FWIW I had the solder flux issue arise with a CD player I'd had for many years. It started saying "no disc" because the drive motor was not turning on. I opened it up and found a small area of residue which I cleaned off and it was working again. Probably 6-9 months later the fault reappeared. I cleaned it again and it has now worked flawlessly for several years. So it takes *very* little residue to cause faults in sensitive parts of a circuit.
In this instance, the residue was perhaps 1/4" or smaller spot at the corner of what I think was the MCU. I concluded from this that I had missed a little bit of flux under the chip the first time. Like most consumer gear, board access was awkward.
My mother's VCR/DVD combo developed problems. The construction was such that I was not about to disassemble it for access to the main board which was on the bottom. For that I got a large plastic container to hold the unit and poured 4 pints of alcohol into the container to bring the level up over the board. As there was no way to see, much less scrub the board I left it to soak over night. The next day I poured the alcohol back in to the bottles (marked to indicate it had been used) and let the unit dry for a day before putting the covers back on. It resumed working for a few weeks, so I repeated the process. Once again after the treatment it worked for a while and then the fault reappeared. I had reused the alcohol from the first treatment which may have been my undoing. In retrospect, I think if I had been extravagant and used fresh alcohol it would have worked. After the last attempt the alcohol had a strong brown tint. But each time I treated the unit it worked for several weeks, so I don't think there is any question that the fault was entirely due to "no clean" flux.
Usually I will use either an artist's brush or an old tooth brush to scrub the board and a pump spray bottle of alcohol. I'll spray and scrub for a bit and then make a couple of rinses where I hold the board vertically and let the alcohol run off, shaking off the liquid between passes. I've also used compressed air to blow off the alcohol between rinses.
I think the primary issue is added capacitive loading on logic lines interfering with timing. though that was obviously not the case with the motion sensitive night light as it is entirely analog.
I wish I had a video of the 34401A as received and after cleaning. I'm beginning to think that there are a lot of "for parts only" units that were repaired, but not cleaned properly afterwards. They then worked for several years, but when the flux effect hit they got scrapped.
But of the limited number of repairs I've done in the last 10 years the vast majority have just required cleaning. At this point, I don't even start circuit tracing until I've made a thorough inspection for traces of flux residue *anywhere* on the board. If I see any, I clean the entire board even if it's just a little residue at the edge of a board.
Hence my interest in constructing a 100 psi pressure washer using a fuel injection pump and a modified airbrush with a gallon or two of alcohol which gets repurified by distillation. That should make cleaning very quick, painless and cheap. The sole effort being the disassembly and reassembly.
