Dan wrote:
>It is very sad that many linear internal bypass relays use voltage and
>current which overload the puny relay contacts in the transceivers,
>particularly when there is a capacitor across the relay line. It is
>very wise to use a power transister to pull the linear relay to gnd
>operated by the transceiver. Harbach Electronics sells a suitable small
>board for about $20 with an optical coupler and pwr xstr to do the job.
>I described a circuit to do this earlier today in an amps post and
>Measures has a similar circuit on his web page.
Agreed... but so far we have been focusing on switching capacity. The
other issue is the effect on T/R timing and RF arcing in the amp's
antenna relay. The open-frame relays on most older amps are too slow to
change over before the RF arrives from the transceiver, so you get RF
arcing.
It's a race against time, to get the relays contacts changed over and
settled in place before the RF arrives. You're counting in
milliseconds... and every millisecond counts!
Inside the transceiver, you *must* get the key/PTT signal out to the amp
as fast as possible, so don't use the internal relay option - it wastes
valuable milliseconds. If the amp requires more switching capacity than
the transceiver's fast solid-state switch can handle, then the problem
is with the amp.
On my website there's a PDF of an interface circuit for the amp that
will solve the switching problem *and* speed up the existing antenna
relay:
http://www.ifwtech.co.uk/g3sek/in-prac/best-of.htm#speedup
I've used that circuit with a number of open-frame antenna relays.
Although it can't promise a complete cure, because those relays are very
slow, it really does help to prevent RF arcing.
The ultimate cure is to use vacuum relays, which will change over in a
very few milliseconds in either Rich's circuit or mine. However, the
original questioner wanted to be ready for the contest this weekend, and
there might not be time to make such radical changes.
On the other hand, the little interface circuit should be do-able - if
you can bring the 12/24V relay supply out the back of the amp, it
doesn't even need to go inside.
The FT-990 (the subject of the original question) has a fixed delay of
10ms between the fastest available PTT signal and the start of the RF.
That means you *have* to solve the switching problem at the amp end of
the PTT link. More modern rigs such as the MP have a menu-selectable
delay (a setting of 0 actually means a 10ms minimum delay).
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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