| Electronics > Repair |
| Help Identifying Vintage Component |
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| Drjaymz:
Future me here - I've been flying this thing for another year or so and occasionally the Over Voltage relay kicked in and no problem I just turned off the ALT switch, turn it back on again and all is well for the rest of the flight. When using the landing light at take off (thats 100W) and selecting flaps in, when the flaps hit the stop (there is no limit switch) it crow-bars the 12V bus due to the heavy load. This causes the charging system to work harder and the voltage occasioanlly overshoots resulting in the Over Voltage relay tripping in. Except now, it will NOT reset until the engine is shut down. So this means that current is still flowing through the over voltage relay thereby latching the SCR. I have supplied the entire charging circuit including the over voltage module we reverse engineered last year. In the cockpit the ALT warning light has now decided to work. It comes on when the ALT switch is turned OFF. I never saw this light before, it didn't used to work with the ALT switch in any position. Looking at the diagram the ALT warning light has one side connected to the 12V bus and the other connected to regulator. That means current is flowing from the main bus to ground via the warning light. I have checked and this is the way its been wired for 50 years but doesn't make a lot of sense to me. The warning light looks like it may prevent the OVR being reset by providing some current preventing the SCR resetting. Secondly how does the warning light work? If the switch is off, then the other side of the bulb must be seeing a low / 0V. Terminal I on the regulator can't pass current to ground because it has a diode (except for the resistor network) - it has to be passing it through the OVR coil. If thats the case - how you supposed to reset it in flight - or perhap you are not meant to/ The flight manual states that you shoudl try cycling the ALT switch and if that doesn't work then shed as much load as you can and land - which is thankfully obvious. Last time this happened I tried turning the master off and that didn't reset it either. I think thats because the alternator has its own ground and therefore if even the tinyest current flows through the fild coils, it will produce an output unless the engines stops. And sure enough when you turn the engine off, it resets. Now - this is Aviation and therefore this is a terrible design with long thin wires carrying large current, its also 1970s and on top of that its French. So what is my actual question? That ALT warning light that has just decided to start working - do you agree with me, that is preventing reset of the over voltage relay? What is that light supposed to indicate - you'd think it means the alternator fault but its a bit ambiguous, because it could be off and the generator isn't generating. Now, I have also attached the actual flight manual in case I have missed something. But seems to me that it would have been better if the over voltage relay output marked REG went to the input of the regulator which is I presume why its marked REG. And you can see by the tenuous OEM diagram that the OVR Doesn't go to the regulator. |
| floobydust:
Kinda quiet but I'll stick my neck out here. You got a few issues. The OVR SCR holding current is typically low ~5mA so the ALT lamp is enough I would say to keep it latched on. Did you replace the SCR or is it stock? Also note SCRs get much more sensitive when hot, if the module is located near the engine. It looks like a bit of a design flaw that you can't unlatch it unless the ALT breaker is open and the lamp pulled out. Or the regulator is also playing a part here and not loading down the I input as usual and stealing current from the OVR module when the lamp is on (so SCR can't latch). I think the regulator (I terminal) voltage drop might be 3-4V so the lamp would be normally a bit dim when on. But the SCR latching on would make it bright. Stopping the jammed flap motor is basically causing a load-dump around 400msec duration (I don't know the alternator size/p/n) but your battery should soak it up. It might be getting old or high resistance in some wiring and so the bus voltage kicks up past 16V. It would be hard on any avionics, have you considered adding bus protection for that? Ultra-lights (with PM generators) they add bulk-capacitance 20,000-100,000uF to smooth out bus power. The Bendix OVR module ideally would have a time-delay, be slow and take a second or two before tripping - but those cap values are too small for that. C3 is in a wrong place and mangles any delay but protects the SCR from +ve voltage spikes and some extra dV/dt coverage- there's no back-EMF diode/TVS on the relay coil. Ick. A bit of a kluge, they must have had a problem with the OVR, it's not the greatest design. |
| Drjaymz:
--- Quote from: floobydust on June 10, 2024, 09:08:14 pm ---Kinda quiet but I'll stick my neck out here. You got a few issues. The OVR SCR holding current is typically low ~5mA so the ALT lamp is enough I would say to keep it latched on. Did you replace the SCR or is it stock? Also note SCRs get much more sensitive when hot, if the module is located near the engine. It looks like a bit of a design flaw that you can't unlatch it unless the ALT breaker is open and the lamp pulled out. Or the regulator is also playing a part here and not loading down the I input as usual and stealing current from the OVR module when the lamp is on (so SCR can't latch). I think the regulator (I terminal) voltage drop might be 3-4V so the lamp would be normally a bit dim when on. But the SCR latching on would make it bright. Stopping the jammed flap motor is basically causing a load-dump around 400msec duration (I don't know the alternator size/p/n) but your battery should soak it up. It might be getting old or high resistance in some wiring and so the bus voltage kicks up past 16V. It would be hard on any avionics, have you considered adding bus protection for that? Ultra-lights (with PM generators) they add bulk-capacitance 20,000-100,000uF to smooth out bus power. The Bendix OVR module ideally would have a time-delay, be slow and take a second or two before tripping - but those cap values are too small for that. C3 is in a wrong place and mangles any delay but protects the SCR from +ve voltage spikes and some extra dV/dt coverage- there's no back-EMF diode/TVS on the relay coil. Ick. A bit of a kluge, they must have had a problem with the OVR, it's not the greatest design. --- End quote --- Thanks for your reply. I can confirm that the light absolutely is why the OVR doesn't reset. The bulb didn't work for years and was recently damaged and replaced (there is a seperate LOW-V light which is mandatory). It now works and this interferes with resetting the OVR. If you look at the terminals one is marked BAT the other REG it would have made far more sense to connect the REG end to the REGulator Input terminal. This is I think the way its meant to be. But you know - French. The SCR is stock. I removed the bulb and now you can reset it no problem. The alternator is an ALY 8403 which is I think 40A, part of the prestolite maintenance manual claims its 60A but all the fuses are 40A and the wires are not much thicker than your multimeter lead, these would be swapped out for a different ALY alternator if overhauled but you're looking at £1500 at least. I checked and all the diodes are working, the windings are roughly even resistance but the slip ring contact was poor. With my meter on continuity across the field terminals when you hand move the prop the beep is scratchy. I removed the brushes, the looked fine about 3/4 inch on both and then I used emry on the dull slip rings. Which improved it a bit but I don't know how they can be improved further without renewing or machining. I also measured resistance from battery +ve to alternator output and to regulator input and wiggled all the cables and panels to see if I could find a poor connection. That all looked ok. The master switch is on the +ve side and not as depicted in the POH - which is no suprise. Looking at the OVR yes, you are correct that C3 will directly feed any supply AC noise directly into the top of the zener which isn't what you want - its really difficult to trace from the photo and I may pull that and have a look at it again. If you remember we guessed the value of C1 and C2 and the originals were more or less open circuit, I couldn't get a reading on any of them out of circuit, my meter just showed erronious values and then I tried a chinese component tested which thought it may be a 137pF cap before decided the others were just resistors of wandering values. I would say worth increasing the value of C1 and C2 with better quality caps. I would say noise immunity is mainly controlled by C1 which would have some time constant. To the battery itself, at annual they will note how well it turns the engine, check the plates are covered and may measure the specific gravity. I look it to an automtive place which measured its cranking current which was in spec - slightly better than spec. The battery is from 2016 and the internal resistance will have risen which will make all of the problems I am having worse with time - like they have done, one cell could easily be much worse than the others and will often still give a good current output but will sag a lot. https://sandalwoodservices.com/battery-dr400-certified---611817010-400605-401-p.asp <-- this is the battery, replacement isn't too expensive. So I think that I have a combination of issues. Aging Battery, sensitive OVR, alternator slip ring contact isn't perfect, french spindly wiring. |
| Drjaymz:
I should just add - if it wasn't a certified aircraft I would swap out the battery for Lithium Phosphate Aviation Replacement, rip out all the wiring for modern better spec wire and replace the alternator / OVR and regulator (which would probably be in the alternator) and put limit switches on the flap position or have it pre-selectable. Replace the landing light with AeroLED. All of which could be done for less than the price of a (s)crap alternator overhaul. Then I guarantee it would be much safer, more reliable and operate without issue for another 20 years. - But because its a certified aircraft it will have to stay with an unsafe battery and charging system that isn't fit for purpose. Is certification for aircraft that are > 40 years old where most parts are not procurable actually counter-productive - discuss? |
| David Hess:
--- Quote from: Drjaymz on June 11, 2024, 07:29:16 am ---Is certification for aircraft that are > 40 years old where most parts are not procurable actually counter-productive - discuss? --- End quote --- Nobody at the FAA every got fired for keeping things as they are. Notice how lawsuits killed all of the civil aviation companies. |
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