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Old 1st May 2010, 9:33 am   #1
Rest in Peace
Join Date: Aug 2009
Location: East Preston, Sussex
Posts: 162
Default AVO VCM163 - Some faults I have had

Hi Guys,
Over the years I have had to repair several VCM163's for my former company and its customers.
The most common faults I have encountered are.
Anode current incorrect. Usually on 100ma range, check shunt resistor R4 (2R4) situated on the meter range switch. Can be checked by removing anode link and inserting AVO , should read 1/2 of the VCM meter reading.
Grid voltage incorrect, check R21 mounted on Grid voltage switch.
Incorrect Gm readings.
If the Gm meter can be set to "cal" position, but valve gives low Gm.
Check R33 (2k4) on Gm switch. Very often looks charred and reads High.
Other Gm faults, but not so common.
Large poly cap on amplifier board goes S/C . 1.99uf is unobtainable, fit 2uF (not electrolytic) in its place. This has very little effect on performance.
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 may help some members with repairs.
How about similar threads for other common test gear.

Regards to all
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