Williams "Black Knight" pinball restoration.
Driver Board Rebuild and CPU board modifications.
Here is another of the critical circuit boards in a typical Williams "System 7" pinball machine, which mainly consists of the circuitry that takes in the switch inputs, then works the coils and CPU controlled lighting circuits. It's really not as complicated as it looks, as most of the circuitry is just duplicated a bunch of times for each coil. Same with the columns and rows for the lamp and switch logic matricies. Of course, it still has a lot of those duplicated circuits, which makes for a lot of work at times. There are three reasons I'm doing a "rebuild" of the driver. 1: As with all the other boards, I cannot stand the IDC style connectors that the wiring harness uses, so they are all getting replaced. While I'm at it, I'm also replacing all the male headers as well. Replacing the headers isn't really necessary in a normal situation unless you have a reason to suspect a problem. Here, I have reasons to suspect problems which I'll explain later. 2: I am having issues with all the circuits on this board in one way or another. I keep blowing the solenoid fuse, some of the switches aren't working, and a couple controlled lights don't work. Problem one is definitely related to the driver board, the others are worth investigating. 3. Since 1980, electronics designs have significantly progressed, and some components are going to be replaced in order to improve reliability as well as reduce the power demands placed on the power supply and other components. As with the power supply board, I have gotten tremendous amout of information from Clay's website at www.pinrepair.com. I consider this to be the single best resource for pinball repair and diagnosis available. The site is amazing, and I'm half tempted to make it the homepage for my web browser. Thanks again Clay! Parts supplies are my two usual suspects Bob Roberts and Ed at Great Plains Electronics. Both guys are great to deal with, very helpful and all around benefits to the hobby in my opinion. I recommend both of them without hesitation.
This is the front side of the removed driver board. Already I can see that work has been done on it in the past. Most obvious is that the PIA chips are in sockets, as is one of the 7408 chips in the solenoid drive area. Looking closer I can spot a few bad areas of solder joint repairs that have to be dealt with as well. You can really see where some of the resistors have been putting out some serious heat over the years. Particularly the bottom right area of the light matrix has gotten baked quite a lot over time.
	Here's the backside of the board. Hard to spot, but again there is evidence of two different people having worked on this board. One, very talented it seems. The other, not so much.
Here is the hack-job work to show you. The top left area of note is where a socket was installed to replace a 7408 IC. The bottom right is an associated TIP42 transistor that controls one of the coils. This transistor drives, surprise surprise, the same melted coil found in the overview page. All of this work needs to be removed, cleaned up, and new components fitted. I am starting to wonder if this guy was using acid-core solder, by the corrosion I'm seeing on essentially all of his work.
	By contrast, the work done to install the sockets for the two main PIA chips is very nice indeed. I'll honestly say it's nicer work than I can currently do, and these sockets done this well is a good thing to find. I was dreading having to ever replace these two chips if they were still directly soldered to the board.
Here is an excellent way to show you why these boards get baked so badly in this area. The older TIP42 transistors have a huge power draw, requiring equally large resistors to run them. Williams upgraded the size of these things a few times in order to reduce the failure rate of them. They still get quite hot, as you can see, in only ten minutes. I can only imagine the temperatures inside the backbox with these things and another 40+ incandecent bulbs. No wonder the backglasses wear so poorly in these machines. This whole area will be upgraded as a result.
	Here's the driver board following stage one of the rebuild and upgrade process. New .156 molex pins all around and a new 40pin interboard connector. The suspect parts from above were replaced with all new components, and I also needed to change out the left 6821 PIA chip that controls the solenoids. This was causing my locked-up solenoid problems, and once that was changed the machine ran great. You may also spot that I added some labels to the wiring connectors, in order to identifiy which ones they are. I haven't done all the IDC connectors yet, but I have done the ones on the circuits which were giving me any hints of a problem. Visibly the board is not very different here. But I didn't do the work here to make it LOOK better, it's all done to make the game RUN better, as well as be more reliable long term. These machines are already getting to be uncommon, so preservation is important in order to keep the hobby existing.
The CPU board also got a minor tweak, but as it's currently running stable I didn't want to go too far into it just yet. The wiring harness got new connectors going into it, but the board was mostly untouched other than re-soldering a few pins that looked like they may be cracked.. I wasn't happy with the remote battery installation, I just don't find the remote AA packs to be a clean install. So, taking a page from more modern electronics systems, I installed a lithium battery on board instead. These don't leak, last longer, are significantly smaller and all sorts of other advantages over alkalines. I figue that if they work without problem in the millions upon millions of PC's around the world, it should work in my pinball machine. Now, as I did need to install at least one jumper wire, I opted to use a diode instead of just a bare wire. Why? Well, just redundancy really. The CPU board already incldues a blocking diode to prevent the system from attempting to charge the memory battery, but diodes can potentially fail. This gives an extra piece of prevention. I happened to have a big roll of 1n4004's for use on the coils, so it only cost me about 4 cents to add it in. Not a critical upgrade by any means. Now, there are also other options out there too, such as no battery at all. Swapping the 5101 RAM to a Simtek 12C68 will remove the need for any battery at all. Upgrade sets are available from a few sources. Should my 5101 ever fail, it's what I'll likely do. But as the current RAM is still functioning, the decade or more that the $4 upgrade to a lithium battery is fine for me.
So, what was the result of all this work? Stability. I started with a machine that had multiple switch issues, a serious solenoid lock-up problem and many other quirks. I finished all this with rock-solid stability and full playfield functionality. Every switch, controlled lamp, solenoid and anything else controlled by the CPU is running at 100%. The only remaining problems, at this point, are a few troublesome GI lights and cosmetic/structrual issues. System wise, though, it's running fantastic now. You may also notice that I still have some IDC type connectors in place in the last couple of photos. Those also are getting steadily replaced, and I already have all the parts to do so. I'm just not in a rush, as the ones that haven't been changed yet are circuits that aren't showing any problems. At the first sign of any issue, these connectors are the first thing that I plan to change. At the latest, they will all get done when I replace the backbox wood and swap over all the components. Either way, I do have all the parts on hand, and can do them at any time.
As promised, I did revisit the driver board one last time in order to do what I consider to be a fantastic upgrade. This board was originally designed in the mid/late 1970's, and transistor technology has advanced considerably in that time. As the above photo with the thermometer shows, it takes a good amount of power to drive the TIP41 and TIP42 transistors in the lamp matrix. Clay's website over at Pinrepair.com documented swapping the TIP42 transistors for newer design IRF9Z34N MOSFET's. This supposedly dropped the temperatures of the 2 watt resistors considerably. Now, rather than just blindly start swapping parts, I looked closer into the design. The TIP41 transistors have larger 3w sand resistors doing the same job, so why can't I do a similar swap on that side too? The TIP41 transistor is just an NPN equivilent to the TIP42 PNP transistors, so I set about to see if there was an N channel equivilent to the P channel IRF9Z34N MOSFET. And you know what? There is! The IRFZ34N.
	So, of course, I had to go ahead with this swap, now that I figured out all the parts I needed. Between Ed at GPE and E-Bay, I managed to get all the MOSFETS that I needed for pretty cheap. The transistors in red got replaced with the IRFZ34N's, and the transistors in blue go the IRF9Z34N's. This was a straight swap, just remove and replace, nothing fancy to it. Then came the moment of truth, which was to fire up the driver board in the machine and see how it did. (Ok, I lied, I tested it first with just ONE transistor on the TIP42 side.)
The results of this modification were almost exactly as I expected to see. The 2w resistors saw a temp drop from over 200 degrees down to just 100. The 3w resistors went from about 150-160 degrees down to 100 as well. The MOSFET's also saw a substantial drop in temperatures, as I assume they were absorbing a lot of radiant heat from the resistors. The pre-swap transistors were seeing temps of 120-150 degrees, and are now all down to 100 degrees. These temperature drops are going to result in big improvements of board reliability in this area, as heat up in the 150-200 degree range is well outside the operating specs of most of the components. There's more improvements though than just the heat drop. Don't forget, heat in electronics is all waste energy. No heat means it's running more efficiently. Measuring the power use I saw a 17 watt reduction in power use for the machine as a whole. It also seems, and this last bit is purely speculative and opinion based, but it appears that the slight ghosting I was seeing on the controlled LED's in the backbox has gone down considerably. As you might have guessed, I'm very happy with this modification so far, and it has been performing flawlessly.
At this point, it's time to close another chapter in this restoration. The circuit boards are all running stable now, and are in fact performing better than new in some areas. Should I ever re-visit these boards in the future for more upgrades or significant repairs, I'll definitely put it in here. But for now, I'm just enjoying some pinball!

Back to Chapter 2

Continue on to Chapter 4

Back to the Main Page