Miscellaneous

NAP monochrome monitor - Power supply - 20190721

I found this new old stock monitor the other day for really cheap, but upon bringing it home and plugging it in, there was no activity - no flyback transformer sound and no power LED. This seems like it is some kind of power supply issue. The first thing I did was check the on board fuses, but both were OK. Inside the case, I found a transformer separate from the main PCB. My initial thought was that some of the low voltage power supplies come from the transformer, but it turns out that all power for the entire monitor is fed by one ~15vac secondary winding on the transformer. From there, I began to trace the power through the main PCB. The first thing that happens is that it goes through a full bridge rectifier and capacitor, making unregulated DC. Then, the power goes into a PNP power transistor, which is the largest heatsink on the board, connected to the base of this transistor was some other small transistors and resistors. My guess was that this was some kind of linear regulator implemented with discrete transistors. The output from the large PNP transistor would be some regulated voltage, which is the main voltage source for the monitor. I could tell where this power bus was because one side of the power LED was connected to it. After studying a bunch of schematics for similar monochrome monitors online, this does indeed seem to be the way that most of them work - while the monitor may run on 110vac, inside it's really running on a low voltage like 12vdc. Indeed, this did seem to be the case with my monitor. As a test, I bridged the power transistor so that unregulated voltage could make it over to the regulated side, then plugged the monitor into a variac and brought it up slowly. I observed that the filament began to glow, and I could hear the flyback transformer operating. After bringing up the monitor power to around 12V, I could see scanlines on the screen. The screen was very wavy from being on unregulated power, but I was happy to know that it worked. A further test with batteries and a video signal from a camera proved that the monitor worked as it should.

Back to the power problem: because I did not have the expertise or a schematic to fix the existing regulator, I decided that I would add in a regulator IC instead (I chose the adjustable LM317). I removed the existing PNP power transistor, and the supporting components used for its feedback. In a coincidence, I found that the footprint of the PNP transistor exactly matched the pinout of my new regulator IC, with the voltage In and Out pins aligning as is, and the Adjust pin being left free due to the removal of the supporting components. This also meant that I was able to reuse the heatsink to keep the regulator cool. Following the application note from the LM317 datasheet, I added supporting decoupling capacitors, and the adjustable voltage divider (fixed resistor and adjustment pot) needed to set the output voltage, to the PCB, although I did need to later move their locations after I discovered it interfered with the case geometry.

Finally, the monitor would work under its own power, but I had still had to set the output voltage from the regulator properly. But, what is the proper voltage? I knew it had to be less than 16V because all capacitors attached to the regulated voltage bus were rated no higher than that. The unregulated input voltage was only about 14-15V, which makes 12V seem too high. Indeed, once I completed it, my regulator would not even put out 12V even when the adjustment pot was turned up all the way. To resolve this, I tried to find the filament voltage of the CRT, which is pretty much wired directly into the regulated voltage supply. The CRT type is an M31-350LA/P, and no data exists about it on google. After a lot more searching, I eventually found a listing for a M31-359 crt from an old test equipment manual, which listed a M31-350 CRT with a filament voltage of 11V. That seems pretty reasonable given what I have experienced so far, so I set my regulator such that the CRT filament had 11V across it. Monitor now works great.