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From: commgrp@silver.ucs.indiana.edu (BACS Data Communications Group)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Compaq Power Supply questions
Message-ID: <1990Dec21.153647.1251@bronze.ucs.indiana.edu>
Date: 21 Dec 90 15:36:47 GMT
Sender: news@bronze.ucs.indiana.edu (USENET News System)
Organization: Indiana University
Lines: 38

>Author: [Michael Zentner]
>I've got a Compaq PC which I'm about 90% sure has a blown power supply.
>3)  OK, it's a three parter.  This is the second time I've had the power
>supply go out in this PC.  Anyone know of similar problems?

Compaq "lovable luggables" are very well-made _except_ for their power 
supplies.  Power supplies are generally the least-reliable part of any 
electronic device; all the power passes through them, and they 
generate the most heat.  Also, the power supply is usually an 
afterthought and its design is assigned to the most junior engineer.  
First rule of troubleshooting:  Measure the power-supply voltages.

A friend who designs switching power-supplies for Collins Radio says 
the Compaq's power supply is the WORST design he has ever seen!

The most common problem in Compaq PC power supplies is cracked solder 
joints around a large power-transistor (in TO-3 package, mounted on 
heat sink):  The transistor is bolted to the circuit board through 
threaded studs which are soldered to the printed circuit.  The 
transistor gets quite hot in normal operation, and thermal cycling 
eventually cracks the solder joints.  Since the transistor case is 
used as a circuit-board jumper (really lousy design!), the broken 
connection causes power-supply failure.  The symptom is usually 
shutdown after a few minutes' warmup.

Late models have power supplies with larger heat-sinks, which are 
somewhat more reliable.

Repair:  Re-solder the transistor mounting-studs (with a large 
soldering iron) and solder an insulated wire jumper between them.

Removing the power supply is tricky--  You must unscrew a hex-head 
bolt near the bottom, which is hard to reach.  DO NOT neglect to 
tighten the bolt upon reinstallation.

--

Frank Reid     reid@ucs.indiana.edu