diagnosing an Intel Series II MDS monitor failure

[Brent Hilpert]

On 2015-Aug-01, at 1:26 PM, Eric Smith wrote:

> On Sat, Aug 1, 2015 at 2:14 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:
>>> 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.
> 
> The circuit used in the PS only ties the reference output (pin 6 of
> the DIP) to the non-inverting input of the amplifier (pin 5), so I
> don't think there's any other likely explanation, though since I
> haven't actually been able to pull the board out yet to inspect it
> more closely, I suppose there might be a very small chance of
> something else being shorted to those pins.

Just a suggestion: you're sure there no ground reference problem on pin 7 (V-) of the 723? (hence raising the Vref).
I've seen small 723-based supplies with sense leads where the 723 V- is on the neg. sense lead rather than neg. output lead.