diagnosing an Intel Series II MDS monitor failure

tony duell ard at p850ug1.demon.co.uk
Sat Aug 1 15:14:08 CDT 2015

> The root cause of the failure appears to be that the internal voltage
> reference of the uA723, which is specified as 7.15V ±0.35V, is now
> 9.4V. The uA723 needs to be replaced. The uA723 is still readily

That's an unsual failure. I've had 723s go internally short-circuit and 
then the output voltage is essentially the input voltage, but never
seen the reference drift.

> available, but due to the construction of the power supply, replacing
> it is relatively difficult. Just probing the pins of the chip was a
> challenge. The pass transistors are mounted to 1/8 inch aluminum
> plate, but the pins are soldered into rivets in the single-sided PCB,

Ouch, that sounds horrible to work on.

One thing... I had a (SMPSU) that seemed impossible to work on, it was built
a bit like that. Then I found that those 'rivets' were actually sockets and the
transistors were plugged in, not soldered. All I had to do was unto the 
mounting screws and it all unplugged. Of course I didn't realise this and 
spent some time desoldering things. 

I assume you have proved your pass transistors are soldered in place.


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