I came into possession of an IBM 5151 MDA monitor which was in rough and non-functional condition. There was a raster on the screen but no video. My sense was that most of the circuitry was working and that the problem would be with something small-signal or digital as opposed to related to a high-voltage circuit.
After probing the power rails I found that the +5V was missing.
The damaged part candidates could be R214, C202, the transistor TR18, the IC201 or the zener diode D0203. I pulled each one in this order and found that it was the zener diode which was shorting to GND. Of course the last device I test..
To confirm the theory that the missing 5V was the only issue with the display I sourced this voltage with a bench supply. Thankfully the display began to work!
I had two options: One was to simply replace the diode but I would have to wait a few days/weeks until the delivery arrived, and the other was to replace this circuit with an LM7805 which is a 5V linear regulator which I could harvest from my parts bin – today!
So this is what I did. I removed the resistor R214 and connected the linear regulator’s input to the 14V at R214’s pad.
This solution worked great!. Here you can see the display working with the regulator mounted on the bottom-side of the PCB.
After making a few adjustments to the internal POTs it was ready to put back together and give a good scrubbing with a Magic Eraser.
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