I have a low-play Sys 11 sample game that spent most of its life in storage. The power supply is original but looks brand new without any sign of heat damage or any other wear. Unfortunately however, the high-voltage section is broken --- F1 (1/4A slo-blo) keeps blowing. According to pinwiki, there are four likely reasons for this symptom: 1) Bad Capacitor at C1 and C3; 2) Shorted Display; 3) Shorted UDN7180; 4) Shorted UDN6118.

I know it's not 2 because I have tested the glass in another game. Given the fact that the game has been mainly in storage, with apparently very few hours of operation, which of the other three reasons do you consider the most likely? Also, can I test the two UDN chips for shorting without desoldering?

Thank you all for the info.

The game is a Bad Cats. Pic of the power supply attached. (More to show off just how nice it looks than any other reason ) The tests I carried out before posting: Zero volts on both + and - 100 at J2 (lower left corner, no display connected) with a broken fuse at F1. Replaced the fuse and same: zero volts and fuse broken again. And I should also mention that the game plays just fine with LED displays, which means that the problem is only with the high-voltage section.

I re-read the information on pinwiki and I think that I was confused before. There are no UDN chips on the power supply, so there's no need to worry about shorts with these chips. From the original list this leaves only bad C1 and C3. I desoldered C1. Unfortunately my multimeter does not go high enough (up to 20uF). However, I can charge the cap slowly with the multimeter set to 200kOhm and the resistance slowly increases as expected. Once charged it holds a voltage of 0.2V. While I do not know whether the cap has full capacitance it looks pristine and behaves like a cap.

This is where I had to stop. My next steps will be to test the transistors Q1-Q4 on the power supply board. Probably this weekend.

powersupply (resized).jpg

Thank you!! That explains the higher capacitance than on the schematic, which should have raised a flag. But I don't think I would have noticed the reversed polarity. (I desoldered C3 and not C1.) Do you mind telling me what damage this problem has likely caused on the board?

[Makes me feel better not to be the first one to have made changes on this beautiful board.]

I re-installed C3 and turned around C1. I also replaced Z2 and Z4 with 91V Zeners (1N4763A) while I was at it. Installed a new fuse at F1 and got -92V on pin1 (good) and initially +88V on pin3 (too low). When I connected the display, it worked but weakly and then dropped out. Re-measuring the voltages got me -92V on pin1 and 84V on pin3.

I am guessing that the low positive voltage may be caused by a damaged C1. I will buy a replacement and report back.

Thank you again, @dumbass!