History and operation
of VFD displays
used by GOTTLIEB.
PANAPLEX and VFD
There are mainly two technologies for the displays used on first generation electronic pinball machines.
The first "PANAPLEX", was installed on BALLY/WILLIAMS pinball machines and can be recognized by its orange color.
The second "VFD", was installed on the GOTTLIEB pinball machines and is recognizable by its beautiful blue color.
PANAPLEX technology is the simplest: to light a segment, all you have to do is circulate a current between two elements. Each digit is carried by an anode and the common segments are connected to a cathode. The voltage required to turn on a segment is of the order of a hundred volts.
VFD technology is a little more complex: to light a segment, it is also necessary to circulate a current between two elements, but in addition, the gas in the tube must be preheated by a filament. Each figure is carried by a grid and the common segments are connected to an anode. The voltage required to turn on a segment is less since 40 to 60 volts is sufficient here.
VFD - The fourth element
The documentations and diagrams of the two main manufacturers of VFD tubes (Futaba and NEC), refer to three electronic elements to drive these displays:
- The grid
- The segments
- The filament
This is indeed enough for the numbers to light up. On the other hand, there is a fourth element barely mentioned in the documentation. As we will see below, this element was forgotten by the Rockwell-Gottlieb engineers for the four-digit displays.
This fourth element also does not exist on very small VFD displays.
The technical documentations of the Futaba tubes used by Gottlieb (4-LT-11 and 6-JS-01) are unfortunately not found. On the other hand, one can refer to the documentation of the 4-LT-46BZ3 tubes, of similar size to those used by Gottlieb. These tubes were used in microwave ovens and differ little from 4-LT-11.
As you can see in the photos below, there is a sort of long pin located inside the tube which seems to be connected to "nothing"! Notice that it's connected to pin 2 and there's something similar, but looks like a little claw, on the 4-LT-11 tubes:
The 4-LT-46BZ3 documentation mentions a "Diffusion Grid" and also specifies the current that can pass through it. It is not the "grid" used for each digit and its function is quite different.
The explanation is actually hidden in the application note of Futaba, we can indeed read this:
In order to guard against the effects of electrostatic discharges which can cause instability of the display, the inner face of the VFD tube is covered with a transparent conductive film. This must be connected to one of the potential of the filament, in order to constitute a protective screen.
The small claw that we saw on the 4-LT-11 display is therefore not connected to "nothing"! It makes contact with the conductive film which covers the inside of the glass. This is also true for the 6 and 7 digit displays where it takes the form of a double claw.
Unfortunately, not all Rockwell-Gottlieb engineers must have read this paragraph! Although this element was taken into account for the 6-digit displays, it was, on the other hand, completely forgotten for the 4-digit ones. It wasn't until many years later that they realized their mistake and made an "on-the-fly" fix.
But that's another story...