Power line weirdness = dead MS
 

:ohnoes:

Here's what happened:
I had my somewhat-newly-built MS connected to my laptop and successfully flashed the firmware from the original B&G to a later version. No problem, laptop sees the MS and the gauges in megatune respond to changes on the stim like they did before. I was just poking through the software familiarizing myself with the many settings. The stim was being powered by the AC adapter from diyautotune. The 120V power in the apartment flickered rapidly on and off a few times in impressive fashion. I don't know why, as it's a nice sunny day with no wind. Most of the rest of my shit at home is working after resetting a couple of breakers, but the MS is dead. :vash:

I can't get the stim & MS powered up with either the AC adapter or a fresh 9V battery. Following megamanual's troubleshooting guide:

"0) Is your stim battery powered? The battery might be dead, as they only last a few minutes while powering MegaSquirt. So replace the battery with a fresh one. CHECK If that doesn't help, remove the processor CHECK, and check the voltage at the voltage regulator (U5). You should get:

* stim voltage (~9 Volts), on the pin nearest the DB9, FAIL, V=0.76
* the middle one should have less than 1 Ohm to ground (use the heat sink, or the center pin of the voltage regulator, as a ground), CHECK
* 5.0 Volts on the pin furthest from the DB9. FAIL, V=0.64

If you don't get 5.0 Volts, there's a short somewhere. The most common cause is the pins of the voltage regulator itself having a 'solder bridge' but it could be a bridge on almost anything you've installed so far......"

The rest of the troubleshooting suggests verifying correct parts installation, however this is extremely unlikely to be the problem as the MS was working just peachy before this power incident.

I noticed that my fresh 9V battery got abnormally warm while I was measuring the above readings. That, and the wording in the troubleshooting guide, appears to suggest a short circuit. I looked over the MS pretty thoroughly and didn't see any gross evidence of smoked/charred parts. Could it just be that the power spike/surge caused the voltage regulator (U5; LM2937ET-5) to fail/short internally?

Anything else I should investigate? I suppose I won't know if anything more expensive (processor) got toasted until I get the power supply section working again. What are the chances that the voltage regulator sacrificed itself to save the rest of the MS?

Here's your first post so someone more knowledgeable might comment. I would 'guess' it's the 5V regulator for the MS. Certainly the problem is in the power supply. I really don't know my MS well, but I do believe something there would have died first if you had power problems.

I suspect it's one of the input protection diodes that blew from a voltage spike. Check the power supply schematic and see if the 12 volt side is shorted to ground.

I followed along with the schematic of the power supply section and took some readings. First I verified that the stim alone was putting out battery voltage of ~9V and it was.

The side of the schematic labeled "12V raw," which includes a common point between MOV1, C15, the banded end of D11 and the unbanded end of D10, measures <1V between that part of the circuit and the heat sink bar. There's not 12V anywhere on the MS board that I can find.

Not sure what the resistance of a zener diode should be without any power applied, but both sides of D12 and D19 measure ~1 ohm to ground, whereas one side of D13 (another zener diode) measures over 1Mohm. So those zener diodes could very well be the culprits.

It's not like Rat Shack has the exact parts I need, so I'll be putting in a small order to Digikey. These parts aren't expensive, so I think I'm going to order replacements for most of the components of the power supply.

I just replaced all the power supply components (diodes, caps, U5). I might have gotten by with just the diodes, but I didn't want to chance leaving any affected parts in there. It was a minor annoyance but the MS is just fine now. Phew. After my HPDE next weekend I'll proceed with the install. :cool: