VCO Adjustment Pot
There are two methods, not using test equipment, and using a frequency counter.
No Test Equipment
1) Hook up the LM4500 board to a tuner, and receive with clear
station. (In fact, try a few different stations). Turn the pot fully
CCW. Rotate the pot CW until the stereo LED light just comes on. Mark
that position. Then keep rotating CW until it goes off. Now come back
CCW and mark where it comes on again (but this time it will be from
the other side). Place a mark in the middle of the 2 observed marks,
this is where it should be set.
With a Frequency Counter
2) Disconnect the LM4500 from the tuner - Short the LM4500 Composite Input to ground.Connect the board test point that says "19kHz TP" to a frequency
counter. Adjust the VCO Pot until the counter reads 19.00 kHz
The first method will get you very close, if not the same result, as
doing it the 2nd way.
Stereo Adjustment Pot
If using an FM stereo generator, set the generator to produce FM Stereo, 10% pilot level, 75kHz FM modulation, at an RF output level that will produce full quieting, such as 1mV into 75 ohms. Then set the generator for L output, and measure the audio response on R. Adjust the 10K “Stereo Sep. Adjust” pot for minimum response on R, using either a scope, AC voltmeter, or PC sound card.
For stereo adjustment with no test equipment, the only way I know of
is to listen with headphones, and adjust for maximum separation.
Another cheap way to adjust separation is to use an IPOD type fm radio
adapter. Make sure it has a stereo mini plug input (not some goofy
IPOD only connector). These are fairly common and cheap, at least in
the US. You may also be able to borrow one from a friend.
Hook the LM4500 unit to your tuner, and adjust the tuner frequency to
the one required by the FM radio adapter. If you normally use a
rooftop antenna, disconnect it and use a dipole placed near the FM
adapter. Use an RCA plug to mini plug adapter cable from one channel
of your preamp out to drive the FM adapter. Play some music, set the
preamp to a very low level - be careful NOT to overdrive the FM
adapter - around 100mv AC should be right. Adjust the separation pot
for minimum audible response while listening to the opposite channel.
So if you drive L, listen to R. Switch channels, and set it to the
best point that gives the minimum response for both channels.