From: rfc@aloft.att.com
Subject: Aligning AM radios
Originator: wdp@gagme
Organization: AT&T
Date: Wed, 30 Sep 1992 02:45:47 GMT

>Has anyone cracked open an AM radio  and realigned it?  If it's a classical 
>(not PLL or varactor tuned) design, there should be several adjustments.
>
>On the AM band, most loopstick receivers are very sensitive to
>the trimmer capacitor tuning for the top end of the band.
>
Generally, the way I've alligned AM radios is something like this:
After you're satisfied that the IF strip is alligned,
tune in a weak station around 1400KHz, and tweak the osc adjustment
cap to get the calibration right.  Go tune in a weak station around
600KHz and tweak the osc coil.  Then go check and readjust the cap
for the station at 1400.  Repeat a few times.  When you're done, you
should be at optimum as far as calibration goes.  Then do the antenna
circuit.  Go to a weak station at near 1400, and tweak the antenna 
trimmer cap.  Then tune a weak station at near 600, and you can adjust
the coil on the ferrite rod by sliding it along the rod.  Avoid putting
your fingers on the coil, use a plastic rod, or some such.  Your fingers
will detune the circuit because you'll add some capacitance to it.  
Usually, there will be some wax to make sliding hard.  Sliding the coil
along the rod changes its inductance.  repeat the 1400, and 600 adjust-
ments a few times, till you get optimum on both.
 
I've even played with a few PLL AM radios.  Usually don't have to touch
the osc circuit, as the PLL circuits automatically adjust it.  The
antenna circuit will still need tweaking with the same process as
above.  If stations sound like they are being tuned slightly off
frequency, the IF strip may be slightly tuned off frequency.  I've
been able to get it tuned better by touching up the IF.  But be
aware that a misadjusted antenna circuit can make a station sound
off frequency.  If the antenna circuit is wrong by 15KHz too low, the
station will sound raspy and distorted.  What does it is that the lower
side band is stronger than the carrier.  And envelope detectors don't
work correctly like that.  Retuning the antenna circuit should fix
it.  Try that before touching the IF strip.  

TIP:  Before you start adjusting anything, use a fine tip marking pen
to make marks on the coil cans and trimmers to indicate original
positions, so you can restore them to there if you decide to.  I 
usually mark one side of the scewdriver slot on the adjustment
on the cap or coil, and a mark on the stationary part of the cap or
coil to indicate precise position.  Count turns of the screwdriver
if you go past one twist.
It also helps a lot if you have a schematic of your radio.  As
you probably won't have that, you can get a good idea of what adjustments do 
what by tracing the circuits.  Stuff electrically close to little boxes
marked "455" probably belong to the IF strip.  Stuff electrically near a
multigang tuning capacitor should be the antenna and osc circuits.  PLL
radios are a little harder to sort out, start with the antenna, and
work your way in.  You should be able to find the antenna trimmer with
no problem, and the IF probably has a small box with "455" written on
it.  The osc circuit will seem somewhat isloated, but there should be
a connection from the osc feeding a counter circuit of some sort to
do the PLL action, in addition to the varactor control line.
There will probably be a coil, a trimmer cap, and a varactor diode 
together (osc circuit).

Practice on a radio you don't care much about before you mess with
your good radios!

Proceed at your own risk!  WA2ISE
Newsgroups: rec.radio.broadcasting