Tuesday, July 25, 2006
This is my first contribution to the group and I hope it helps some
frustrated Pro-83 owners out there. About a year ago I see there was
quite a bit of discussion about the Pro-83 suffering from roaming or
random birdies in the VHF LO band. My unit fell into that catagory, so
much so it had been nearly impossible for me to listen between
30-50Mhz without it stopping on a frequency for a duration of a few
seconds up to 15 minutes before I had to press scan again. I live in a
rural area and the VHF-LO band is still heavily used by the local fire
department, and I was damned if I was going to miss out on any of it.
Today I had a few hours to burn and experiment with my Pro-83 and I
have solved the problem. I'm not an engineer so I can't explain why it
works, I've just used some "old time" techniques from years ago when I
built radio kits as a kid. What you'll need is a small piece of hookup
wire, a .0001uf (100pf) mica disc capacitor and basic soldering skills.
1. Remove the back off of the Pro-83 by unscrewing the 6 phillips
screws, don't forget there's two inside the battery compartment.
2. Carefully separate the front and back and lay face down on a
cushioned surface or piece of cloth (We don't want to scratch our nice
scanners up do we?) Be extra careful not to stress the power leads
coming from the battery compartment to the PCB either.
3. Locate where the antenna BNC connector is soldered to the printed
circuit board. Obviously, it's at the top where the volume and squelch
are located. Take note there are 3 solder points on the BNC connector,
the one in the middle is the actual connection to the antenna and the
other two on the outer most left and right are ground.
4. While looking straight down on the circuit board with the BNC,
volume and squelch at the top, solder one end of your hookup wire to
the LEFT ground point of the BNC. Solder the other end of your wire to
the closest corner of one of the tin shields covering the oscillators
and other components. This should be easy to do since there's plenty
of solder blobs on the tin shields already and keep your wire as short
as possible. The idea here is for us to have "positive" contact from
the antenna ground and the PCB ground bus and we're sure there's no
resistance of any kind.
5. Solder one lead from your .0001uf (100pf) capacitor to the center
connection of the BNC (or antenna connection) where it connects to the
printed circuit board, and the other lead to the RIGHT ground
connection of the BNC, keeping the leads as short as possible on the
capacitor without burning it up with your soldering iron.
Ta-da!, you're done. Carefully slide the front and back together
making sure not to pinch or crush the battery leads, replace and
tighten the screws and enjoy birdie free VHF-LO scanning.
Now, before you all ask why this works I'll truthfully say I don't
know. Maybe the engineers can come out of the woodwork. I do know that
nearly every radio kit I built as a kid involved some sort of 100pf or
1000pf mica capacitor connected at the antenna as described here to
act as a small RF choke...and essentially that's what we want to
achieve, choke the RF birdie from getting into the attached portable
antenna. As for the hookup wire, there's an added bonus other then
making sure our antenna has a "positive" connection to the ground bus.
It also appears to improve reception slightly most likely because
there's no chance of ground resistance and the tin shields now act as
a small ground plane.
After making this simple modification I have now been listening to my
Pro-83 for three hours straight without one birdie lockup on VHF-LO
using a 3 foot telescopic whip. The local fire deparment comes in loud
and clear as well as all signals on VHF-HI, UHF and 800Mhz.
May the force be with you.
Eric / N3KMJ