[Icom] 765 output spike

Wed, 5 Jun 2002 16:35:52 -0400

Thanks for your thoughtful reply, Adam.  At present, I'm physically unable
to look into the ALC circuit as you suggest, but I will do so as soon as
possible.  I believe that the ALC circuit was involved in other transceivers
with this problem, which has been well documented in the past.  Responses
from others indicate that I must have described the problem in such a way
that I was assumed to be rather ignorant.  To respond to them briefly: One
does not need an oscilloscope to observe the presence of a signal that
should not be there, but clearly is.  With mic gain set at zero in SSB mode
(in fact the mic is turned off), THERE SHOULD BE NO RF OUTPUT WHEN THE PTT
LINE IS KEYED.  Two peak reading wattmeters in the line to the dummy load
show the output at around 50 watts for a brief period.  While it may be
interesting to observe the waveform of this signal, of more interest is why
it's present in the first place.  While one should not totally trust meter
readings - particularly involving transient events, in 43 years of
electronics work I've yet to see a meter indicate an event without an input.
And - the problem I described made no mention of CW.  Since the 765 RF power
control can't be used to reduce the output to zero, one indeed would not
necessarily be able to judge a spike on the CW waveform without a scope.

K8AC

----- Original Message -----
From: "Adam Farson" <[email protected]>
To: <[email protected]>
Sent: Wednesday, June 05, 2002 1:39 PM
Subject: RE: [Icom] 765 output spike

> Hi Floyd,
>
> This initial output spike is caused by an excessively long ALC attack
time; a
> similar problem exists in the IC-781.
>
> From a look at the schematic, I believe that you may be able to decrease
the ALC
> attack (and decay) times by reducing the value of C92 (now 1 uF). You
could also
> increase the value of R149 (now 100 ohms). Refer to IC-765 Service manual,
p.
> 9-1, MAIN Unit.
>
> If you do not have a service manual, I can scan the relevant pages and
e-mail
> them to you.
>
> Best 73,
> Adam, VA7OJ/AB4OJ
> North Vancouver, BC, Canada
> http://www.qsl.net/ab4oj/
> Note new e-mail address:
> mailto:[email protected]
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]On
> Behalf Of Floyd Sense
> Sent: Wednesday, June 05, 2002 06:44
> To: [email protected]
> Subject: [Icom] 765 output spike
>
>
> I've seen this problem documented for other HF rigs, but have been unable
to
> locate anything for the 765.  Has anyone found a cure for this ?
>
> Problem: In SSB mode with the power control at maximum, keying the PTT
> switch results in a short output spike of around 50 watts as seen on a
peak
> reading wattmeter.  The 765 power meter shows only about 10 watts owing to
> the short duration of the spike.  This occurs with no audio input, mic
gain
> at full CCW.  Reduction of the spike appears to be relatively linear as
the
> power control position is moved CCW.
>
> I've read through the mods for the 765 and found nothing describing this
> condition, although there is one item that addresses  improved T/R
> switching.  Any ideas greatly appreciated.
>
> K8AC
>
>
>
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