Hi, this is my first post here, always been into electronics and love Dave and the EEVblog. Recently started to get back into hobby electronics since the kids are starting to get older and it's easier to find the time and joy for it now
I'm going to be pretty thorough with this post so bear with me and I hope you feel it's worth your time to have read it when you are done I'm going to be a bit much some places with the designators but that is to help people find this information with a search engine if they ever have a similar issue. Not sure if to put this post in Beginners or Repair forum but going for Repair.
Background:I bought a Pinball machine, advertised as not working. The machine is a Revenge from Mars (RFM) using the Pinball 2000 platform released in 1999 (Star Wars Episode I (SWE1) uses the same system and components). When I started to take it apart this odd looking fuse poked me in the eye, soldered jumper over the fuse?
trying to remember what Dave would have said about such a thing (That's terrible, Muriel!?, nha probably something else). That is the Power Driver Board (part nr# 04-12329) or PDB for short. This PDB is controlled by a computer though a parallel cable on which the Pinball software is running. Computer boots from a proprietary card called a Prism card which contains all the game roms. Screen is a standard arcade monitor, CGA I believe.
Troubleshooting:Ordered a hardcopy of the Pinball 2000 Schematic Manual and downloaded an online scanned copy to get started right away. You can find the schematic at ipdb.org (
https://www.ipdb.org/files/4446/Bally_1999_Revenge_From_Mars_Pinball_2000_Schematic_Manual_February_1999_OCR_searchable.pdf) on page 10 to follow along.
I Used some Lab grade distilled water and a toothbrush to clean up the board, finished up with some IPA, almost got all the dirt and grime of the board. Removed all the fuses from the board since I won't trust they used the correct fuses if they did that hacked fuse job to get the machine running. As the board was drying upp I got to have a look at the transformer and everything checked out, correctly configured for my power system (For Europe high line 240 VAC) and looks like it left the correct voltages when measuring, or a bit high for 50V and also 20V if I recall correctly. The line fuse was the correct one so that was all good.
Connected the dried PDB to the power harness coming from the transformer and started to put in the fuses one by one after checking I used the correct ones. Power was cycled between each fuse insert since there are 18, 20 & 50 volt lines, and they are actually a bit higher than that, don't want to get shocked. Everything checked out and no issues so far, fuse F100 thru F107 (F100, F101, F102, F103, F104, F105, F106 & F107) are turned on by a relay from the main computer so they could not be checked until I connected the computer to the PDB via it's parallel cable.
Luckily I had already confirmed the computer was working so I got to work connecting it and tried out the monitor, success, computer AND tube is good
After successfully booting the computer and seeing it communicate with the PDB properly I was pretty happy, F100-F107 LEDs lit up (LED5, LED6, LED7, LED11, LED12, LED13 and LED14), or actually all but one. F104 did not have it's status LED working (LED4), and checking the fuse showed it was good and I had the expected voltage on that rail. It says 50V DC in the schematic but I actually get ~66V DC when measuring.
Looking at the schematic revealed that the fuse status LEDs was connected to a voltage divider, R176 10k and R285 47k, with the LED in parallel with the 47k resistor. This divider also feed an LM339 (U37 & U40) comparator though R191 1k resistor (duplicate dividers use R193, R194, R197, R201, R204, R206 & R207) to detect that there is power on that rail and the fuse is good. Comparator compares this with a ~1.25V reference according to the schematic. After the comparator there is a 10k resistor pulling the value high and feeding it into a 74HCT240 buffer which is ultimately fed to the computer via the parallel cable.
Measuring the divider for the F104 rail showed about 55 V in the middle, which is pretty sane if there is no diode (this one seemed broken, so open loop then) next to the 47k resistor with a supply voltage of ~66V. Measuring all the other fuse rails and their resistor dividers (R177, R178, R179, R182, R183, R184, R185, R286, R287, R288, R292, R293, R294, R295) which are identical that had working LEDs showed about 1.7V in the middle, sane considering there is a LED with a forward voltage of about 1.7V there.
I messed around with some other wire harnesses and started connecting these to the PDB, some went to the front of the cabinet and some was just lose harnesses that should go to the playfield once I get to that stage. Sometime when doing all this and messing around the fuse that previously had that odd jumper soldered to it blew, this is fuse F108 with a green status LED and supplies 12V DC which is then regulated though a 7805 to supply 5V VCC, this in turn has it's own red status LED.
Ok, so we have a broken LED on one of the 50V supply rails, this makes the divider go really high and feed this into the LM339. And the LM339 is fed power from VCC, starting to feel this could somehow be connected if the LM339 is faulty. And that 55V seem awfully high. Checking the datasheet for this part (BA10339), to be clear I'm not that experienced but I think I can read and get the most important parts. In short the datasheet states the absolute maximum rating for differential input voltage is +36V, even with a reference voltage on the inverting input of ~1.25V my head math says me 55 is higher than 35.75, and by quite a substantial amount here. So if the LM339 got fed this over voltage one can start to imagine what has happened inside it. I'm thinking VCC is being shorted and this is causing the fuse F108 to blow whenever the LM339 feels a bit sad. Changing the fuse and I measured the LM339 inputs and outputs, doesn't look stable, values jumping up and down.
Solution:So I guess the LM339 is damaged then. I replaced the broken LED and removed the suspect LM339. Now the computer thinks the fuses F104-F107 are good no matter what, only the fuse status LEDs give them away. I've ordered new LM339's but there should be no issue running without it I believe.
Connected the playfield and have used the machine for a few hours without F108 blowing, So I guess that's a success.
Follow-up:Since I was about to look into simulating circuits I figured this could be as good a thing as any to try. I wanted to see if this was a poor design and figure out what would have made it not fail like this. Looking around for what sim tool to use I found out that KiCad has Spice supplied with it. Unfortunately it took me way much more time than I would have liked to get it working but now I believe i get the basics and can simulate with KiCad
I was considering sharing my KiCad project in this part of the post and the simulation results but I'm going to spare you that part for now. Anyone interested to look at it and see the results and maybe spot my inevitable errors leave a reply.
From my findings the short story is that had the resistor next to the failed LED been 4.7k instead of 47k the voltage divider would never had gone above ~21V (instead of ~55V) with a supply of 66V. This is way inside the boundaries of the LM339 specs and a failed status LED with open loop only makes the LED go out, not the whole machine.
Design flaw?:So do you consider this a flaw in the design using this divider with 10k and 47k? That's what I am thinking. Or am I missing something here?
If you read all the way here, thank you so much for doing that. I hope my story was easy to read and I hope you think I am a sane person, or do you think I'm figuring this wrong?
Cheers!
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