Power transformer or rectifier blowing fuses

[Apexelectric]

First time on the forum and hoping for a little guidance.

I am working on a friends home built amp and have an issue with what appears to be a short in the power transformer or in the rectifier. It is a AX84 P1 kit that was not put together too well and I am taking care of some of the build issues as well as trying to get this once working amp working again. I rebuilt the board with the original components and cleaned up the wiring as a practice project. I figured the original problem had to do with the original board assembly but found nothing alarming in the disassembly and bench tested all of the components when they were removed with no issues found using my multimeter. 

Here is the current scenario.

I plugged it into a light bulb current limiter and I get a steady, half bright, glow from the bulb. This is with the standby off and the power switch on. With the HV leads disconnected from the rectifier, the transformer powers up without an issue and does not blow the fuse. The voltage readings are normal on the HV secondary. With the leads connected to the rectifier and not to the power rail, or anything else, it blows the fuse. The diodes test fine with a meter.

Not sure which one is the culprit. Any guidance would be greatly appreciated.

Thanks

[shooter]

The voltage readings are normal on the HV secondary. With the leads connected to the rectifier and not to the power rail, or anything else, it blows the fuse. The diodes test fine with a meter.

for a $ I'd replace the diodes.
but before you fire back up, with power off and new diodes in, ohm from the diodes output to ground and post.

[Apexelectric]

Thx For the reply.

That was going to be the first choice but I couldn't come up with a scenario in my mind why the PT would be kicking out the proper voltages if a winding went bad. Wanted to know if there might have been something I was missing there.

Hopefully it's just the diodes.

Thx

[kagliostro]

How is arranged the PT + diodes scheme ?

Is there a PT with CT ?

Are you using Full wave rectify with CT (2 diodes + CT ?)

Are you using a PT without CT and a diode Full Bridge ?

It will be better to know for those who can give you help

Franco

[John]

I'd also just replace the diodes. Make sure they're installed with the anode towards the PT. Like Shooter says, take ohm reading from the end of diode to ground. Should be infinity. All it takes is a thread of solder touching to light up the bulb (short). When I'm building, I wire up the PT, test. Connect it to diodes, test. Connect first filter cap, test. Connect heater filaments, test.

[sluckey]

With the leads connected to the rectifier and not to the power rail, or anything else, it blows the fuse. The diodes test fine with a meter.

Are the diodes connected properly? How did you test the diodes with your meter?

[PRR]

> half bright, glow from the bulb

What size bulb? (Related: what size amplifier?)

How about rect connected but rect load (the rest of the amp) disconnected?

[Apexelectric]

Diodes are in the right direction. Tested with a meter in both directions. Full wave rectifier. Yes on the center tap which is switched through the standby switch. 60W bulb. One EL 84 output tube single ended.

The amp was powered up with no connection to the filter capacitors after the rectifier. The only power connected was the filament circuit. No tubes installed.

I have some new diodes coming but wanted to reach out for some help before continuing further.

Good idea John! I'll be sure to keep that procedure in mind. Transformer tests fine when not connected to the rectifier.

Thanks all!

[sluckey]

Everything you have said points to the diodes either bad, or connected wrong, or wrong type.

What type diodes do you have? Can you post a hi-rez pic showing the diodes, board, standby switch, and first filter cap?

[Apexelectric]

I pulled the diodes already but they are N5406 3A 600V. The transformer puts out 380VCT.

Center tap goes through the standby switch to the filter cap ground/rail.

[shooter]

600V

you want something north of twice B+, most guys use 1000V piv diodes

Center tap goes through the standby switch to the filter cap ground/rail.

the schematic I looked at has the standby switch connecting to the 2 diodes cathodes, then to the B+ power rail, NOT ground.  can you post the schematic you're looking at?

[PRR]

> you want something north of twice B+

He said "transformer puts out 380VCT". If true, B+ max is 266VDC. A 600V rectifier is safe day-to-day, though has no leeway for power line spikes.

If he mis-typed and meant 380-0-380 (760VCT), he has 537V max B+ and 1,075V across each rectifier. This is clearly way-over the 600V spec, and would explain trouble getting it up.

As the "AX84 P1" seems to call for 250V main supply, the 266V (before sag) seems more likely than 537V, and the 600V crystals should be OK, at least until lightning season.

[shooter]

If true, B+ max is 266VDC

I need new bi-focals for the fine print

[Apexelectric]

It was built off the P1 revision 11 which was over 15 years ago. The latest revision standby switches the B+ after the rectifier. The older version switched the center tap. 
PT is a Hammond 269EX OT is a Hammond 125 ESE
Planning on replacing the 600V diodes for 1000V

[Colas LeGrippa]

The first thing I do when I service an amp that blows fuses is taking off the power tubes and fire it again. If it blows another fuse, I isolate the power supply from the rest of the amp and see what's happening. Gradually, I can pinpoint the issue.


Colas

[Apexelectric]

Put in the new rectifier diodes last night and it fired right up.

Thanks guys!

[Colas LeGrippa]

You' re a champ !