AVO VCM MkIII

[John_BS]

Does anyone have intimate knowledge (or any kind of experience come to that!) of this particular valve-tester. I've found what look like schematic errors, but it may just be my waning powers.....

For instance, when setting the mains voltage on ~ setting, it seems to show the meter not connected to anything?

John

[Dekatron]

If you look closely at the leakage switch you can just make out number 9 on the inner rotor, this then is connected to the outer numbers via the rotor, shorting all but one pin which is connected to 9.

It is not as clear as one would like it but later editions (1966) of the Mk IV circuit diagram shows this as two separate switch wafers making it easier to understand.

[John_BS]

Thanks Martin. I can't make any sense of the original schematic. Could you possibly direct me to where i can find the second version you posted?
Another question: do you happen to know if the 30uA movement has a magnetic shunt which can be adjusted to trim the sensitivity?

Thanks again
John

[ms660]

The schematic should be easy to follow once you've "cracked the code" for the leakage switch diagram....

.....In that diagram, switch wiper No.9 is shown in the gap in the circle, it rotates as the that circle rotates, eg: it's ganged with it, that wiper is actually wiper No.9 on the circuit selector switch.

Imagine the circle with the gap as a disc, the disc being a conductor.

Another thing that can confuse some is that wiper No.3 (referenced as A1 on the circuit selector switch) could be mistaken as being connected to heater -ve and wiper No.10 on the leakage switch, which of course it isn't, also Avo missed out a connection dot just to the left of the A1 reference on the schematic.

Lawrence.

[Dekatron]

The circuit diagram for the AVO Mk IV can be found in any of the instruction manuals for the Mk IV.

All meters I've seen in a Mk III has had a magnetic shunt but I've heard of some without but can't say if those were original movements or not.

[ms660]

A better leakage switch diagram from another Avo VCM MKIII schematic.

Lawrence.

[John_BS]

Thanks for all the help in decoding the rather badly-drawn schematic!

I'm working my way through sorting one of these, and I've discovered that the Vg pot has an issue near the -1V end. Opening the pot I discovered the wirewound section in that area had been cooked and the winding had "sunk" such that the wiper went o/c a that point.

With a valve mis-connected or faulty, a large surrent can flow down the grid wiring to the cathode 0V common.

Now, another question; is there any reason not to put a resistor in series with the grid feed, say 10k to 100k? And why didn't AVO do that?

John

[Dekatron]

Depending on what model of the Grid Volts potentiometer that you have in your tester it is sometimes possible to remove the whole track and put it back in reverse, turning it over 180 degrees, sometimes you can also flex it the other way and put it back to get at an unused portion of the track - but as always you'll have to be careful while bending and flexing the track as it easily breaks, if you can't go this way they, or similar types, are available at auction sites.

Putting a series resistor with the grid would mean a voltage drop that is current dependent so you would not know exactly what voltage you will end up with at the grid. Sometimes you can use grid stopper resistors and capacitors in a socket adapter but then you'll have to measure the grid voltage at the tube pin.

[Craig Sawyers]

The grid pot in the MkIII is linear. Unlike the CT160 which is a weird law.

Craig

[Pamphonica]

OT, but interestingly, the CT160 pot is actually "tri-linear"! Three successive 90 degree wirewound linear sections of 1250, 2500 and 6250 ohms (total 10K). That equates to Vg of 0-5, 5-15 and 15-40V on the scale.
Replacements are definitely "Unobtainium".
-Jeremy

[John_BS]

Not being a valve guru, I thought the grid would normally not require any significant bias current?

John

[Craig Sawyers]

Correct. If the valve is devoid of shorts. But the test protocol on AVO valve testers is to test for shorts cold, test for shorts hot, and then check cathode heater leakage. Then proceed to testing gm.

Craig