[AMPS] Hot Switching

[Ian White, G3SEK]

Dick Green wrote:
>
>> That's a good point, Dick - did you measure the 7ms delay in the FT-990
>> using the transistor remote switching option, or the relay option?
>
>I measured the timing of the transistor remote switching option. I didn't
>see any provision in the 990 schematic for keying the amp on key closure
>when the relay option is selected -- basically the switch just transfers the
>signal from the transistor to the relay. So, I have to assume there's a 7ms
>delay for the relay, too. I didn't think it was worth testing because the
>relay isn't rated for QSK and it's noisy, so there was no way I wanted to
>use it.
>
OK, so it really is a delay imposed by the rig's T/R controller. 

What we're trying to do, then, is to bypass the controller's 7ms delay
on key-DOWN, but to leave the controller to handle the key-UP events as
normal (or the mic PTT release, or VOX delay). 

>> Another possibility for the 990 might be to route the key closure
>> directly from the key jack to the rear ACC socket. A couple of steering
>> diodes should ensure that the amp gets the PTT signal in real time,
>> while the 990 can still hold the PTT line down for a few extra ms on un-
>> key.
>
>Not a bad idea for QSK, but it won't work in VOX mode (don't want to key the
>amp for every element in that case.) In VOX we  need a signal that does what
>PTT is supposed to do -- switch on when the key closes and stay on until the
>end of the VOX delay time. 

Sorry, I'm not making myself clear enough. I didn't mean to interfere in
any way with the rig's control of the PTT/VOX *release* timing, on any
mode.

What I meant was to add a circuit to monitor the key jack and the
microphone PTT line, and watch for any of them to be switched to ground.
It would then ground the external PTT output to the amp immediately,
avoiding the 7ms delay. 

7ms later, the rig grounds the external PTT output as well. The amp
doesn't notice this - it already got its PTT signal, 7ms sooner than it
would have. 

When you release the key/mic, the rig still  keeps the PTT output
grounded, and will add the normal short delay before unkeying the amp.
If VOX or CW semi-BKIN is being used, the rig will add a longer delay,
again just like normal.

It would take a few transistors and diodes, but it seems completely do-
able. 

(The 13.8V power supply for my FT-990DC just "resigned" this morning, so
I'm even further away from actually trying the mod!)
 
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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