So for a long time I have been saving up for a Fluke 9010a, an in-circuit debugger for systems that utilize microprocessors such as the 6502, z80, and many others. It is a popular peice of test equipment for folk who do repairs on vintage arcade boards as it is super useful for debugging hardware problems.
So I had finally won one in an auction, only problem is that it was having display problems.
problem was that segments were stuck on, which tells me the display tube was good. (At least until I dropped the display module, broke the tube, then ended up replacing the tube with the only close one on ebay, bringing it back to the condition it was before I broke the tube). So I noticed that basically only some of the middle segments were stuck on. In the service manual, it has a chart that shows how the segments are categorized.
Since all the characters were working, I could guess that the gate controller side of the VFD circuit was working. And being that it was only some of the middle segments, it was on the segment controller.
Thankfully, the service manual shows that the segments in question are only controlled by two chips: a shift register (U14) and a VFD driver chip (U12). In all reality, it could have been either of these chips, so I socketed both of them with the intent of replacing both. However, I took a closer look at the shift register, being a CMOS chip, I figured it had a higher chance of failure. From there I hooked up the shift register to my logic analyzer to see what it's up to.
Taking a closer look at the signals in the shift register. We can see that it's clock, strobe, and it's output enable are all toggling as they should, this is fantastic. We could also see that it was getting data input, so that means the microcontroller is feeding it data. Only problem here, is that that the data output, labeled as D_OUT, was not changing, even though that the data input did (as shown in the middle D_IN pulses). This was our problem here. After changing it, all our display segments were working perfectly.
The overall lesson though, is be careful, I'm still kicking myself for dropping the display module. I got super lucky that I was able to source a replacement. At the end of the day, I was able it working and even decorate it with some stickers of art drawn by a super cool artist here on VGS.