AVO Valve Tester (VCM) Mk 3 leakage problem

[rhowgego]

I have recently acquired an AVO Mk 3 VCM in excellent clean condition. With the Circuit Selector switched to Test, the expected readings of Ia and Gm are obtained for a variety of valves, so this function appears to be working as it should.
However, when the Circuit Selector is set to Check (C) (cold leakage), a high degree of leakage (about 80% full scale deflection) is obtained for all positions of the Leakage switch. This reading persists even if the valve is removed, suggesting that the leakage is within the tester itself.
With the Circuit Selector switched to Check (H) (hot leakage) or C/H INS (heater-cathode breakdown), the meter sits at about half fsd, regardless of whether the valve is inserted or not. I've checked the switches themselves and tried random resistance measurements which have left me even more confused. I'd very much appreciate some suggestions.
From Ray in Caterham, Surrey

[Dekatron]

Does anything change if you rotate the Electrode Selector switch or the Meter switch?

[rhowgego]

Hello Martin
No, nothing changes when any other switches are operated.
Ray

[Dekatron]

What happens if you set all roller selectors on the top to zero (0)?

[rhowgego]

The problem still exists when the rollers are all set to zero. I see that you are following the same logical sequence as I adopted, and which began to drive me mad!
The problem is that it’s so difficult to isolate parts of the circuit without widespread desoldering. Can you suggest any measurements that might be made? I’ve tried making resistance measurements between the plug sockets on the top, even between the pins on the valve holders, but I never get the same reading twice!
Thank you for your interest in this fault; I do appreciate it. I could still use the tester for measuring the characteristics, but it would be nice to have it all working properly.
Ray
Ray

[Dekatron]

I would check the leakage switch first of all, remove it from the tester by removing the knob and then unscrew it so you can have a good look at both sides of the wafer. one side has a center piece that shorts all pins except one and the other side is inverted, only connecting that pin that is not connected on the other side. Make sure that it is working properly and that it isn't the cause for the high resistance. When you have it out you can easily slide in two pieces of paper underneath the wiper on the side that only has one connection, that way you will isolate it completely from the circuit (effectively opening the circuit) so that the meter should fall back to its resting position indicating no current flow. If some current still flows you'll have to follow the wiring to se where it comes from.

[rhowgego]

Dear Martin, problem solved! To narrow down the field I disconnected the top panel by separating the two 9-way tag strips (quite easy with a 150-Watt soldering iron). The problem remained, so the leakage was in the switches.
The previous owner must have been a heavy smoker. When I drenched the Leakage and Circuit Selector switches in isopropyl alcohol, dark brown droplets dripped from the switches. Repeated soakings with alcohol, then drying with a hair drier until the drops were clear, restored the leakage to zero on all settings of the switches.
Many thanks again for your helpful thoughts.
Ray

[Dekatron]

Nice, always good when you find the problem.

It could also have been someone who sprayed the switches with some lubricating/cleaning spray that made them conductive.

[trh01uk]

Very interesting. Presumably these "dark brown droplets" cleared out with alcohol were some kind of tar deposits emitted by tobacco of some kind?

I always wondered why equipment "from a non-smoking home" have been thought to be worth more.

Richard

[MotorBikeLes]

Richard, forty years ago I bought ex rental TV sets. You would get the odd set that was full of "bedroom dust". I used a compressor outside to blow them clear. That set usually had no problems and was easy to work on.
At the other end of the scale, some would be sticky to touch. My first action would then be to spray the cabinet with a diluted cleaner, watching gravy run down the cabinet and tube and onto the floor. Later, with back removed, expect to find it just as bad. I remember one set, with a blue "skeleton" tripler,where the holes were completely blocked with brown whiskers.
The chassis of such a set would go into the sink, more of the cleaner plus plenty of hot water would have the gravy go down the drain. Next I would hang it up in my boiler room and leave it for about two weeks. Usually most of the skeleton presets would need replacing.
Oh yes, tobacco tar was real!
Les.