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

Steve Katz stevek at jmr.com
Tue Oct 5 13:36:52 EDT 2004


The sequencer is not intended for real VOX operation.  Normally you'd use
the PTT line output from your microphone, or a footswitch which would allow
either voice or CW mode operation.  If VOX causes your transmitter to
generate RF, that's not a good thing for a sequenced operation intended to
protect a receiving preamplifier.  -WB2WIK/6


"There is no reason anyone would want a computer in their home." - Ken
Olson, Chairman & Founder, Digital Equipment Corporation (1977)

> -----Original Message-----
> From:	j4976 at juno.com [SMTP:j4976 at juno.com]
> Sent:	Tuesday, October 05, 2004 10:42 AM
> To:	Steve Katz
> Subject:	Re: [BULK] - Re: [TowerTalk] ARR Preamp Failures
> 
> Just a quick question about the sequencer:  Do you need to key manually
> with one of these, or are they ok with a vox setup?
> 
> - Thanks.
> 
> On Tue, 5 Oct 2004 10:01:35 -0700 Steve Katz <stevek at jmr.com> writes:
> > What's commonly done with VHF masthead preamps and such, and I've 
> > used these
> > for decades, is to simply install electronic sequencing in the shack 
> > and use
> > control-line keying (not RF sensed keying).  ARR used to sell a 
> > sequencer
> > for this, maybe they still do; but the circuit's been in the 
> > Handbook since
> > the 1970's, and of course designing one's own sequencer isn't 
> > difficult.  My
> > 4-stage sequencer uses sealed, small, mercury-wetted relays that 
> > switch
> > quickly, have no contact bounce, and are good for millions of 
> > operations;
> > they cost about $10 each.  When I push the footswitch or mike PTT 
> > bar, the
> > first thing to switch is the preamp, from RX to bypass; the second 
> > thing to
> > key is the power amplifier T-R relay, from RX to TX; the next thing 
> > is the
> > power amp's bias key line to switch it from cutoff to operate; the 
> > last
> > thing is the exciter, to produce RF.  Releasing the switch makes 
> > them unkey
> > in reverse order.  The sequence time needs to be more than the relay 
> > contact
> > settling time; I use 30 mS.  So, pressing the button causes four 
> > relays to
> > key in sequence, taking up about a tenth of a second; releasing it 
> > causes
> > them to unkey in reverse sequence, also taking up about a tenth of a 
> > second.
> > I built my sequencers per plans from Chip, N6CA, that appeared in 
> > the
> > Handbook maybe 20 years ago.  Still work fine, and I use one on each 
> > VHF
> > band.  -WB2WIK/6
> > 
> > "There is no reason anyone would want a computer in their home." - 
> > Ken
> > Olson, Chairman & Founder, Digital Equipment Corporation (1977)
> > 
> > > -----Original Message-----
> > > From:        Tom Rauch [SMTP:w8ji at contesting.com]
> > > Sent:        Tuesday, October 05, 2004 10:10 AM
> > > To:        Jim Brown; Tower Talk List
> > > Subject:        [BULK] - Re: [TowerTalk] ARR Preamp Failures 
> > > 
> > > > So the question is, is this a common problem?  Am I doing
> > > anything wrong? Could I do
> > > > anything better?
> > > 
> > > This is a common problem. The problem is the system
> > > configuration requires the radio output RF before the preamp
> > > switches out of line.
> > > 
> > > This means the preamp has to handle nearly the full output
> > > power of the radio or any amplifier BEFORE the preamp relay
> > > drops and bypasses the preamp. If they did anything like add
> > > a diode across the relay or didn't do there homework in the
> > > switching system, it can be even worse!
> > > 
> > > You need to switch the preamp out of line before
> > > transmitting and switch it back in line after a safe delay.
> > > RF detected circuits can't do this.
> > > 
> > > In some designs, I had to add an intentional MMIC buffer and
> > > attenuator pad to allow RF switching the preamp out. The
> > > MMIC is rugged, and a pad of several dB limits the power
> > > applied to protection diodes and the GAsFET drain. It is a
> > > real PITA thing to design a preamp that won't fail from
> > > being banged with high power RF while waiting for the relay
> > > to transfer.
> > > 
> > > 73 Tom
> > > 
> > > 
> > > _______________________________________________
> > > 
> > > See: http://www.mscomputer.com  for "Self Supporting Towers", 
> > "Wireless
> > > Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 
> > with
> > > any questions and ask for Sherman, W2FLA.
> > > 
> > > _______________________________________________
> > > TowerTalk mailing list
> > > TowerTalk at contesting.com
> > > http://lists.contesting.com/mailman/listinfo/towertalk
> > _______________________________________________
> > 
> > See: http://www.mscomputer.com  for "Self Supporting Towers", 
> > "Wireless Weather Stations", and lot's more.  Call Toll Free, 
> > 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
> > 
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk at contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> > 
> > 
> 
> ________________________________________________________________
> Get your name as your email address.
> Includes spam protection, 1GB storage, no ads and more
> Only $1.99/ month - visit http://www.mysite.com/name today!


More information about the TowerTalk mailing list