bizarre power supply issue

[vaughandy]

So as I continue my maiden voyage of amp repair, I've encountered a strange phenomenon with my Ace Tone 601. The original power transformer, which has 220V primaries, only provides me the 175V output. I've had to use additional 6.3V and 15V transformers to provide those voltages.

That said, I **thought** I was providing the correct power to everything, but then all of a sudden (with no apparent changes to anything on my end) my external step-up transformer blows a fuse. When I replace it (VERY briefly) with a wire and turn it on, I get a big humm and one of my filter caps (see thread below) heats up like it's ready to explode. This had been a problem for me before, but I thought I fixed it by replacing my rectifier diodes.

On a whim, I reversed the orientation of the plug into the 220V outlet and, voila, no problems. The amp isn't working, but everything seems stable. If I switch the orientation, I'm back to the blowing fuses / capacitor state.

I can't conceive of what's going on here. How could the orientation possibly matter from an AC source? I'm definitely a novice at electronics but this just doesn't make any sense to me. Any ideas??


See these thread for more pertinent info:

http://music-electronics-forum.com/t23864/
http://music-electronics-forum.com/t23729/
http://music-electronics-forum.com/t23603/

[Mars Amp Repair]

well, I'm not certain but years ago as a kid I built an tesla coil. I didn't have one transformer to get up to the b+ voltage they required so I put 2 in series. I found, as you might suspect, that the polarity of the ac phasing of those secondaries had to be matched or mayhem would result. Keep in mind I was about 13yrs old. I was very lucky I didn't short myself out at that age.
I wonder if there is any issue you have like this with any common secondary connections between the transformers. just a thought...glen

[Enzo]

Oh yeah. I recall as a kid trying to make a 1300v supply for a CRT scope tube project. I took a 12v transformer and applied the 120VAC to its secondary. it worked for a while.

Why does the original transformer only provide the 175VAC? Is it defective? Any chance then that there is an internal short in it? So it might matter whether something is closer to ground?

[vaughandy]

Oh yeah. I recall as a kid trying to make a 1300v supply for a CRT scope tube project. I took a 12v transformer and applied the 120VAC to its secondary. it worked for a while.

Why does the original transformer only provide the 175VAC? Is it defective? Any chance then that there is an internal short in it? So it might matter whether something is closer to ground?

dunno why the transformer only provides 175. the other windings don't seem to be shorted. it's from like 1968, but i don't really know what would be liable to breaking down in a transformer.