[Amps] Alpha 87A/TenTec Omni VI Plus QSK Problem

[Paul Christensen]

> Has anyone used the W6RU wiring scheme and eliminated this spike in the
> Omni's SWR?

Yes, John.  That published scheme will not work correctly between the Omni
VI/VI+ and the Alpha 87A/89.  It's been twelve years since I tried the "Y"
adapter approach and it leads to truncation of the transmitted CW waveform.
I was part of the 1998 on-line discussion when the idea was first presented:

http://lists.contesting.com/archives//html/TenTec/1998-09/msg00363.html

W6RU was pushing for the "Y" adapter.  I countered with use of Ten Tec's "T"
voltage.  Many folks took to the "Y" adapter as the "easy way out" and
unfortunately, many of us have heard the resulting key clicks on the air.

The TX OUT and TX EN lines were never meant to be summed together, then
forwarded to an amp key line, even with diode isolation.  As I recall,
hot-switching and VSWR spike occurs on the trailing edge of the CW envelope
but that should be verified.  A single-input scope is fine, although precise
timing measurements require use of a dual-trace scope, triggered externally
or following one of the two scope channels.

In 1998, my fix solved the interfacing problem by sampling the Omni VI's "T"
voltage through a 10K resistor to a 2N5551 transistor with the collector
wired to the amp key line RCA connector.  The "T" voltage is a buss that
appears at multiple locations in the Omni VI.  I simply cut a small PCB
trace from the existing external amp frame relay and brought the collector
to that point for interfacing to the existing RCA panel connector.  The
modification is minimally invasive and results in excellent QSK timing with
the Alpha 87A/89.  The 2N5551 is rated for +140VDC, but there are even
better MOSFET switching devices available today that offer better
return-isolation to the rig in the event of transistor failure caused by a
high DC voltage key line.

There's just no substitute for purchasing a good, used oscilloscope when
trying to mix-'n-match transceivers with amplifiers across vendors.  We
think nothing about purchasing a $500 wattmeter, yet decline the purchase of
a $200 used scope.  Is it all the knobs and buttons that scares folks?
There's really little added value when going form $100 to $500 HF band
wattmeters.   Moreover, the scope can be used for many other measurements
when not being used as a station monitor.

Paul, W9AC