Well, gents, I can now report full success in getting my cruise working again!
This was one of those non-trivial cases, where multiple things combined.
0. Door locks got new diaphragms; main heater valve got replaced - this solved most of the vacuum leak problems (some still persist). Also, the actuator cable slack was cancelled.
1. The cruise control unit was not keeping speed because of the two dried out 47uF electrolytic caps - these got replaced first.
2. Once #1 was done, the unit was surging like crazy and still losing a few mph every few miles (hard to notice on short distances) - all caps got de-soldered, tested and replaced. In total 15 got replaced, but all bar one didn't really need to be. More on that below.
3. With #2 out of the way, there was no surging any more, but still some speed loss.
4. All major components got replaced with new parts - TAA765A (op-amp), LM2901N (quad-comparator), BD437 (power-transistor) and BSV81 (IG-MOSFET)
5. All major joints have been re-flowed (not all, as I was fed up with soldering at that point).
It cost me €70 in total in terms of parts, and only because I was rebuilding two units and some components had minimum order quantities. The other unit is as dead as it gets despite all the work (relays).
Some observations made along the way:- Surging is most likely caused by two things - the power transistor (the one stuck to the housing) is probably burned out, or close to. This points at two things - poor cooling, and/or to much power draw from the actuator. Poor cooling is due to daft design by VDO (plastic as radiator, really?). Replace the transistor even if it's OK. In my 1979 unit this was a BD437, which costs about €0.44. The 1977 unit had a 2N5190, which is about €3.20. If the actuator is drawing to much current from the amp the power transistor will burn out quickly, so look for a working actuator. You're in for some big expense - new units are available but cost in excess of 400EUR.
- The polyester film caps are rock-solid despite their age. The ceramic 68pF caps are good as well. I've tested all on both my units and had just one dud (the 4700pF 400V one). I've only replaced some of mine because they were dirt cheap.
- If your unit does not engage at all (no response to CC stick), it's most likely one or both the relays. Good luck finding replacements...
- Do not replace the polyester film caps with electrolytic ones.
This is how my unit looks like post-repair. I still need to find a way to cool that power transistor properly.
