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Old 3rd Apr 2015, 10:34 am   #17
Dekatron
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Join Date: May 2009
Location: Linkoping, Sweden
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Default Re: AVO VCM163 - Some faults I have had

Hi Graham,

Thank you for the information. Please make the information available here in this thread, with photographs if you have the possibility.

Have you read what Swordholder wrote on this in the first post: "I once had an EL34 giving very strange Gm readings (low) and none of the usual suspects cleared the fault. This was traced to C1,C2,C3 & C4 on the mains transformer going O/C. Can't see why this should cause the problem, but replacing all four cleared the fault." - this is the only case that I know of where these capacitors have affected the testing. I checked this by unsoldering one side and then the whole tester became susceptible to interference from the wall warts and other signals on the mains voltage. By using a DC-power supply with a pure sine wave inverter these problems went away, without these capacitors in place so I think that they are there to dampen/remove interference as well as having a stabilising effect. So maybe you should consider using capacitors on all taps like AVO did.

In the three VCM163s that I own and one that a friend owns the capacitors are wired to the connections that I changed them to in the circuit diagram, I made a full reverse engineering of my VCM163s when I corrected the schematics, by measuring and checking every connection and also asked my friend to check his.

I guess that this is one more of the famous AVO changes or errors in their schematics and how they actually wired the VCM163s - either an error in the wiring or an error in the schematic, or both.

I can't say which one is the correct one but checking the specification datasheet for the transformer the approximate voltages 100V, 200V and 300V are found on tags 14, 18 and 22, but on all three of my VCM163s those voltages are found on 15, 19 and 23. AVO or the person winding the transformer might have changed that over time due to updated specifications, so on my transformers the voltages are found on different pins, or it might be an error in the design specification.

Could you check your VCM163 and see how it is wired, or do you just own the transformer?

If you still have contact with the person who gave you the transformer it would be very good if you could ask if he remembers anything on this matter!

When we know more on this I can make a new schematic that reflects this.

/Martin
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Last edited by Dekatron; 3rd Apr 2015 at 10:45 am. Reason: Missed one comment.
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