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Cheap Protection for your Downconverter



>From K5GNA:

There has been a lot of input on protecting your downconverter from an 
inadvertent 
key-up from your 2 meter transceiver -- some are simple circuit modifications 
and 
some use equipment that cost money.  I have keyed up into my downconverter 
with no harm done, but I have repaired a few downconverters that have been 
blown up 
because of keying up a transmitter into them.  A simple solution is available 
from 
Radio Shack (and others).

I have been one of the proponents of reducing excess noise generated by the 
downconverter before it gets into the receiver.  A no-signal S meter reading 
of 6 to 9 
or greater should be reduced and a 6 to 12 dB attenuator is a good way to do 
it. 
However, I do think we need a couple of S units of noise to work with here.   
The 
attenuator also affords some protection to your downconverter, as it will 
probably 
blow up before your downconverter does.

I bought two Radio Shack model 15 - 1257A, 6 dB attenuators for $2.99 each. A 
single attenuator reduced my no signal S - 5 meter reading to lower than S - 
3.   I 
have 75' of RG-59 from the downconverter for attenuation also.  The 
attenuators were 
bought on the same day from different stores, and were entirely different.  
Both were 
Pi type attenuators with a choke to couple the DC through.  One used some 
chip 
resistors and the other used all 1/8 watt leaded resistors.  Actually, the 
one with the 
chip resistors had chip resistors in the input and output to ground, but used 
an 1/8 
watt leaded resistor for the input to output element.

Now for the scientific test:  I keyed up my IC 821H into the attenuator with 
the chip 
resistors, using 40 watts into an indicating dummy load and timed it.  It 
took 45 
seconds before it opened and the power dropped from 10 watts to 0 in the 
dummy 
load.  That is kind of long.

Now for the unscientific test:  I was using the other attenuator last night, 
the one with 
the leaded resistors, and inadvertently keyed up into it without noticing.  I 
soon found 
that my no signal noise went down to S - 0, but the beacon was still 
readable.  The 
attenuator was open (it burned the 56 ohm resistor) and my downconverter was 
working fine when I checked it with no attenuator.

Check out my picture of the attenuators (after letting the smoke out) and the 
schematic:

http://members.aol.com/k5gna/6dBattenuator.jpg

These attenuators, or ones like them, will protect your downconverter from 
your 
transmitter (and you).  The 1/8 watt resistors are much cheaper to replace 
than the 
downcovnerters.  They are fairly simple to open and replace the resisitors.  
In fact, 
they would make a good housing for any attenuators you may design.  The choke 
could be cut out and the capacitor eliminated (straight to ground here), if 
you like.  
Other attenuators do not pass DC and do not have the choke circuit.

For the purist, a 1/16 amp (or smaller) pico fuse in series with the 56 ohm 
resistor 
would probably provide instant protection from transmitting through it. There 
is plenty 
of room in the housing for a pico fuse.  I use my attenuator at the 
transceiver side where it is easy to get to, so the DC path is not needed.  
The low current fuse does 
not carry any DC current for the downconverter anyhow, because the choke will 
provide a DC path and bypass the fuse.

 
Bob Seydler
K5GNA


 

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