Jim Reid wrote:
>Ok, is it possible to hot switch the amp when
>using the SSB mode? I don't think so, unless you
>"switch" right in the middle of a loud vowel sound?
>Correct?
It depends on the combination of the transceiver, the amp and the voice.
Basically it's a race against time between the transceiver and the amp.
The amp gets its PTT from the transceiver, and it then has to change the
relays over before the RF arrives.
Rich's solution is to make the relays switch much faster, and that
usually buys you enough time. Rich is correct to say that even if you
only use semi-breakin, something still needs to be done because you
always get arcing on the first key-down. The only difference for full
(between-the-dots) breakin is that the arcing happens much more often.
Rich uses fast vacuum relays, and speeds them up even more by forcing
current through them from a high-voltage supply. There's also an
excellent relay speed-up circuit by K1KP that runs off the normal supply
rail and achieves much the same as the high-voltage method. There's a
description on W6XX's site, and also in my RadCom column a few months
ago.
If your setup is already nearly OK, then either of these circuits can
move your amp well ahead in the race against the transceiver, and often
can completely cure the relay arcing.
However, the amp cannot even start to switch until it gets the PTT
signal from the transceiver. If the transceiver wastes too much valuable
time in getting the PTT signal out to the amp, there will be problems
that even the fastest amp can't handle. If the PTT output involves a
relay inside the transceiver, much time is wasted and then it's
impossible to give the amp enough time to switch... in extreme cases the
RF output has started before the PTT relay closes, so your amp doesn't
have a chance!
If the transceiver has a solid-state PTT switching output, that's the
fast one, so *use it*. You may also need to build a transistor interface
into the amp, so that it doesn't overload the small switching
transistors that most rig manufacturers seem to use. This should be part
of any amplifier upgrade.
Many transceivers with a slow PTT output can be easily modified to pick
up the key and/or mic contact closure, and send it instantly to the
rear-panel PTT output. A new solid-state switch (open collector) is
connected directly in parallel with the existing one, so that either
will command the amp.
To repeat: there must be *no* mechanical relays - and no other time
delays - in the entire path between your morse key and the circuit that
switches the RF relays in your amp.
But VOX is a problem, because there's no way to speed-up the reaction of
the VOX circuit itself. If you're using a DVK for contesting, with lots
of compression, you may be hitting the amp with almost full power in the
first few milliseconds - which is almost as bad as the CW situation. The
only simple answer I can think of is to use a contest logging program
with a built-in DVK facility and a hard-wired PTT output - the program
will have a user-configurable delay between PTT and the start of audio
output (same for CW keying also). FSK data can be bad too, but again
most PC software can key the rig and then delay the start of the AFSK
output.
[Hmmm... I seem to be writing the outline for a talk that I'm scheduled
to give next year! If it's useful, carry on reading...]
There are other solutions too. A good solution for CW-only operators is
to use an amp that allows you to plug the key directly into it. The amp
has a sequenced key-out line that does to the transceiver's normal
morse-key input. Then the amp has full control, and doesn't key the
transceiver until it's good and ready. But that doesn't solve any of the
problems on phone or other modes.
Another solution is to use a transceiver that has a separate control
input, so you can press the key or the PTT and key the amp, but the
transceiver won't actually deliver RF until the control line goes low.
The transceiver keys the amp in the normal way, but there is no RF drive
until the amp sends a 'ready' signal back through this control line.
Recent Yaesu rigs have this on the band-data output, mainly for use with
their own solid-state amp. But then my FT-990 messes everything up, by
delaying the PTT output by several milliseconds, and then delaying the
RF by a *further* 10 milliseconds from the time the control line goes
low. In total, that's enough to ruin it for fast QSK - with
semi-breakin, the first dot will be cut in half; and with full-breakin,
*every* dot will be cut in half. (Question: are the FT1000 series any
better?)
[Well, that's been useful for me, to get my thoughts together for the
talk. I hope it's been useful for AMPs too.]
--
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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