Adjusting Bally/Stern MPU Clock Speed

18 August, 2010 (04:13) | Pinball | By: admin

Update: I found some instructions online to replace the original 470pf capacitors on -17, -13 and MPU-100 MPUs with 220pf to convert them to the MPU-200 clock speed. I tried this conversion but measured the clock speed at close to 1Mhz. That’s too fast for MPU-200. I was able to get close with two 50uf caps but it’s probably easiest to go with the original values, replacing the two caps and two resistors to convert to MPU-200.

I was inspired by this thread on RGP to do some hacking on the Bally/Stern MPU clock circuit with the intention of converting -17, -35 and MPU-100 MPUs to the MPU-200 clock circuit. This in conjunction with the 5101-6116 conversion would effectively convert these MPUs to an MPU-200. You would also need to convert the MPU to use either 2732 EPROMs or a single 27c256.

The difference between the 2 circuits is actually pretty minimal. Only 2 resistors and 2 capacitor values need to be changed. Here are the steps to convert a -17, -35 or MPU-100 clock circuit to MPU-200:

1. Replace R4 and R10 with 13.7K 1% resistors (1/4 watt)
2. Replace C14 and C15 with 100pf capacitors (at least 6V)

This will convert the MPU to the MPU-200 circuit. Alternatively you can just replace C14 and C15 with 220pf capacitors.

The following clock circuit schematic shows values for all Bally/Stern MPUs.

Click “Read more” for more detail on the Bally/Stern clock circuit with some examples of this conversion in action.

This is what phase 2 of the clock looks like on the stock Stern MPU-100. You can see that it’s running at approximately 500kHz. Bally -17 and -35 MPUs share the same clock circuit.

The MPU-200 has a faster clock. This is what 02 looks like on a stock Stern MPU-200. We can see it runs at approximately 850kHz.

Here’s an example of an MPU-100 running at close to the MPU-200 speed. I didn’t have any 13.7K resistors on hand today when I decided to start hacking on the clock circuit so I used 10K and 3.9K resistors in series. This made for a slightly lower clock but the image below shows what phase 2 of the clock looked like on the MPU-100 board I was using. This is another one of those “I’ll do a better tutorial when I have more time” scenarios. And again… it works!

I figured I might as well try running an MPU-200 at the slower speed just for grins. Converting an MPU-200 to run at Bally/MPU-100 speed is as easy as removing the jumpers between E32-E33 and E34-E35.

This is what the MPU-200 clock looks like with the E32-E33 and E34-E35 jumpers removed.