Saturday May 18, 2024

More Stern MPU-100 Repairs
  Posted by: AceBHound on Aug 6th, 2011 10:44 PM
Spent most of the day cleaning up a few more Stern MPU-100 pinball boards. One of the boards is for a Nugent pinball and had light corrosion by the battery, but the CPU chip & 6821 chips had literally black pins. I've seen that on a few other boards I've repaired and I'd imagine it's from being stored in a damp basement / climate. What's weird is in this case none of the other ICs on the board had black legs, but someone had soldered one of the PIA's directly to the board (usually I think they're all socketed) so my guess is the ICs came from another board that had sat somewhere damp. In any case, replaced these chips & resocketed the non-socketed PIA. Cleaned up the corrosion and added a plug for a remote battery. I'm not 100% sure I'm done with the board yet but good progress made on it anyway!

The other MPU-100 board was out of a Hot Hand that had a hole blown through the J4 connector, not by me luckily :) I'm guessing the 43v shorted as the hole was in this area. I think I had replaced some of the ICs on this board last year and got the board booting, but it didn't pass the self-test since the battery corrosion did its damage at the bottom of the board. Most people would probably just throw it or harvest the chips, but I figured if I clean it up I can use it as a test / project board -- maybe solder some ZIF sockets on it. Then I don't have to worry about killing a useful board if an experiment goes bad!

I also took another look at this severely hacked Stern SB-100 sound card. As soon as you turn the machine on, you get a loud THUD and constant background hum. The board was non-working at one point but I replaced a few ICs and got the tones working. Just had this loud background hum that I figured was due to the old capacitors. Well, last week I replaced all the caps and the hum did not go away. So today I replaced a few more ICs on the board, but still no-go. At this point I'm thinking one of the LM380's is probably bad -- they're expensive and shouldn't really be socketed with having a heat sink on them, so this board will go on the back-burner for a bit longer.

I also looked at a display from a Flight 2000 that was flickering on all digits. Sometimes it's the connector, but in this case wiggling the connector didn't make the problem better or worse. I knew the 4543 IC was involved with the latching / strobing of the digits so suspected that IC. A quick Google search confirmed it might cause flickering on the digits so I removed the MC1453 from the board and replaced with a socket & a 4543BE IC. I'll probably write a quick repair log up and maybe shoot some video of the issue -- since I think one of the other displays is has the same problem.

Well that's it, time to collapse! =)

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