Failure of two Mac SMPS in quick succession - coincidence or not?

[mac_enthusiast]

Hi all,

Joining the forum to ask for some thoughts on a bit of a mystery that I've encountered:

TL;DR: my old Mac booted and ran for 30 minutes but then its SMPS failed, likely due to a blown PWM switch driving the control/standby power transformer. Bought another working second hand Mac, which failed instantly in exactly the same way. Trying to work out if this is a coincidence or whether anything else like the monitor could somehow be to blame.

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I decided to fire up my old Power Mac that's not been run for about 10-15 years but kept in dry conditions. Connected it to an Apple CRT monitor, powered from a separate power lead but plugged into the same multiway socket. The computer started up fine and ran for about 30 minutes until it suddenly switched off and wouldn't turn back on. No loud bang but the classic smell of recently-deceased electronic components...

A quick nose-based diagnosis confirmed the problem lay in the Delta Electronics DPS-150GB ATX power supply. Here is probably a good time to admit that, while a long-time electronics hobbyist, switched mode power supplies are not something I've had to work on before. Everything I've learned has been since I've started researching this issue.

After safely discharging the capacitors I took apart the supply and found some scorch marks around a ceramic capacitor near a PWM switch (TOP200YAI) that drives a small transformer, which I think handles the control/standby power. This appears to have shorted internally and, while there is still the correct voltage coming out of the bridge rectifier, a 1 ohm resistor between this and the transformer has also blown, probably due to the failure of the PWM switch.

My order of a replacement PWM switch - now obsolete - was going to take a while. In the meantime, I was lucky enough to pick up a working Power Mac in the local classifieds. I connected this up to the same monitor and - as I was plugging the power cable into the computer - there was an instantaneous bang, spark, and the same smell. The monitor was not powered on at this point. The replacement Mac's power supply (this one an Astec AA19360) had failed even more spectacularly in the same place with a blown PWM IC (AS2214F).

What I am trying to work out is: is this just a coincidence - I know that SMPS components will wear out over time - or could another factor be at play here, like the CRT monitor? I struggle to see how, given it is powered separately and is only connected to the Mac via the monitor cable.

Any thoughts anyone can provide would be greatly appreciated. Many thanks in advance and happy to provide more details, if helpful.


John

[tooki]

Well, for sure I wouldn’t connect that display to anything else!! While random coincidence can’t be ruled out, it’d be one hell of a coincidence. It’s far more likely that the display is somehow to blame.

Can you share exactly what Mac models and display it is?

[kripton2035]

somewhere I can't see how an external display with it's own power supply could blow up the power supply of the computer it is connected to with a low level voltage cable...
if anyone can explain ?

[mac_enthusiast]

Well, for sure I wouldn’t connect that display to anything else!! While random coincidence can’t be ruled out, it’d be one hell of a coincidence. It’s far more likely that the display is somehow to blame.

I completely agree! I won't be connecting it to anything else in a hurry...

Can you share exactly what Mac models and display it is?

Of course, it's a Power Macintosh 7200/90 (as is the replacement I bought) and the display is a Macintosh Color Display (model no. M1212). The monitor connection is the standard Apple DB-15.

[TheMG]

Degraded electrolytic capacitors would be my guess. I'd go through the whole thing with a capacitance/ESR meter, don't overlook the small ones either. I've seen a few SMPS fail catastrophically due to bad capacitors in the primary side control circuits.