Michael,

I did not intend to turn this thread into a philosophical debate (I had
enough with the "no progress in decades" thread" !) but your are bringing a
good point.

I should have stated more clearly that my intend is what you describe as
"providing a kit to the builder". We may debate (please don't !) on the
value that this brings to the builder. On a 1 to 10 scale, buying a
transceiver is a 0 and building it from raw silicon, carbon and copper is a
10+. Where does building a "kit" from an article stand on that scale ? I
guess somewhere between 1 and 9. But I am going to firmly defend the fact
that it is more than zero. I can find many projects that anybody can build
with spares (like many "one transistor radio") but I also believe that to go
beyond that you need a PCB and an orderable parts list... When this is done,
some may chose a path that will teach them a lot. More than 25 years ago, I
did many mods on the first 2-meter FM transceiver that I owned. It was fun
but finding the spare Japanese IC's that I burned in the process was a
nightmare ! I whish I had done these mods on a XCVR that I built, even if I
had to buy the PCB and the key components.

Good point on the 49 MHz intercom. I have a pair of them. And you are right,
they have a wideband ceramic filter at 10.7 MHz. I guess they need to
because they share the second conversion Xtal with the PLL (10.240 MHz) so
the IF is really at 10.695 MHz (2nd if is at 455 KHz). This is confirmed by
looking at the PLL datasheet. The bad news is that the PLL has a 60 MHz max
input with preprogrammed channels. By the way, my intercom is not
full-duplex but the phones are. May be somewhere someone is running a
repeater with modified 49 MHz phone !

Thanks again !
Bernard


"Michael Black" wrote in message
...

"Bernard" ) writes:


1) I recently bought a dual conversion 72MHz RC receiver. For $45 (and
13
grams !), one can get one Toshiba TA31136FN, a ceramic discriminator, a
11.155 MHz (10.7 + .455) crystal, one 10.7 MHz crystal and one 455 kHz
ceramic filters, one IF transformer, 3 adjustable inductors, one
dual-gate
mosfet (unidentified), a 3.3V regulator and more. I am not sure what to
think except that this might be a non traditional way to source 1/2 of
the
RX section !
But amateur radio has always been about scrounging. Once you know enough,
you can make do in a lot of cases with what's available rather than
what's specified. Indeed, over the decades many an article has
been based on what the author had at hand (or in some cases what he
thought would be readily available) rather than because there was
something special about the parts. A beginner suffers from not
knowing enough, and going down to the parts store with the list of
parts and having no choice but to live with the prices, or if
the parts aren't available. But once they have a more extensive
understanding, they know that they can pull those bypass capacitors
off that VCR, and use those generic transistors instead of the generic
transistors specified in the article.

$45 is kind of expensive. Take any older cordless phone, and
one is bound to find a double conversion (10.7MHz and 455KHz)
FM receiver, often using one of those Motorola ICs. You can
get them cheap at garage sales over here, I've stopped bothering
with them because they are still plentiful (though I suppose
a later generation will be less useful, being more compact and
maybe using different IF frequencies). Indeed, buy the handset
and the base unit, and you get two IF strips. The ones I've
taken apart have a ceramic 10.7MHz filter, of the type you'd
find in an FM broadcast receiver, but that happens in a lot of
stuff. (Note there are some exceptions, where the units
are single converstion dropping from 49MHz to 455KHz, but the
one I picked up like that did still use one of those Motorola
ICs.)

Before FRS took off, there were license free 49MHz superhet
walkie talkies, that had dual conversion receivers with
the better ones, and they could be adapted with a change of
front end coils. These have faded away, so I don't know how
available they are (I once bought a pair for twenty dollars
at a garage sale), but they're is nothing forbidding their
use.

Baby monitors maybe, but I've never taken one apart and
because of their use I have my suspcions that they may be
single conversion, and perhaps even superregen.

Old cellphones, the bigger and clunkier the better, offer
up narrow band FM IF filters, and even complete IF strips.
The first IF will usually be in the 45MHz or higher range,
but given that they do include the conversion crystal to
455KHz, it doesn't really matter. The older ones are cheap
now, and the further back you go the more discrete components
you'll see. They'll be through-hole ICs, and identifiable,
rather than more recent ones that are surface mount and much
higher density where you can only use the ICs for the intended
purpose.

One problem that has arisen is that construction articles
in recent decades have become more about providing a kit to
the builder. Expect the builder to copy it perfectly, buy
the exact parts, and maybe even buy it as a kit from the author
or a third party. Once that happens, the author focuses on
using parts that are easy to come by, and the article is about
"building it like this" instead of explaining the philosophy and
possible alternatives to the parts. But it's only through understanding
that people can look at a schematic and realize they can use
that filter they have. The more they are spoon fed the more dependent
they become.

Michael VE2BVW