| Electronics > Repair |
| Help Identifying Vintage Component |
| << < (7/10) > >> |
| Drjaymz:
--- Quote from: floobydust on May 22, 2023, 08:58:31 pm ---It won't work as drawn unless it's using an SCR. C3 is not on the schematic, part values would help. --- End quote --- I agree, because it latches on. (I think I mentioned that in previous post). The mystery component which I thought was a zener has the markings 4545 in blue on a black case and no other markings except a ring for the anode. I identified it as a zener because it doesn't do anything until just over 11.1V where it appears to clamp and as you raise the supply up to 20V its breakdown voltage only rises slightly to 11.4V. Its common to have a zener reference that turns on a transistor in these type of systems, either over-voltage or the regulator (they are both the same). But as drawn it wouldn't work because once you turned on the transistor then the transistor biasing circuit would turn it off again because its supply is now Vce which is ~0.8V. And yet, here it is on my desk working. Hence why I'm perplexed. Quite often when you look at automotive electronics from the 60's and 70's the circuit isn't as obvious as it may seem until you figure it out and realise its very cleverly done. The regulator someone else posted is very similar but that would be non-latching. I noticed that I omitted C3 after I took the photo. Its actually between Relay -ve and top of C1. Its value isn't readable but its a high voltage small capacitance polyprop. Its not measurable with my meter out of circuit so its less than 1n at this age, I think its supposed to be 4.7nF. The other odd thing is that R2 measures 1K but its colours are Pink Black Red Gold - now I have seen Pink before and I looked up a weird series where pink is 7. But it could also just be age as its not uncommon for white to fade (although usually looks yellowish). This unit would not get warm as it should only activate under fault conditions. |
| Drjaymz:
If we ignore the relay for a moment, then the junction between R1 and VR1 would scale with the supply voltage. Current does not flow through ZD1 until Vr1 > ~ 11.1V. Then Vbe raises to about 0.7V at which point the Relay activates. Then it stays activated. And thats where I am confused. If R3 took its supply from the input then that would work, but then the relay would de-activate when you reduced the supply voltage - and it doesn't, TR1 stays hard on until the relay lets go at 3v. If you then raise again it activates again at about 7V demonstrating that the transistor has never switched off. So its latched. Now, to get further into it I could dismatle it further, but since it now works and all I have to do is replace C1, I'm probably not going to risk breaking it. So it may end up keeping its secrets. I have thought about this a bit more, T1 Must be an SCR. I don't come across those much these days and all the more important that the capacitors have some capacitance because otherwise it could trigger on spikes and will by definition latch on. I'll see if I can find markings before I stick it all back together. I could also easily test this by desoldering the "base" and triggering it manually, if it stays on then thats what it is. So whilst I know what an SCR is and how its triggered, what I don't know is what voltage will be present on the gate once its triggered (if its not connected). I mean that we know you need a threshold to trigger it but we see 0.8V after the device latches on. Now, since its a stack of silicon that may well be correct. Hopefully someone understands what I meant there. I had a look at whats available today and T092 case style Thyristors with a gate voltage of 800mV do exist and perhaps the part number is irrelevant from the 60's anyway. |
| Drjaymz:
--- Quote from: fzabkar on May 22, 2023, 04:48:27 am --- --- Quote from: floobydust on May 22, 2023, 04:38:32 am ---Logo "H" OK I found the manufacturer is Hilton Capacitor... --- End quote --- https://web.archive.org/web/20050210214206/http://www.tantalum-pellet.com/ --- End quote --- This is it. 4070 is the date code. Thats week 40 1970 which is correct because the aircraft was built in 1974. So 75 would not have been it. The rating capacitance and voltage rating then must be 15V75. And it cannot be 15 volts so it must be 75V and that leave 15 which is wither 1.5uF or 15uF. 15uF at 75V I think would be physically larger, this is 13mm long, 3.4mm diameter. So that makes it most probable that its 1.5uF. There is another cap next to it and I cannot see its markings but its slightly less diameter and length and its measures just under 1uF. So its a Tantalum Pellet 1.5uF 75V capacitor. And the weird circuitry - my diagram is correct, its not a transistor its a Thyristor, what threw me was that I can't access the markings and that in the on and off state the voltages were very close to an NPN. Its a T092 800mV gate voltage and would latch as it does. I may just change both tants for a 1.5uF, I mean the tolerance of -10 / + 30 means they could overlap! I can't understand why they are so expensive and why nobody has no stock unless as someone has said, they are indeed still hand made. |
| Drjaymz:
This is fixed, tested and working nicely. Now, going back to the circuit diagram. Why does it use the zener at all? You could just arrange the potential divider such that it was biased around the Thyristor trigger. One reason could be that the zener is a stable voltage reference. I don't really see it being any better than a simple divider and transistor. |
| David Hess:
--- Quote from: Drjaymz on May 25, 2023, 10:40:16 am ---One reason could be that the zener is a stable voltage reference. I don't really see it being any better than a simple divider and transistor. --- End quote --- It is much more stable than just the Vbe of the transistor. Placing a zener in series with the base or emitter is an excellent way to make an accurate reference and error amplifier for a regulator or reference. Pick the right zener voltage and the temperature coefficients of the zener diode and Vbe cancel. RCA used to sell transistors with the zener built in. |
| Navigation |
| Message Index |
| Next page |
| Previous page |