C & R Tester

[Hunts smoothing bomb]

Hi Everyone,

I have just got back from picking a load of vintage TV test equipment up.
amongst it all (unfortunately most is only useful for parts) is a C & R bridge. You can see how small the unit is and it is transistorized. My question is... What battery does it take? I need to know what voltage it is.
Looking at the battery holder it uses the same connectors as a PP9 and it also looks by the positioning of the leads that the contacts are on each side of the battery.

The unit whistles when fed 7Vdc and it is some kind of inverter as there is a DM71/70 and a neon in there. How does it work? how do you operate it? what am I looking for with respect to the DM70 & neon? (neon doesn't light by the way, but then I don't know if I'm feeding it the correct supply voltage.

Yours eagerly awaiting replies!

Lee

[wave solder]

The unit is for measuring Capacitance and resistance.

It can measure resistance and Capacitance from 1 ohm to 100 Megohms and 1 picofarad to 100 microfarads in 6 ranges.

Usually a bridge with a single lamp on it, the lamp usually lights, but then dims as you are getting the bridge towards the "Null" point (by turning the null knob). as you pass this point the lamp will go out and then then steadily increase in brightness again. Back the control off again until the lamp goes out then using the multiplier shown on the range switch read the value off the scale of the null knob and with the multiplier this should give your value.

[ppppenguin]

The battery needed is a PP4 which looks like a C cell except it's 9V and has PP9 style snap connectors. No longer available. Use a PP3 and fit appropriate snap connector.

[Mike Phelan]

Nombrex were usually quite well made and not too expensive. The DM70 will be to show a null and would need HT for the anode and target.

The transformer visible is probably for an inverter - maybe the neon is for capacitor leakage?

[Hunts smoothing bomb]

Thanks everyone,

I don't think it is working properly because if I connect a cap or resistor to it, I can't get the unit to really do anything. The DM70 lights by the way but the neon doesn't.

Thanks

Lee

[Mike Phelan]

Lee
Looks as if the inverter is OK if the DM70 is giving a green "exclamation mark".
Does the neon light if you short the capacitor terminals together on "C"?

[Hunts smoothing bomb]

Hi Everyone,

Just found out that the neon only lights when the top left knob is turned to L and the probes shorted.

Anyone think that L stands for leakage? I just tested a known leaky cap and the neon lit up, On testing a known good cap the neon does not light.

On testing a resistor of 1.2k I can get the exclamation mark to shorten, trouble is that on reading the scale it isn't telling me it is 1.2k.

Am I using this correctly? who knows

Thanks

Lee

[Station X]

Lee.

I'm probably telling you what you've already worked out.

To test capacitors using a bridge there must be an AC supply available. Given that the box is battery powered there must be an oscillator in it somewhere. Possibly the same one as is used for the invertor which generates a high voltage for leakage testing? Do you get a high voltage at the test terminals when set to L?

The DC bridge for testing resistors should be fairly simple apart from the magic eye indicating arrangement. It's probably worth checking all the precision resistors and cleaning the selector switch contacts.

What I don't understand is how the bridge is balanced when testing capacitors for value. I can't see a variable capacitor anywhere.

[Hunts smoothing bomb]

Hi Graham,

Yes, when set to "L" there is 150Vac at the test terminals.

No, there isn't a variable cap. There is another knob which is marked "PF %" and has a scale from 0 to 70 I do not know how to use this.

Thanks
Lee

[Station X]

I'd concentrate on the resistance measuring facility. It's obviously possible to balance the bridge as indicated by the magic eye indicating a null. It's just a case of why it balances in the wrong place. Determining whether an incorrect null is given on all resistance ranges or just some of them might give a clue as to where the fault is.

When testing capacitors for capacitance there should be an AC voltage on the test terminals, possibly at high frequency.

[igranic]

Lee,

A direct appeal for a copy of the manual may be a good move at this point. Make sure to mention the Manufacture and model number of the bridge in the subject line.

When you have a copy of the manual you will be able to check if the component values in the bridge itself are within specification, which may lead you to the cause of the incorrect balance. The manual may even have a calibration procedure.

CR bridges rarely use a variable capacitor, the 'known' arm of the ratio bridge is usually made up of fixed and variable resistors, as I can see is yours from the large 5,000 ohm Colvern pot.

If you would like to gain a really good understanding of how bridges work, and the different types, then take a look at the Radio Laboratory Handbook, by M G Scroggie (later editions may be titled Radio & Electronic Laboratory Handbook). This is an excellent book, having been in print almost continuously (with frequent revisions) since 1936. Sensibly priced copies are often available from stallholders at BVWS meetings.

HTH
Edward

[Ray Cooper]

What I don't understand is how the bridge is balanced when testing capacitors for value. I can't see a variable capacitor anywhere.

You don't need a variable C to implement a capacitance bridge - use the variable R as two legs of the bridge, and balance your unknown C against a (switchable) standard one. Here's a drawing of a home-made one:-

http://www.electronicsforu.com/elect...tance%20Bridge

There is another knob which is marked "PF %" and has a scale from 0 to 70 I do not know how to use this.

This is probably a 'power factor' measuring device, useful for electrolytics. A reading of 0 indicates good power factor: 70(%) is pretty diabolical. To use, you tweak the bridge balance pot and the PF pot alternately, to give the deepest balance indication that you can get on the DM70. For ordinary caps, I'd leave it set to 0 (unless it has a click-off switch at the zero end, in which case use that instead).

[Mike Phelan]

Lee
The Taylor version here may be useful even though it's a valve one.
Sounds as if the neon is what I said it is (leakage) and is working as it should be.

[GMB]

If the scale is non-linear then it may be like the Lafayette TE-46, see my web site for a couple of bridge circuits: http://home2.btconnect.com/gmb

[Chris55000]

Hi!

A quickie - from the info I remember the DM70 had two published sets of operationg conditions, one for mains and one for battery. For battery, the anode voltage recommended was 67.5V, with approx -8V to blank off the exclamation mark, and filament pin 4 earthed. For mains an anode voltage of 90V was recommended, with -10 to -12V to blank off the exclamation mark with filament pin 5 earthed.

In either case, the filament rating was 1.4V 25mA and known equivalents to this indicator were the Mazda 1M1/1M3, and the GEC/Osram Y25.

The power factor control scaled 0-70% was intended to evaluate the "goodness" (what would today be called ESR) of a capacitor. If you were unable to get a good null on the balance indicator, you could try advancing the power factor control a bit to see if a better null was obtainable. This was understandably a bit of a trial-and-error procedure, but as a rule, if you had to advance the control much above 5% a capacitor was considered to be suspect. I've got a lot of PW and RC articles covering bridges, I'll have a root to see if I can find you gospel information!

In these days of tiddly Peak LCR Analysers, its easy to forget bridges were all we had once!

Hope this'll help you check the indicator etc., out in your bridge!

Chris Williams

[Hunts smoothing bomb]

Hi Chris,

Thanks for the info!
I too have the Peak Atlas LCR, brilliant! but I would love to just get this vintage way of doing the same job working!

Cheers
Lee