28-05-2017, 03:32 AM
Here is the schematic for the JTV2:
As may be seen, the Fireball turret was substantially modified, with ECF80 and ECC81 valves in place of say ECC84 and ECF80.
The ECF80 triode served as a grounded grid RF amplifier, unusually for that type with AGC. It fed the ECF80 pentode mixer, its customary role. But a triode RF amplifier feeding a pentode mixer was generally considered to be an undesirable combination in VHF practice, as the triode would not have enough gain to offset mixer noise. One ECC81 triode was the oscillator, and the other was the AFC reactance valve. One may wonder why Jason did not retain the standard turret circuitry with the addition of a varicap diode for AFC. That was established practice in the USA and Germany for example.
The three-valve IF strip with a Foster-Seeley discriminator was conventional when looked at in FM terms, athough for FM only one might have expected the second stage to be configured as an amplifier/limiter, rather than an amplifier only, the latter being required for its AM function. AGC bias was taken from the limiter grid back to the 1st IF stage and also to the RF stage without the additional delay typically found in TV receiver practice. I’d guess that its FM quieting curve was not quite as good as might be obtained in an FM tuner with say an ECC85 front end and a three-stage IF strip, although its limiting sensitivity might not be too much different.
AM was taken from the limiter grid, the diode formed by the grid and cathode acting as a demodulator. (That same rectifying action produced the AGC bias, after appropriate smoothing.) And there was a noise limiting diode, as was found in the sound channel of many UK TV receivers.
That though does not give too much joy in trying to derive the JTV circuit, and in particular its two-stage IF strip. Possibly that was derived from the IF strip for the Prefect switched FM tuner. That used a pair of EF80s, and quite unusually for a two-stage circuit, had a Foster-Seeley discriminator. I think though that provided a bigger AFC voltage as compared with a ratio detector, so might have been the reason for its choice. As far as I know the JTV lacked AFC, so probably could have used a ratio detector to advantage.
Cheers,
Steve
As may be seen, the Fireball turret was substantially modified, with ECF80 and ECC81 valves in place of say ECC84 and ECF80.
The ECF80 triode served as a grounded grid RF amplifier, unusually for that type with AGC. It fed the ECF80 pentode mixer, its customary role. But a triode RF amplifier feeding a pentode mixer was generally considered to be an undesirable combination in VHF practice, as the triode would not have enough gain to offset mixer noise. One ECC81 triode was the oscillator, and the other was the AFC reactance valve. One may wonder why Jason did not retain the standard turret circuitry with the addition of a varicap diode for AFC. That was established practice in the USA and Germany for example.
The three-valve IF strip with a Foster-Seeley discriminator was conventional when looked at in FM terms, athough for FM only one might have expected the second stage to be configured as an amplifier/limiter, rather than an amplifier only, the latter being required for its AM function. AGC bias was taken from the limiter grid back to the 1st IF stage and also to the RF stage without the additional delay typically found in TV receiver practice. I’d guess that its FM quieting curve was not quite as good as might be obtained in an FM tuner with say an ECC85 front end and a three-stage IF strip, although its limiting sensitivity might not be too much different.
AM was taken from the limiter grid, the diode formed by the grid and cathode acting as a demodulator. (That same rectifying action produced the AGC bias, after appropriate smoothing.) And there was a noise limiting diode, as was found in the sound channel of many UK TV receivers.
That though does not give too much joy in trying to derive the JTV circuit, and in particular its two-stage IF strip. Possibly that was derived from the IF strip for the Prefect switched FM tuner. That used a pair of EF80s, and quite unusually for a two-stage circuit, had a Foster-Seeley discriminator. I think though that provided a bigger AFC voltage as compared with a ratio detector, so might have been the reason for its choice. As far as I know the JTV lacked AFC, so probably could have used a ratio detector to advantage.
Cheers,
Steve
