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So I have a power supply for an old Molar Mac that repeatedly chirps and does not power on. One issue is that the thing looks like it is in perfect condition (no bloated caps or anything). I cannot try to power it on without load because it has to be connected to the analog board (as far as i can tell, i need to look at it a little more) to even try to power on. I have heard if it is a power supply issue (not a connected device shorted/pulling too much) it is usually the caps or diodes. I am a noob, so how would I go about testing something like this?

Thanks!

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  • \$\begingroup\$ Look for a current sensing resistor or fuse and measure it the look for DC OK or OVP protection signal. Monitor main Voltage on power-up and look for a spike or any sign of voltage on the ACDC converter. It could be a bad choke, diode ,transistor etc \$\endgroup\$
    – D.A.S.
    Commented Mar 28, 2021 at 2:30
  • \$\begingroup\$ Could you share a picture if how the power supply looks like? Since it needs to be connected to the machine to turn on, are you sure its the power supply and not the machine? \$\endgroup\$
    – Christianidis Vasilis
    Commented Mar 28, 2021 at 13:38

1 Answer 1

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Look for a current sensing resistor or fuse and measure it the look for DC OK or OVP protection signal. Monitor main Voltage on power-up and look for a spike or any sign of voltage on the ACDC converter. It could be a bad choke, diode ,transistor etc.

I recall Apple big screens were notorious for bad solder joints on heavy flyback transformers causing PTH via fatigue. (invisible microcracks) The chirp tells me the flyback switcher was working then failed due to overvoltage. (> 25kV?) Then retries after settling time.

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  • \$\begingroup\$ Would reflowing the direct connections help or is this cracks in the traces? \$\endgroup\$
    – ISAAC BUCKINGHAM
    Commented Mar 28, 2021 at 3:11
  • \$\begingroup\$ No It's an annular ring in a large circular plated thru hole for heavy parts like transformer \$\endgroup\$
    – D.A.S.
    Commented Mar 28, 2021 at 3:51
  • \$\begingroup\$ What mac is it specifically? With electrolytics, they don’t need to be bloated to be faulty. Chirping could be caused by a number of problems. Use a multimeter to measure each of the rails when it is chirping. Whilst they won’t reach their specified voltage they should register something unless the short is on that circuit. \$\endgroup\$
    – Kartman
    Commented Mar 28, 2021 at 6:10
  • \$\begingroup\$ If it is a first generation Mac c1984, look at caps C39 and C43. \$\endgroup\$
    – Kartman
    Commented Mar 28, 2021 at 6:19
  • \$\begingroup\$ @kartman all the rails that i have checked are at ~0volts... not excalty that but somewhere around there, nothing above .1 though \$\endgroup\$
    – ISAAC BUCKINGHAM
    Commented Mar 28, 2021 at 13:57

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