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Old 7th May 2022, 4:00 am   #1
Lionaird
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Default AVO VCM 163 - manufacture year

Good Day,

I just acquired a beautiful AVO VCM 163 valve meter with the serial 1617. Can one tell the manufacture year based on the serial?

Thank you,
Tobi

https://www.vintage-radio.net/forum/...1&d=1651892218
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Old 8th May 2022, 8:03 pm   #2
Longespee
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Default Re: AVO VCM 163 - manufacture year

Welcome to the Forum Tobi, I think the best idea you will get of a date is from the capacitor date codes etc, yours is an early version and after this one they went to collet knobs ( easier to adjust ) and a Black selector switch unit which was apparently stronger.

But it's a 163 and if it's working well date will have little bearing on value if that's what you're after, they are fantastic machines and that's it. The meters are the main problem and if dates were involved ( which they aren't ) an early one would be superior, but unfortunately all these meters are far cry from anything that was supplied in any AVO VCM previously.
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Old 9th May 2022, 2:07 am   #3
Lionaird
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Default Re: AVO VCM 163 - manufacture year

Thank you for your reply and for the welcome. I didn’t realize that it was one of the earlier units. I think I read somewhere that AVO initially produced the meters for 163 themselves and later outsourced them to some Italian company? Not that it really matters because at the moment it appears to be working flawlessly. It allegedly was calibrated last year but I might get some pre-measured tubes to confirm that.

And yes, I am aware of the meters being the Achilles heel of the 163 and I am a bit concerned about longevity. But I suppose I will cross that bridge if/when the meters start playing up. There is probably not much I can do in regards to preventive measures apart from treating it gently.
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Old 9th May 2022, 10:40 am   #4
Chrispy57
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Default Re: AVO VCM 163 - manufacture year

Hi Tobi - fortunately there is something that you can do to safeguard the meter - several posts on here go into great detail, worth a search. Basically, you connect a combination of diodes/zeners/capacitors across the meter itself.

Cheers
Chris
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Old 9th May 2022, 10:52 am   #5
Longespee
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Default Re: AVO VCM 163 - manufacture year

Ah I didn't know that, interesting and certainly the first batch would be desirable indeed.

Calibration is very straight forward and the service manual is available online. Below is the best copy I have found.

http://bee.mif.pg.gda.pl/ciasteczkow...o/sm-ct163.pdf
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Old 9th May 2022, 10:26 pm   #6
Lionaird
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Default Re: AVO VCM 163 - manufacture year

Thanks for the advice. I will definitely look into measures to safeguard the meters. Would adding diodes etc. affect the accuracy of the measurements in any way? Might also try to buy some standardized tubes (or alternatively make one myself if that’s not too hard to do)…
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Old 10th May 2022, 11:34 am   #7
Dekatron
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Default Re: AVO VCM 163 - manufacture year

The left Anode current meter (Ia) should have two diodes in parallel unless it is a very early model where there sometimes only was one diode. I've never seen one without diodes.

The right mA/V (gm) meter shouldn't have any extra diodes added as it will affect the amplifier slightly. The amplifier circuit already uses a few diodes (and also capcitors) to limit the swing and to provide the correct polarity. In some cases the protective diodes found on the secondary side of the Ferrite pot core (connected to the transistor VT1 via a capacitor C2) are missing, these seem to be a later addition as they are soldered directly to the connections to the Ferrite pot core on the underside of the circuit board.

Longevity of the original meters is usually affected by humidity/moisture as that affects the metals used in the path the current takes to/from the moving coil. Several different metals used there gives rise to galvanic effects and corrosion which results in poor contact and also high and variable contact resistance. You can remove the bolts connecting the meter to the solder lugs and clean them as well as the solder lugs if really necessary but that might give rise to other problems or in the worst case you might damage the meters.

As always don't try to fix what's not already broken.
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Old 10th May 2022, 11:41 am   #8
Lionaird
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Default Re: AVO VCM 163 - manufacture year

Thanks Martin. You are right, less is probably more. I won’t fiddle with the meters as long as they are still functional. It’s a shame that there aren’t any companies anymore that are specialized in meter restoration…
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Old 10th May 2022, 4:19 pm   #9
Longespee
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Default Re: AVO VCM 163 - manufacture year

I had to take my meters apart just to clean the bolts, washers and nuts, as Martin mentions best left alone.
They didn't appear corroded but they were so bad when I measured them for continuity the nuts were Open Circuit!!!!!!
I re-cut the threads on the bolts and dip cleaned the nuts and threw away the washers and replaced them.

That was not the end of my troubles, I'll write about it when I get chance.
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