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Old 30th Dec 2020, 1:49 pm   #1
cathoderay57
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Default VCM163 Anomaly

Hi folks, testing valves today on my VCM163 I made a switch selection in a sequence I wouldn't normally do. Having tested a valve hot I turned the Function Switch back to "Check(C)" (check cold) and rechecked the leakages by rotating the leakage switch. The heater gave continuity as expected at position h(C) but the cathode position c(C) also showed short circuit. After about 30 seconds the meter needle slowly fell back to zero as though a capacitor was discharging. I have had a look at the circuit and I presume the gradual voltage decay was the discharging of C6. It is clear that in the h(C) position there is continuity from Meter 1+ via S5O, S6B/2, S6A/2, S5B/1, through the valve heater to chassis. However, in the case of the cathode position c(C), the circuit goes from Meter 1+ via S5O, S6B/3, S6A/3, S5A/1 to the valve cathode. Unless the cathode has leakage to heater I cannot see any other path back to chassis. The same condition was repeatable on 3 different valves and yet, testing using the ch(INS) function valves showed all 3 valves to have infinite heater/cathode resistance. Is this a fault condition or am I missing something? Cheers, Jerry
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Old 30th Dec 2020, 7:25 pm   #2
Ed_Dinning
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Default Re: VCM163 Anomaly

Hi Jerry, I normally perform a similar test when using the HSVT; rechecking grid current, electrode shorts an H/C insulation with the valve hot.
I have seen a few valves that test OK on the way up, but when retested hot showed a fail, so it may not be the tester

Ed
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Old 17th Jan 2021, 11:46 am   #3
cathoderay57
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Default Re: VCM163 Anomaly

I'm bumping this to add a request. I am still finding that the cathode shows a short circuit in c(C) leakage position) BUT only immediately after the valve has been heated - any valve will give the same result and therefore either it is a feature or a fault on the VCM163. The only explanation that I can think of is that while the heater retains residual heat, and after the heater power has been removed, maybe there is diode action between the inner heater wires and the cathode causing conduction in the c(C) test position? This would be consistent with the symptom because the short circuit reading decays as the valve cools. Please would another member owning a VCM163 run the same test outlined in Post #1 and let me know if they get the same result? Thanks, Jerry
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Old 17th Jan 2021, 2:05 pm   #4
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Default Re: VCM163 Anomaly

What happens if you quickly replace a hot valve with a cold valve in the CHECK(C) position after you have finished testing the hot valve?

What measurement do you get then, does it fall to zero immediately when you put in the cold valve?
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Old 17th Jan 2021, 4:42 pm   #5
cathoderay57
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Default Re: VCM163 Anomaly

Hi Martin, thanks for your response. I did your test with a pair of EF91s. When I removed the hot valve and immediately put in the cold valve there was no continuity indication. Quickly putting the warm (by then tepid) valve back in showed continuity briefly again but decayed until the valve had fully cooled. I don't think my entire stock of valves all have thermally dependent heater-cathode shorts, so unless I've gone stir crazy in the lockdown (always a possibility) it looks like the continuity is definitely happening within the valve. Cheers, Jerry
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Old 17th Jan 2021, 5:11 pm   #6
Leon Crampin
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Default Re: VCM163 Anomaly

I've noticed this with my Mk 4 tester. Without looking closely at the heater and cathode potentials under these conditions, I've come to the conclusion that the heater itself is emitting electrons to the cathode - within it. This would explain why the effect disappears as the heater cools. One could play with the heater voltage whilst observing this effect.

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Old 17th Jan 2021, 5:23 pm   #7
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Default Re: VCM163 Anomaly

Your observation with a cold valve contra a hot valve was just what I wanted you to see as it rules out any fault in your VCM 163.

I agree with Leon regarding what is happening. The capacitor in the circuit, C6, is not discharged as it is constantly supplied from the grid volts winding all the time. The effect you see can then only be due to the electron flow decreasing in the valve itself due to the heater becoming colder and colder emitting less and less electrons.

You can possibly check this by just heating the valve for a short time and then switching back to the CHECK(C) position, it will probably cool faster then and probably start at a lower temperature and emit less electrons while cooling, so the meter should reach zero faster. This effect might be easier to see with some valves than with others depending on the heater construction.
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Old 17th Jan 2021, 7:14 pm   #8
cathoderay57
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Default Re: VCM163 Anomaly

Thanks for the replies - makes perfect sense. I found with a high thermal capacity valve heater such as a TH30c the cooling effect and continuity lasted a lot longer. I am reassured that the trusty VCM163 remains in good shape. Cheers, Jerry
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Old 17th Jan 2021, 9:59 pm   #9
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Default Re: VCM163 Anomaly

ISTR a warning in the information on my MKIV VCM not to use the continuity/leakage test once the valve is hot because the result will be meaningless. I’ve certainly observed the effects that you report and simply attributed them to a surplus of free electrons in the bottle.

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