Now you have me confused because you are saying its nothing to do with the computer, that is actually do with the telemetry, which is what I said originally
I was making a general comparison. Lots of electronic problems can be fixed with a power cycle that properly triggers a power-on reset. Computers are just the most well-known example, but they're not the
only example. The SCE undoubtedly contained its own logic to, e.g., cycle through sensors in a defined order, and it was probably sent into a disallowed state by the power glitch.
Sometimes (e.g., on generic PCs but not most Macs) there's an explicit reset button that does the same thing without actually cycling power. Many modern systems, especially those intended for unattended and/or high reliability operation, have "watchdog" timers that force a system reset unless they are periodically "tickled" by the software to indicate that they're working properly.
In the specific example of Apollo 12, the AGC (computer) and the SCE were separate modules both powered (like everything else on the CSM, either directly or indirectly via the AC inverters) from +28V DC buses A and B. Both were affected by the momentary severe drop in bus voltages when the fuel cells dropped off and threw the entire load onto the entry batteries. The computer reset and restarted itself correctly but the SCE did not. Aaron's call "SCE to AUX" momentarily interrupted power to the SCE as it was switched to its backup power supply, thus forcing the unit to reset itself correctly.
What did the Flight Controllers think might be the cause when they lost all their telemetry?
I don't know, do you know of any interviews with Aaron in which he describes his thinking at the time?
The crew were indicating that the fuel cells had tripped off and the main bus voltage was down to 24v approx, not the 30v it was supposed to be, so they were on battery power..
Yes, the crew knew the DC bus voltages were low when Bean read them but they did
not immediately realize that they had momentarily fallen much lower after the fuel cells dropped off and before the entry batteries picked up the load. This was a property of silver-zinc battery chemistry, not any electrical circuits.
This momentary (milliseconds) drop is what caused the SCE to malfunction, and the AGC to independently reset and restart.
With SCE to Normal, they were never going to get their telemetry back because of the low bus voltage,
I don't think that's right. The momentary loss of power put the SCE into a disallowed, malfunctioning state that was most easily fixed by turning its power off for a longer period and then turning it back on so it could do a proper power-on reset. I suspect Aaron, already being familiar with the behavior of an SCE in that malfunctioning state, knew the easiest (and perhaps only) way to do this was to switch the SCE to its backup power supply. So he made the "SCE to AUX" call that saved the day.
As near as I can work out, the crew didn't reset the fuel cells until after Alan Bean switched SCE to Aux. (approx 000:01:50), At 000:02:19 Houston told them to reset their fuel cells and Bean was going to but was told by Conrad to "wait for staging".
Right, and this is fully consistent with my understanding: the problem wasn't the low but steady bus voltage with the fuel cells offline and the batteries supplying the load, but with the much lower bus voltage that had existed for a very short time when the fuel cells first dropped offline.
So they went through staging with no fuel cells and running on batteries.
Yes, and this was a wise call by Conrad. The Saturn V's own guidance system was still working fine, so there was no real urgency in getting all the CSM systems running again. Acceleration builds to 4g's twice during S-IC boost (shortly before inboard cutoff and again just before outboard cutoff) and staging itself is pretty violent, so you'd run the risk of hitting the wrong switch and/or making erroneous readings.
The standard Apollo rule for emergencies was "If you don't know what to do, do nothing". This seemed like wise advice since ill-considered action could easily make something far worse.
