A clock using six digit neon gas plasma displays

These displays were used between 1977 until 1980 on a variety of Bally and Stern pinball machines.

Space Invaders

The displays made by a variety of manufacturers. Below is a variety of different versions.

Fixing up the displays

I had bought a large quantity of displays at the auction of the Robert Thomson collection of pinball machines on 11th july 2004. The displays were still on their driver PCBs. All used and in a condition unknown. Someone who had done a lot of repair work for Robert sidled up to me and told me they were all junk and some even raided for parts, so I had to assume most had faults of some kind. I bought them banking on the fact that some of the display envelopes would be very good as I could see many with no burn marks.

In a 4 player pinball machine, the player 4 display gets the least wear, as it is rare that 4 player games are actually played. Conversely, the player one position gets the most wear and the last two digits of this display the matching number replay feature and so show the most signs of burn and silvering.

Testing displays using a plasma ball proved to be a quick and easy way to establish if there was any gas left in the display. This weeded out some of the junk so that I could concentrate on the rest of them. Dead displays could provide donor boards for other boards which had issues. Some boards had suffered bad burnouts so I had a bunch of good displays on bad boards. Some boards had trivial faults such as a transistor out or a bad decoder chip. The pile of good displays soon added up.

After making a simple test circuit, I was able to start fixing any faults. There were some common faults that I soon learned to recognise. Others were trickier. On one I found a shorted copper track on the circuit board that must have been there since it was manufactured – the board was mint and had never been worked.

I had bought one large box of these used displays. There was a second box I didn’t buy. This eventually found its way with much of the rest of the unsold pinball spares to an avid pinball enthusiast and turned up in an Ebay auction a couple of years later. I bought those too. On picking them up I was asked if I wanted to go through a large bin containing yet more of these Bally and Some Williams and Zaccaria displays. I bought even more of them.

Needless to say, any refurbished displays I supply are nothing like you might see in this junk!

Burn baby, burn!

At the end of its useful life I think

It was a deliberate Bally design ploy for the high voltage regulation circuit to fail leaving the displays still lit. The argument was that a machine with a blank display would take no money at all, whereas a display that was crackling and fizzing away at maximum voltage was still playable until the display was replaced. Arcades in London never seemed to bother and I often saw pyrotechnics in the displays for weeks on end. The end result would be extreme silvering on the glass leading to hot spots.

Designing a clock

I’d sent 10 displays as a thank you for previous help to a friend in Wales. He fired them up and very soon had some clock code working which he emailed me. This code formed the basis for some tweaking and improvement.

Six digits – ideal for hours, minutes and seconds. My artwork separates them into sets of two digits. The artwork is sandwiched between two sheets of glass and it can be changed for custom artwork and various designs have been produced to date.

I wanted to make my clock board completely non-destructive of the original display. As all the connections to the display were on one set of sturdy pins, I decided my board could simply plug on top of these. There are two main types of these Bally circuit boards and fortunately the connector is the same on both, although the position is different. The picture below has one of each type. Stern boards are a third variety. The circuit is the same but the boards are different.

The electronics

12 volts in. My clock driver board provides the necessary 5 volts and 180 volts out to the displays. The existing Bally boards already have seven segment decoders and digit select lines and in addition all the high voltage and keep-alive circuitry. It was an easy matter to interface directly to them from the microcontroller.

Bespoke designs

The most popular design to date is the Addams Family version. I have sold a number of these to proud owners of Addams Family pinball machines.

Sean Mills of The Pinball Palace commissioned a one off design:

A fun little project and one that seems to have impressed those in the pinball community that I have shown it to. A number of complete clocks have been sold, however I no longer have any for sale.

Schematics for Bally display boards

6 digit Bally PCB schematic PDF
7 digit Bally PCB schematic PDF

This Post Has 8 Comments

  1. Jon

    I would like to buy one of these Bally boards. Are you selling them?

  2. Dennis Kuchar

    Hello, is it possible to purchase the clock driver board that you made for the Bally 6 digit display? If not would you be willing to share the information needed to produce my own. I’m afraid that I am just not bright enough to design my own board and would appreciate any help toward getting my Bally display working.

    Much Thanks, Dennis Kuchar

    1. clock-it

      Hello. This project is now retired. I am sorry, I no longer have PCBs and coded PICs available for this clock.

  3. Nap Pepin

    Hi! Are you still selling any spare displays and would you have any Stern units?

    1. clock-it

      I am sorry, I don’t have any Stern units for sale. Are you after 6 digit or 7 digit displays? I amy be able to provide something suitable. I think the only difference is that the Stern units use the commas.

  4. Koen

    Could you share the schematics for your test circuit?

    Greetings Koen

    1. clock-it

      My test circuit involved programming a PIC from Microchip. This has to sweep all six digits rapidly to give the illusion of them all being on at once. My test circuit ran through all the digits on the display in turn, to show up any bad numbers or segments. I gave away my tester to a friend of mine at Pinball Palace who repairs pinball machines. Alas the documentation and code is lost in the mists of time now.

      If all you need is to see whether the display has gas, then a plasma ball will do, or you can make a simple mains tester with wire, a diode and a dropper resistor, whose value will depend on whether you are in 220-240 volt land or 110 volt. Reference to the circuit diagram will let you know which pins on the display to test. If the display lights up at all, then the gas is good and it is then simply a matter of fault-finding if it doesn’t work as expected.

      The main cause of failure is broken solder joints on the connector pins. Resolder these anyway. I take the old solder off and replace with new.
      The second cause of failure are the 100k resistors. Replace these with 1/2 watt types, as the 1/4 watt originals tended to cook.
      If a single segment on all display fails or one digit fails to light, it is a transistor at fault. Refer to the diagram for the transistor at fault.
      If the segment pattern is wrong or the display keep alive is glowing but you have no numbers, then the problem is the decoder chip. Replace it, adding an IC socket for easy future replacement if necessary.

  5. Shannon Pezzolla

    First of all, let me say that these clocks are fantastic. I would LOVE to proudly display one of the pinball clocks in our game room. My husband and I collect and own 9 pinball machines. I love the generic one you did and the Addams Family clock. Do you do any others that you haven’t shown here? What do these clocks cost?

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