Arcade PSU repair - Page 1
Forum section: electronics / beginners

author: naujoks
author_rank: Regular Contributor
timestamp: December 13, 2022, 07:53:27 pm
content: I got PSU with an isolation transformer for a Hantarex monitor on my bench.I'd like to use this project to learn about kinds of things.The PSU has the isolation transfomer (from 220v to 150v, I'm in Germany) and a part to generate +5, -5, +12 and -12v.There is no model number or brand on the PSU.It was known to be under water for some time. There wasn't all that much corrosion to clear, luckily. The iso transfomer coils and PCB are largely in good condition.I haven't powered up the unit yet, instead I measure through many parts and found some issues, and I also have some questions.The first picture shows two parts which I think are tantalum capacitors. They give erratic results in my tester, variously showing around 1000nF to being a diode to "damaged part". Are these indeed tantalum caps? Is their value 1000nF?I noticed two bridge rectifiers on the board - why does it have two? Would the transformer coil possibily output 150v DC and not AC?There is a row of power transistors on a thin heat sink.The first one rests directly in the board. There is no mica insulator under it, the screw does not have an insulator either. The other two transistors both have mica insulators and insulaturs on the screws. There is conituity between the screws of every transistor.My question: what is the expected behaviour of these transistors? I can never figure out if there is supposed to be continuity between the back of the transisor/the screw, or not. How can I tell?Lastly, on the PSU there's only feeds for 220V AC, but I can't see any place where to connect a PE wire. I know the idea of an isolation transformer is to achieve galvanic separation between the two circuits, but can I not have PE connected to the transformer's primary side? And what about the low voltage part of the PSU?Very grateful for your input!

author: wasedadoc
author_rank: Super Contributor
timestamp: December 13, 2022, 08:28:38 pm
content: Yes those are tantalum capacitors. 1uF 35volt.The voltage regulators (not transistors), left to right, are +12, -12 and -5 volt.79xx regulators have the heatsink tab connected to the input pin. 78xx regulators have the heatsink tab connected to the common pin, ie 0 volts. That is why the 7812 doesn't need an insulator.

author: rstofer
author_rank: Super Contributor
timestamp: December 13, 2022, 08:40:13 pm
content: Quote from: naujoks on December 13, 2022, 07:53:27 pmIt looks to me like those are voltage regulators. Left to right: 7812 (+12V), 7912 (-12V), 7905 (-5V). I don't know where the +5V regulator is mounted but it will likely also have some kind of heatsink. Or, there could be several spread all over the motherboard.

author: naujoks
author_rank: Regular Contributor
timestamp: December 13, 2022, 08:50:19 pm
content: Thank you!How about PE?

author: rstofer
author_rank: Super Contributor
timestamp: December 13, 2022, 09:22:27 pm
content: Earth grounding is too complicated to get into without knowing what was done at the factory.Did the original mains cord include a ground? Well, it went somewhere. That's where you should make the connection as well.