Strange display problem on Williams System 11b (Taxi)…

[pb1592]

Hello,

I’m having a very strange problem with my displays on a System 11b Taxi machine. Here is the situation:

1) Purchased the machine used and noticed that the upper main display was beginning to outgas and flicker, most likely due to its age. Still functioned adequately and got better as the machine warmed up.

2) I performed the Pinrepair.com mod on the high voltage section of power supply, swapping in new 39k resistors at R1 and R4 and 91V zener diodes at ZR2 and ZR4. (Note: I’ve done this mod to four other machines and it has worked flawlessly.)

3) After swapping R1 and R4 there was no change and everything still worked as it did before. After swapping ZR2 and ZR4 however, the upper main display would not work at all. I figured the lower voltage (91V vs the original 100V) wasn’t sufficient for the already failing (outgasing) upper display.

4) HERE is where it gets strange: I decided to reverse the mod and put the original 100V diodes back in ZR2 and ZR4. When I did this however, the upper display still would not work…

5) After playing around and double checking solder connections, I noticed that if I completely removed ZR2 from the board and put nothing there, then the displays worked perfectly. This worked with ZR4 being both either the 100V or 91V diode. The displays were dimmed as expected when ZR4 was a 91V diode and ZR2 was not present.

So my questions are…

1) Does anyone have a logical explanation behind this? My knowledge of this display circuit is very limited.

2) Is this causing an over-voltage of the whole display circuit?

3) Is it safe to keep it this way? No other mods were made to the power supply or display circuit. The only change at this point is fresh new 39k 1-watt R1/R4 resistors and a missing ZR2 diode in the high voltage circuit of the power supply board.

Thanks!

-Brian