> Recently there have been reports of a number of PA failures in recent
> manufactures of the ICOM IC-7800. Reasons are unclear but one concern is
> poor sequencing when the IC-7800 may be keying an amp and the IC-7800.
Timing is always a concern between any amp and amplifier. However, I key my
7800 from its MOSFET switching transistor into an old Alpha 70V with a
vacuum relay speed-up circuit, and an Alpha 77Dx. Neither of these amps
uses extraordinary key line switching methods and my 7800 keys them both
without hot-switching to more than 60 WPM. The 7800 keys my Alpha 86 to
more than 100 WPM (the fastest I can generate) and the leading/trailing
waveform remains perfectly intact. I am using no special sequencing
circuit, nor IN/OUT handshaking lines. The 7800 key line connects directly
to the Alpha "Relay In" with one wire pair. Interestingly, beyond 70 WPM,
the IC-7800 pulse-stretches the amp key line slighty to ensure that the
majority of amp T/R circuits can keep up with it. This has the effect of
disabling super fast QSK between keyed elements, but at 70 WPM, I can live
with it. <g>
To monitor my keying, I modified a microHam CW keyer and added a switching
transistor to produce a current-limited, positive voltage at the external
triggering input of my Tektronix SC504 scope. Triggering the scope this
way allows me to keep from having to chase the waveform around by adjusting
the scope's time base. An adjustable Bird RF sampler feeds one channel of
the scope. The second channel is used to monitor receive CW keying
envelopes.
That said, I would probably look for other causes, if in fact there's really
an issue at all with the reliability of the 7800 PA.
Paul, W9AC
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