I have not worked on a 412 for many years. Is it possible the hum balance is NOT power supply hum but the residue of the chopper?
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On 11/30/2024 1:00 PM, Dan via groups.io wrote:
Hello all, I am finishing up on a restoration of a HP 412A VTVM. ?It¡¯s quite accurate from the get go after the restoration.
I replaced every capacitor minus ceramics, I replaced pretty much every carbon composite resistor with 1% metal film of good quality, installed nos tubes that tested quite strong. ?I¡¯m not sure if I¡¯m doing the hum balance correctly.
It says to connect a scope to the amplifier terminals which happen to be on the back of the meter and adjust for minimal 10 hz signal. ? I¡¯m not seeing any 10 hz, only seeing stuff in the kHz.
If I adjust the hum balance pot it does change the waveform seen and it does lower the frequency, but I get nowhere near 10 hz and the frequency is jumping all over the place.
I did also notice that the hum balance adjustment also affects the zero point of the meter fairly greatly. ?After playing around with it I had to zero the meter using the small screw directly below the meter face.
How do I do this correctly? When adjusting hum balance in a tube amplifier I would adjust for lowest noise/hum. Would adjusting for the lowest point where the needle sits on the meter work? Equating that the needle is higher due to noise, this is a total guess here. ? I¡¯m sure there are others that have aligned this meter so I¡¯m hoping for some help.
Thank you,
Dan
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Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
