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Re: 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 fact
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 and
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 preamp
off.  So I can't just pull a ground line low and key them that way.  It has
to be a little more sophisticated.
2.) I have a long lead of DC feed to the shack, so I don't want to regulate
to the voltages I use in the box inside my shack.  Also my relays are 26Volt
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 the
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 is
a lot of voltage drop and dissipation, but the preamps draw minimal current
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 and
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.  A
pair of relays in what ends up being a DPDT configuration are used for each
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 energize
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 thus
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 the
preamps are bypassed.

Note that I chose to switch the voltage to the RF relays and NOT the ground.
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 out
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 won't
dekey at all.  So what I have done was put a diode in series with the keying
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 bricks
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 ARR
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 J
I am not transmitting on 432, but I have successfully switched it in and out
with no ill effects when transmitting.

To be ulitmately safe, you may want to add a sequencer circuit to your rig.
DEM sells these and a lot of folks use them.  They do require some rig
modifications though.

I hope this helps. 



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 is
> 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) before
> 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.  But,
> I don't think that the results would be any different unless there is a way
> 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)



"A life lived in fear is a life half lived."

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