Hello Everyone,
This is about a "simple tune-up" of an FM-tuner stage of a broadcast FM receiver (Selena B-210:
https://www.radiomuseum.org/r/minsk_radi_selena_b210b_21.html).
Assuming the IF and FM decoder is properly tuned up, one first adjust the oscillator coil at the bottom of the FM band, then the oscillator trimmer cap at the top of the band, repeated a few times.
This assumes a tuner that is tuned by a variable cap on FM, that the Selena radio has.
Then comes the tuning up of the tracking, similarly starting at the bottom of the band (well, a bit above that) with the modulator coil, then at (close to) the top with the modulator trimmer, again, repeated.
While I was dong this, an interesting but not that surprising phenomena presented itself. tuning the modulator circuit trimmer cap also mis-tuned the oscillator, so the output could not be peaked. The tuner circuit is a "regular" grounded base RF-amplifier stage and followed by a grounded base mixer-oscillator stage.
(The radio has very decent IF-filters, both on AM and FM.)
A simulation of the tuner stage confirmed that a 4pF change in the modulator circuit trimmer cap cause about 120kHz offset in the FM-oscillator frequency.
Previously I tuned up the modulator circuit with the oscillator disabled, but now as the radio is fully assembled and the tuner shield installed this would be a rather involved process.
I am doing this as part of re-tuning the tuner from the OIRT FM band to the CCIR FM band.
Any comment on the above?
Thanks, Peter