|
On John's advice, I executed a 'just tweak it' alignment of the sound IF and discriminator inductors. The result is quite acceptable - except that things only seem to line up on channel 3. So either the tuner or the entire IF chain is still out of alignment. Its good enough for now, since the TV is living it's life w/ an RF output on channel 3 anyhow.
I also picked up a Heathkit IG-57A post marker/sweep generator (minus the attenuator and RF probes - but these can be refabricated), and thanks to a fellow hobbyist, I have attained the schematics and instructions. It actually seems to be in working condition -- the markers are set around 45Mhz IF. However, the marker circuits are pretty simple tuned circuits with crystals for reference in the base of a miller oscillator. I forsee no issues in swapping out the crystals w/ the closest match for 21Mhz IF and adding some capacitance to re-center the tank circuits.
The sweep width and center frequencies are set via a variable saturable inductor. DC current is run through the inductor primary to drive it partially into saturation and lower the effective inductance, changing the center frequency. to get the sweep, some AC current is injected at line freqeuncy to modulate the inductance. I haven't measured much directly, since my scope is only good to 20Mhz, but it looks like its possible to sweep down to the needed frequencies, as-is. I think I'm in good shape, since the Heathkit also has provisions for 10Mhz FM IF.
If and when I get around to it all, I'll post some pictures of the true IF curves Vs Sams.
|