VFD THEORY OF OPERATION
Ensoniq used an NEC FIP (Fluorescent Indicator Panel) series display, the type of which is known generically as a vacuum fluorescent display (VFD).
The VFD can roughly be conceptualized as a combination of grid position and character segments. Each grid corresponds to the horizontal location of a character, and since this is a two-line (80 character) display then each grid actually controls two characters -- one for the top line, one for the bottom. The shape of a character is determined by 14-segments which can be either lit or not lit. In other words, which segments are designated to be on or off at any given time will determine the shape of the character displayed.
Note: Even though it is classified as 14-segment, there are actually 16 controllable features per character, so it's a slight misnomer when examining how it works.
This particular VFD has 40 grids which means there are 40 distinct character positions from left-to-right. There are two TL5812 driver ICs and each 5812 handles half of the grids, so 20 grids per driver. One TL5812 takes the left half of the display and one 5812 takes the right, but it should be noted that information is relayed from the left 5812 to the right 5812 so if the left 5812 dies then the right will not function either.
Note: Since this is a two-line display, there are two characters per grid. Since there are 40 grids positions available then a total of 80 characters are controllable.
There are four TL 4810 driver chips which control the segments for the pair of characters currently being written to (determined by which grid is enabled) -- it's a two-line display so both the top and bottom line characters are selected simultaneously by any given grid. Each character segment has been given a letter designation of either A, B, C, D, E, F, G, H, J, K, M, N, P, R and two additional controls called CR and DP. There are two characters per grid so a total of 32 signals are needed (8 per TL4810).
Note: The 4810 chip signals are not clearly split between display or character lines so any given dead 4810 will affect both lines evenly and not totally wipe out an entire line.
The segments are alternately referred to as either anodes or phospors depending upon the technical context.
The VFD also needs a filament (cathode in a technical context), in this case supplied with 9VAC. Uneven illumination can usually be traced back to a problem with the filament voltage. This voltage comes in from the PSU on display/keypad power connector pins 5 and 6.
The grid and segments are run at high voltage (+50VDC) the source of which is called VBB on the schematics. Even if the digital logic on display board are working, if the VFD driver chips do not have proper VBB then the VFD cannot work. This is specifically mentioned because VBB is carried in on its own line from the PSU over to the display via display/keypad power connector pin 3.
These display schematics should help: http://www.buchty.net/ensoniq/files/sch ... isplay.jpg