, it's past the PT.
- resolder the PT heater windings ONLY; try the bulb. If it lights, it's in the heater wiring. If not, it's somewhere else.
- resolder the PT B+ windings; disconnect the rectifiers from the first filter cap. Try the limiter. Lighting means it's a problem in the rectifier or first filter cap, otherwise, continue down the power chain

In general, you head from the AC power side down the power chain, reconnecting ONE section at a time until you find the first section that reliably causes the limter to light. The trouble is likely to be in there.

The sneaky problems are (a) internal shorts in the PT; find these with the geofex internal short finder; and (b) an internal short to frame in an inductor.

I am figuring theres a rectifier tube in there, if the tubes are pulled does that not eliminate 95% of the circuit?

Nosaj

[singaporerock]

I am figuring theres a rectifier tube in there, if the tubes are pulled does that not eliminate 95% of the circuit?

Nosaj

Hello,

not if theres a short in the rect's socket. even intermittent. then removing the tube would not remove most of the circuit.

I think RG's Suggestions are spot on as far as making the simplest approach.

[madkatb]

Well, I pulled the transformer so there are no connections to anything. Ran the variac from the current limiter into the two primary leads. Starts to draw current (bulb lights up) at about 30 volts AC. Guess the primary has an internal short.
So, a new tranny, new electrolytics and this should be playing as well as it ever did.
Thanks for the help guys.

[Tom Phillips]

...Guess the primary has an internal short...

It could be the primary OR one of the secondaries but no matter, you have isolated the problem to be a bad transformer.

[Enzo]

Agree, in fact I think it MORE likely a secondary short, but it is a transformer so all windings affect one another. And bad is bad.

[mozz]

No meter to measure resistance? It does sound like a primary short.

[Enzo]

A shorted primary of course draws current. But a shorted secondary appears to the mains as a short as well.