Old Lafayette combo amp popping fuse... should be simple

[disorder]

This old Lafayette combo uses a half-wave rectifier into a cap can with two 40uF 350V caps in it. It was blowing fuses so I removed the tubes, and then the output transformer, and then lifted various filter caps and it wasn't getting me anywhere. So I disconnected everything but the power transformer and it was OK again. I then added just one of the filter caps to the power transformer secondary and it started pulling a lot of current again? Tried two other filter caps (also 40u 350V+) and they do the same thing. Replaced the single diode that acts as rectifier with a brand new 1N4007 and it still draws crazy current even with just a filter cap attached. There is .01u ceramic cap attached across the rect diode... I just used the old one again when I tried the newer diode because I dont have any else in stock... could that cap be the cause? not sure why it's there in the first place?

[Jazz P Bass]

Temporarily remove the .01 cap.

[disorder]

Removing the .01 does not stop the excessive current draw. Could it be that the PT is shot and it starts drawing excessive current once any load is connected (ie even a single filter cap) ?

[Chuck H]

Yup. This Q is sort of like seeing a fly in your soup, so you carefully spoon all the soup from around the fly into another bowl.

Well, not that that's what's been done already. But why go through all this wondering when the cap is right there just asking for it?

[disorder]

Just trying to understand why the amp is fine with nothing attached to the PT (the secondary measures close to what it should be)....

[Chuck H]

Just attaching a cap to the secondary shouldn't cause current draw. Something is fishy. Check for voltage betwen each leg of the PT secondary and the chassis.

[J M Fahey]

Could it be that the PT is shot and it starts drawing excessive current once any load is connected

NO

Just trying to understand why the amp is fine with nothing attached to the PT

Does an amp work without being attached to its Power Transformer?
NO
How can you say it's "fine" then?
Beats me.

Your question should be: why a capacitor , connected to a power transformer through a suitable diode , with the proper polarity (hope you checked that) , makes said PT to draw excessive current, while said PT, on its own, did not?
Good luck.

[disorder]

Perhaps I did not articulate myself correctly. When there is nothing attached to the PT secondary, there is not excessive current draw. When there is a cap connected between the higher voltage secondary winding of the PT there is excessive current draw.

edit - yes that is what I meant J M

[disorder]

Just attaching a cap to the secondary shouldn't cause current draw. Something is fishy. Check for voltage betwen each leg of the PT secondary and the chassis.

no voltage between either leg of PT secondary and chassis.

[olddawg]

no voltage between either leg of PT secondary and chassis.

Does it draw excessive current with only the diode installed and everything else floating? Are the filter caps old?

[Chuck H]

The OP was good about playing those cards already. It's written earlier in the thread.

[J M Fahey]

Please draw a simple schematic showing your power transformer, the capacitor and the diode, showing how they are connected.
I'm guessing a couple things but a drawing is better than 1000 words ... or so they say.
Thanks.

[olddawg]

The OP was good about playing those cards already. It's written earlier in the thread.

Yeah your right. I would pull the transformer and jig it up. I would also use a variac with an amp meter. If that is unavailable at least use a lightbulb limiter. Dude, you didn't rewire the power cord for a 3 wire cord did you? Anyway, one should be able to test the PT out of circuit by attatching a rectifier and a load.

[Jazz P Bass]

O/K, how about this.
We need some numbers.
What is the current draw (in amps or watts) from the mains with no load on the secondary?
What is the draw with a diode & cap attached?
Numbers, man.
We need some numbers.
I do believe that you know what "excessive" means.
In this case, anything over 4 watts is excessive.
Aside: I have an old Gibson GA-35RVT on the bench.
No tubes installed.
None.
It is pulling 136 watts from the mains.
Bad caps.

[disorder]

I appreciate the help so far. I will report back tomorrow with current reading

I've been using a light bulb limiter to determine "excessive" current draw. To double check that I was reading the brightness of the lightbulb correctly I hooked up just a filter cap after the rect diode without the limiter and it popped the fuse. So it is in fact pulling enough current to pop the fuse.