Extra 7905 regulator in Aten KH1516i KVM switch - why?

[hanakp]

About a year ago, a system admin in my workplace asked me to fix old Aten KH1516i 16-input VGA KVM switch. It repeatedly rebooted and beeped, some LEDs blinked during the process. It was the classic problem, bad primary-side caps in its power supply.

Two months ago a different problem emerged, the switch didn't route video from any slave ports, but keyboard and mouse signals worked fine. At first I tought the master port was dead, but it wasn't the case, because it displayed OSD and service menu as it should. But it showed a black screen from all slave ports. So I assumed the analogue switches for the VGA signals must not work properly. I couldn't find any service manual, but I assumed it must be some power problem. The power supply has 3 output voltages: standalone +5 V for digital circuits and symmetrical +-5 V for analog ones. The +5 analog voltage was fed to the main board directly, but the -5 V one (black wire in the first photo) went through a 7905 regulator, which was "guerrila-style" mounted on one of the screw posts. I measured about -4.2 V at its output. When I shorted it, the switch immediately started to work. So as a fix, I disconnected the regulator completely and made a bypass wire that I installed directly into the original connectors (3,96 mm pitch Molex KK series). It has been running for two months now with no apparent ill effects.

The reason why I'm writing is that I don't understand these things:

1. Why was the regulator there in the first place? In essence, it just worked as a really expensive diode, there was around 0.8 V drop on it. And it wasn't faulty, either - when I connected its input to -9 V external PSU, -5 V appeared on its output and the switch started to work.

2. Why the switch worked fine for 12+ years, but then suddenly stopped? Could something change on 7905 chip over time when it's not working in the active region? Electromigration, perhaps?

The only explanation I can think of is that some PSUs produced higher negative voltages than they should and Aten added the 7905 as a precaution/protection.

[PKTKS]

apparently you answered yourself

3pin 7xxx  or equivalents are meant to 3V differential minimum
for proper regulation

If for some weird reason you have less than that ..
the supply pin of the regulator is bad.

Your problem is there - and is not the regulator.

for the purists the quiescent dropout is 2V
and 2.5 should be enough although poorly regulated.

Paul