Re: power supplies

[Rubywand <rubywand@swbell.net>]

tb writes ...
> 
> how can you tell if your power supply is bad?
 ....

     Aside from a completely 'dead' machine, one quick indicator of a defunct
power supply may the absence of some sound-- like you hear no high-pitched
whine-- when power is turned ON.

     Unusual sounds can indicate a short (you may hear clicks) or no-load from
a broken or bad connection (you may hear clicks and/or squealing). Before
declaring a power supply dead it's a good idea to check for things like a poor
power supply connector connection, a blown ON/OFF switch, and an AC line cord
which has developed a break.


     One approach to checking is to measure voltages in the computer or
whatever is supposed to be powered by the power supply.

     For example, in an Apple IIe, you could turn ON the computer and check
voltages across a few 74xxxx series IC's-- like a 74LS02, 74LS139, etc.. Your
Voltmeter's Positive lead would touch pin 14 (on 14-pin 74xxxx IC's) and
Negative would go to pin 7 (again, on 14-pin 74xxxx IC's). The reading should
be 5 Volts DC.

     If the reading is below 4.8V (and your Voltmeter reads accurately), you
are into 'problem' territory. Possibly, the power supply is being overloaded by
Slot cards; or, it may have developed a defect.

     A different approach is to disconnect the power supply from whatever it's
powering. Connect or clip on a load-- like, say, a 20 watt 3 Ohm resistor--
from +5V to GND and measure Voltage across where the clips connect to the power
supply. (3 Ohms connected with good heavy clips will draw about 1.7 Amps.)  If
the power supply is a typical Apple II power supply it should have no problem
delivering +5V into this sort of load.

Note: In general, you want to measure output as close to the power supply as
possible. Clips often introduce resistance and so measuring across the load
resistor could indicate a falsely low output.

Note: Switching power supplies should not be turned ON with no load. Something
like at least 10% of rated load should be connected across the the high-current
output (usually +5V) whatever other outputs you are checking. For an Apple II
power supply, this would be something like a 20 watt 10 Ohm resistor. For
larger switching power supplies such as used in a PC, you may need to place a
load across other outputs, too. (Usually, for a computer power supply, this
would be the +12V output.)

     A really good load test needs to pull current reasonably close to the
rated output of the power supply for the Voltage leads you are checking. Like,
for a power supply rated at 4 Amps on the +5V output, you would want something
like a 1.5 Ohm load connected from the +5V output to GND.



Rubywand