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Re: Preamp bypass circuit

This has been an often discussed subject on the EME list group.  The FT847
is notorious for killing preamps if not sequenced properly.  Keying the Mic
will send RF up to the preamp if not sequenced properly.

The most common approach has been to modify the rig and split the RX/TX
lines internally.  With this modification, there is no possiblity of
transmitting into the preamp but you will need a line for both RX and TX  to
the antenna.  There are sites on the internet with descriptions and pictures
of this modification.

I use the DEM sequencer and have not split the lines internally.  The only
probelm is that keying the Mic or the keyer will still send RF into the
preamp.  I do not use the radio for SSB on EME so I simply put a switch on
the Mic line that is turned off most of the time.  I installed a line with a
relay to the keyer and the DEM sequencer is "keyed" with a footswtich.
Therefore, if I accidently bump the keyer it will not key the rig unless I
have pushed down the foot pedal to activate the sequencer first.  I have not
blown the preamp running this system with 1500 watts.  This same type of
relay line could be run to the Mic PTT line as well so that the  Mic can not
key the rig unless the preamp is first turned off by the sequencer through a


Scott Olitsky
Buffalo, NY


144 MHz EME
4 x M2 9SSB
8877 PA by antennaworks
MHP - 145 preamp
----- Original Message -----
From: "Jon Ogden" <na9d@mindspring.com>
To: "Duffin, Jon" <jduffin@dbxpro.com>; "'amsat-bb@amsat.org'"
Sent: Thursday, February 15, 2001 12:08 AM
Subject: Re: [amsat-bb] Preamp bypass circuit

> Hi Jon (and nice way to spell your name! :-) ),
> I recently went through this same mode of thought and came up with the
> that I did NOT want RF switching.  Any pause during SSB could knock the
> preamps back in line and so forth.  Don't need that mess.  I hate it on
> bricks and amplifiers and I hate it on preamps.  I prefer to hard key all
> the way.
> I built a relay control circuit that consists of the following:
> Four SPDT relays (or easier 2 DPDT relays would work), a control circuit
> a power supply.
> OK, here's how I thought about things:
> 1.) Want the preamps OUT of line when I am keyed AND when I turn the
> off.  So I can't just pull a ground line low and key them that way.  It
> to be a little more sophisticated.
> 2.) I have a long lead of DC feed to the shack, so I don't want to
> to the voltages I use in the box inside my shack.  Also my relays are
> relays, my preamps, 5 Volts, etc.
> OK, I built a full wave bridge rectifier in the shack that gives me an
> unregulated voltage output of about 35 volts converted DC (peak DC since
> it's a converted AC waveform is a little higher).  I run that unregulated
> voltage up to the box on the mast.  In the mast box, I convert that
> unregulated 35 volts plus line loss to 26 volts using an LM317T (if I
> remember right - simple TO220 3 terminal adjustable regulator).  This is
> voltage that will drive my relays.  I also have a second regulator where I
> take the 26 volts down to 5 volts for the preamps.  Yes, I know that this
> a lot of voltage drop and dissipation, but the preamps draw minimal
> and I am using a TO220 regulator on a significant size heatsink.
> Now, I want the RF relays to be energized when I turn the preamp box on
> I want that energized state to be through the preamps.  In other words, I
> use the bypass path when the preamp box is off and when I key the radio.
> pair of relays in what ends up being a DPDT configuration are used for
> preamp (VHF and UHF).
> Now, in order to do this, I need another set of SPDT reed relays that will
> control the voltage to the RF relays.  These are 26 volt reeds I bought at
> Radio Shack (like most of the control circuitry).   The reed relays are
> de-energized when I am either turned off or am in receive mode.  I
> them when I transmit.
> The supply voltage (26V) for the RF relays is fed into the Common terminal
> on the reed relay.  The "normally closed" pin goes to the RF relay's
> positive supply terminal.
> The "normally open" pin has nothing attached to it.  The supply pin on the
> reed relay is connected to the 26 Volt supply.  The ground pin on those
> relays is my PTT line from the shack.
> So when the PTT line is drawn low (to ground), the Reed relay energizes
> removing voltage from the RF relay.  Since the RF relay has no voltage on
> it, the preamps are put off line.  When power is turned off to the preamp
> box, the RF relays drop since they aren't being supplied then either and
> preamps are bypassed.
> Note that I chose to switch the voltage to the RF relays and NOT the
> This is because the ground pin on the relay is acutally connected to the
> relay case which is connected to the heatsink and the relay case is also
> connected to the shields of the RF cable.  If I switch the ground in and
> it does nothing.  The only way is to switch the voltage.  I can get away
> with switching ground on the reed relays because I can isolate that ground
> from everything else.
> Now, the FT-847 has keying lines built in through one of the DIN ports on
> back.  I use these to key my bricks as well.  However, the brick put out a
> voltage on the keying port when they are not keyed.  It screws up the
> circuitry I described above for obvious reasons.  In fact, the preamps
> dekey at all.  So what I have done was put a diode in series with the
> line going to the anode and the cathode going to the junction point where
> the keying line from the radio, the keying line from the bricks and the
> keying line for the preamps all meet.  This way, the voltage from the
> is blocked from going up the line to the preamps.  It works great.
> So far, this system works very well.  I have a DEM preamp on 432 and an
> preamp on 2m.  I will say I blew the ARR preamp up.  I don't know why
> though.  It was right before I got married in October and I did not have
> time to fix it.  Since then, the WX has been bad or I've had no time.  I
> will need to wait for spring until I fix it.  But I use the UHF preamp
> whenever I work mode J and I have NO problems with it.  Of course in mode
> I am not transmitting on 432, but I have successfully switched it in and
> with no ill effects when transmitting.
> To be ulitmately safe, you may want to add a sequencer circuit to your
> DEM sells these and a lot of folks use them.  They do require some rig
> modifications though.
> I hope this helps.
> 73,
> Jon
> NA9D
> on 2/14/01 1:15 PM, Duffin, Jon at jduffin@dbxpro.com wrote:
> > I have recently built up a GasFET preamp for use on the 2 meter band on
> > AO-10.  To switch the preamp out during transmit, I have built up a RF
> > sensed circuit which bypasses the preamp through a relay.  The problem
> > that I can observe a breif RF transient that would go into the preamp (I
> > actually have not hooked up the preamp to the switch circuitry yet)
> > the relay switches into the bypass mode.  I am fearful that this RF
> > transient would be enough to blow out the GasFET.  Does anybody have any
> > hints or ideas how to supress this transient so I won't be replacing the
> > FET?
> >
> > My rig is a FT847, and so I though another idea would be to switch the
> > preamp out of the transmit path by using the 847's PTT accessory jack.
> > I don't think that the results would be any different unless there is a
> > to delay the power output stage from coming up at the same time as the
> > switching of the PTT accessory jack signal.
> >
> > How does everyone else get this to work on their homebrew preamps?
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
> http://www.qsl.net/ke9na
> "A life lived in fear is a life half lived."
> ----
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