First thing I decided to do is remove the display and all the resistors from the MDB. I want to remove the display to clean the glass and board from any residual tape so that I can use new double-sided tape to properly mount the display.
I want to remove all the resistors because I’m going to replace them all with new ones so I won’t have any problems with any old ones going open. If you have missing digits or segments on displays, the resistors can be the problem. They obviously were on this board and best thing to do is just replace them all.
I have an old Radio Shack temp controlled soldering iron and a Hakko 808 desolder that I use for all my electronic repairs. These tools have worked well for me. I wouldn’t call myself a master solderer – sometimes it’s not the prettiest job you can ask for but I usually don’t lift traces or damage boards. I did enough of that in my younger days. The most important thing is a temperature-controlled iron. You’re least likely to do damage with that. A solderpult is also very useful. Other tools to suck solder are advised. Solder wick is not. I won’t get into the debate about which works best. Save that for another thread.
I desoldered the leads on the display to remove it and two things happen: more mounting pads come off the board, likely loosened when the last person worked on the display. And then I notice this with the display leads:
I can move the leads inside the display and make the tabs touch each other. That’s not good. I suspect this was done when someone took the Weller to the board and they heated the leads so much that they detached from their connection. If I pull hard enough I think I can pull a lead out entirely. Ugh again – I’m going to need a new display glass.
I proceed to remove all the resistors – both the 10K and 3M ones from the board and notice that one of the traces from one of the resistor mounting pads to the 7180 was damaged where the jumper wire was. Ugh x3. Another one to fix.
Pulling the display and resistors exposed the problems with the existing display and traces. If I was using the existing display it would have made it easy to remove and clean the tape from it as well as clean up the board. Another benefit is that it allows you to also check the 6184s and 7180s effectively. Which I did following this procedure outlined on PinWiki: http://www.pinwiki.com/wiki/index.php?title=Williams_System_3_-_7#Master_Display_Drivers_for_System_3_to_6. If you leave the glass in and it is shorted, you may get a false reading on the 6184 test. As it says in PinWiki: "A shorted display glass can show up during the UDNxxxx 11 through 17 test." So without removing the glass, you won't know if the problem is the UDN chip or the glass itself.
Once I completed the testing, it was time for Ugh x4.
I suspect with the bad leads in the score glass, the old display somehow shorted and as a result took out one of the 6184s which tested bad. Guess I’ll be needing one of those.
My needed part tally:
14 10K ohm 1/2w resistors
5 3M ohm 1/4w resistors
1 18 pin DIP socket
1 6184/6118 transistor array
1 six-digit score display glass
The resistors and DIP socket are cheap and easy. Our good friend Ed@GPE has these. The resistors are $.05 each with a minimum purchase of five of each required. That’s about $1 in total to replace all the resistors on the board.
The DIP socket is another $.50 so we’re up to about $1.50
The 6184/6118 is a bit tricky. As Ed states on his site:
“BEWARE! The UDN6118A and UDN6118A-1 transistor arrays are heavily counterfeited. Allegro stopped making these in February of 2002. Easiest way to spot counterfeits - always look at the four digit date code on the second line. The Chinese counterfeiter's -love- to provide seemingly new parts to customers so the date codes always reflect relatively recent dates on them. If your date code is higher than 0210 (10th week of 2002) -- you can be assured that the part is counterfeit.
Also beware of sellers providing parts with a "-2" suffix. These are the low voltage version of the 6118 and have an absolute maximum rating of 60V. Only place these will safely work is on a Gottlieb 4-digit display.”
You see these all over ebay. Do a search on “6118 IC” and you’ll see all these counterfeit units being sold from Hong Kong/China. If you buy one of these be prepared for fireworks. You have been warned.
Ed does sell a replacement – the uPA6118C from NEC for $5. The only issue with it as Ed states:
“Note -- these are identical to the Sprague UDN6118A's but have a lower breakdown voltage than the UDN6118A-1's. If using this in a Williams display driver application that requires the UDN6118A-1, make sure the voltage at the display driver is below 85 volts. Often these machines use a 1N4764 to derive 100Volts. This 100Volts is then reduced by an addtional 10 volts on the display driver board to provide 90 volts to the display. It is recommended to replace the 1N4764A with a 1N4763A to reduce this voltage to just under the 85 volt threshold.”
This means that in order to use this part on the MDB, you have to lower the voltage on the power supply for the displays. This is always a good practice to extend the life of the displays. I’m planning on doing that on my Flash anyway BUT, I didn’t want to have a display board that potentially would be swapped out down the line into another machine where the HV was factory standard. Not only that, you may have some displays that won’t work at a lower voltage so this solution may not work for your machine.
There are NOS 6118s out there to be had but they are starting to get scarce. I wound up ordering a few from Robert @ twobits.com. Cost is $5 + shipping.
Here is the new 6118 installed in a new DIP socket. Notice the date code of 9741 which identifies this as a legitimate part.
Now I’m into the repair for about $6.50 in parts + shipping but I still have one problem left – I need a new display glass.
I could get one from Steve Young @ PBR – he sells new ones for about $59+shipping – less if I bundle my order and get it to $100 for the 10% discount – but that’s just a little too rich for me.
I see that jewboyflowerhead posted that he had Williams Sys6 displays. After some checking on his end, he sells me a working display for $10 – an ideal alternative. And he happens to be a great guy to deal with and a great Pinsider.
There was one problem with the display he sent me, which wasn’t really a problem at all. The display was on a slave board (player display) and once again the double-sided tape was dried out. Perfect! This saved me the trouble of having to pry off the display from the tape/slave board. All I need to do is desolder the display from the slave board and clean it up. The good thing is that the six digit displays are mostly interchangeable with each other, as long as the leads on the display are long enough and haven’t been cut too short, so it was fine this display came from a slave board.
For $16.50 in parts I now have everything I need to fix the MDB.
After installing the new 6184, I install the new resistors. I actually put 1/2w rated resistors for the 3m parts instead of 1/4w – it’s overkill but there’s no harm in doing so.
As I install the resistors I clip and save the excess leads. The reason I do this is to use them to fix the broken traces like this:
Here’s the repair for the trace from one of the 10K resistors to the 7180. This is a common method to repair broken traces. There are other more sophisticated ways to do this but for this application this is fine.
Post edited by viperrwk : Added PinWiki link for testing UDN6xxx.