iMac G3 does not power on

[feltel]

I got two early 2000 iMac G3. Both of them are in different need of repair. One powers up perfectly but has a cracked case and a scratched screen. The other one looks way better but does not power on. I could make one good machine out of the two but rather would try to repair the non-working machine. The machine does not power on. Only a light scratching noise from the speakers while I press the power button. Nothing else, no screen output, no chime.

According to https://de.ifixit.com/Antworten/Ansehen/173281/its+not+powering+on I measured voltages on the logic board. On J9 I get 5V as it should be. On the power connector to the PSU I get nothing (as the machine does not power on). The PSU looks okay regarding capacitors. No visible leakage. I did not disassemled the machine further to take a look at the analog board. Which is more likely to cause such issues?

Another side question: Which of the two machines is the "faster" one? Both have a 400 MHz CPU, but the difference is probably the graphics card. Or is this just different labeling done in different factories.

"IN/400/64/10/CD/128P/56K" vs. "IMAC DV AB 400/64/10G/RAGE128PRO4XL/512K"

 

[feltel]

I now freed the analog board in the hopes to spot an bulging or leaking cap. But nothing. All looks exceptional clean. The machine must have been died right after warranty expired. Also the CRT and the anode cap are nearly free of the usual black sud.



Are there any caps to look after? There are a ton on them on the board.

 

[MaCJaX]

I had same problem and just replaced the logic board. I never could figure it out without a schematic and pulling these things apart is a royal pain.

 

[feltel]

I'm not sure if the problem is the logic board. I disassembled the remaining bits further. On the back side of the analog board I spotted some suspicious areas. Right under the flyback and under the large transformer it looks like cap leakage. Maybe this is only flux, but why is this not on the whole board. There are areas where caps are installed which look totally fine. I repaired some Mac PSUs which were prone to leaky caps. When caps leak they do this on both sides of the board. This time it is only on the under side. Strange. I'll desolder some caps from these areas and check them with an component tester.

 

[bzyzny]

If you haven't already, use your multimeter to test your fuse has continuity, and test the pins of the mosfets, bridge rectifier etc for shorts. Look for any spots on the PCB or components that are darkened from excessive heat. Check for loose and/or cracked solder joints.

Hard to tell for sure from the pictures, but I think that looks like flux residue, although an unusually large amount. My guess is someone reflowed the flyback transformer pins, or maybe even replaced it. If you can clean it off with alcohol and and there's no stains, corrosion, or damage left behind then it was flux. You are right that capacitor leakage can make its way to the underside but the majority would be on the top normally.

I'm also currently working on a G3 iMac, hopefully we both manage to fix them I found a shorted BJT on mine, not sure yet if that was the only problem though. Good luck.

 

[sangelov]

hi all,

my first post. lots of good info around that helped me revive many macs and i really enjoyed the process -- keep up the great spirit!

joining the "imac g3 no power" club -- mine is iMac G3/350, slot loading (Summer 2000 - Indigo).
when i press the power-on button, the led lights and stays green (DS11 led on logic board stays green as well) , i can hear hissing like high voltage/slight brum (as far as i can tell from the video neck board area?) but not the usual high voltage sound when a working CRT would initialise. when the cd and disk were connected i can hear as if the disk is trying to start spinning for a second but gives up.

i've gone trough the service guide troubleshooting steps:
-- battery was dead so i replaced with a new one
-- performed the PMU reset steps
-- J9 trickle power is +5.03V (cable in, powered off)
-- DCD voltage at C10: -1.14V (when powered off), -18.57V (when powered on)
-- J7 voltage: pin2 +10.01V (expected +12V), pin 4 +3.21V (expected +5V), pin 14 +2.24V (expected +3.3V)
-- disk molex connector: red +3.65V, yellow +10.54V

across the board (besides J9 trickle power) all voltages seem lower than expected. is it correct to assume that if voltages are already low on the J7 connector that connects the down converter board to the logic board then the down converter board should be ok?

i've red many post describing the known issues with the flyback transformer but i am not really sure how to continue troubleshooting narrowing down the faulty area more precisely. power/analog board would not start if the logic board is not connected (wanted to measure voltage on the connectors), power/analog board and flyback look quite clean, no leaks, no cracks, no visible darkened components, etc.

is there a way to measure power fed to the video neck board or some other troubleshooting measurements on the power/analog board itself?


thanks & best regards,
Stoyan