Dead PSU

From: dave dameron <>
Date: Sat Jul 4 13:19:32 1998

At 08:23 AM 7/4/98 PDT, you wrote:
>This is not quite on topic. I want to know if it's practical to repair a
>Mac Quadra's PSU. The problem is that it runs fine, when not plugged
>into anything. When some given amount of stuff is connected (just the
>motherboard does it), the fan starts oscillationg back and forth. When I
>plug in speakers they make a beating sound. I counted it to be about 120
>HZ. This seems to me like a single component, possibly a diode.
Is it a switching PSU? I would guess so. Many do not regulate well under no
load, as there is a minimum current and switching on time required.

It may be also that a main high-voltage filter capacitor is bad, feeding
more raw AC into the switching stage. The 120Hz points to this. Under light
loads, the output low voltage
filter capacitors are enough. Under heavier loads, the line AC ripple is seen
in the outputs which now cannot hold the voltages.
Use caution on the high voltage side.
Received on Sat Jul 04 1998 - 13:19:32 BST

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