Testing for leaky caps

[Craig Sawyers]

The General Radio 1863 manual is here https://www.ietlabs.com/pdf/Manuals/1863-64_im.pdf . Semiconductor, apart from a valve as a series regulator. I guess that high voltage semiconductors were not available at the time it was designed.

Craig

[trobbins]

The cautionary aspect of using a 'megger'/insulation resistance meter is that they typically only have a few voltage settings (my handheld has 100, 250, 500, 1kV) and they typically don't discharge the capacitor at the end of the test (which means having to connect a voltmeter or load R, or both, to avoid any remnant stored charge).

[Ed_Dinning]

Hi Folks, although I have my own design of HV cap tester, I find that an AVO8, on the R*100 (15v) range is the next best thing.
There is also, on most 8 series AVO's the ability to use a higher voltage for insulation testing. This requires a low power transformer (a few mA) at, I believe, 240v.
Details are given in the AVO handbook and it could be adapted for a switched voltage range.
Discharge capabilities could easily be incorporated.

This could form a nice project and most folk will have a suitable transformer.

Ed

[cathoderay57]

I use the Avo analogue meter on the MegOhms range and if the DC resistance doesn't go to infinity then the cap is useless. You can get away with a cap that shows a DC resistance of, say, > 1 MOhm in some circuits such as cathode decoupling capacitors where the cap is in parallel with a resistor of a few hundred Ohms. I once read an article about reforming leaky paper caps but IMHO that would be a complete waste of time. If the cap shows virtually infinite DC resistance on the low-voltage Avo test, it still is not necessarily OK at full working voltage so in critical applications such as grid couplers it is worth applying HT to one end and a voltmeter across the open end and HT negative. There should be an initial pulse then the voltage should drop to very close to zero. As said above, in these applications, unless you are a stickler for using original components, it's just simpler to replace it with a new one. In less critical applications I usually rely solely on the low-voltage Avo leakage test and check the valve electrode voltages against those quoted in the Service data when I power up the set. As said many times before on this Forum, the service sheet voltages were often measured using a low impedance voltmeter and so, using a DVM, you should often expect higher voltages than those quoted, especially in high impedance circuits. Jerry

[stevehertz]

I concur with that, a lot of common sense there.

[GMB]

There are in-circuit situations where a simple test is possible. The classic example is the coupling capacitor between audio stages in valve circuits, from anode to grid.

If you don't have a high voltage source for the test (without which the results are not reliable) then simply power the circuit and measure the voltage that appears on the grid using a modern high-impedance voltmeter (so not an old AVO). The smallest positive voltage is a very bad sign.

With parallel heaters you can remove the valve as it too can be a source of this kind of leakage current.

Another in-circuit test is to check the whole HT. Ideally you need a circuit diagram as sometimes you find a resistive path from HT to earth. If so you could break it with perhaps only one disconnection and then do a leakage test on the whole HT with valves cold. Be careful to use the lowest working voltage that the HT "sees". This is often a way to bulk check all the screen decouplings.

[Craig Sawyers]

Originally Posted by trobbins

The cautionary aspect of using a 'megger'/insulation resistance meter is that they typically only have a few voltage settings (my handheld has 100, 250, 500, 1kV) and they typically don't discharge the capacitor at the end of the test (which means having to connect a voltmeter or load R, or both, to avoid any remnant stored charge).

The ones I mentioned in my post have a discharge button. And yes, some of them only have a few voltage settings. The exception is the General Radio 1864, which has three decade dials. 1V steps from 10V to 109V, and 10V steps from there to 1090V. The GR 1863 and 1864 have a 3 position switch, measure/charge/discharge.

Lafayette CR-46 CR analyzer discharges too - which has and electrolytic leakage test that is ideal for reforming. The only setting on that you have to be careful of is Insulation Test, that uses 600V - so using that setting for most paper or film capacitors will destroy them.

So any device that is intended for measurement purposes rather than a brute force megger, has a discharge setting to prevent people from killing themselves.

Craig

[Craig Sawyers]

Originally Posted by Ed_Dinning

Hi Folks, although I have my own design of HV cap tester, I find that an AVO8, on the R*100 (15v) range is the next best thing.
There is also, on most 8 series AVO's the ability to use a higher voltage for insulation testing. This requires a low power transformer (a few mA) at, I believe, 240v.
Details are given in the AVO handbook and it could be adapted for a switched voltage range.
Discharge capabilities could easily be incorporated.

This could form a nice project and most folk will have a suitable transformer.

Ed

There is also the somewhat rare "Model 8 Univeral Avometer Resistance Range Extension Unit".

I have one.

But here is a picture of the one in the Science Museum https://collection.sciencemuseumgrou...meter-avometer

There is a thread here too https://www.vintage-radio.net/forum/...ad.php?t=96679

Craig