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 Post subject: Re: AM transmitter prototype
PostPosted: Jan Wed 24, 2018 6:29 pm 
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Very nice.

Would be nice if an auto tuning unit could be designed and added to the transmitter so that the antenna is auto tuned much like how the talking house transmitters do.


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 Post subject: Re: AM transmitter prototype
PostPosted: Feb Tue 13, 2018 3:15 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Arlington, TX, USA
I made a short audio recording of the transmitter from an old Jackson Bell 62 radio connected to the computer at the speaker terminals. It actually sounds quite good apart from some distortion in the radio itself, but has a wide audio bandwidth.

https://drive.google.com/file/d/1NmoWCDvupzDfbLMb54V0cyQZg4EG2N1d/view?usp=sharing


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 Post subject: Re: AM transmitter prototype
PostPosted: Feb Tue 13, 2018 4:18 am 
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Erich Loepke wrote:
I made a short audio recording of the transmitter from an old Jackson Bell 62 radio connected to the computer at the speaker terminals. It actually sounds quite good apart from some distortion in the radio itself, but has a wide audio bandwidth.

https://drive.google.com/file/d/1NmoWCDvupzDfbLMb54V0cyQZg4EG2N1d/view?usp=sharing


Lovely sound. You did well. The Jackson Bell is TRF, right? It sure has nice highs. Do you know how wide the bandwidth is?


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 Post subject: Re: AM transmitter prototype
PostPosted: Feb Tue 13, 2018 2:23 pm 
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Location: Arlington, TX, USA
Thanks! The radio is a straight ahead, no-frills TRF. When I was setting up to record the audio I was surprised just how wide the bandwidth was at the high end of the tuning range, and that's why I used this radio to do the recording. That was for the most part always a feature/bug of TRF (bandwidth dependent on frequency) that made the superheterodyne circuit the standard. I didn't do much in the way of optimizing the whole thing, and a plasma TV was on in the next room. However, this audio clip shows that high quality sound was available all those years ago. I'd say that the quality of the speakers in the radios and the audio sources at the transmitter were the bottlenecks at the time. I'll have to measure the audio response of the transmitter/radio chain later on just to see how it is. I also have another old TRF to try out as far as sound quality; an American Bosch 48AA. It has pin jacks for the speaker, so it will be easy to connect to the sound card.


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 Post subject: Re: AM transmitter prototype
PostPosted: Feb Wed 14, 2018 1:13 am 
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Location: Arlington, TX, USA
I did a frequency response measurement on the Jackson Bell 62 at 1500kHz on the dial just to see what it was... it goes all the way out to 20kHz! The big peaks on the left of the plot are due to AC hum. The response has a broad peak around 7-9kHz and slowly drops off either side of that. This is with only a potentiometer and the analyzer as a load on the audio output transformer, however, no speaker voice coil is connected. Distortion, isn't very good as I expected since it was visible on a scope screen at the detector output. I measured around 12%THD below the obvious clipping point, but that's mostly in the radio itself, not the transmitter.

I guess the wide bandwidth is expected since this radio sometimes will receive more than one station at the upper end of the tuning dial.

The screen shot is a bit difficult to see due to picture size limitations.


Attachments:
File comment: Jackson Bell 62 frequency response
jb62FRplot.png
jb62FRplot.png [ 85.49 KiB | Viewed 40 times ]
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 Post subject: Re: AM transmitter prototype
PostPosted: Feb Wed 14, 2018 1:31 am 
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Location: Arlington, TX, USA
Just so I could be sure of what I measured, I did the same thing with a Zenith Royal 500E transistor radio. This radio has a peak at 700Hz, half power points at 400Hz and 1.2kHz. Distortion was about 1.5%.


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File comment: Zenith Royal 500 response
R500FR.png
R500FR.png [ 108.5 KiB | Viewed 38 times ]
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 Post subject: Re: AM transmitter prototype
PostPosted: Feb Wed 14, 2018 1:44 am 
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Location: Livermore, CA
Jackson Bell 62 is a TRF radio with no audio interstage transformers. Frequency response on this type of radio can be much better than superheterodyne.

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