Replacement for diodes marked D/31 (Circa 1970)

[marinedalek]

I've inherited a Marconi TF2415 Counter/Timer which I believe was manufactured around 1970 from a few date codes I've seen.
I fixed an issue with one of the nixie driver cards by tracking down open circuit failures of two diodes in the decoder logic. I've been able to get it operating normally by substituting in some 1N4148 signal diodes for now, but I'm not sure if that's a great idea in the long run as I'm not sure of the current/reverse voltage requirements.
The diodes I replaced are just marked D/31, with no other markings. Does this ring any bells with anyone?

[SeanB]

Probably will work fine, the originals might have been germanium diodes, used as logic, but silicon will work there well enough. Picture of one of the diodes will help though, as if it has a green band it probably is an OA81 germanium diode.

[factory]

The TF2415 is a Venner design that was rebranded by Marconi, I'm fairly certain they use Fairchild uLogic decoder/driver ICs for the display tubes. Do you have a diagram for the relevant area?

David

[BrokenYugo]

Pictures may be helpful.

Why not check the operating voltage and current? If I'm understanding right this is display decoder stuff that more or less runs at DC, should be easy enough to check with a DMM.

Are there any functional, identical, diodes on the board? Could at least check forward voltage to tell silicon from germanium.

[marinedalek]

Sorry! I should have posted pictures earlier, but I was about to head out.

Unfortunately I've not been able to find a schematic for this model, or even a similar model in the range. It is indeed Venner, as that's the name on the individual cards. Photo #2 is the card in question; each card drives a single nixie tube.

The mystery diodes appear to be silicon, as they test at around 0.6V forward voltage in circuit (using DMM diode test). Photo #3 is a close-up of the diodes:

I've done a bit of a burn-in test and the 1N4148s don't appear to be getting warm at all, so I'm going to go out on a limb and say it's probably OK. There doesn't appear to be anything in the way of large reverse voltages either, so that's good. Thanks for the responses though!

[George Edmonds]

Hi

This is the best schematic I have

G Edmonds

[james_s]

I bet you'll be fine, those don't look like anything special. If you're really paranoid, measure the voltage across them but diodes are pretty robust.

[factory]

Not quite what I was expecting, my lower spec Venner counter in a similar height case, uses plug-in boards with ICs, also have one of the later smaller ones from AMF Venner.

Anyway the diodes in that low speed counter board, are listed as X20639/31, which is a rebranded Mullard 1N4009 (or 1S44), a high speed diode. Power supply is +/-6V and the HT for the display digits is switched by the ten transistors next to the tube.

David

[FIXITNOW2003]

https://english.electronica-pt.com/components-cross-reference?ref=1N4009

cross ref gives 1n4148 as suitable

[marinedalek]

Thanks again everyone for the input! Glad to hear I inadvertently stumbled upon the correct diode by virtue of it being the only one within reach at the time. I ended up having to replace one of the divider transistors too, which now has a slightly exotic gold-leaded 500MHz 2N2369 in its place. Again, because it was the only TO-18 transistor near my workbench at the time... I get the feeling that any general purpose NPN transistor would work though. The only place a more specific part would be needed is (I'd assume) in the switching array for the nixie segements/neon indicators.

[factory]

Apparently they used Fairchild U15718/1 (2N2369) a lot, aka Marconi part #X20638/55.

The display drivers are Fairchild C407 (NPN), if one fails, MPSA42 with higher voltage rating, or similar would probably work fine.
I've used the PNP version, MPSA92 in place of failed display drivers in other test gear.

David