Capacitor (Condenser) Checker alternative?

[SurreyNick]

I have a whole load of mica condensers to check if they are still good, some dating back to the 1920s/1930s. I could just re-stuff them with modern caps (as illustrated in the last photo) but I know that mica capacitors are generally pretty reliable. But some of these ones are coming up to their centenary. Some of the pitch-filled ones also have missing values.

I have read that the capacitance function of modern DMMs is no good for checking these very old condensers and that it is only any good for modern low-voltage types. Is this true? Likewise I have read there are serious shortcomings to voltage/spark tests and to an ohms test using an AVO8. Clearly these latter two methods will be no good for establishing the capacitance value either.

I don't have a Heathkit or other make of capacitor checker and, other than buying one, I wondered what my alternatives are?

Thanks

Nick

[Radio Wrangler]

The capacitance measurement in most DVMs is a bit cheap and cheerful and is aimed at larger capacitance values than RF mica parts usually show.

You could ask if anyone has a Q-meter in working order and the matching box of test inductors so they could check them for you. It would probably be a shame to restuff these things. If they were buildings, they'd be listed

David

[julie_m]

Use your "unknown" capacitor to build an oscillator, and measure the frequency? That's pretty much what all capacitance meters do, anyway.

[Diabolical Artificer]

It's possible to measure a cap with a scope using a known resistor, as AJS says.See PDF.
The maths is a bit scary but with practice it's not too hard.

Andy.

[SurreyNick]

I agree it would be a shame to re-stuff them and that's what I want to avoid. My lack of test kit is beginning to become a problem. Time I think to correct that.

In the meantime I have had a very kind offer from one of this forum's members to test them for me

I will also place a wanted post for a decent capacitor tester.

Nick

[hansomcommon]

The cheap Japanese multi testers are superb, eg LCR-T5 for about £20 from ebay, mine goes down accurately to 20pf and with the transistor test + lead out function I wouldent be without it for vintage & unmarked components.

[PETERg0rsq]

Dont forget that cheap capacitor testers dont test at anywhere near the operating voltage of the capacitor.

You need to test for three main parameters.

Max voltage to ensure they dont break down in use.
Capacitance, so they do what they should.
ESR (not entirely necessary, but good to establish no hidden problems).

Capacitance is easily checked with a cheap tester, bridge or oscillator circuit.

Voltage means getting a supply that will go as high as the capacitor voltage (I note some are 3kV, so safest way to get near that is an insulation tester).

ESR needs testing with an ESR meter, which can be bought or made.

Peter

[Radio Wrangler]

RF mica capacitors tend to be smaller in capacitance than ESR meter go down to. They're aimed at testing electrolytic capacitors in the microfarads range.

Smaller capacitors can be measured on a bridge with a loss component facility, but these are limited in the Q range they are good to. High-Q RF capacitors get tested by the lossiness of a resonant circuit using them. This needs low loss inductors to work and the inductors still tend to dominate the Q reading.

David

[G8UWM-MildMartin]

Putting "neon capacitor tester" into the search box above comes up with many posts including several relevant ones.

[Diabolical Artificer]

There doesn't appear to be a one off cap tester homebuild project out there, IE one that tests leakage, value and ESR. I use a DMM for the value, an ESR meter and a simple Megger.

Someone over on VRAT made a nifty tester using a through away camera, which might be worth a look at. They used the flash circuitry.

I suggested using a scope to test the value of a cap as it teaches you about oscillators and delves a bit deeper into electronic theory, and yes maths. As a beginner myself I shy away from maths sometimes,but once you practice at it, it gets easier.

Andy.

[SurreyNick]

I like the ideas put forward and if I were more confident about the whole matter I might have tackled building an oscillator or one of the other ideas presented. They all look and sound a little intimidating though so I think my best bet is to save up and then try to obtain a suitable tester. Sorry if that sounds like a cop-out, especially as I asked for alternatives, but I don't think I'm ready yet.
Nick

[G8HQP Dave]

Originally Posted by SurreyNick

I have read that the capacitance function of modern DMMs is no good for checking these very old condensers and that it is only any good for modern low-voltage types. Is this true?

No. The capacitance function of a DMM measures capacitance; it does not distinguish between old and new capacitors. It uses low voltage, but to measure capacitance any voltage which is within the cap ratings is fine. Some dielectrics (such as high-k ceramics) vary significantly with voltage, but that is not an age problem. Mica is fairly stable.

[SurreyNick]

Oh. So are you saying a DMM meter (such as a Mastech MS8268 or MS8250C) will accurately read the capacitance but just won't be able to tell me if the capacitor will hold up under maximum voltage. Yes?

If that's the case could I put it under a (voltage) stress test for a period of time and then check it again?

Thanks
Nick

[ms660]

For measuring small value (picofarad range) capacitors a simple homebrew or manufactured device called a Grid Dip Meter (aka Grid Dip Oscillator or G.D.O) is a handy bit of kit to have, it was one of the first things I built many moons ago, with it you can ascertain inductance and capacitance values for those used in typical radio tuned circuits, they are straight forward to make, lot's of schematics on the web.

Here are some applications for one:

https://g4rvh.files.wordpress.com/20...-dip-meter.pdf

If you want a bridge then as has been said the Marconi TF2700 is a good one to go for if it is in fully working condition, it's powered from a 9 volt battery and is plenty accurate enough and doesn't take up too much bench/workshop space, I have one, they are easy to use once you have mastered the art of finding the sharpest minimum null.

To test for leakage a capacitor should be subjected to the voltage it's rated at in order to get a go/no go in most cases.

From what I understand most DMM's with capacitance measurement facilities are based on the CR time constant principal, leakage in that instance may give a false reading.

Lawrence.

[SurreyNick]

OK. Thanks Lawrence. I will keep a look out for a Marconi TF2700 (presently none available anywhere), along with a high-voltage capacitor tester.
I will also do some research on homebrew Grid Dip Meters to see if building one is within my scope.
Nick

[ms660]

Nick, most GDO's use just one active device, commonly a valve or FET etc, there's not a lot to them but they are handy if you are into homebrew radios or checking L and C values and resonant frequency etc. if you need to do simple antenna measurements then a simple noise bridge is also handy to have, plenty of homebrew designs on the web.

If the pocket will stretch then the TF2700 is well worth it, they go for reasonable money on Ebay quite often, failing that radio rallies/vintage radio events etc.

Lawrence

[Radio Wrangler]

The Marconi TF2700 is great little bridge. It covers a useful range of components and it's as accurate as it needs to be. As Lawrence says they fetch reasonable money.

You could also keep your eyes open for almost any of the many bridges made by Wayne Kerr. These are substantially more comprehensive, measure over wider ranges and have significantly greater accuracy... far better accuracy than almost everyone needs.

However, sometimes they fetch high prices because some people know how good they are, sometimes they fetch very low prices because either no-one knows what it can do, or because everyone is scared silly of all the knobs and buttons.

I said it earlier today, in another thread; these bridges scare people away and that drops their prices. I've seen standards lab grade bridges go for less than a TF2700 because no-one dare tale it on. The irony is that they are quicker and easier to drive. There are several users of themonhere and if one of these things turns up, don't miss it. We'll talk you through making some measurements until you're going properly.

It's as if full Leak systems were fetching lower prices much of the time than Pye black boxes. Either will do you, it's just a matter of what turns up.

David

[boxdoctor]

Just had a quick google and found this http://www.ebay.co.uk/itm/NEW-LCR-Me...item5b04ce1aa8 At the price, worth a go I would think. There are many others too. That one would probably be reasonably useable down to 100pf at a guess
You would still have to devise a leakage/voltage tester. Perhaps a neon in series with a 100k resistor and the cap. under test across a variable D.C. supply? Don't forget to be sure the cap. is discharged before putting it on the testmeter! Tony.