AVO VCM MKII restoration starts

[Dekatron]

I've had a similar problem with a to short peg, and I didn't think clearly on how to fix that properly then as I tried to glue a new peg onto the rotating piece and it would always break off and then I thought of making soem arrangement to raise the sides of the zero setting piece on the meter itself so that the short peg would reach into that groove - what I didn't think of first was that I could drill a new thin hole in the rotating piece and glue a brass pin in that hole and then cut it to length. Took me some time to think of that solution, sometimes you get stuck in the wrong way of solving things.

[retailer]

Spent some more time on the VCM - soldered up the meter amplifier onto a bit of vero board using a circuit from the Wex millivolt meter pdf, the components can be selected to provide a reasonable degree of meter protection by limiting the current through the meter, I had to manufacture a meter mount as the original was missing and then wait a day for the paint to dry, meantime I bent up some 2mm Al and made a bracket for my meter amplifier and small transformer, I experimented with various locations for it and eventually settled on the top right hand rear corner, I wanted the dc offset and cal adjustments easily accessible so I had to mount it on the back of the bracket with a cutout for the components and adjustment. All of the original lugs/terminals were retained so if an original meter was ever available it would be easy to reinstate it.

All finished and calibrated I gave it good test yet again only to find that known good valves showed an internal short, a bit more testing and I realised a short was showing without a valve in the socket, removing the top valve panel again I narrowed it down to pin 7 on the valve sockets had a partial short to ground, it turned out to be the old shielded cable used to wire up the sockets. With the offending cable replaced it all seemed to be working except - that when I select Check(H) the meter gradually moves negative a very small amount, only a mm or so with small signal valves but worse with power valves such as EL34, looking at the circuit I can see that MR4 forms part of the shorts circuit - it's the original component and I'm hoping this has some leakage allowing reverse current to flow from the hot fillament and cathode, tomorrow will tell.

[retailer]

I replaced MR4 with a schottky diode I found in my spares box - SR304 - forward voltage drop according to the diode test function of my Fluke 87 was 165mV, the result was dramatic - in the wrong direction - meter only managed to make 50% of scale on all functions that use that diode ie mains voltage set, and shorts function, a strange result. I checked the MR4 diode that I had removed with the Fluke, it tested as open circuit on the diode test function but on ohms, 6meg in one direction and 2meg in the other, so it was sort of working as a diode with no obvious issues other than the slight negative meter indication when testing for shorts with hot filament. I didn't bother trying to work out why the meter only made 50% of scale and just installed a standard 1N5404 in place of the SR304 - meter movement was back to full scale, sadly so was the slight negative meter indication, perhaps this is just a quirk of these testers.

Looking at the EMER circuit I see 5 diodes - my tester seems to have only 4 - I'll check that out later, I suspect it is the diode that is in series with the screen supply, a note in the EMER documents states that it was fitted to prevent possible issues when testing beam power valves.