So we're going to need/use a repeater receiver for the project
using a converted commercial two-way radio. In some cases the
same radio will duplex (transmit and receive) within the same
case/chassis. Many radios duplex with various modifications,
which are available on web pages.
Some radios don't duplex do you would need to use two radios
to make up a standard repeater. The Motorola Maxtrac is one
typical example.
I normally would not use an Amateur Radio for a repeater receiver.
The typical consumer ham radio (for the two-meter band as an
example) doesn't have enough front-end protection to survive in
more than minimal transmit power repeater service. Such is also
the case when trying to use scanners as repeater receivers. There
are some amateur radios that do/would work in repeater service. You
would need to do your homework and get informed technical opinions
from people who have actual technical knowledge about a specific
radio or receiver.
There are way too many armchair repeater engineers with really
narrow viewfinders posting unsupported opinions about equipment
they don't favor. It's easy to get a project quickly derailed if
you listen to the opinions only crowd when some really creative
projects have already been completed.
****
At this point we want to talk about just to received signal logic
or circuit. The common label for this function is COR or COS,
which is carrier operated relay (an older term) or squelch (most
often meaning a newer solid state circuit... ie transistor). We
will often see the two labels interchanged so don't be confused
when you see them on paper.
So some type of received signal indication comes out of our
example receiver. Sometimes the squelch or cos logic is available
at a place/point on the receiver/radios accessory jack/plug. On
some radios we have to go in and bring wires out.
Some people don't like the original squelch logic circuit and they
choose to add an external board/circuit. People can chime in with
the various external squelch board options found around the net.
It's fairly simple to measure & judge the performance of your
receiver squelch circuit. If the normal operation of the receiver
as-built seems to perform pretty well... why not try to keep it
in service.
The receiver we have for a local project here is a vhf band mobile,
which has been tested working to spec before any mods or conversion
work is done. This radio is old enough that no available cos logic
is available without going inside the radio. We do have the service
manual for this radio.
We followed the manual and service information to reprogram and
align the radio down to our 2-meter band repeater receive frequency.
Depending on your choice of radio... you might have to mod your
radio choice to operate out of band as in the example case of our
radio down almost 10 MHz. Sometimes you even have to order crystals
for the project, which has been a popular past thread on this group.
The project radio does not provide any external received signal
or squelch open logic (electronic indication of a received signal).
With the Service Manual in hand we find a location in the receiver
squelch or audio amplifier control circuit to interface some type
of interface to the outside world. Sometimes we find people using
radio front panel receive LED or Lamp circuits/voltages to "tap-
in".
More on the actual interface circuit in a short time as this
saga goes forward. Maybe some pictures...
cheers,
s.
