Help with fuses blowing at startup

[NightFlight]

Well, in the power supply, there are just caps and resistors left. I woudn't have a clue how to ensure the transformer is still good.  If you like toys, it would be a good excuse (for me) to purchase a cap testing equipment.

Or you can put a small charge on the caps and see if they charge/discharge. They would have to be removed from the circuit to test. A PITA, but possibly necessary.  They also can't maintain heat on their legs for a terribly long time. The datasheet might have more in that regard. If  you do pull them, make sure everything is discharged. I found several residual voltages the other day while I was poking around.  I found 183V on my one output cap the other day. Don't know how it got there - possibly the multimeter, but I certainly found it when I touched post #6 by accident.

[fullheadofnothing]

Far more useful then testing capacitors would be doing the resistance checks from the manual. If there is a terminal that reads less than 1Ω and is not specified as being 0Ω, that would be the problem.

If that fails to find the problem, go through the wiring section of the manual, carefully, step by step. Confirm that every single component and wire is soldered EXACTLY where and how it is specified in the manual. Close is not close enough.

[Paul Birkeland]

Josh is quite correct.  If you pass the resistance checks, let us know, as I have another idea about how to systematically eliminate potential issues.

[Ankaret]

Ive been through resistance checks and they all check out perfectly. Last time I went through it all was just after replacing the four rectifier diodes.

I have also been through the manual step by step again, but I guess Ill spend some time doing that again.

Any other ideas?

Thanks all

[Doc B.]

You probably need to disconnect the power transformer from the rest of the high voltage circuit first and run it by itself to be sure it wasn't damaged.

[Ankaret]

How would I go about doing that? Just unsolder everything connected to the power transformer minus the LED coming from the on switch?

And how would I know that it's running correctly?

[NightFlight]

Sorry guys. Don't listen to me. I was attempting to be helpful, but might have missed the mark entirely.
Watching this thread... its an interesting bug.

[Doc B.]

The power switch we supply does not have an LED so you have lost me there. You want to disconnect the red wire that goes from power transformer terminal 9 to t-strip terminal 19L. Disconnect it at the power transformer. Fire up the amp and see if the fuse holds. If it does, turn tthe am off, set your meter for AC volts, at least 250VAC. Connect the red meter test lead to power transformer terminal 9 and the black test lead to power transformer terminal 10. Turn the amp on an measure the AC voltage across those two terminals.

[Ankaret]

So I disconnected the wire that goes from power transformer terminal 9 to 19L, at the power transformer. Note: I left the other red wire connected to 9, the one that is part of the double wind that goes to terminal 6 on the transformer, correct? Then tried to fire it up and fuse still blew...does this mean the transformer is acting up?

[Doc B.]

My hunch is that the 6 ohm reading instead of 12 indicates that the high voltage secondary has some shorted turns from the backwards diode. To be thorough I should have mentioned removing the tubes to take them out of the equation too. If you had the tubes in the sockets you might want to try it again with the tubes out. I suspect it will still blow the fuse. The only other possibility left after that is a short somewhere around the IEC power inlet. But that would be more likely to blow the fuse as soon as the amp was plugged in whether it was switched on or not.

[Ankaret]

I did have the tubes out actually, so those are ruled out.

Anything else I should test before throwing my hands up and looking at how expensive it's gonna be to buy a replacement transformer?

[Paul Birkeland]

Be sure the twisted pair of wires that jumpers the two power transformer windings together is installed properly.

If you switched them on one end, you will blow fuses indefinitely. 

[Ankaret]

These look good, red goes from 6 to 9, and black goes from 7 to 10.

[Ankaret]

Also, I did follow this revision: "REVISION 5/6/10:
Making  this  wire  3”  long  and   connecting it to Terminal 14U instead of terminal 22L may yield more quiet operation."

Should I try 22L instead, or that wouldn't have much if anything to do with my issue?

[Doc B.]

One last thing that is a bit of a stretch would be to ask that you are sure the fuse is blowing when you throw the power switch. If it is instead blowing before you power up - right when you plug in the power cord with the amp switch set to off position - it is conceivable that there could be a short in the IEC receptacle wiring