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

[Mick Bailey]

My Fluke DMM has a Lo-Z function for this purpose - it changes the impedance from 10M to 10K ohm. Perhaps this requirement is overlooked with benchtop electronics because ghost voltages aren't so common, though I've been 'had' a few times with capacitor-coupled output stages where I've read a high DC on the output with no load connected only to find it disappears with the meter on the low impedance range.

[Helmholtz]

To my knowledge ghost voltage (aka Phantom Voltage) is always AC.
It is due to stray capacitance between the parts probed.
A capacitance means an AC impedance.
The capacitance between winding and core can be as high as 1nF, meaning an impedance around 3M at 60Hz.
So we have a HV source in series with an impedance of 3M.
Connecting a meter causes a voltage dividing effect, so a 10M meter might read 80% of the winding voltage while a 1M meter might read only 20% (no exact calculation).

In circuit the PT secondaries typically have some ground reference, so ghost voltages will essentially be shorted to ground.

https://www.nema.org/docs/default-so...v_aug_2020.pdf

[bsco]

My Fluke DMM has a Lo-Z function for this purpose - it changes the impedance from 10M to 10K ohm. Perhaps this requirement is overlooked with benchtop electronics because ghost voltages aren't so common, though I've been 'had' a few times with capacitor-coupled output stages where I've read a high DC on the output with no load connected only to find it disappears with the meter on the low impedance range.

Actually, I never knew that...this is a great tip for sure.....and yes, the couple of meters I have do not have that function....I will use this 10K trick from now on.....

[bsco]

To my knowledge ghost voltage (aka Phantom Voltage) is always AC.
It is due to stray capacitance between the parts probed.
A capacitance means an AC impedance.
The capacitance between winding and core can be as high as 1nF, meaning an impedance around 3M at 60Hz.
So we have a HV source in series with an impedance of 3M.
Connecting a meter causes a voltage dividing effect, so a 10M meter might read 80% of the winding voltage while a 1M meter might read only 20% (no exact calculation).

In circuit the PT secondaries typically have some ground reference, so ghost voltages will essentially be shorted to ground.

https://www.nema.org/docs/default-so...v_aug_2020.pdf

great info there.....so I have a question....what about an analog VOM?? they usually have a low impedance come to think of it......the digital meters are usually 10 meg or higher?
the analog ones are rated 20K ohms per volt??? I must check out the one that I have here and see how it preforms...that was an excellent article......thanks for posting it.....

[Helmholtz]

Actually, I never knew that...this is a great tip for sure.....and yes, the couple of meters I have do not have that function....I will use this 10K trick from now on.....

Just make sure to not connect a low power resistor to a "real" voltage .

You could also try a 0.1µ cap of sufficient voltage rating instead of the 10k resistor.
Should make ghost voltage collapse to a few volts.

[Helmholtz]

great info there.....so I have a question....what about an analog VOM?? they usually have a low impedance come to think of it......the digital meters are usually 10 meg or higher?
the analog ones are rated 20K ohms per volt??? I must check out the one that I have here and see how it preforms...that was an excellent article......thanks for posting it.....

To find the actual input resistance of an analog meter you need to multiply the Ohms per volt rating by the range selected.
So with 20k/V in 1000V range the input resistance would be 20M(!). Higher than any handheld DMM.

Often the Ohm/V for AC is much lower than 20k/V, though.

[bsco]

To find the actual input resistance of an analog meter you need to multiply the Ohms per volt rating by the range selected.
So with 20k/V in 1000V range the input resistance would be 20M(!). Higher than any handheld DMM.

Often the Ohm/V for AC is much lower than 20k/V, though.

I'll check my meter out later this evening and let you know.........thanks for the info...much appreciated......

[bsco]

@ Helmholtz. checked out my old analog meter....10k ohms/volt on the AC range and 30K ohms/volt on the DC range....so I will take your advice and use the 10K resistor across the meter leads when checking for leakage between a transformer winding and the metal core.......thank you for this tip......

[Helmholtz]

@ Helmholtz. checked out my old analog meter....10k ohms/volt on the AC range and 30K ohms/volt on the DC range....so I will take your advice and use the 10K resistor across the meter leads when checking for leakage between a transformer winding and the metal core.......thank you for this tip......

I mentioned the 0.1µ cap method because there would be no risk of cap overheating with real voltage.
If the reading significantly drops with the cap it should be ghost voltage.

Also the 10k resistor needs to have sufficient voltage rating.