Bally/Stern MPU Repair #2

13 September, 2010 (10:25) | Pinball | By: admin

A fellow collector from the KLOV forum acquired a Xenon machine recently and needed an MPU. I offered to sell him one and he accepted so I pulled one out of my huge pile of MPUs needing repair and did a bunch of work to get it working and make it reliable.

This is the MPU I started with. It started life as a Bally -17 but I added my EPROM replacement board which will easily let me use Xenon ROMs making it basically a -35. I picked an MPU with virtually no corrosion damage thinking that it would be a quick repair but I ended up replacing all of the sockets so it took some time anyway. This inspired me to start going through a bunch of these MPUs and selling them. I’ve got way more than I’ll ever need and I would like to buy a better playfield rotisserie so you can expect a few more MPU repair logs in the coming weeks.

You can see a label on it stating “Boots with test ROM. No PIA LED”. When I first built my MPU test boards I went through all of my MPUs and made some basic notes of what was wrong with them.

I decided pretty quickly that I would be replacing all the sockets. The originals probably would have worked but this is the kind of thing that can cause intermittent operation and I wanted this MPU to perform perfectly. I also decided that I would install a new Molex header at connector J5 so my EPROM replacement board would make good contact.

The first thing to do when replacing this type of socket is pry up all of the socket housings leaving the pins behind. These sockets have some kind of heat resistant paper under the housings. This paper should be removed before you start desoldering the pins.

This picture shows all of the original sockets removed. Once the housings and paper were removed I heated and removed all of the pins then desoldered the holes with a soldapult. I’m really beginning to think I need a better desoldering solution so I’ll probably buy some kind of desoldering station before I go too crazy with MPU repairs. You can see that I’ve already replaced the Molex header at connector J5.

Here’s a close-up of the CPU socket position.

Once all of the original sockets were removed and the holes cleared I installed new dual-wipe sockets.

I don’t know if this diode was damaged when I started or if I damaged it prying the socket housings off the board but it was replaced. It’s only used when you’re checking TP6 anyway.

Shiny new diode installed.

The next problem I decided to tackle was a gross repair that someone had done at U20. It seemed to be working but I didn’t want to send the board out to a fellow KLOVer with this hack on it.

This picture shows the component side of the board. Someone globbed all kinds of solder on the pins. I had to use a bunch of solder wick to remove it.

This is what the solder side of the board looked like. Pretty ugly.

Removing this IC was a real pain but I managed to get it out without any permanent damage.

There were a bunch of solder pads missing from the solder side of the board.

I re-tinned the pads on the component side of the board and cleared the holes again. This gave me nice pads to solder work with.

Finally ready to install the socket.

I used machine pin strips so I could solder to both the component and solder sides of the board. I don’t usually like to do this and will use dual-wipe sockets because it’s a real pain to remove them should you ever need to but I didn’t have a lot of choice this time.

The pins on the IC were pretty mangled. I didn’t have another one on hand and wanted to ship this board out so I soldered machine pin strips to what was left of the original IC legs. Not exactly pretty but it works well.

Here we can see the IC before soldering.

This shows the IC after soldering the legs to machine pins.

It’s a machine pin Big Mac!

Next I installed a 5101 RAM adapter to eliminate the batteries.

The final step was to install one of my EPROM replacement boards, burn the Xenon ROMs and ship it out.

This is what the board looked like with all the repairs completed.