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Old 14th Jan 2020, 9:09 pm   #1
sven_swe
Tetrode
 
Join Date: May 2018
Location: Cologne, Germany
Posts: 88
Default AVO VCM MK3 - Restored with new meter finalized

Hi enthusiasts!

Just wanted to share a few words about the restoration of my AVO VCM MK3 that I bought last year in Germany in a terrible condition. Story was that it was brought over to Germany from England by a English officer.
Wen I picked up the machine it was clear that it had been stored in a garage/barn or similar. The inside and front was full of dust and the screws and bottom panel was a bit rusty. When I picked it up I started it fast just to see that the meter was moving and not completely dead. It moved and I hoped that the transformers were ok, the rest I thought can be fixed.
When I finally had it cleaned from all the dust I decided to check the meter. I took it out and it had the correct specs in form of current FSD and internal resistance, I cleaned the worst dust from it (it was full) since the glass in the back was completely loose.
What was not ok was that the meter tended to be stuck close to zero, even after cleaning. I then decided to send it to Herts meter for a service.
They did an excellent job as usual and provided me with a service report, and after talking to Robert at Herts it was clear what the problem was. When mounted in the MK3 at a 60 degrees angle, it tends to stick. This is due to the pivots are worn and expressed as a "pivoty" meter. It functions ok when being set flat horizontally though, but when mounted in the machine it needs a small "tap" to go the last millimeters back to zero. Since no spare parts are available for tis error at the moment I decided to go again for a replacement meter from Martin, Sweden (Dekatron).

The project expanded not to just having a new meter, it also finalized with having the following extras:
  • Meter protection (board behind the meter)
  • LED for visual overload display
  • Summer for overload audio display

Thanks to Martins great design of the meter replacement and LED PCB I now have a very functional machine, and I consider the meter replacement to be a upgrade to the original meter. It has a very nice "swing" and it's slightly bigger for reading. Very nice!

Apart from the meter faults i had the following major errors:
  • Neg. Grid volts potentiometer burned
  • 2 volts heater cable was not soldered correctly to transformer
  • Various resistors were out of spec. Especially the 10K meter resistor

To fix the neg. grid volts potentiometer I bought a Clarostat pot of internet and replaced the "track" only and kept the original pot. Now works fine.
Heater issue must have been already from factory, I simply connected it.
Resistors I have changed a few for Vishay high grade resistors.

Now the machine performs really well, I have my CT160 (also new meter, see other post) and RPM370 to compare with with. Thanks especially to Dekatron(Martin,Sweden) and David Simpson for supporting with ideas and standard valves.

Please see pictures of the new meter, the circuit board for the LED driver and summer and the overload "Light-up" from the LED board. For some reason I couln't find my own picture of the overload in action, so I used Martins. It really lights up the front!

What is still on the ToDo-list is conversion from the tube diodes to Silicone and forging new plastic transparent dial scales that were cracked and not possible to see through. It's ongoing!

It has been a fun project.
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Old 15th Jan 2020, 11:33 am   #2
David Simpson
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Default Re: AVO VCM MK3 - Restored with new meter finalized

A great job, well done min Ven. And, although your original meter doesn't like working at 60 deg., it'll still do as a standby spare for your horizontal CT160. Veckert arbeter, if you'll excuse my poor Svenska skriva.

Regards, David
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Old 15th Jan 2020, 4:16 pm   #3
merlinmaxwell
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Default Re: AVO VCM MK3 - Restored with new meter finalized

What is the pigmy bulb for?
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Old 15th Jan 2020, 6:48 pm   #4
sven_swe
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Join Date: May 2018
Location: Cologne, Germany
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Default Re: AVO VCM MK3 - Restored with new meter finalized

Quote:
Originally Posted by David Simpson View Post
A great job, well done min Ven. And, although your original meter doesn't like working at 60 deg., it'll still do as a standby spare for your horizontal CT160. Veckert arbeter, if you'll excuse my poor Svenska skriva.

Regards, David
Tack David! Your Swedish is getting better every month True with the exchange, but now I have a new meter in my CT160 as well.

Merlin do you mean the big red bulb? That's the old overload lamp, the picture is from partly finished mounting of the new meter. The bulb is in a different position at the moment.

I might remove it completely and try a led like in the MK4 as Martin hinted for me, but for now the new LED circuit from Martin is working just fine.
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