[AMPS] Output Pulse from ICOM 706

[Ian White, G3SEK]

Peter Chadwick wrote:
>
>Is this phenomena peculiar only to Icom rigs, or do Yaesu and Kenwood do it
>also?
>
>Funny, these rigs - slow attack on ALC, too fast a rise on the rising edge of
>the keying waveform...........somebody has reversed priorities!

They seem to have solved it on the IC-746. That's as far as I can see,
on a non-storage scope but with the brightness turned way up to make
even a very short spike visible.

It's certainly very visible on the FT-990. As W8JI and others have
noted, the design mistake was to use the ALC to vary the RF power. It
could have been done much more effectively by manually controlling the
gain in the TX IF stages (one day, I'll work out how to re-wire the RF
PWR pot on the '990 to make it do just that).

As K6NDV has also noted, a competent amplifier control circuit like the
ACOM will detect this spike and may signal an "overdrive" fault. To
avoid the same problem, I've reluctantly had to advise users of the
Triode Board and Tetrode Boards to slow down the response of the grid
current detector circuit slightly... reluctantly, because the source of
the problem is in the transceiver, not the amp.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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