Went away for the weekend with friends, and there was a mains surge, associated with the lovely rain Friday night. Killed a phone charger and the PSU for a CPAP machine. Well the power supply, after a few minutes work with a hacksaw blade, is open, mains fuses are fine (surprise surprise, lovely input filtering, 2 stages of common mode filtering, and NTC, plus a LC noise filter, with 2 fuses, line and neutral, but no MOV at all, you would think they only ever tested with pure sine wave filtered mains) but input side PFC smells somewhat crispy. well, it is a 24v 3A supply, something you should be able to get right. Look at the 3 wires leading to the cable, and there is a red wire, marked ground, a white wire, marked +24V, and a third black wire marked ResMedBus, vanishing into the secondary side to a 8 pin device with no visible part numbers, and a few transistors and resistors.
So Saturday morning go to the town, hard to find ant info, because typing the name brings you up bacon, the main factory in the town, or "adult service" places. hit the China mall, and there is a single universal laptop power supply, switchable from anything from 12V to 24V. Test equipment to hand is very limited, a 12v SLA battery, and a panel mount 3 digit DVM module, and some fishing kit. PSU grafted onto the cable, and then plug in and test. Screen powers on, followed by a error message that there is a lack of communication with the PSU. DRM via that black lead strikes.
So a solution is needed, secondary side seems fine, so spliced the 3 wires back in, so the old PSU is now also connected, but no primary side, and power is once again applied. CPAP powers on, goes through the self test, and is happy with power supply now. so now it has a mid cable wart on it, to provide the DRM to make it work, with now an aftermarket supply, which cost $25 AUD, instead of the original, which is close to $200AUD, and a week to get. We were not going to drive for 4 hours to fetch the home power supply for it.
Then got hit by a typhoon that night.....