[Amps] Sequencing an Alpha 87a

Dick Green wc1m at msn.com
Wed May 23 21:50:20 EDT 2007

I had some off-reflector discussion with Larry and found that his concern
isn't with the 87A T/R switching. After all, the 87A uses PIN diodes and
should be able to switch fast enough to not require a keying loop unless the
rig has serious bugs (besides, the 87A keying loop doesn't work very well
and the manual says don't use it.)

Larry is concerned about the mechanical relay that bypasses input RF during
an automatic band change. He's worried that the 7800 will see a high-SWR
transient as the relay is switching. Probably true, but I've never run into
a rig that had a problem with it.

Paul, does the 7800 really have delicate finals and/or a fold-back circuit
that can't protect them?

73, Dick WC1M

> -----Original Message-----
> From: Paul Christensen [mailto:w9ac at arrl.net]
> Sent: Tuesday, May 22, 2007 7:44 PM
> To: jsternmd; Amps Amps
> Subject: Re: [Amps] Sequencing an Alpha 87a
> > 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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