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Resistance/Capacitance Meter (with valves (tubes))

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Ash Small

Sat May 28 2011, 02:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I picked this up the other day when I collected an AVO model 8.

Looks like it's a resistance/capacitance meter. It has ranges from 1pF to 1000uF and 1 Ohm to 100 MOhm.

It has an EM34 dual shadow magic eye tuning indicator, and also has a Cossor V7 valve inside. From what I can find out online, the V7 is a series of valves, they had a paper label apparently which gave the serial number. (It has four pins)

There are two terminals on the front marked Live and Earth, and a co-axial connector marked 'IND'.

Can anyone give me any advice as to what the co-axial connector is for, or give me any other advice before I plug it in?

It has a hand drawn scale, etc, and the knob in the middle adjusts what appears to be a potentiometer mounted behind the capacitor in the photo.

Proud Mary

Sat May 28 2011, 03:32PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It's a home-made LCR Wheatstone Bridge, Ash, using a 'magic eye' as the null indicator.

'IND' stands for inductance - the inductance to be measured.

I'd guess it to have been built in the 1960s, using cannibalised parts, some of them like the body-end-spot resistors and the Ferranti transformer perhaps dating back to the 1930s.

Safest to assume that the capacitor on the rectifier tag strip is duff, and replace it before firing up for the first time.

Check that the mains fuse is present, and of a suitable size - not more than 3A - before powering up.

If you have a variac, I'd suggest using it to very slowly bring up the supply voltage, while listening for crackling and abnormal mains transformer hum, and sniffing for any smoke.

Looks fun, I must say!

Ash Small

Sat May 28 2011, 04:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I take it this is the capacitor you were referring to? (there is also another that appears to be wired in parallel (but I'll have to remove the assembly from the box to be sure) that wasn't visible in the first photo)

Am I correct in assuming that the valve is a rectifier (diode)?

The left hand rotary switch does have a position marked IND, but how would I read off the value of an inductor? (the scales are only marked in Farads and Ohms)

The right hand knob seems to be connected to a potentiometer, and is marked 'P.F.%'

(It's really made my day being told it's also an inductance meter, but how does the inductance part work (how do I read the inductance)?

It is also fitted with an old two pin bakelite plug (unfused). I'm planning to fit a three pin fused plug and to earth the case before plugging it in.)

Proud Mary

Sat May 28 2011, 04:34PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

OK, I see the second capacitor now. I'd replace both of them, if you are able.

See that white dust inside the case? That can be associated with gradual out-gassing of electrolyte.

The valve is a rectifier - probably an indirectly heated half-wave type, which accounts for
the four pins on the base ( a, k, h, h)

There are many explanations of the Wheatstone Bridge and its variations on the web.

Ash Small

Sat May 28 2011, 05:08PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm familiar with the original wheatstone bridge circuit comprising four resistors, and I'm aware of bridges comprising four capacitors, etc.

I assume you're referring to Wien bridges, Schering bridges, Maxwell bridges and Hay bridges?

Proud Mary

Sat May 28 2011, 05:32PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

So far as I can see, the Belling Lee coax socket is intended for connecting in yer actual calibration inductors. The front panel terminals E and L are for the component to be measured.

I think it possible that the basic design may have been modified one or more times. Doesn't the Belling Lee socket right at the edge of the panel look wrong, as do E with an L only?

Here is the Mullard datasheet for the EM34 Electron Beam Indicator.

If the rectifier valve heater lights up, but the 'magic eye' doesn't work, first check for the presence of 6.3V AC across the heater base pins.

Afterthought: You must prepare yourself for the possibility that this contraption may not work very well, and that the null points may be less than clear on the 'magic eye.' Do you see two small, modern resistors across the EM34 base? Someone has been trying to introduce more symmetry into the design, and not too long ago, I'd guess.

Ash Small

Sat May 28 2011, 06:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Ahh, OK. That could make a lot of sense.

The IND position on the left hand rotary switch looks to be in keeping with the rest, lettering wise, etc.

The IND next to the co-axial socket looks like it might be in a different pen, but I'm not sure. The screws holding the co-ax socket are identical to all the others.

It might take an external inductor for calibration, but some of the bridge circuits I looked at online only have one inductor, so I'm not sure.

I don't suppose I'd damage anything if I plug it in and try it.

Thanks for the EM34 datasheet.

Would the L and E be for testing electrolytics? (would this not make sense, L=+ve, E=-ve?)

Proud Mary

Sat May 28 2011, 06:19PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...
It might take an external inductor for calibration, but some of the bridge circuits I looked at online only have one inductor, so I'm not sure.

If you had only the one reference inductor, which is all you should need in theory, you will find the readings become impossibly bunched up at one end of the scale, were you to try to cover a range such as 10 Î¼H to 10 H.

The sloppy layout will limit the contraption's accuracy in measuring low inductance and capacitance.

Ash Small wrote ...

Would the L and E be for testing electrolytics? (would this not make sense, L=+ve, E=-ve?)

Maybe or maybe not. Try it and see.

Nah

Sat May 28 2011, 06:32PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

My two bits-

1. In my opinion it is a full wave type, as it has two plates from the looks of it.
2. Mary's null poin it this-you conect a cap, turn it on, and mover the value selecter to different values. When the magic eye fully closes,that is the value. Also, it can test for leakage by sending a pulse of around 300-600 volts and seing if it is good. (Used back then to check paper caps)
3. The em-34 sells for around 40-50 dollars new so it may be worth more than the meter if it is bright.

Ash Small

Sat May 28 2011, 06:35PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

You don't think connecting a signal generator to the co-axial input is necessary then? I know the General Radio GR650a, etc require an external signal generator, and that might explain the prescence of the co-ax socket (just a thought).

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