01-06-2017, 02:57 AM
Yes, taking the demodulated AM from the limiter grid was quite a neat idea. There was prior and significant validation of the technique for high quality purposes in the form of the Ambassador/BBC VHF AM/FM Comparator receiver, used during the UK VHF AM vs. FM trials. This receiver was briefly described, with mention of its AM detector, in Radio-Electronics for 1951 October, page 51ff. A generally more detailed description was provided in Wireless World (WW) for 1952 January, page 35ff, although the AM detector was not mentioned.
A little more information on the JTV that I have since found:
It had a ratio detector. Its sensitivity was quoted as 80 μV for 40 dB quieting. This same number was also quoted for the Prefect switched FM tuner (although I have also seen 60 μV), which was a local-area model with a two-stage IF strip. The FMT3 fringe area FM tuner with a three-stage IF strip had a sensitivity of 5 μV for 40 dB quieting, whilst for the JTV2 it was 10 μV. So, the original JTV fell into Jason’s local area group of models) whereas the JTV2 fell into the fringe area group.
All models in the local area group had two-stage IF strips consisting of an amplifier and a limiter. The fringe area group all had three-stage IF strips, either two amplifiers and one limiter, or one amplifier and two limiters. Initially Jason favoured the ratio detector, but later designs used the Foster-Seeley discriminator in order to obtain a larger AFC bias voltage. The JTV was from the later era in this regard, but as it did not have AFC, the ratio detector could be and was used.
From looking at the information that I have on hand on the various Jason tuner models and their respective circuits, my best guess is that the JTV IF strip looked very much like that of the JTV2 except that the second IF amplifier (EF89) was missing. Thus, demodulated AM would have been taken from the limiter grid as it was on the JTV2. Whereas the FM tuners used the Z77 or Z719/EF80 as 1st IF amplifier, the EF89 was probably chosen for the JTV (and JTV2) because whereas with FM (and TV vision), it was not unusual to apply AGC bias to sharp cutoff valves, with AM it was normal to use remote cutoff valves. Jason FM tuner practice was to take the AGC bias from the limiter (or 1st limiter) and apply it to either the RF amplifier (early designs) or the 1st IF stage (later designs), but not both. For the JTV2, and I’d guess for the JTV, it was applied to both the RF amplifier and the 1st IF amplifier, perhaps in part because RF stage AGC was normal for TV front ends.
Returning to the Hi-Fi News magazine connection, it would appear from the attached that the article on the Jason JTV was in the 1958 September issue, as may be deduced from the attached page from Hi Fi Year Book 1959. I imagine that that article contained the full circuit and constructional details. I also understand that the JTV (probably the built version) was reviewed in Hi-Fi News for 1959 June.
Cheers,
Steve
A little more information on the JTV that I have since found:
It had a ratio detector. Its sensitivity was quoted as 80 μV for 40 dB quieting. This same number was also quoted for the Prefect switched FM tuner (although I have also seen 60 μV), which was a local-area model with a two-stage IF strip. The FMT3 fringe area FM tuner with a three-stage IF strip had a sensitivity of 5 μV for 40 dB quieting, whilst for the JTV2 it was 10 μV. So, the original JTV fell into Jason’s local area group of models) whereas the JTV2 fell into the fringe area group.
All models in the local area group had two-stage IF strips consisting of an amplifier and a limiter. The fringe area group all had three-stage IF strips, either two amplifiers and one limiter, or one amplifier and two limiters. Initially Jason favoured the ratio detector, but later designs used the Foster-Seeley discriminator in order to obtain a larger AFC bias voltage. The JTV was from the later era in this regard, but as it did not have AFC, the ratio detector could be and was used.
From looking at the information that I have on hand on the various Jason tuner models and their respective circuits, my best guess is that the JTV IF strip looked very much like that of the JTV2 except that the second IF amplifier (EF89) was missing. Thus, demodulated AM would have been taken from the limiter grid as it was on the JTV2. Whereas the FM tuners used the Z77 or Z719/EF80 as 1st IF amplifier, the EF89 was probably chosen for the JTV (and JTV2) because whereas with FM (and TV vision), it was not unusual to apply AGC bias to sharp cutoff valves, with AM it was normal to use remote cutoff valves. Jason FM tuner practice was to take the AGC bias from the limiter (or 1st limiter) and apply it to either the RF amplifier (early designs) or the 1st IF stage (later designs), but not both. For the JTV2, and I’d guess for the JTV, it was applied to both the RF amplifier and the 1st IF amplifier, perhaps in part because RF stage AGC was normal for TV front ends.
Returning to the Hi-Fi News magazine connection, it would appear from the attached that the article on the Jason JTV was in the 1958 September issue, as may be deduced from the attached page from Hi Fi Year Book 1959. I imagine that that article contained the full circuit and constructional details. I also understand that the JTV (probably the built version) was reviewed in Hi-Fi News for 1959 June.
Cheers,
Steve
