Electronics > Repair
Help Identifying Vintage Component
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Drjaymz:
Reverse Engineering Complete (Big Clive would be proud).

But, the bit I don't understand is that if R3 really does take its supply from the -ve relay coil end then when the relay is energized doesn't that then turn itself off again.  Because TR1 is pulling R3+ve to ground.  I thought there might be a wire behind, but probing it suggests its as shown.

When it actuates what you actually see is around Vbe  ~ 0.8V T1 and about 0.7 Vce.  And it latches on.

If you then reduce the voltage to about 4V then the relay lets go but if you increase comes back on again.  Until you turn the circuit off again.
So could it be that ZD1 is actually not a zenner?  How does it stay on?  Its markings are 4545.

None of this is really relevant as I just need to replace c1 and c2 and get on with life, but I still wanted to work it out.
David Hess:
It looks like a hermetically sealed axial tantalum capacitor to me.  It could be either wet or dry but this can be determined from what type of seal it has.

Without the epoxy coating causing strain in the tantalum pellet, these axial capacitors support up to 75 volts without derating and the risk of failing short so they are common in high reliability applications.  Based on the part number I suspect it really is a 75 volt part but 1.5 microfarads instead of 15, which is consistent with the size.

Drjaymz:
That sounds about right to me, and 75V should be ok for the intended application.  You can in a 12V system see really huge spikes of any polarity but its not directly across the supply.  I would say its done ok for 50 years old.  But then again, we don't know when it actually failed because the relay will fail closed which is the non-fault condition.

So question comes to what to replace it with?  I have 2.2uF 100V multilayer - but they still feel far too modern for me and 2.2uF electrolytic, I think you just have to reckon that these will gradually get larger ESR and lower capacitance over time.  Capacitors - still the dilemma they always were.

On films they start up a million year old alien space ships and everything works fine, but we can't get capacitors to work for a couple of decades.  Good job those aliens didn't use rifa caps.
Swainster:
Hilton SWT wet tantalums appear to be still in production, now made by QuanticTPC (formerly the tantalum pellet company). You can even "tour" the inside of their office/lab (factory?) via google maps. To be honest, it looks like each cap is being made by hand - you could probably get them signed by the operator :-DD

It's probably worth checking with them about the specs of the cap, given the application.
Drjaymz:

--- Quote from: Swainster on May 22, 2023, 09:43:43 am ---Hilton SWT wet tantalums appear to be still in production, now made by QuanticTPC (formerly the tantalum pellet company). You can even "tour" the inside of their office/lab (factory?) via google maps. To be honest, it looks like each cap is being made by hand - you could probably get them signed by the operator :-DD

It's probably worth checking with them about the specs of the cap, given the application.

--- End quote ---

Nice!  but I can't see it being necessary to chase down the exact parts.

I looked up the relay and they did make them by hand until about 1990 about £400 and now they are cylindrical construction and advertised as with "integrated circuit" for reliability.  As if measuring a voltage and activating a relay was something difficult requiring pixie dust and that being able to trip within a millivolt is somehow superior when you consider that the system is already 2 full Volts over-voltage at this point.

And by the way, that relay isn't anything fancy, the field current is up to 4A and in reality a couple of amps.  But for anyone who doesn't know, DC is sometimes difficult to interrupt because the electric field never falls to 0 like it does for AC.
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