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Re: [BULK] - Re: [TowerTalk] ARR Preamp Failures

To: "Steve Katz" <>,"Jim Brown" <>,"Tower Talk List" <>
Subject: Re: [BULK] - Re: [TowerTalk] ARR Preamp Failures
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 5 Oct 2004 14:02:09 -0400
List-post: <>
I have some stuff about sequencing at:

I don't really have a schematic on site for multi-stage
sequencing systems, but it really only takes *one delay* to
do a system.

The only delay you need is to inhibit the transmitter while
all other things are activated as fast as possible. That
means you close all relay transfers in parallel with the key
closure while you hold off the transmitter for xx
milliseconds while all the systems transfer. The transfer
order of external devices is not important so long as the RF
is applied last with proper lag. You also have to be
absolutely sure any break in transmit knocks the TX off
first and requires full TX inhibit once the accessories

In most preamps, the relay is energized when in the RX
preamp on mode. This means the relay has to drop before RF
is applied. The single most common mistake people make is
adding a diode across the relay coil in this application.  A
back-pulse cancelling diode will substantially slow the
relay release time.

My advice would be to do the preamp and all other devices as
rapidly as possible, and then only add a single hold-off for
the radio activation. For CW, the MFJ 495 (I think that is
what it is, they sometimes change numbers after I last see a
product before production) keyer has a programable holdoff
delay for a PTT output and it delays the data you are
sending by the same amount. This means you won't have
leading edge chopping.

I was after them to build a similar thing for SSB that
included an internal VOX, but they weren't interested.

By the way, be careful to look at any sequencing system with
a dual channel scope. Someone brought over a system using a
footswitch sold or manufactured by a big name in microphones
that flatly just did not work. It had the silliest excuse
for a delay I have ever seen. It used two switches that were
staggered in PTT actuator movement. If you consumed enough
Qualudes and drank enough wine to slow your foot travel time
to about 750mS per inch distance, it added enough delay. The
drawback is anyone whose foot moves that slow would be too
sedated to talk.

73 Tom


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