Rotation VIII restoration project
Today I received a Midway Rotation VIII for restoration. The person who originally purchased this machine was an experienced pinball repair person who had done a very good job restoring the cabinet but unfortunately was unable to finish the job due to a lack of experience in the electronic aspects of this machine.
It’s my job to pick up where he has left off and bring this rare beast back to full working functionality.
Before picking up the machine I made some quick checks on the power supply for the correct voltages to give me an idea of what damage may have occurred.
Everything seemed fine, except the -5 V DC logic which was at -7V DC. So this is where I will begin;
the electronic schematic of the power supply shows that the -5V DC is obtained by a 79M05 voltage regulator. Luckily, I have an NTE961 which is a direct replacement.
So, getting started consisted of getting familiar with the machine’s wiring and schematics and creating a test environment. I repaired all of the voltages on the power supply board and checked all of the values with an oscilloscope. After stabilizing the power supply board, I disconnected all of the connectors of the logic board and only connected J0. J0 brings the +5, -5 and +12VDC to the logic board. To facilitate troubleshooting and provide a means to connect test equipment, I created 4 test points on the logic board labeled +5, -5, +12 and Ground. I would advise anybody serious about bringing a sick board back to life to do the same. This allows connecting a logic probe and easily checking the board’s ICs. Firstly, I checked the reset was high on J8-pin 2. This was high.. but the CPU is still not booting.. Next, I checked the clock signal was present, A1 pin 10.. hum… this is not working… I found my first problem on the logic board.. I will attempt to fix this tomorrow.
Troubleshooting the clock signal;
The clock signal is located in the upper right corner of the logic board and is composed of a quartz crystal (XTAL) and a feedback circuit. I removed all components, checked them, and put them back. I don’t have any means of testing the crystal.. maybe someone knows how; not me.
Still no pulse.. I suspect the crystal but have no spare one that runs at 14.138 MHz.. So I used what I had to see if this would generate a pulse. Lo and behold, it works!
Okay… Now I will order the crystal and some capacitors, wait a few days to receive them, then move forward.
Troubleshooting the clock signal in the lab environment has been quite a challenge. I decided to build the circuit on a breadboard and test all the components and their values. However, I haven’t received the new XTAL (quartz crystal) yet, which I ordered from the USA. I received one from Poland, but it’s not the same size as the old one, and I fear that there is something I haven’t understood about crystals yet. I rebuilt the circuit with all the original components except the crystal, and as you can see from the image, the crystal is oscillating, but the frequency is running at 6 MHz, and the amplitude seems to be out of scope. This frequency is expected because I am using a 6 MHz crystal. I can’t make any progress until I have a new 14.1381 MHz crystal.