FM-tuner tune-up - Page 1
Forum section: electronics / rf-microwave

author: orbanp
author_rank: Regular Contributor
timestamp: May 09, 2021, 09:32:33 pm
content: 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

author: szoftveres
author_rank: Regular Contributor
timestamp: May 10, 2021, 05:14:46 am
content: As you mentioned, the phenomena is not that surprising - these tuners were designed in the 1960's and they are pretty basic.The RF filter is at the emitter side of the mixer-oscillator transistor, while the oscillator LC tank is at the collector side - and there is a direct (although very loose) capacitive coupling between these two tanks (C8, C13, C14). So adjusting one tank will always affect the other.Good luck!

author: orbanp
author_rank: Regular Contributor
timestamp: May 10, 2021, 02:39:21 pm
content: Hi Szoftveres,Thanks for the reply!I will have another go at tuning up the tuner.As I have been thinking about it, the AFC probably should have taken care of the pulling of the oscillator frequency!I do not recall how was the AFC switch set, it was not a deliberate decision to set it one way or the other.As for the conversion, contrary to your solution, as you posted in another forum, I have not modified anything on the IF-board so far.I have not even changed the de-emphasis filter capacitor.Neither have I implement that "frequency-deviation compression" circuit either that you found necessary.I did not notice any harsh sounding in the reception, though granted, I have not tested the conversion that exhaustively.Thanks, Peter

author: szoftveres
author_rank: Regular Contributor
timestamp: May 10, 2021, 08:08:46 pm
content: I live in the U.S. where different pre- and de-emphasis constants (75us) are being used on FM. This is country-specific, European-made radios (not made for the US market) sound pretty bad over here, even the ones that are originally capable of receiving 88-108Mhz. Time constant in Europe was 50us on both FM bands (66-74Mhz and 88-108Mhz).

author: orbanp
author_rank: Regular Contributor
timestamp: May 11, 2021, 01:18:48 pm
content: I also live in an area where 75us time constant is used in FM broadcasting.Using 50us de-emphasis on 75us broadcast barely makes 3.5dB boost at higher frequencies, well within the treble tone control of the radio to correct for that.Here is a good discussion on this:https://www.radiomuseum.org/forum/fm_pre_emphasis_and_de_emphasis.htmlRegards, Peter