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power supply

Posted: Thu Mar 03, 2005 9:53 am
by dinrough
I have 2 APC9258 KVM switches that both suffer from the same problem, the power supply has blown a zenner diode (NTE137A
# ZENER DIODE-6.2 VOLT, 1 WATT)I replaced it w/ the same part, but the power supply still does not work properly.
I took a known good power supply and checked the voltages, i get +12v, gnd, -5v on the outputs (ironically, this is a slightly different design where the zenner is not present, but still same output). On the repaired power supply, i get +7v, gnd, -5v.
I suspect a TL431 9B1 voltage regulator to be the culprit.
Can anyone give me some pointers on where to look. I am pretty much an newbe to 120v ac electronic circuits, but reasonably familiar in the 12v dc automotive area.

Re: power supply

Posted: Thu Mar 03, 2005 3:47 pm
by sofaspud
I'm not familiar with the specific equipment or regulator you have, but...
If that is a regulated +7 volts you are getting than the TL431 is likely to be good. Is the input voltage to the regulator of sufficient amplitude? It also seems possible that a support component(s) common to the zener and regulator has gone bad, causing the zener failure and the 5 volt output error.
You can check the regulator by lifting the input pin and applying a variable DC voltage while monitoring the output.

Re: power supply

Posted: Thu Mar 03, 2005 4:57 pm
by Enzo
Are these actually constructed with the NTE diode in them, or is that the sub you looked up? What is the original part that was blown? Your 7v looks suspiciously like one junction drop above the 6.2v.