At 06:47 PM 12/6/04, you wrote:
>Been running an 850 here for 19 years with no trouble frim
>RFI. Is this a new install? Anything else recently changed
>with your repeater when it started?
>
>The electrolytic caps will dry out on the boards. Seems like
>I heard this could cause problems, although we have not
>changed caps on our unit.
The 850 design uses a 15khz signal from the master
microprocessor clock, feeds it into an audio power amp (an
LM386 if I remember correctly) and then rectifies that to
make the negative power supply for the audio stages. When
the caps dry out the negative buss gets 15khz power supply
ripple on it.
>I'm sure the day is fast approaching though....
Yep. Use a scope to check the negative supply for 15khz
ripple. If your TX has sufficient audio filtering you may have
the problem but just don't see it.
Every 850 gets it sooner or later - the caps mostly came
from the same vendor and in the same time period.
The worst case scenario is that the ripple rolls right
into the TX and creates spurs every 15khz up and
down a chunk of the band...
My solution is to simply shotgun the negative
supply - I replace every electrolytic, and once that
is done I think seriously about doing everything in
the positive supply as well.
Mike WA6ILQ
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