diagnosing an Intel Series II MDS monitor failure

Jay jaeger cube1 at charter.net
Sat Aug 1 23:23:34 CDT 2015

Sounds just like the supply on my Altos. What a pain.

Eric Smith <spacewar at gmail.com> wrote:

>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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