wa2ise
Super Member
I did an intermediate to high skill level (you should have experience in aligning radios and RF circuit building) modification to a Kenwood tuner I picked up recently. This one, a KT56 is a digital tuned tuner, has the usual ceramic filter AM IF filter that yields about up to 5KHz of audio. Contrary to popular opinion, AM stations can broadcast up to 10KHz audio, for a bandwidth of 20KHz on the RF spectra. Yes, AM stations in the USA are spaced 10KHz channel spacing, but not in the same town. So 20Hz to 10KHz audio can be had from a local AM station. Sure, for talk radio who cares, but some AM stations play music not found on FM. Like Big Band music. Anyway...
The mod involves removing the AM IF ceramic filter, CF3 in the diagram I got from a datasheet of the chip the Kenwood tuner uses. A 0.01uF cap replaces it (at this point, the reception becomes very wide (about 60KHz) and audio becomes close to full range for AM, and depending on your local AM dial and if all you're after is a strong local station, you could quit at this point). And adding another IF transformer. The new (actually I salvaged it from a scrap tuner board) goes between the old IF transformer and pin 17 of the LA1265 chip. I cut the trace from that pin that went to the old transformer. And there's a jumper wire that connects that old transformer to the power supply that I removed. The new 3pF cap (see after diagram for the new parts marked with red *'s) connects to this old IF transformer tank circuit (that's the full LC resonant circuit, its primary) and to the new transformer's LC circuit, its primary. The other end of the old transformer LC circuit now goes to ground. The other end of the new transformer goes to the above mentioned power supply, and the tap will now connect to pin 17 of the chip. See the after diagram.
Once you have this installed (I drilled a few small holes in the empty area off one end of the LA1265 chip to mount the new transformer) fire up the tuner and you should hear a strong local AM station, and FM should work as it did before. Now to tune the transformers. I stagger tuned the two transformers to get a reasonably broad passband centered around the IF frequency, 460KHz. The Kenwood IF transformer allowed me to unscrew its slug upwards, shifting its resonant frequency higher. The new transformer is a different physical style, and limits upward tuning, but permits downward shifting of the resonant infrequency. The 3pF coupling cap should create a but of double humped passband, but one can do this transformer tuning to broaden this. Something like 453 and 467KHz. But be careful you don't depress the station's carrier too much or you'll get distortion. Very little margin there. I tuned in a local strong station playing music (and one I know does 10KHz audio, not the Disney outlet that limited themselves to 5KHz to permit IBOC for HD radio). Using Cooledit on the computer I connected the soundcard to this tuner, and putting cooledit into spectra mode. twiddled the transformers to get as much audio highs without distortion due to insufficient carrier. Tune the tuner up and down one AM channel, 10KHz and ideally you'd hear slightly distorted audio like what you get when you are not tuned properly, hopefully the same sound for above and below the station frequency.
Once you get done, you should be able to listen to music on AM and have it not sound so horrible. Oh, it won't compete with FM, but at least you won't want to barf.
10KHz audio isn't that horrible, and for older music they didn't record much above 12KHz anyway. And they don't play big band on FM in this town.
The mod involves removing the AM IF ceramic filter, CF3 in the diagram I got from a datasheet of the chip the Kenwood tuner uses. A 0.01uF cap replaces it (at this point, the reception becomes very wide (about 60KHz) and audio becomes close to full range for AM, and depending on your local AM dial and if all you're after is a strong local station, you could quit at this point). And adding another IF transformer. The new (actually I salvaged it from a scrap tuner board) goes between the old IF transformer and pin 17 of the LA1265 chip. I cut the trace from that pin that went to the old transformer. And there's a jumper wire that connects that old transformer to the power supply that I removed. The new 3pF cap (see after diagram for the new parts marked with red *'s) connects to this old IF transformer tank circuit (that's the full LC resonant circuit, its primary) and to the new transformer's LC circuit, its primary. The other end of the old transformer LC circuit now goes to ground. The other end of the new transformer goes to the above mentioned power supply, and the tap will now connect to pin 17 of the chip. See the after diagram.
Once you have this installed (I drilled a few small holes in the empty area off one end of the LA1265 chip to mount the new transformer) fire up the tuner and you should hear a strong local AM station, and FM should work as it did before. Now to tune the transformers. I stagger tuned the two transformers to get a reasonably broad passband centered around the IF frequency, 460KHz. The Kenwood IF transformer allowed me to unscrew its slug upwards, shifting its resonant frequency higher. The new transformer is a different physical style, and limits upward tuning, but permits downward shifting of the resonant infrequency. The 3pF coupling cap should create a but of double humped passband, but one can do this transformer tuning to broaden this. Something like 453 and 467KHz. But be careful you don't depress the station's carrier too much or you'll get distortion. Very little margin there. I tuned in a local strong station playing music (and one I know does 10KHz audio, not the Disney outlet that limited themselves to 5KHz to permit IBOC for HD radio). Using Cooledit on the computer I connected the soundcard to this tuner, and putting cooledit into spectra mode. twiddled the transformers to get as much audio highs without distortion due to insufficient carrier. Tune the tuner up and down one AM channel, 10KHz and ideally you'd hear slightly distorted audio like what you get when you are not tuned properly, hopefully the same sound for above and below the station frequency.
Once you get done, you should be able to listen to music on AM and have it not sound so horrible. Oh, it won't compete with FM, but at least you won't want to barf.
10KHz audio isn't that horrible, and for older music they didn't record much above 12KHz anyway. And they don't play big band on FM in this town.Attachments
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