Nombex Type 62 Cap/Res Bridge.

[Keithstv]

David

I can confirm that the DM70 display does shorten at the null and it is quite distinctive. The output of the bridge, connected to TR2 is an AC signal which is amplified by TR2 and "rectified" by TR3. The problem may be that there is too much signal being presented to TR2. Mine is actually the 32 which has a pot on the input to the amplifier to control the sensitivity. I'll power mine up and check on the voltages etc and report back later. As I said earlier I virtually rebuilt mine replacing all the non precision resistors capacitors and diodes.

Keith

[Yorkie]

Many thanks for your speedy response and helpful comments Keith.

It wouldn't of course be a big deal to replace all the Rs, Cs and the two diodes if need be, but I  didn't want to just pull components out on the off-chance that they might be duff, when a more restrained  and logical approach would be a better option. I don't think those little tubular caps and resistors are noted for being unreliable or drifting in value. Initially, I'd wondered if the diodes could be suspect, but I think that given the 118V at D2, they seem quite healthy. I could always sub them for 1N4001s to see what change if any that it makes. I've not checked the voltage on the bases of TR2 & TR3, but what's puzzling me is the lack of any change in voltage on the grid of the DM70 when the calibration dial is swung round. I think I'll have to remove the PCB again to get at some inaccessible points on the circuit to take a few more voltage checks, and perhaps remove TR2 & 3 to test them out of circuit.

Quite an innovative little firm and something of a mystery. Brings to mind the likes of Codar and Jasonkits who had their moment in the sun before it set on them.

[Refugee]

I have seen a number of tubular capacitors go leaky or short circuit. The most notable example being in a Philips FM front end.

[Keithstv]

Attached is the additional control on my Nombrex 32 which controls the sensitivity. The pot is 5k linear.

I've measured the voltages on my bridge. With a supply voltage of 8.6v and a 10k resistor across the measurement terminals and set to the 10k range. All voltages measured relative to the ground connection

TR2 base - 0.15V

TR2 Collector 0.11V at null & 3.5V to 4.5V at extreme positions of the bridge pot.

TR3 Collector 8.3V at null & 4.4V 50 5.9V at extreme positions.

If the sensitivity control is set to maximum I cannot get a null.

HTH

Keith

[Yorkie]

Thanks for going to that trouble Keith - much appreciated!

[Nick]

I have a lot of NIB DM70s if you need one...

EDIT: rats - I've only a couple of those - lots of DM160s/6977s though...

[Yorkie]

Thanks for reading the thread Nick - good to know that you have a couple of DM70s, should I need one.

The one in the Nombrex seems quite healthy, as does the one on a Pye radio I have. They don't seem to loose their luminescence as do many magic eyes. I think it's just that for whatever reason, theres no null on the bridge so the voltage on the grid doesn't vary and the display just stays constant. Really, the compoenent coun't is so low that I can see me simply taking each component out to test and replace as necessary, checking for dry joints etc.

Id rhather hoped that changing the two electrolytic caps and two Hunts caps that it might have restored it to full health, but not so!

[Keithstv]

I suspect you are getting too much signal from the bridge. Connect a 10k resistor across the test terminals, set the range to 10k and monitor the signal at the input of the amplifier as you swing the balance pot from one extreme to the other. You should see a dip in the amplitude around the null point. This would show if the bridge is OK. Next monitor the collector of TR2 and check the amplitude changes. If it does check the voltage at the collector of TR3. This should vary considerably rising to around 8V at the null point. If it doesn't vary it may be worth adding a sensitivity control.

I think one reason why the DM70 is in good condition is the lack of use after all these are not turned on for hours at a time.

Keith

[Yorkie]

Thanks for the advice Keith - very helpful.

I think you're right about the lack of use keeping the DM70 in good shape. I have a Heathkit C3-U bridge with has an EM34 in it, the display of which is just like new.

[AlanBeckett]

A good EM34 is probably worth more than the Bridge David.

Alan