Once the receiver is determined to be working properly, alignment is required.
To properly align the discriminator coil, two pieces of test equipment are essential - an accurate frequency counter with resolution to 1 KHz and a high impedance voltmeter, either a digital VOM, or a VTVM.
To align the discriminator:
1) place the voltmeter across the 5K6 resistor between pins 7 and 10 of the LM3089;
2) Using the frequency counter to measure local oscillator frequency, tune the radio to exactly the center frequency of a strong broadcast signal;
3) With the receiver tuned to exact center frequency, adjust the discriminator coil so that the meter reads exactly zero volts. The AFC will fight this process, so it will be necessary to repeatedly retune to center frequency and readjust the discriminator to zero volts until zero volts is realized at exactly center frequency of the broadcast channel.
Unfortunately, I could not properly adjust the discriminator coil by ear. I tried it several times by adjusting the discriminator coil for what I perceived to be the correct response, and it was off when I checked with the voltmeter. The IF transformer I am using for a coil has more than one response peak, and I picked the wrong one every time by ear. The receiver will work with an alignment by ear, but it won't work as well as it is capable of.
The commercial front end is pre aligned, and the coils should not be touched. The IF transformer might benefit from adjustment to the load presented to it, but it probably won’t make much difference. Align for maximum received signal at exactly the center of the IF channel with a scope or spectrum analyzer. It might be possible to align by ear by tuning to center channel of a noisy broadcast station and tuning for maximum quieting.
When properly aligned, the radio should be very sensitive, tune cleanly, and have very clear and undistorted recovered audio.
Alignment
Once the receiver is determined to be working properly, alignment is required.
To properly align the discriminator coil, two pieces of test equipment are essential - an accurate frequency counter with resolution to 1 KHz and a high impedance voltmeter, either a digital VOM, or a VTVM.
To align the discriminator:
1) place the voltmeter across the 5K6 resistor between pins 7 and 10 of the LM3089;
2) Using the frequency counter to measure local oscillator frequency, tune the radio to exactly the center frequency of a strong broadcast signal;
3) With the receiver tuned to exact center frequency, adjust the discriminator coil so that the meter reads exactly zero volts. The AFC will fight this process, so it will be necessary to repeatedly retune to center frequency and readjust the discriminator to zero volts until zero volts is realized at exactly center frequency of the broadcast channel.
Unfortunately, I could not properly adjust the discriminator coil by ear. I tried it several times by adjusting the discriminator coil for what I perceived to be the correct response, and it was off when I checked with the voltmeter. The IF transformer I am using for a coil has more than one response peak, and I picked the wrong one every time by ear. The receiver will work with an alignment by ear, but it won't work as well as it is capable of.
The commercial front end is pre aligned, and the coils should not be touched. The IF transformer might benefit from adjustment to the load presented to it, but it probably won’t make much difference. Align for maximum received signal at exactly the center of the IF channel with a scope or spectrum analyzer. It might be possible to align by ear by tuning to center channel of a noisy broadcast station and tuning for maximum quieting.
When properly aligned, the radio should be very sensitive, tune cleanly, and have very clear and undistorted recovered audio.
73,
Win W5JAG