RF -- Welcome to AK! By the time the units were developed (among all manufacturers) that employed on board MPX decoders, all of the IF circuitry was changed to "wide band" operation -- which by that time was primarily concerned with the discriminator/ ratio detector stage. By the beginning of the 60's most manufacturers had abandoned the stagger tuned type of IF design in favor of a more traditional type that could simply be peaked. The older stagger tuned transformers had a much narrower bandwidth, requiring the stagger alignment to get the flattest results. The newer transformer (from about 58-59 on) were generally all of the peaking type, so that the need for stagger alignment went by the wayside. Most of the early "stereo ready" tuners included transformers that would pass about 260 kHz between 6 db down points, which you would certainly need all of for good MPX stereo. You couldn't really expand this any further since the FM stations are separated by just 200 kHz. Any greater bandwidth in the IF strip would certainly impair good selectivity. Max deviation for FM is 75 kHz on either side of the IF frequency, so a flat response is needed over at least 150 kHz, centered on the IF frequency. The extra bandwidth beyond that was simply the roll off on either side of the center frequency.
The alignment of the IF strip is crucial to the proper operation of MPX decoder, because as you can see, there's not a lot of wiggle room for the strip to be off if the full stereo channel is to be faithfully reproduced. Of particular importance is the discriminator stage transformer's alignment, so that zero output corresponds directly with 10.7 mHz, with linear deviation on either side of this frequency with modulation.
The Scott ratio detector is an unusual animal, producing a very low output. This then requires a pentode pilot signal amplification stage in the decoder, as well as the additional audio output stage that most examples of their decoder contains. For alignment of the MPX section without an FM Stereo generator, once tuned, the pilot signal of most any strong station can be used to peak the 19 kHz and 38 kHz sub-carrier transformers, watching that you don't accidentally reverse the channel identity in the process. The difference between the L and R stereo channels is simply matter of the 38 kHz sub-carrier's relationship to the 19 kHz pilot signal: it can be peaked for great operation -- with the channels perfectly reversed.
For the tuner section, the FC is your friend if using a sweep generator, so that a very good job can in fact be done with that equipment. Without the counter, it's tough to know if the frequency you're aligning the strip to is accurate or not if the generator does not include an accurate digital readout as to frequency.
I have been winding down the work I do considerably, but if you'd like to discuss that it would be best to do so via PM.
I hope this helps!
Dave
