"Jeffrey D Angus" wrote in message
...

Or more specifically, it's to present a uniform high impedance
from the generator to the radio. At the time, with the exception
of a few lab grade instruments, signal generator outputs were all
over the map with regards to output impedance. The "dummy antenna"
presented a fairly uniform, and high, impedance to the radio and
kept from detuning the front end while doing an alignment while
connected to a signal generator.

Jeff


Ahhh, so for the very first time, I finally hear an answer to one of those
things that I've scratched my head about for years!

I've often wondered why the literature always says to stick a capacitor
inline from the signal generator -- I figured if I had to do this all the
time, then why didn't the signal generator maker put one in there? Turns
out, usually they do, but it's probably not an impedance match. Still, for
most radios I just stick the probe to the antenna and go to town. It's fine
for the IF -- enough of it gets past the RF to give me a workable signal.
Then for RF I use whatever frequency I need to check/adjust the LO. For
final RF adjustments I plug in the antenna and tweak it up on a weak
station.

Again, I probably differ from most of you because I work mainly with car
radios. I already have a pretty decent antenna at the bench -- some $10
special from Auto Zone -- that does a great job. On a conversion there are
no RF adjustments; on a repair/rebuild, I can do every adjustment except
one, the one which must be done inside the car anyway -- the antenna
trimmer. However even this gets checked -- I've noticed that with my bench
antenna, the trimmer usually adjusts toward the "high" end, with the screw
fairly loose. So if the trimmer tweaks in this area I'm fine. If it tweaks
"tight", or not at all, I'll adjust the slug as needed -- or troubleshoot.

Actually it is rare that I ever have such a problem -- with one exception:
the 58-60 T-Bird radios. Even these are just fine, usually, but I've had a
number of them whose front ends are way off. I've spent many hours trying
to figure out why, and come up with nothing. I've replaced RF caps, checked
the range of trimmers, checked resistor tolerances, and basically gone over
the circuit with a fine tooth comb. Everything checks perfect, but the
alignment is out. So I adjust the tuning slug -- a pretty fair distance --
to bring it in line. I've had a couple that were so far off that I had to
tinker with the others as well. It's really fun because those slugs are
WAA-AAY up in there, and difficult to reach. You can't do it with a
non-ferocious screwdriver, or even a ferocious one -- it's too tight. You
have to reach the shaft with needle-nose. In any case, all this fun aside,
I can tinker and bring it into line -- but I still wonder why I have to do
it in the first place.

I think my next step in this "investigation" is to spend time with the next
58-60 T-Bird I get for conversion. If it's a good radio, when I disassemble
I'll measure the tuning slugs, trimmers, fixed coils, etc., and write them
down for reference.

--
Gary E. Tayman/Tayman Electrical
Sound Solutions For Classic Cars
http://www.taymanelectrical.com