02-14-2021, 01:01 AM
I finished the RF section Friday. Most of the bands line up pretty good, though mid band on a couple of bands are off by about 4 kHz. Not bad but on a radio like this, I would like it spot on. To do that I would have to increase or decrease coil inductance by spreading or pushing coil windings together, a Scott described process that is evident on some coils from original Scott tuning. I don't really want to mess with that. Today (actually yesterday - it's after midnight!) I got the FM all aligned. I followed the Scott recommended procedure and verified with the spectrum analyzer. The discriminator took the most time. Getting the output as linear as possible took a bit of back and forth tweaking. It seams to be very close from +80 kHz to -80 kHz modulation. Beyond that, linearity suffers a bit. But FM is limited to +-75 kHz so should be ok. Scott says +-80 is good, but it's nice to to get +-100 kHz in case a station likes to over modulate. A little non-linearity over +-75 kHz translates to a bit of distortion on over modulated signals.
One of the guys on the Scott forum has a converter available to use the FM band. I have one coming. It's really an FM receiver whose output is at 45 mHz and ties directly to the FM antenna connections. The alternative that I've seen is a TV tuner converted to receive in the FM band with a fixed local oscillator and a wide band output. This allows one to use the Scott tuner from 42 to 50 mHz. The problem is that the converter has to use 3 "channels" because you cannot get the the wide band output wide enough to cover the whole FM band. So you have 1/3 of the FM band with each TV tuner channel. A chart of conversion frequencies is necessary to know where on the Scott tuner you would tune to get a certain frequency. This is how the original converter systems worked that were provided when the FCC changed the FM band in about 1946. With the converter I bought, that process won't be necessary. Yet, I won't be twisting the Scott knob to change stations! Doesn't seem quite right. With the unit I purchased, one could easily ask, "Why not just purchase an FM tuner with audio out and connect it to the phone in?" A legitimate question that I considered. But, for some less than fully reasonable reason, I want to utilize the full FM section of the Scott receiver, just because I rebuilt it so I want to use it! It should sound great.
So far, all is very good. The switch for the "noise filter" is broken internally, and the volume control has a bit of static at very low volumes. The noise filter is fairly useless so I'm not to worried about replacing it, and the volume control can be replaced, though it really doesn't bother much.
One of the guys on the Scott forum has a converter available to use the FM band. I have one coming. It's really an FM receiver whose output is at 45 mHz and ties directly to the FM antenna connections. The alternative that I've seen is a TV tuner converted to receive in the FM band with a fixed local oscillator and a wide band output. This allows one to use the Scott tuner from 42 to 50 mHz. The problem is that the converter has to use 3 "channels" because you cannot get the the wide band output wide enough to cover the whole FM band. So you have 1/3 of the FM band with each TV tuner channel. A chart of conversion frequencies is necessary to know where on the Scott tuner you would tune to get a certain frequency. This is how the original converter systems worked that were provided when the FCC changed the FM band in about 1946. With the converter I bought, that process won't be necessary. Yet, I won't be twisting the Scott knob to change stations! Doesn't seem quite right. With the unit I purchased, one could easily ask, "Why not just purchase an FM tuner with audio out and connect it to the phone in?" A legitimate question that I considered. But, for some less than fully reasonable reason, I want to utilize the full FM section of the Scott receiver, just because I rebuilt it so I want to use it! It should sound great.
So far, all is very good. The switch for the "noise filter" is broken internally, and the volume control has a bit of static at very low volumes. The noise filter is fairly useless so I'm not to worried about replacing it, and the volume control can be replaced, though it really doesn't bother much.
