Nombex Type 62 Cap/Res Bridge.

[pwdrive]

The osc frequency might affect the PF calibration scale, just a thought.

Lawrence.

[Bushbaby]

Hi David,
If I recall correctly I replaced the dodgy diodes with 1N4004s and the unit does appear to work correctly. I also had a couple of resistors that were well out of spec that I replaced.

Regards

Rob

[Keithstv]

The '32' circuit shows 3 x 5uF electrolytics, but I can only see two on the PCB of this '62' Bridge. I'll start by replacing those, and a couple of smaller value Hunts (0.1 and 0.01uF).

I've just realised why there are only two 5uF electrolytics in your 62 and three on the 32 circuit. C9 and VR3 are missing from the 62 circuit. They must have changed the amplifier circuit for the 62 to remove those components. All VR3 appears to do is alter the sensitivity of the null indicator. It also makes the front panel look untidy due to its location which makes me think it was a bit of an afterthought.

I'll post a picture of the 32 front panel tonight.

Keith

[Yorkie]

Thanks for your observations Keith - I'll look forward to seeing the pic.

Needless to say, mine is still on my 'to do' list!

Interesting to look back at adverts in in mags such as PW, RC etc, at firms such as Nombrex, Codar et al, who all had their 'moment in the sun' then disappeared after a few years, leaving little trace. All small scale 'cottage industry' labour-intensive equipment, with little prospect of long-term survival, as the passage of time has shown.

[jimmc101]

The Power Factor (PF) of a capacitor is equal to the equivalent series resistance (R ) divided by the impedance (Z).
For a perfect capacitor R = 0 and so PF = 0. For a resistor Z = R and so PF = 1 or 100%.

Also for a capacitor of reactance (X), Z = SQRT(R^2 +X^2) so a capacitor with a PF of 70% would have R = X which is a pretty poor capacitor.
The only type of capacitor to show anywhere near this sort of loss is the electrolytic which usually has a value well above 100nF, so only the top range requires a PF control. Paper, film, ceramic etc types should all have PFs much less than 1%


Looking at the circuit with PF set at 70%, R2 = 100ohms (ie at max) which is in series with C6 = 1uF.
So to get a PF of 70% (1/SQRT(2) to be exact) the reactance of C6 must be 100ohm this would require a frequency of 1.5918kHz (2*Pi*F = 10^4). Is it possible that the bridge should run at this frequency?

Incidentally it is more usual to measure Q = X/R or D (Dissipation Factor) = R/X = 1/Q. Certainly the TF2700 does this.

Jim

[Keithstv]

Here are the pictures. As you can see it looks as if the pot was an afterthought.

Another difference is that the 62 looks to be built into the case whereas the 32 is all mounted on the front panel for fitting in a different case.

You can see I've replaced all the electrolytics and most of the resistors as they were all way out of tolerance, and the two diodes. Everything else was OK.

Keith

[Yorkie]

Thanks for posting the pics Keith.

Nice neat front panel. I think that panels such as those were made by a photographic technique which was popular for a while. I have a variable voltage bench power supply with a front panel made in that fashion.

I was never keen on SRBP as a substrate for printed circuits as was the norm for many years on radios, TVs, test gear and so on, until glass fibre took over. The copper tracks quickly lift from the SRBP when heat is applied however much care is taken.

Thanks to you too Jim, for your erudite explanation of 'PF' - Power Factor. When I initially saw that on the front panel I thought it alluded to picoFarad until it's real meaning was explained.

[Yorkie]

Someone on the VRF posted a rather indistinct copy of the original leaflet, which included the operating instructions and circuit.

It re-awakened my interest and has prompted me to re-draw the circuit and re-write the instructions (verbatim) to make them legible. I've attached the re-drawn circuit and the re-written instructions, which might help someone who has one of these dinky little jobbies. I've also attached a pic of the PCB, on which I've more clearly annotated the test points of the connections to the DM70 'magic eye' balance indicator.

I've got mine in bits just now, with two Hunts mouldseals replaced and the two electrolytic removed, ready for replacement.

Hope that might be of interest and use to someone.

Since Joe  - who used to look after the archive/library - sadly left the forum, I'm not sure what happens about placing data in that section.

Something we can do ourselves as the mood takes us, or what?

[Mark R]

Thanks David,
the data you supplied is now here http://golbornevintageradio.co.uk/forum/...p?tid=4676

Mark

[Yorkie]

Though the DM70 'magic eye' shines brightly on the display, no null occurs on any of the capacitance or resistance ranges with a component under test, so I've now got round to furtling with it!  

I've replaced the two Hunts caps, and two electrolytics I referred to in post #18, which I'd hoped might produce an improvement but it hasn't. All of the transistors test OK, and the oscillator is running (I can even hear it!).  I'd wondered if the two diodes in the voltage multiplier might be defective, but I doubt it, because at the junction of C3/D1, the voltage is a respectable 84 Volts, which  - after D2, is raised to 118 Volts.  The voltage on the anode of the DM70 is 92V, and the grid is minus 7.8 Volts, which doesn't vary when the Balance Control knob is swung round with a component under test. Those Voltages seem about right to me, and suggest that the two diodes in the Voltage multiplier which lift the Voltage from 9V  aren't suspect. The range switch looks fine, and I've cleaned the contacts with Servisol.

I don't really understand the operation of the circuit, though I do understand the 'Wheatstone Bridge' principle, in which - when all four arms of the bridge are in balance, no current will flow across the bridge.  I assume that in the circuit when the bridge is in balance with a component under test, a negative voltage is presented to the base of TR2, which TR2 & TR3 then amplify to make the Voltage on the grid of the DM70 go more negative, thus causing the 'exclamation mark' display to shorten to give an unmistakable 'null'.

To make the circuit more understandable (to me anyway!) I've redrawn it without the range switchery and have shown just what I've termed a 'Standard Resistor' - that being the one for the range which suits a resistor under test. (I could just as easily of course, shown a 'Standard Capacitor' and shown a 'capacitor under test' at the test terminals).

Firstly, I'd be grateful if anyone who has a working Nombrex Bridge could confirm that yes - there is an unmistakable null when the bridge is in balance with a component under test, causing the DM70 display to shorten almost to nothing. Secondly, any thoughts as to where to look next please?

I try not to just pull components out on a lucky dip basis to test them in the hope that I might come across one that's duff. The four caps I replaced were likely suspects, the diodes and transistors seem OK, and short of taking the rest of the components out one at a time to test and replace if found faulty - methodical though that might be - I'd prefer a more rational approach.

Waffled and dribbled 'Confused, of Cottingham'!