@nottinghack And that's the cleaning DONE!

Just have electrics/electronics to test and all being well, I should make a start on reassembly.

@nottinghack Update:

Who knew that cleaning 40 years worth of grime would be really hard?

Yesterday I was able to get everything stripped down, and briefly checked, mechanically it seems sound apart from the chain, which was rather slack despite ample pressure from the tensioner and also a few of the links were seized, I was able to free all bar one of these so it 100% needs a new chain.

Also the weird cone nut tool, perfect bottle opener.

The actual cleaning is a long and slow process, I'd say overall I'm probably only 20% of the way there, as I have read in various places Cif cream does work best but wow does it take a while to scrub in, also using green scowling pads leaves behind a green tint in the white plastic which was fun to then scrub out, it looks more like this is going to be a multi day cleaning job before I actually get it reassembled again; well, now back to work

@nottinghack And the C5 springs into life for the first time in over 3 decades!

The POD once again showing minimal load as the wheels are off the ground this is all great!

Just now need to tidy up the wiring and get this thing out for a ride.

@nottinghack I was a little concerned; after a few minutes the POD LEDs for battery status would loose an LED, but after soak testing for half an hour it just remained at that, I realise I had the PSU set exactly for 12v, and the battery I have measures at more 12.5v, so in theory if there were losses the POD could be seeing 11.5v and assume declining battery, setting the PSU for 12.5v and I never lost an LED - the POD and ULA appear to be working perfectly!

I then disassembled the test setup; and started putting the control box back together; this was a little awkward, the plastic case had been welded together and I had to cut these welds, but there were tabs for 3mm self tapping screws, I which I was able to fit some black plastic 3mm screws and nuts in to keep it together and also it kinda looked the part.

Quickly attaching all the wiring in the C5 and remounting the control box, I was ready to give it a test with the real motor.

@nottinghack After making these repairs, I wanted to test absolutely everything to be sure nothing else is about to blow.

I initially started simple, I connected the control box to a power supply, current draw was small and all the voltage lines looked OK.

Moving a little further, I wanted to make sure it could switch a load without issue, but I didn't want to do that with the POD attached in case of damage to that; looking at the circuit diagram for the control box, transistor 3 appears to connect from the coil of the power relay at the collector, and 0 volts at the emitter, with the base coming from the pod from a line named "RELAY DRIVE", so this must be how the pod can disable the motor.

A simple dupont wire between pins 1 (+5Va) and 7 (relay drive) on header X9 (POD connector) ensured the relay was always enabled, also shorting pins 4 and 5 on X6 (thermal cutout switch), then just adding a small 12v motor to the motor terminals, and shorting pins 5 and 6 (motor switch feed/return) on connector X8 resulted in the relay making a nice clear click and current draw at around half an amp - perfectly fine.

Then I just quickly popped the POD off and with some dupont wires connected it to the control box, with a 10k ohm resistor between pins 1 and 2 of X6 to simulate the thermocouple in a good state, and everything worked as expected, the POD LEDs behaved just as expected, showing full battery and low load.

@nottinghack And repair/PCB pics!

There was some solder bridges on the PCB, but after checking the schematic, these actually seem intentional

@nottinghack After that, I again connected up the lights and the POD and then applied the battery and everything lit up as expected, whew.

This is good, the control box on the C5 is a simple circuit with all readily available parts and is the best thing to go wrong, to see the POD still working which uses a custom ULA chip is a relief.

I have just popped back home from Nottinghack to write this update and also grab my laptop and some tools I intentionally left at home so I could carry the battery; my next steps are to get food for once today, and also connect the control box, isolated, up to a bench power supply just to check everything is working as it should, no components are an unexpected current draw, and maybe even connect the POD to do a full dress rehearsal with a small 12 volt lamp or small motor, once I am happy with that, I will do a test with the main motor.

@nottinghack After consulting the schematic further, it looks like the POD uses a +5Va line in driving the LEDs, this is generated in the control box by a LM342 off of the larger +12V line which was of course, still lifted form the overcurrent; that might explain how the LEDs went off but the lights didn't, I put another bodge wire in there but I also thought I should check R1 and D4...

R1, which was supposed to be at 2.2ohms was more like 500k, not good, and D4, which was incorrectly listed in the schematic as D3 (there are 2 D3s, the one connected to the coil of the relay is the real D3, they are both 1N4001s), was open in both directions, and connected between the smaller and larger +12V lines, and thus explaining why the traces blew;

When the power relay connected D1 to the negative terminal of the battery, D4 also being a dead short would have started pulling current from the smaller +12V line, damaging the quite large trace to the smaller +12V line and obliterating the small trace from the LM342 to the larger +12V line.

I had a direct rep for R1, bit D4 I could only find a 1N4002, which as far as I can tell from the datasheet is basically the same thing but rated for a higher voltage.

Whilst I was at that, I did the same with D1, similarly again replacing the original 1N5401 for a higher rated 1N5406.

@nottinghack Diode D1 sits between the positive line coming straight in from the battery and the negative terminal of the motor in reverse bias, this is a problem, as when the motor relay clicks closed, it connects the negative terminal of the motor directly to the negative terminal of the battery...

D1 was directly shorting the battery, and was probably why I saw smoke, I subsequently removed D1

For any C5 owners out there, check D1 (the big diode when you remove the upper potion of the control box cover where all the connections are), if it's shorted, just go in there with a pair of pliers and cut it.

Now time to look at what other damage I had done; the C5 has two +12v lines from the battery, one of these is intended to power the auxiliary circuitry like the lights and POD and is fairly low current, the trace for this looked to have been lifting and might be why the POD LEDs were turned off, after putting a bodge a small wire in there, I plugged the POD and lights back in and applied power, lights came on all as before but no POD LEDs :/

@nottinghack Sooo, that was a wild ride.

Just after this video I pressed the power on button one more time (dunno what I was expecting), and all the LEDs went off and only the lights remained on.

THAT'S NOT GOOD

So, first and most obvious place to start is to take off the control box, after removing the cables and 2 screws it came straight off.

Unfortunately the lid on this one appears to be plastic welded on to the base, but it still had tabs where it looks like you can screw them together, so I had to cut those welded joints to get it off.

Inspecting the board, apart from 2 burnt traces going towards the power in (explains why the LEDs in the control box went off at least) there wasn't much I could see that would have blown, other than diode D1.

I had read about D1, it's a common diode to go on it, simply pushing it backwards at more than a snails pace will kill it; and I had measured last week when it was on the bench to be a dead short in both directions, for those screaming why did I not replace it there and then; everything I had read emphasized how unimportant D1 is and how it can be left in temporarily if blown; then I read the schematic...

@nottinghack Update, the motor runs!