Thoughts on whether I've blown my Allied 6K56VG Transformer?

[dtonthept]

Hello all,

I finally finished up my Drip REDD47 today (it's been a busy few years). It fired up just fine, reasonable amount of hum with a mic plugged in with gain maxed. I had reduced this a bit with some additional connections of earths at terminals to the star ground by the transformer (pad, to amp, vu meter, and audio ground by the xlr terminal).

I ordered a phantom power unit as recommended by another board member:

www.digikey.com/catalog/en/partgroup/psa065m-series/15727

This had been installed through early testing, but it didn't seem to be working so I pulled it out while I de-hummed. Got my hum to a good place then popped it back in. Phantom still wasn't working so I kept it connected but removed it from the case so that I could check voltage from the pcb connection itself - I got a reading of 40V.

Anyways, I was looking at something else then out of the corner of my eye saw a spark on this near the power hookup of the phantom board (which was connected to one of the Primary terminals). One of my monitor speakers (which wasn't in any active monitoring circuit, and actually connected to a totally different power circuit!) made a funny wubwubwub sound for a second or so. I looked back down at the REDD and all the lights and tubes were out and no audio was passing anymore. Dead.

The onboard fuse is totally fine. Same with the fuses in the IEC connector. I've tried a couple of different IEC cables, no difference. Other gear on the same power circuit is fine - though this is just my soldering iron and some lights.

I'm getting voltage readings at the AC in terminal (118V on positive, about 1.3 on Neutral) and primary (1.3V on both terminals) but now nothing coming back out of my transformer - the Allied 6K56VG.

Sooooo... Is it possible to fry one of those transformers somehow? I'm wondering if there was a massive power spike in the building that just happened coincidentally while I was working on this - though I've never experienced anything else like this is in the few years I've been in the building. Is there anything else I should check? I've tried switching the 5Y3GT tube but no change, tried switching fuses just incase - nothing.

So - is 1.3V a normal reading on Neutral? And is it possible to blow a transformer like this? It seems pretty heavy duty. I've been unable to find a schematic which would confirm whether or not the fuse on the PCB is pre-transformer or not. Otherwise do you think that the phantom board could somehow have caused a problem? I don't know if a component may have somehow shorted against the case, but it was hanging on the outside and all the metal around it is painted, I'd also guess if something shorted there it would have got to earth...

Please help! It was sounding great and I'd love to use it!

Thanks,

D

 

[PRR]

> is 1.3V a normal reading on Neutral?

A bit high. "Neutral" is never Zero unless you only have balanced loads (US 240V) or have no load at all. With ample wiring, less than a Volt is common. With my too-long feeder I can get 2V-3V with microwave or coffeepot.

> voltage readings at the AC in terminal (118V on positive, about 1.3 on Neutral) and primary (1.3V on both terminals)

Hot-Neutral checks should be made line-to-line.

However if *both* are 1.3V to ground, you probably have ZERO across the primary.

And zero in is zero out.

Since you do have ~~118V at these "in" terminals, but zero at the primary, there's a break in between. Switch? Fuse? Bad joint?

> unable to find a schematic which would confirm whether or not the fuse on the PCB is pre-transformer or not.

If it is a reasonable PCB, *UN-PLUG* and trace a pencil from the wall-cord through the PCB switch fuse primary and back.

This is usually no big deal because the wall-wiring is supposed to be well separated from the secondary wiring.

If the PCB is obscure, *UN-PLUG* and use the continuity buzzer to figure what connects to what. You may need to disconnect primary for this since on DC test it has very low resistance.

 

[dtonthept]

Hi there PRR,

Thank you so much for the info and troubleshooting steps to follow - I'll give all this a shot now.

Really appreciate your input,

D

 

[dtonthept]

Hello again,

PRR I owe you a really big beer. I'm pretty good at troubleshooting in my studio in general, but haven't figured out so well how to do it once you take the lid off a case!

Anyways I'm profoundly relieved to say, that having followed your instructions, I discovered that a fuse which I thought was fine (just by looking at it rather than testing it) had indeed blown. A quick switch and all was fine. PHEW! THANK YOU!

I guess this is the analogue circuitry equivalent of you patiently asking if I've tried switching my computer off then back on again ;-)

All the best,

D

 

[PRR]

> I'm pretty good at troubleshooting in my studio in general, but haven't figured out so well how to do it once you take the lid off a case!
> I guess this is the analogue circuitry equivalent....

It's equivalent to "I have signal from mike-amp to patchbay, but no signal into my console."

So you jiggle the patchcord, then look in the back for buggered wiring.

Only difference is: you can put fingers on a live mike/line signal. NOT on a live wall-power circuit. So yes, there is a knack in deducing from meter readings.