Dead PSU

From: <(>
Date: Sat Jul 4 09:51:26 1998

> Date: Sat, 4 Jul 1998 11:19:32 -0700 (PDT)
> Reply-to:
> From: dave dameron <>
> To: "Discussion re-collecting of classic computers" <>
> Subject: Re: Dead PSU

> 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.
> >Suggestions?
> >
> 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.
> -Dave

Work on it without any power and close that PSU up before doing any
testings under load.

You need to remove that main transformer first so that will not
confuse your meter readings on diodes and other components.
The fluttering fan points to either decayed capacitors in primary
side or dud diode in your secondary side. Use your meter in ohm
in around 200K to 2000K mode on capacitors, as it charges up, you're
reading a voltage drop until it stops charging up then you read no
voltage. Longer on larger ones and fast on small ones. Judge these
speed of charging up by other good capacitors. Do that on
individually removed capacitors and put em back in right polarity!!

For any secondary caps, you want hi frenquency at 105C rating but you
can use normal grade at 105C on primary side because this is only
function to filter the mains power. For Diodes, they're high speed
recovery and at hi frequency types in large current ratings.

Don't worry about the complex circuit with thround resistors and
little capacitors, it's working becasuse of that fluttering fan and
it did power up when you commanded it to turn on by the keyboard.

Jason D.
Jason D.
Received on Sat Jul 04 1998 - 09:51:26 BST

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