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vintage_8bit · 18th May 2020, 1:07 pm

I had the option of a Mullard recently but not got the room here.

Back to the MkIII.

So after checking the meter I have gone through all the resistors and 1 cap, I changed any out of spec.
My question now is about the "Set mA/V" calibration. The grid volts cal appears spot on.
In the service manual Section 2(e) and with the grid circuit link removed, I performed the following.

I am reading the grid voltage with a
keithley 2000 DVM or a cheapy hand held DVM. With the meter switch to 2.5mA I am adjusting the the Neg grid volts controls to 105mV I am then setting meter switch to mA/V and nulling out the grid volts with the set mA/V control. The result should be 5. I'm getting 5, so good. If I do the same with the grid volts at 0.52V I get a result of 1, which is also right. However the spec also asks you to perform this with a grid volts of 52.5V. this would mean producing a +ve grid volts to null out? I am obviously doing something very wrong here, but what is it?

Just had another go at it.... Is it a typo? should it be 52.5mV. This would be very difficult to achieve on my tester as the grid volts Add control is on the edge of its track.

18th May 2020, 1:07 pm	#5
vintage_8bit Heptode Join Date: Feb 2013 Location: Rainham, Kent, UK. Posts: 525	Re: AVO VCM Mk 3. I had the option of a Mullard recently but not got the room here. Back to the MkIII. So after checking the meter I have gone through all the resistors and 1 cap, I changed any out of spec. My question now is about the "Set mA/V" calibration. The grid volts cal appears spot on. In the service manual Section 2(e) and with the grid circuit link removed, I performed the following. I am reading the grid voltage with a keithley 2000 DVM or a cheapy hand held DVM. With the meter switch to 2.5mA I am adjusting the the Neg grid volts controls to 105mV I am then setting meter switch to mA/V and nulling out the grid volts with the set mA/V control. The result should be 5. I'm getting 5, so good. If I do the same with the grid volts at 0.52V I get a result of 1, which is also right. However the spec also asks you to perform this with a grid volts of 52.5V. this would mean producing a +ve grid volts to null out? I am obviously doing something very wrong here, but what is it? Just had another go at it.... Is it a typo? should it be 52.5mV. This would be very difficult to achieve on my tester as the grid volts Add control is on the edge of its track.