[Packet] fpllow up to question on KPC-2

[Dr. Gerald N. Johnson]

On Tue, 2006-09-26 at 17:11 -0400, John Hagle wrote:
> 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.
> 
> 
Go back and check the polarity applied to that capacitor. Many a TNC has
fried the negative filter capacitor because the negative lead was
grounded when it should have been the positive. Sometimes the component
silkscreen was wrong too.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer