14-06-2017, 08:03 AM
Here is the Hi-Fi News review of the Jason JTV, from the 1959 June issue:
The review was undertaken by R.S. Roberts, whom I recall was pretty searching when it came to RF equipment. It was done on a Jasonkit JTV. Some salient points:
* The JTV used a TV turret type front end. The RF stage, using a double-triode, was described as being of the “driven grounded grid” amplifier. This was RCA’s original terminology for the series cascode circuit.
* The Demodulated AM was taken from the limiter grid, as on the following JTV2 model. The limiter grid was also the source of AGC bias for the RF stage and the 1st IF stage. It did have an AM noise limiter diode.
* For FM, a ratio detector was used, which also supplied the bias for the magic eye tuning indicator.
* The JTV was not intended for fringe area use.
Contrary to my earlier speculation, the JTV, at least as tested, did not have a drift problem, although the reviewer suspected that without AFC, it might have done. Thus, Jason’s reasons for adding AFC to the JTV2 probably lay elsewhere.
An interesting comment in the review was that more accurate tuning was available by audible means than from the tuning indicator. Audible tuning was done by switching to AM for FM signals, and FM for AM signals, and then tuning for minimum output. Of course, it was well-known that conventional magic eye tuning indicators were not really satisfactory for FM receivers because of the relatively wide, flat-topped nature of their IF passbands. Jason itself had provided a commentary to this effect in its earliest literature on its FM tuner kitsets.
With that background, one may adduce Jason’s reasons for moving from the JTV to the JTV2 as being: (1) to provide fringe area reception capability; and (2) to simplify tuning, by using AFC effectively as AFT, presumably with a lower residual error than could be obtained with a conventional magic eye. In respect of the latter, one may note that the Jason/Jasonkit FMT3 fringe area FM tuner, released at about the same time as the JTV, had variable AFC, but not a tuning indicator. The variable AFC allowed reduction of its effect when manually tuning say a weak station next to a strong station, whilst after tuning increasing its effect to provide both fine tuning and station hold. With a preset tuner, like the JTV, AFC variation was less necessary since initial station tuning was a setup event, not a recurring event.
Cheers,
Steve
The review was undertaken by R.S. Roberts, whom I recall was pretty searching when it came to RF equipment. It was done on a Jasonkit JTV. Some salient points:
* The JTV used a TV turret type front end. The RF stage, using a double-triode, was described as being of the “driven grounded grid” amplifier. This was RCA’s original terminology for the series cascode circuit.
* The Demodulated AM was taken from the limiter grid, as on the following JTV2 model. The limiter grid was also the source of AGC bias for the RF stage and the 1st IF stage. It did have an AM noise limiter diode.
* For FM, a ratio detector was used, which also supplied the bias for the magic eye tuning indicator.
* The JTV was not intended for fringe area use.
Contrary to my earlier speculation, the JTV, at least as tested, did not have a drift problem, although the reviewer suspected that without AFC, it might have done. Thus, Jason’s reasons for adding AFC to the JTV2 probably lay elsewhere.
An interesting comment in the review was that more accurate tuning was available by audible means than from the tuning indicator. Audible tuning was done by switching to AM for FM signals, and FM for AM signals, and then tuning for minimum output. Of course, it was well-known that conventional magic eye tuning indicators were not really satisfactory for FM receivers because of the relatively wide, flat-topped nature of their IF passbands. Jason itself had provided a commentary to this effect in its earliest literature on its FM tuner kitsets.
With that background, one may adduce Jason’s reasons for moving from the JTV to the JTV2 as being: (1) to provide fringe area reception capability; and (2) to simplify tuning, by using AFC effectively as AFT, presumably with a lower residual error than could be obtained with a conventional magic eye. In respect of the latter, one may note that the Jason/Jasonkit FMT3 fringe area FM tuner, released at about the same time as the JTV, had variable AFC, but not a tuning indicator. The variable AFC allowed reduction of its effect when manually tuning say a weak station next to a strong station, whilst after tuning increasing its effect to provide both fine tuning and station hold. With a preset tuner, like the JTV, AFC variation was less necessary since initial station tuning was a setup event, not a recurring event.
Cheers,
Steve
