diagnosing an Intel Series II MDS monitor failure

Eric Smith spacewar at gmail.com
Sat Aug 1 15:09:32 CDT 2015

I previously wrote about the monitor of my Intel Series II MDS going
out, which was because the +15V DC supply tripped the crowbar. The
voltage adjust was all the way to the minimum, and the voltage was
still about 17.4V with no load, and trips the crowbar with even a
small load. I think someone previously encountered this problem and
adjusted the pot rather than fixing it, and the problem has gotten
worse since.

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
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,
entering from the etch side (which is closest to the aluminum), so
they will have to be desoldered to get at the etch side to replace the
723. I considered just cutting the pins of the 723 and soldering
another (or a socket) onto it from the component side, but pins 1-7 of
the 723 are right next to a big axial electrolytic so there's no way I
can solder that side. Input and output wires to the board are soldered
into single-pin cup thingies on two opposite edges of the board, so I
can't even get the board into an orientation suitable for working on
it without desoldering or cutting some of the wires.

I haven't bench-tested the monitor, because I haven't kludged up a
suitable signal source yet, but I don't think there's anything wrong
with it.

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