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NeXT 400dpi laser printer restoration

So, after replacing the IC and the voltage regulator in the daughter board, nothing has changed.
At a further check, it seems as though it is in fact the 24V psu which is not working. It temporarily provides sufficient voltage for triggering the relais and spinning the fan, but not enough for the 5V PSU to come alive. With my multimeter I read around 4-5 V at the 24V lines, so way too low.
I am just speculating, though. The brief moment during which something happens is so short (less than 3 seconds) that it's impossible for me to gather additional solid evidence for narrowing down the troubleshooting. Without a working unit at hand, it's indeed difficult.
I will try to find a way to trigger the unit even without connecting it to the printer, in such a way to rule out the logic board and speed up the investigation on the PSU alone.
 
You can just try feeding a current limited 24v supply into the 24v rail on the printer and see what happens.

I suspect you have a short somewhere, which is why the rail is down to 4-5v.
 
So, something strange but interesting happened. Today I dedicated some 30 minutes to the printer. I would like to unbundle the power supplies from the the printer, I need to understand if the PSU is per se good or not. When the printer starts up, it seems to me the only trigger is a 3.5-ish volt on rail 7 of the DC PSU. I could not catch any other activity on any rails during this initial step. Rail 7 seems to be linked to opto couplers, which are in turn linked to an obscure Sanken "SI-9509" component. Quite unbelievably, there seems to be no documentation at all on line about this very component. Seems to be an IC for controlling the switching PSU 115V > 24V, though. Anyhow, I tried to soft start the PSU by providing a 3.5 V signal on rail 7, but without success, nothing was happening.
Curiously enough, though, as I reconnected all the rails of the PSU to the printer and attempted to start the printer again using the main CPU, quite surprisingly the PSU sprung back to life and remained operative (you can tell it, as the fan spins, first at full speed, then it goes low) for some ten seconds or something. It was then switched off, as the fuser was not connected and the logic has it that without response from the fuser the unit is switched off.
Any further attempt to restart the unit, though, miserably failed, displaying again the same behaviour (the fan spins, then ito goes dead after 3 seconds).
 
Based on that, you can rule out any kind of major fault like a shorted power transistor, or bad power supply driver IC.

Depending on how much capacitor leakage there was, there could be some subtle damage from that. Maybe an open resistor, or electrical leakage on the board if it wasn't cleaned well enough. I had a similar experience recently with a Mac LC power supply. After cleaning, and recapping it, it worked fine for a while, but then started shutting down. In the end, I had to remove some components to clean under them more thoroughly.
 
It can be a huge problem. One check I like to do is to probe various spots on the board with an ohm meter. Any reading at all means it's not clean.
 
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