[HECnet] My VAX lost its mind!

[Ian McLaughlin]

The best tool for repairing switching supplies (or monitors, for that matter) - an ESR meter.   Out of all of the test gear I've purchased over the years, this one probably gets the most use!   They use such a low voltage, you can usually use them in-circuit without having to desolder the component off the board before testing.   Only takes a few minutes to go through a power supply and identify the suspect electrolytics.

Ian

On Feb 5, 2014, at 12:05 PM, Dave McGuire <mcguire at neurotica.com> wrote:

On 02/05/2014 04:17 AM, Mark Wickens wrote:
Fans would click for a second and then kick right back off.   It
stopped doing that until I removed all the drives.   Plugged 'em back
in after opening the PSU to look for obvious faults and found none. 
Everything is working fine now...Strange.

I had a hard disk taken out by a power spike. I also had firmware
corruption that required reflashing the EEPROM, but I don't remember if
that was related to the PSU. They draw 30 watts when switched 'off' so
it's definitely worth turning them off when not in use.

Twitching fans might indicate over-current protection kicking in.

If it's a repetitive twitching, it's probably the error amplifier or
the voltage divider feeding it.   (those will be difficult to test
individually unless they're discretes, of course)

One other thing I've seen recently with older switching power supplies
is the output capacitors' ESR going up due to age, causing their time
constant to exceed that of the regulation loop...creating, you guessed
it, an oscillator.

I know one thing however - switched mode PSU experts are few and far
between!

This is painfully true.   I *design* the damn things and I shy away
from working on them most of the time.   Direct AC-driven ones are RIGHT
OUT in my book.

                            -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA


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