Well during the course of making the replacement Bally MPU board I bought a few 'dead' boards from a Fleabay seller in the USA for comparisons and checking. When they arrived I was very impressed with the condition of them all and I was astounded that someone would even write these boards off....very little battery corrosion at all.
Most were missing some or all of the plug in chips but after getting the two 'best ones' going I decided to have a look at the third. It had no plug in chips and the obsolete clock chip was also gone and there was a socket in its place.
I am now far more familier with these boards and exactly what makes them tick than I have ever been (maybe TOO familier....hahaha). I had a closer look at this part of the circuit and thought that surely it can't be too hard to provide a clock pulse at around 500Khz??? It was then the penny dropped - this board (more specifically this chip along with its mate next to it) actually generate TWO clock pulses that are inverted from each other. OK, that still can't be too hard...so I lashed up a breadboard circuit using a bog standard 74LS00 and a few caps and resistors chosen pretty much by guestimation. Stuck the CRO on it and BINGO - I had two clock signals, one an inversion of the other - so far so good.
Here are some pics of my test setup:
Plugged into the socket on the MPU board:
Switched on and VOILA! I was met with the flashing green LED thanks to Leons test ROM. That was all good so as I was running a batch of PCB's I though I would whip up a couple and make it 'properly' - here are some pics:
Assembled and plugged into the socket in place of the original chip - no soldering, flyleads or any mods at all to the original board!
And here the CRO shows the two clock signals being generated - the frequency is a little low as you can observe on the CRO but that will just be the juggling of a capacitor value on the board - enough for today - that deserves a beer I think - even if I do say so myself - Hic.....…
This is actually so simple and so inexpensive I don't know why it hasn't been done before??? Probably $5 of parts plus the PCB but Veroboard (perfboard) would work just as well.
