AEG component identification

[Uncle Bulgaria]

I can't find a reference to these things. Any ideas before I get out the tiny pliers and try to winkle them out?

[eddie_ce]

At a guess I would suggest that it is a one way (Einweg) rectifier diode, 360V @ 15mA, red presumably being the cathode.

[AC/HL]

A similar format was used for selenium rectifiers, so I'd hazard a guess at a 360 volt 15 mA rectifier. What is it's application?

[Joe_Lorenz]

You are right, folks! These are 360V/15mA rectifiers. I remember those as most unreliable components, I'd get rid of them.

Joe

[Nickthedentist]

They look nice though! Maybe re-stuff them?

[Uncle Bulgaria]

Thanks all. I think I've managed to draw out the circuit they're in. This is another of the watch-timing machines I'm repairing. They mark dots on a paper roll in time to the tick-tock and the interpretation thereof can describe what's wrong with the watch.

This one's method of marking the paper is different from the inked rollers I've seen so far, and these components are part of it. The helical roller is on a synchronous motor linked to the beat rate of the escapement under investigation. Its outer rim has a wire around it. A ceramic element with a wire stretched along its summit mounts just below it, and is the final piece of the circuit the AEG parts make up. My guess is that these wires arc or fire in some way to mark the paper.

If the AEG parts are faulty then that could be good news. The mains transformer is making bubbling sounds and as access is very awkward I am not looking forward to unsoldering all the secondaries to see if the fault remains - I'm certain to melt a load of insulation by bumping it with the iron...

The attached diagram shows the AEG parts (ends marked R[ed] and B[lue]) connected to one of the secondaries on the transformer. The two 1µF capacitors are NOS Soviet film types that test perfectly. The other cap is labelled. The part marked 'coil' is the ribbed object at the top of the photograph. The ceramic piece topped with the 'marker wire' is shown next to its mounting point beneath the helical roller in the other photograph.

[Lucien Nunes]

That is a conventional voltage doubler circuit. The two capacitors are charged alternately on the two opposite AC half-cycles, and discharge in series to give twice the DC output voltage compared to a conventional bridge rectifier albeit at lower current.

The existing selenium rectifiers can be replaced with ordinary silicon diodes but the HT might be somewhat higher than originally intended. The normal cure for this in selenium-to-silicon conversions is to add some series resistance to substitute for the resistance that was inherent in the selenium rectifiers, but whether it is actually necessary here depends on how great the drop originally was. Is there any indication of what the DC voltage should be, other than the absolute limit of the capacitor voltage rating?

They look nice though! Maybe re-stuff them?

They do look nice, but I would not re-stuff them. Partly because I now have a policy of preserving unusual vintage parts instead of mutilating them with drills and knives, and partly because if stuffed with a diode, they will have different characteristics which will conflict with what they claim to be (unlike a restuffed capacitor that is pretty similar to its original spec). I would put them in a little ziploc and tape it inside the bottom cover.

I have a similar-but-different chart recorder of the spark-trace type: A Farnboro engine indicator as developed by the RAE in the 1920s to study the combustion conditions inside the cylinders of aero engines.

[Uncle Bulgaria]

Thank you Lucien - I've not come across a 'doubler' before, and it's good to hear that my inkling the contacts arc isn't completely outrageous. If these diodes are likely to be faulty, as Joe_Lorenz thinks they might be, there could be excessive current in this secondary...

The only indication of the voltage is the capacitor values, as I haven't a circuit. I can't think it'll make too much difference - perhaps a more vigorous spark? I'll keep looking for the mains transformer fault, and bear in mind the values given above for these AEG diodes.

[Lucien Nunes]

The selenium rectifiers are indeed likely to be faulty; I probably wouldn't have powered the unit up before changing them. In the past I have had generally good luck with selenium, and used to recommend against changing on sight. But selenium does not age well; the entire world inventory is failing part by part and amongst other things even my trusty 1960s fork-lift charger, which has a selenium bridge rated at 100 amps, has started to heat excessively.

Preserve them as an example of a technology that was once widespread and important but is completely unloved and unwanted today.

[Nickthedentist]

Originally Posted by Lucien Nunes

They do look nice, but I would not re-stuff them. Partly because I now have a policy of preserving unusual vintage parts instead of mutilating them with drills and knives, and partly because if stuffed with a diode, they will have different characteristics which will conflict with what they claim to be (unlike a restuffed capacitor that is pretty similar to its original spec). I would put them in a little ziploc and tape it inside the bottom cover.

Very sensible, thank you!

[Uncle Bulgaria]

Thanks all. Is a 1N400 type OK, or would a modern fast recovery be better?

[Lucien Nunes]

Any rectifier will do, although choose a nice high voltage rating like a 4007. Ultrafast are better at not producing RF noise and now my usual stock items, but here you have a small spark-transmitter so I think the diodes are the least of your concerns with generating interference.

[Uncle Bulgaria]

I happen to have some UF4007s to hand so will try those. Tiny compared to the originals! I've desoldered them and one tests as a diode (red cathode, as eddie_ce surmised), the other as a ~200pF capacitor.

Why is the cathode of one of them direct to ground? So the final potential difference on the marker wire is several hundred negative volts?

[Lucien Nunes]

Apparently. If the capacitors are not polarised, the spark electrode polarity might not matter, but perhaps they found one way was better due to differences between the electrodes, and obviously it makes sense for the helix to be earthed to the chassis.

Even if the selenium rectifiers were working I wouldn't trust a computerised automatic component tester to identify them as it probably doesn't know anything about selenium. They will have much higher slope resistance, forward drop and poorer forward / reverse ratio than the kind of diodes it's been programmed to recognise. Any component that is just barely open-circuit is likely to be identified as a capacitor, but it might also mean that the inter-plate capacitance of a working rectifier has fooled it.

An AVO will probably test them, a digital multimeter resistance-range or diode test voltage might be too low. Or a bench power supply, resistor and milliammeter. Obviously you can only prove that they are not leaking with a suitably high voltage from e.g. a 250V megger.

[Uncle Bulgaria]

The capacitors are indeed non-polarised. Drawing the first bit out again, is this what's happening? I was finding it hard to get over the idea of current just flowing to ground, as I find this conceptually tricky.

Checking the rectifiers on an AVO gives a random combination of a couple of hundred kohms and shorts. My Megger is in storage, but using my capacitor tester at 250V one has microamp leakage when reverse biased; with the other both polarities draw more current than the tester can supply so it never rises above a few tens of volts. I think it's safe to assume faulty.

[Lucien Nunes]

It's not really flowing to ground, it's just passing through the chassis metalwork to the other end of the spark circuit, which is unavoidably already in contact with the chassis.

What kind of coil is that in the spark circuit, and what triggers the spark? And how is the helix drive synchronised with the watch under investigation? Sorry, but if you're going to show us rectifiers in unusual pieces of kit you're going to get questions!

Your dwg is correct but the doubler configuration is most self-explanatory when drawn thus;

[Uncle Bulgaria]

Thank you Lucien. I think I'm the one who ought to apologise for always bringing the intellectual level down with wee laddie questions!

The motor can be run at three speeds which are each multiples of the beat rate for particular escapements, driven by an EL84 and a crystal oscillator. Triggering for the other examples I have is via a thyratron connected to the microphone input which fires a solenoid to bang the paper against the inked helix. I haven't drawn this version out yet, but I guess the principle is the same.

The bit I've called the coil is this ribbed object, which has several wires going into its casing. Perhaps the yellow/blue are to do with triggering as the remaining one looks the same type and size as the other end which comes from the circuit we've been discussing and goes to the marking wire.