-=> Roy J. Tellason said to Greg Mayman
 -=> about "POWER SUPPLY" on 11-05-03  12:06.....

 RJT> I'm thinking that this is a startup problem of some sort.  I keep
 RJT> reading posts about higher-value resistors that go bad,  stuff like
 RJT> that... 

Yes, it could be a start up problem.

I went back and read that Silicon Chip article in more detail. It
is concerned with modifying an AT power supply for a single high
current +13.5v output, but the description of the operation is
mainly for an unmodified power supply.

First hint: scan the track side of the board and print it out as
large as possibe -- or photocopy it and enlarge it to the
maximum. Then draw in and label the components. This will help in
tracing the circuit.

Second hint is as follows:

Pre-test Before Modification
============================

Some more recent PC power supplies derive their control circuit
power from the +12v output. This feature allows the control
circuit of these supplies to be powered and checked without
connection to the 240vAC mains.

Connect a 33 ohm 5w resistor between the +5v output (red) and
ground (black) and a 47 ohm 5w resistor between the +12v output
(yellow) and the 5v output (red). This will maintain an
approximate 5v to 12v ratio between the respective outputs.

A variable DC power supply (8 to 14v range) is connected across
the +12v output and ground. Check for power at pin 12 of the
TL494. It should be about 0.6v less than the supplied voltage.

In the absence of power a jumper needs to be placed between pin
12 and the +12v line.

An oscilloscope is used to view the waveforms and operation of
the TL494 and LM339 as the applied voltage is slowly raised from
8v to 14v (no higher than 14v). A 30KHz (or higher) sawtooth
waveform should be present at pin 5 and square waves should be
visible on the unbgrounded output pins 8 and 11 (or 9 and 10).

[MY COMMENT: The block diagram shows that these pins are
 connected to the two output transistors: 8 = Q1 collector, 9 =
 Q1 emitter, 10 is Q2 emitter, and 11 = Q2 collector]

These oscillations should stop as the voltage is raised to a
level equivalent to the design output. The waveform should
reappear as the voltage is reduced. If the over-voltage circuit
has been activated, the waveforms will not reappear until the
circuit is reset by removeing the power.

Careful adjustment of the power supply is necessary to
demonstrate these two very similar voltage levels.

If no oscillations are observed, pin 4 of the TL494 [block
diagram shows this is associated with "dead time control"] will
need to be isolated and connected directly to ground (follow the
track from pin 4, desolder and lift one leg of each component
connected to this track, the track can then be grounded by a
jumper wire). The overvoltage protection circuit will now be
inoperative.

Reconnect the power and a sawtooth waveform should now be visible
at pin 5 and square waves at pins 8 and 11 (or 9 and 10). Do not
exceed 14v in an attempt to demonstrate the over-voltage
protection mode -- you have just disabled this circuit!

.....

If your PC power supply cannot be tested with an external DC
supply, you can still modify it but it will be far more difficult
(and dangerous) to do any initial testing.

[ Why? It should be possible with a second DC power supply for ]
[ the TL494... but that's just a guess.                        ]

Further Reading: Making use of an old PC power supply, Silicon
Chip, Dec 1998.
===================================================================

There is quite a bit more about testing the circuit with the
mains input disconnected, but the above should get you started.

If you want the whole article, I suggest you contact Silicon Chip
magazine and see if you can get a copy of the October 2003 issue
with this article.

Their telephone number from outside Australia is 061-2-9979-5644.
Normal office hours would be 2300 to 0600 UTC during the present
daylight saving.

Price of the magazine is Aus$7.25. I don't know what the postage
would be. They do take Visa and probably most major cards.

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... Experience varies directly with equipment ruined.
___ Blue Wave/386 v2.30

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