Fender 68 Super reverb amp...power transformer issues?????

[bsco]

Good morning...I haven't posted a repair question for quite some time......but here goes.....a friend of mine dropped off this amp for repair......he was complaining about getting shocks from the amp.....it has been in storage for years and full of corrosion and rust inside the chassis....I had to remove the transformer and had to repair one of the primary winding wires as it was practically rotted away from corrosion....I repaired it and insulated it with some shrink wrap and then did a continunity check and everything seemed ok...I measured voltages for the different windings and everything seems good....however, when checking for leakage between the transformer frame and the main B+ winding I am getting a voltage reading of 60 something volts AC.....if I go to a low voltage winding i get around 7 VAC.....I did a resistance check from all the transformer windings to the frame and there is no indication of any leakage...at least on the meter and I used to different meters to double check.....so...the transformer is faulty??? how will I know for sure as I really don not want to install this if there is a problem??? any comments, advice , etc, greatly appreciated........

[glebert]

Presumably the chassis wasn't earth grounded? I'm super frugal and hate having to replace the expensive bits of an amp, but in this case it seems like it is necessary. Your friend was already getting shocked by it, you see AC voltage on the frame, so I say it needs a new PT. I don't want anyone's headstone to read "He Saved $300."

I would probably pop the bell covers off the PT though and see what it looked like inside, just because I'd want to see if there was an obvious cause.

[Helmholtz]

I guess you're measuring a "ghost voltage".
This would be a voltage due to the capacitance between winding and frame/core.
Try measuring again with a 10k resistor connected between meter leads.
A ghost voltage will collapse when loaded.

[Chuck H]

Maybe there's moisture in the PT? You could pop the bell covers and bake it in the oven at 170F for an hour. Then open the oven and blow a fan in there until every is cool and re test. Nothing to lose.

[Helmholtz]

I'm pretty sure all PTs show some ghost voltage when using a high impedance meter.

[Mick Bailey]

Measuring resistance between a winding and core with a DMM can be misleading. It can tell you if the transformer is bad, but won't necessarily tell you if its good. This is down to the meter's test voltage and to overcome this an insulation tester is need that applies a high enough voltage to check for breakdown leakage.

Ghost voltages are a bit of trap and I often forget to put my DMM into low impedance mode when checking.

[bsco]

Presumably the chassis wasn't earth grounded? I'm super frugal and hate having to replace the expensive bits of an amp, but in this case it seems like it is necessary. Your friend was already getting shocked by it, you see AC voltage on the frame, so I say it needs a new PT. I don't want anyone's headstone to read "He Saved $300."

I would probably pop the bell covers off the PT though and see what it looked like inside, just because I'd want to see if there was an obvious cause.

I am pretty sure the chassis was earth grounded as it does have a 3 prong power cord there......that reminds me as it looks as if somebody installed that...I will recheck....it does have the death cap still installed...but i tested the transformer out of the amp.....there was a corroded wire in the primary lead inside the bell cover...I removed that and repaired the wire...now I am able to get a contuninity reading on the primary....I will retest the windings again and post the voltages I am reading.....

[bsco]

I guess you're measuring a "ghost voltage".
This would be a voltage due to the capacitance between winding and frame/core.
Try measuring again with a 10k resistor connected between meter leads.
A ghost voltage will collapse when loaded.

OK. I did test it unloaded...I will try the 10K resistor across the meter leads.......

[bsco]

Maybe there's moisture in the PT? You could pop the bell covers and bake it in the oven at 170F for an hour. Then open the oven and blow a fan in there until every is cool and re test. Nothing to lose.

it looks ok...I'll check into that one....

[bsco]

I will post the voltage readings here later as I have to run out for awhile.......
Cheers.....

[bsco]

I guess you're measuring a "ghost voltage".
This would be a voltage due to the capacitance between winding and frame/core.
Try measuring again with a 10k resistor connected between meter leads.
A ghost voltage will collapse when loaded.

Ok. did a quick check before i go out.....with no resistor across the meter leads I get a voltage reading on 235VAC between one side of the high voltage B+ winding and the transformer frame....with a 10K 2W resistor connected across the meter leads, I get .435VAC between one side of the high voltage winding and the transformer frame...with the HV winding unloaded and no resistor across the meter, I read over 700VAC.....but that is an unloaded voltage measurement of the high voltage winding....that 10K resistor gets damm hot....and I am going through a dim bulb...100W....got to run out and will check back later,,

[Helmholtz]

Be careful, a ghost voltage will not heat the resistor.
Will elaborate later.

[bsco]

Be careful, a ghost voltage will not heat the resistor.
Will elaborate later.

ok. The resistor got hot when I measured the voltage across the high voltage winding....I wanted to see what the reading would be when compared to a non loaded measurement.....when I measured from one side of the high voltage winding to the transformer frame with no resistor connected across the meter leads, it was 235VAC approx.......when the 10K resistor was connected across the meter leads and voltage measured between one wire of the high voltage winding and the transformer frame it measured .435VAC......hope this clarifies any confusion.....

[Helmholtz]

Oh well, the 10k load resistor was only meant for measuring between frame/core and winding.
If placed across the HV winding the resistor will dissipate 50W. No sense doing so.
What about some electronics basics?

And please do spend a 0 before a decimal point. Otherwise I tend to misinterpret the number especially when there are that many useless dots in a text.
No one would leave out the zero before the decimal point over here, so I'm not used to it.
Also weak eyes.

[Chuck H]

So a measure of 0.435VAC would indicate sufficient collapse of the suspected "ghost voltage" for an accurate diagnosis??? This is a great tip by the way and thanks for that.