[Icom] Spike on transmit: Pro and L-7

Fri, 11 Oct 2002 09:50:55 -0700

Joe,

A possible explanation for the SWR spike is the switching time of the T/R
relay in the L7. Whilst the relay is switching the RF input from the receive
bypass to the amplifier input networks, the exciter is transmitting into an
open circuit. This will show up as a momentary SWR spike on the on-screen
display, which has a fast attack time. There is no risk to the Pro, as the
output reflectometer in the radio will fold back the drive. The T/R relay in
the amplifier may be sluggish, and require servicing or replacement.

There is a short delay in the Pro between keying-line closure and the
application of RF to the output, to minimise hot-switching. The switching
time of the L7 keying relay exceeds this delay. One workaround is to operate
non-break-in on CW (press the Transmit button at the beginning of each
"over".)

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Icom FAQ site:
http://www.qsl.net/icom/

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Joe Moraine
Sent: Friday, October 11, 2002 09:16
To: [email protected]
Subject: [Icom] Spike on transmit: Pro and L-7

I received the following message from a fellow ham in Finland who is
using an N8MIS amplifier interface between his IC-756Pro and Drake L-7
amplifier.  I tried the test on my identical setup and notice no SWR
spike at all on initial key up.  Does anyone have any ideas what might
be causing his problem that I can pass on to him?
Thanks for any help,
Joe, W5LX
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Hello, Joe.

Ref. our discussions 6 months ago I acquired N8MIS outboard relay and
it seems to work OK.

I wonder, if you still have IC-756 PRO + Drake L7 combination,
so that you could make a following test?

Only recently I found out (poor reading of the 756 PRO II manual!)
that I could have all the meter readings as bar-graphs. All concurrent
meter readings looked good except, when using the IC-756 and the L7
on CW. Pressing the key would momentarily bring the SWR reading up
to more than 3:1 and then quickly return to a normal value .

Do you see the same phenomenon? As if there would no full connection
in the circuit IC-756 - N8MIS - L7 before the transmission starts?

Funny thing is that I do not notice it on SSB when pressing the tangent
of the SM-20 microphone.

With the plain IC-756 PRO II on-line, no such action of the SWR reading
occurs on either mode. It goes directly to the normal SWR reading.

If you still have the 756 PRO and L7 and N8MIS relay in between with
no ALC line connected in service, could you make a similar test? And
what your bar graph meter shows?

What could be the explanation and possible cure? The L7 is unmodified
(except the fan changed) and works since 1981 here.

73 and have good week-end.
Tapani/OH2LU
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