[Packet] fpllow up to question on KPC-2

[kd7nm]

This is a fairly common problem where inexpensive capacitors
are used as part of a DC/DC converter to generate a negative
power supply from a positive power supply, as for RS-232
interfaces, and some modem chips.  Unless the capacitors
have a low effective series resistance (ESR), they heat
internally and dry out.  And I'm sure it was exacerbated by
the heating from the 7905 VR.  That, and a KPC-2 isn't a
spring chicken any more!

73, Bob, KD7NM



----- Original Message Follows -----
From: John Hagle <jhagle at madbbs.com>
To: packet at mailman.qth.net
Subject: [Packet] fpllow up to question on KPC-2
Date: Tue, 26 Sep 2006 17:11:12 -0400

>I found the trouble. It was a bad electrolytic capacitor.
>C3, a  4.7mfd at 35VDC.
>
>It showed signs of a little bit of overheating. It was
>pressed up  against an 7905 voltage regulator. That may
>have contributed to its  failure. That cap acts as a ripple
>filter on the negative 12.5volt line  to the 7905 regulator
>input. That voltage was down to around 10.45 VDC,  and the
>result was that the KPC-2 didn't work as described before.
>
>It was a little frustrating because I mislead myself into
>thinking the  regulated circuits were okay because the
>negative 5 volts was right on  the money. But with the
>regulator input filtering being bad, the raw  -12.5volts
>was way off spec.
>
>73
>
>de N2JH
>
>ps: I hope this may help someone else someday.
>
>
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