Capacitor testing.

[Darren-UK]

The subject of testing and/or replacing capacitors comes up frequently, but what do you guys use for testing ?

I make do with intuition, the assumption that waxy TCCs and black/brown Hunts jobs are probably duff anyway and an Avometer, but have looked at commercially available capacitor testers - only to be put off by price.

[audiomagpie]

I purchased a capacitance meter from CPC earlier this year (£30 ish +vat as I remember) and have found it very useful. Obviously you have to take the cap out of circuit to test it. Total O/C or S/C you can find with an avo, but the cap meter has occassionally found a component that still has capacitance but has altered in value.

Disadvantage is that the meter is battery powered so it won't stress any capacitor that may only become leaky or breakdown under high voltage.

Good for checking the value of a component where the value numbers have faded off!

[Norm_Leal]

Hi Darren

I don't find many caps that have changed value but most old ones are leaky.

I use the highest resistance scale on a Triplett 630 meter to check for leakage. This meter uses a 30 volt battery on the highest ohm scale. Any leakage and a paper type cap gets replaced.

Some mention you need higher voltage but this meter has worked well for me.

Norm

[Alan_Douglas]

Many (most?) digital cap meters can't distinguish between capacitance and leakage. Try an experiment: measure a known good cap, while bridging a resistor of a few megohms across it. The capacitance reading will increase.

I built a leakage tester, a variable 700-volt supply at up to 10mA, with a panel meter in series. The meter is shunted to give ranges of 10mA, 1mA, and 100µA (and diode-protected against shorts). The schematic is in my book " Tube Testers and Classic Electronic Test Gear " (excuse the commercial plug). I use it for reforming electrolytics too.

All paper capacitors leak. The very best ones pass a few microamps, maybe not a problem, maybe just a matter of time before they short. Your call, depending on the expected use, and the damage that a shorted or leaky cap might cause, from none to severe.

[Station X]

Work on the basis that all paper capacitors either leak now or will leak in the future and you won't go far wrong. I change them all. It helps prevent future problems.

However I like to show myself that I am capable of finding faults rather than changing components wholesale. A leaking capacitor will pass current. That current will have to pass through a resistor which will change the voltage across that resistor. Look for voltages on valve pins that differ widely from those shown in the service sheet. Suspect any capacitor connected to a pin which has the wrong voltage on it. Disconnect one end of the capacitor. If the voltage changes there's you fault.

Another method is to apply HT with all valves apart from the rectifier removed. There should be no current drawn from the HT supply and no voltage across any resistor. If you see a voltage across any resistor suspect any capacitor connected to that resistor. Disconnect that capacitor and see if the voltage returns to zero.

The exception to the above is a set where the screen voltage is derived from a potential divider. In that case there will be a voltage across the potential divider resistors.

The valve removal technique won't show up faulty cathode bypass capacitors. These will result in incorrect cathode voltage with the set powered up.

Don't forget that resistors which have been passing excess current may have changed value. Check with a meter and replace if necessary.

Graham.

[wireless_paul]

I am now on my second Capacitor Tester and have found them very useful. If I knew where it was (am in the process of sorting out my workshop!!) I would tell you what make and model. It would be a boring world if we all did the same things. Personally I do not change capacitors unless I believe there is a problem with them, damaged or leaking etc. I aim to get a radio running and working reasonably well with as many original components as possible. Does it matter if it breaks down later on, part of my enjoyment of the hobby is to strip it down and sort out the problem when it happens??
Paul E

[Aerodyne]

I use a venerable old Avo capacitor tester. I find it is surprisingly accurate. I test 'new' (perhaps old stock) caps before use. The tester will show considerable variation from any handful of nominally the same value.
An Avo 8 on ohms X 100 will give a reasonable indication of leakage but as already mentioned, all paper caps of the old style will usually show some leakage and it's a matter of judgement. I find it safest to change screen grid decouplers, RF bypass caps, tone-correction caps and any others with HT across them, even if they read 'OK'
Tony

[jim_beacon]

I have a nice circuit for a capacitor reformer / leakage tester, that appeared in " Radcom " some years ago. It gives an ouput variable between 0 and 500V, using an 807 as a grid controlled rectifier (single silicon diode to give the bias supply), and has a meter shunted for various levels of leakage.

Anyone with a reasonably stocked junkbox should be able to knock one up in a couple of hours, I think the hardest part to find would be an 807 base (but you could use a 6L6 instead....).

If anyone is interested, I'll dig out the diagram.

Jim.

For anyone else who is interested, I've posted the details of the Radio Costructor / Radcom capacitor re-former here:

www.g1jbg.co.uk/capref.htm

Jim.

Anyone with access to old editions of Practical Wireless can find a constructional article for a " condenser " condition tester in the February 1958 and April 1958 editions.

It wasn't anything very complex - essentially a 600 volt DC power supply, isolated from the mains, with a switched potential divider to give outputs 12 - 600 V. These were applied to the capacitor under test via a neon or a moving coil meter to indicate leakage at the rated voltage. Current limiting resistors and a capacitor discharge switch were incorporated.

I built one, and it's useful not only for checking paper capacitors, but also for reforming electrolytics. I also use it for quick checks on the HT line of radios (disconnect radio from mains or batteries, remove all valves, connect tester output to HT line, ramp up the voltage to measure overall leakage, and also condition of smoothing capacitors. It's also possible to check screen decoupling capacitors by measuring the voltage at the valveholders - there should be no voltage drop across screen feed resistors if the capacitors are good).

Robert

[Richard]

Hi all.

I test wax/paper caps first for leakage with a high voltage power supply, at the rated voltage, and if they pass that test I check them with a digital cap meter.

Electrolytics get reformed with the high voltage power supply, and kept in if at all possible.

The cap meter is a " Caltek CM3230 " .
9 ranges 200pF > 20000uF.
This was about £35 from CPC, cheap and works well and will cover anything you will find in a radio.

If one or two fail in a set, I tend to change the lot.

All Hunts moulded caps tend to go straight in the bin.

Richard

[Neil F]

I'm surprised no-one's mentioned using a Megger to test for leakage. I've got one of those battery-operated jobs that allows you to test using a variety of voltages from 50v to 1000v and displays leakage in megohms. As a rough rule, if a cap is showing less than about 5 megs at its rated voltage, and if it's in a high voltage (or high impedance) part of the circuit, it takes a quick trip to the bin! I also have a Maplin capacitance tester which is a generally useful piece of kit: apart from capacitance it measures inductance, temperature (surprisingly useful) resistance and has a 15MHz frequency counter chucked in for good measure. And all for around 30 quid, if I recall correctly!

Neil F.

[radioman]

I use a digital LCR meter made by Tenma (same as Megger B131). This has a dual display which also shows the dissipation factor which is indicative of leakage. It's very useful for testing inductors too, but not so good with resistors for some reason.

Andy

[Paul Stenning]

For proper testing it would be necessary to disconnect the capacitor from the rest of the circuit, and probably to remove it completely. Having done that it's easier to just put a new one in than testing then trying to refit the old one. More reliable too...

I don't bother to replace caps; I prefer to just replace them all and have done with it! Just my £0.02 worth, and I know others will not agree!!