Repair of Power Macintosh 5500 / 275 Black

[tonata]

I am trying to repair a Power Macintosh 5500 / 275 Black. The main problem is that nothing shows on the screen. There is no HDD, but I thought that something should still show up. The screen never blinks or gives any sign of life. There was a chime before and the floppy drive seems to work. It did not like any of the 1.44 System 7.6 images I tried. The CD does not seem to be powered on (looks connected though).

Next I found a swollen capacitor on the power supply. No idea if that was the real problem. It was 25V 220 mF. I changed it with the a good desoldering pump and solder iron.

And it all got worse. The fan did not start. That was the first sign something was going wrong. The weird noises from before kept going though. Next I inserted the motherboard drawer and ... I got electroshocked. So this might have damaged the motherboard? The power supply seems to be shutting itself down. It does not provide any voltages to anything including the fan. It makes some shutting down noise in a few seconds I think after power on.

Any help will be appreciated. Yeah, I should be wearing gloves, discharge the CRT , etc ... I was counting on the fact that the metal part that I was touching (while the computer is off) is grounded after all. And I was isolated well from the floor as well.

I do not know what to do next. I double checked the polarity of the new capacitor and it is OK. Now I am trying to test the power supply while disconnected from the computer, but maybe it needs a load? No voltages are produced. No home circuit breaker including the differential circuit breaker are ever activated.

 

[GiGaBiTe]

Pretty much every Mac from the G5 era and older by now has failing or failed capacitors. You'll need to completely recap the entire machine before you do any further troubleshooting. A capacitor does not have to look physically bad to be very bad, just replace everything. When removing the old capacitors, check for wetness under the caps. If you find any, this is capacitor electrolyte that needs to be cleaned off the board. It's both corrosive and conductive, and can cause weird shorts and erratic behavior. If you also find any yellow glue on components, remove that as well. That glue has a habit of becoming conductive as it degrades and absorbs moisture from the air.

If the power supply is not starting up, and making a ticking noise, that's definitely a sign of bad capacitors. It can also be other things, but it's usually bad capacitors in Macs.

 

[NeXT]

The few photos I can find seem to imply the PSU is independent from the video analog board, so I'll assume that initially you had an "ok" PSU (it's gonna have cap issues but still it WAS working, so lets rule them out for the moment) but you were running blind because the analog board was shutting down. Though running blind, a chime and the fact the floppy was responsive meant the logic board was working and the only variable between then and now is when the power supply was reinstalled either you have connectors mixed up or something is now shorting out. Unfortunately you are the variable at this point.