Sean,
Most Class C VHF power amplifiers have DC voltage provided continuously,
and the TX keying is performed earlier in the chain at the exciter or
one of the drivers. What you describe is a very serious problem:
self-oscillation that is generating spurious signals at full power.
Since this oscillation is not what the PA was designed for, destruction
of the PA can result.
What event happened around the time this problem began? Were any
changes made to the repeater? Could the tower have been struck by
lightning? Has another station been placed into service nearby? If a
high-power paging station is nearby, and your PA does not have an
isolator/circulator on its output, your PA may be suffering from
intermodulation.
If no outside events coincide with the instant problem, then it is
likely that a component within your exciter or one of the driver stages
has failed. You might also look closely at the forward power control
circuitry for proper operation. Does the problem occur when the TX is
feeding a dummy load?
73, Eric Lemmon WB6FLY
maxjam_99 wrote:
>
I have a VHF Mitrek that has been split for repeater operation. It has
worked flawlessly until now. When the TX is unkeyed the transmitter
will stay keyed for approx 10 seconds. During this time it generates a
wide signal +-1 MHz centered on the TX frequency with full power out.
In 10 seconds or so it will unkey and cease tx'ing. If you key the TX
during the time it is generating this wide signal then it will TX a
proper clean signal, unkey, and start over.
>
> Any thoughts?
>
> Thanks...
>
> Sean / N�PBA
>
>
> Yahoo! Groups Links
>
>
>
>
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