Blowing Fuses

[Alonzo]

In close up picture number 2, is the wire bridging the 220 caps touching the terminal strip ground tab (the one screwed to the chassis)?

[Paul Birkeland]

You'll have to very carefully run the tests on page 47.  You might want to use a pair of clip leads and power up the amp very briefly to get a look at those voltages.  Alonzo could be correct that you have half of a capacitor bank shorted to ground.  If things are getting hot enough to start smoking in the amp, this will alter the voltages a bit. 

It would also help to see a photo of where the UF4007s connect to the PT and terminal strip.

-PB

[Sawgolf4me]

Ok. I started it up one more time to try to measure the voltages and it immediately shorted (with a 3 amp fuse).

I added a couple pictures of the rectifiers and the caps.

Here is a recap of everything i've seen:
When I first built the unit, the power transformer was humming pretty good. I read on the forums that usually this is from the screws not being tight enough. So I tightened them down and then the humming went down quite a bit. This may or may not have been the day before the first fuse blew. I can't remember, but I tightened them before the fuse blew.

Then I blew the 1.6 amp fuse after listening for 30 minutes.

Then I put in a 3 amp fuse and the power transformer started smoking after 30 seconds.

Now I immediately burned a 3 amp fuse.

When I measure the resistances of the power transformer, I get these:
14-90 ohm
13-90 ohm
12-90 ohm
11-90 ohm
10-90 ohm
9-90 ohm
8-1 M ohm
7-1 M ohm

[Sawgolf4me]

Also, when I measure the power transformer mounting screws to ground, they measure 0.3 ohms. I thought maybe with the fabric washers, the power transformer was likely supposed to be isolated from ground?

[Adrian]

0.3 ohms is pretty small - could this be the normal resistance of your leads?

[Paul Birkeland]

14-90 ohm
13-90 ohm
12-90 ohm
11-90 ohm
10-90 ohm
9-90 ohm
8-1 M ohm
7-1 M ohm

If you're measuring from each terminal to ground, that wont tell you anything.  One must measure the DC resistance of each winding, which is a pair of terminals. 

[Paul Birkeland]

Also, when I measure the power transformer mounting screws to ground, they measure 0.3 ohms. I thought maybe with the fabric washers, the power transformer was likely supposed to be isolated from ground?

They are isolated from the chassis but then hard grounded to the safety ground.

[Paul Birkeland]

The next step for you is to disconnect each UF4007 where it connects to the power transformer.  After these are disconnected (the disconnected ends can just be left sticking up), then test the amp with no tubes installed to see if it blows a fuse.

-PB

[Sawgolf4me]

Ok. I desoldered those two from terminal 8 and I turned it on with no tubes.

3 amp fuse blew after 1 second.

[Paul Birkeland]

Red and black wires connect to power transformer lugs 1-6.  Remove them from the power transformer and pull them so they are sticking up but not touching each other, then retest.

[Sawgolf4me]

I removed the black wire from 1 and the red from 6. When I turned it on, nothing happened. No power coming from transformer.

[Sawgolf4me]

Could a theory be that there was something with the power transformer internals?

Things worked ok for the first 10 hours or so and then a couple heat and cool cycles shorted something inside? Then it self healed a bit when it cooled after the fuse blew. And thats why it worked for a couple minutes when i put a new fuse in, until it shorted again. Then when the 3 amp fuse went in, the higher amperage really shorted something inside and thats why it started smoking.

[Paul Joppa]

Could a theory be that there was something with the power transformer internals?

Things worked ok for the first 10 hours or so and then a couple heat and cool cycles shorted something inside? Then it self healed a bit when it cooled after the fuse blew. And thats why it worked for a couple minutes when i put a new fuse in, until it shorted again. Then when the 3 amp fuse went in, the higher amperage really shorted something inside and thats why it started smoking.

That is one of many, possible scenarios. PB is leading you through a sequence of tests to determine what the actual problem is; each step of the sequence is chosen using the results of the previous one. It seems very meticulous, but it's much, much faster than any other approach we've found. Patience!

[Paul Birkeland]

I removed the black wire from 1 and the red from 6. When I turned it on, nothing happened. No power coming from transformer.

Did the fuse hold?

[Sawgolf4me]

Did the fuse hold?

Fuse held. And I am using a 1.6amp time delay because I ran out of 3 amp.