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Old 16th Apr 2021, 11:16 am   #1
SteveCG
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Join Date: Jun 2009
Location: Herefordshire, UK.
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Default Advance VM77c Valve Millivoltmeter

This was a car boot purchase from quite a few years ago - so what its prior life was I've no idea. At the time I checked it worked which it did of sorts but other things 'to do' took precedence.

I decided to finally look at it again a few days ago and it has proved to be a 'Sherlock 3 pipe problem' for me. I powered it up in stages gently using a variac and found it worked of a fashion.

I searched on the forum and found the following thread:

https://www.vintage-radio.net/forum/...d.php?t=164726

which has a link to the VM77b information which I downloaded. I gather the 77b and 77c are very similar apart from the front panel layout. Fortunately for me the capacitors in my 77c are Wima clear yellow types and not the Hunts listed in the 77b part list, with two exceptions. One of these is the input isolator capacitor marked as C1 on the 77b circuit diagram, a Plesseal 0.05 uF, 500V. This was quite an electrical leaker and was replaced. The other is marked as C21 connected across the meter, a 0.02 uF brown Hunts another replacement! By the way, I checked the voltages in the unit with those marked on the 77b circuit diagram and found, using a DMM, that they were in good agreement.

It is an aspect of the calibration for which which I'm requesting help . In brief, there is an adjustment shown on the 77b which also exists on the 77c but with no explanation of what to do. It is marked as as a preset pot, R33, in series with C25, a 0.001uF capacitor.

So does anybody have a 77c manual and if so does it describe how to adjust R33?

Thanks
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Old 16th Apr 2021, 12:02 pm   #2
barrymagrec
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Default Re: Advance VM77c Valve Millivoltmeter

According to the 77B circuit it`s an HF adjustment - I`d be inclined to set it for a flat response at 1Mhz compared with 1Khz then check how it looks over the range.

(I only have a VM77B instruction manual and an original large meter VM77)
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Old 16th Apr 2021, 5:26 pm   #3
SteveCG
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Default Re: Advance VM77c Valve Millivoltmeter

Thanks barrymagrec,

I'll have to sort out a 1MHz source. I can measure its amplitude using my 40 MHz 'scope.
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