Thanks to the moderators for allowing this cross-post
to two different reflectors. I know that lots of contest
operators still use the IC-781 radios. I first posted a
message to the Icom list regarding this problem in July
2005. There have been several suggestions over the last
few years, yet non of them has worked. I'm still searching
for a possible solution.
I am having a problem on CW when using IC-781 radios with
an Ameritron AL-1200 amplifier. The problem only occurs
with IC-781 radios, and it only occurs when running semi
break-in CW with the amp on-line. Two different IC-765
radios with the same AL-1200 amplifier do not cause the
symptom. The symptom only occurs with IC-781 radios, and
it occurs with two different IC-781's tested here. Following
is a description of the problem and the symptoms observed.
Barefoot, both radios reach full power instantly, whether using
MOX, semi break-in, or full break-in CW. A scope shows no power
spike on the leading break-in character, nor any other anomalies
that have plagued some IC-781 radios with ALC component problems.
When using the AL-1200 with the IC-781 in semi break-in mode
(I don't have full QSK capability on the amp) a very strange
thing happens. It takes up to a full second for the amp to
reach full output power. I can see the power slowly rise on
the wattmeter in the rig, as well as the wattmeter after the amp.
I can also see the envelope slowly rise on the scope (which is of
course, after the amp.) Here is a five-second video of the scope
waveform:
http://www.k4ro.net/k4ro/misc/IC781-AL1200.MPG
The first 3 key closures are with the amp PTT manually held down
before the radio is keyed (MOX.) The last two key closures are
running semi break-in. You can see that the waveform takes longer
to reach full amplitude when in semi break-in mode.
If I use a forced PTT closure, either by manually flipping the TX/RX
front panel switch, or by using a computer-toggled PTT key-ahead of
approximately 10 milliseconds with a contest keying program, I get
full output instantly, with no delayed power ramp-up. The radio
exhibits the slow rise output symptom only when driving the AL-1200
in semi break-in mode. I am not seeing any strange SWR problems on the
rig looking into the amp. Also as stated above, two different IC-765
radios work normally into the same amplifier.
The cause might well have to do with ALC action. I note that the ALC
meter on the IC-781 reaches a higher initial peak level at key down
when in semi break-in mode. When PTT is forced ahead of time (MOX)
the ALC does not peak as high, and full output is reached instantly.
The ALC component values are correct (some IC-781's have incorrect
or swapped components) and there is no evidence of a leading spike
on the scope that I can detect. I have tried manually controlling the
ALC directly via the jack on the radio, but the problem still occurs
in semi break-in mode, no matter where I set the ALC manually.
Thanks for any assistance with this perplexing problem.
73
-Kirk K4RO
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