20-04-2018, 09:59 PM
The Chapman S6BS/FM Mk II tuner had separate AM and FM sections (except for a shared tuning indicator and a common power supply), which was unusual in British practice, although reasonably common in American tuners.
The AM side looks to be the regular S6BS circuit, which was Chapman’s piece-de-resistance, the original version dating from early 1953.
When the original S6BS/FM was introduced, c.1958-59, it was said to be a combination of the S6BS and FM91 units. It does look as if with the S6BS/FM Mk II, there was some improvement. (The FM91 itself was discontinued at about the time that the Mk II version was introduced, so probably never received the same improvement.
As best I can determine, Chapman initially used the ECC85 front end in its FM81 tuner of 1953, but switched to pentodes, including a pentode self-oscillating mixer with the FM81 Mk II in 1956, and stayed with them thereafter. Evidently it thought that two pentodes were better than a single ECC85. AFC was used on the FM80 switched tuner of c.1957, and adopted on the variable-tuned model with the FM91 iteration in 1958. This required a triode-pentode in place of the pentode, still self-oscillating, with the triode as reactance modulator. Several British makers retroactively added AFC to their variable FM tuners, probably having discovered that however stable they thought their oscillators were, it wasn’t quite enough. This group included Armstrong, Chapman, Jason, Lowther, Pye and Quad. Leak used AFC from the start with its Troughline series. In some cases, such as Quad, the AFC action was relatively mild, intended simply to correct minor drift, and not to be a substitute for initial correct tuning.
Two-gang front ends, with an aperiodic input to the RF amplifier, were the norm in British valved FM tuners, although there were some exceptions where three gangs with tuned inputs were used. Leak moved up to three gangs (and a cascode RF amplifier) with the Troughline II of 1959. Chapman had hitherto followed the norm, using two-gangs. But then came the previously mentioned improvement to the FM section of the S6BS/FM Mk II. It had three gangs, but the third gang was not used to provide a tuned input, but rather a bandpass interstage. One may only guess at the reasoning behind this. Presumably the aperiodic input was not seen to be a problem; the RF stage was not AGC’d and likely an aerial input attenuator would be used in excessively strong signal conditions. The tuned interstage would provide greater RF selectivity than a tuned input stage anyway, so using the third gang to provide a bandpass interstage would have provided the biggest benefit in terms of pre-mixer RF selectivity.
The two-stage IF strip and ratio detector was characteristic of the Chapman FM tuners throughout, although some of the lesser AM-FM models with combined IF strips did have a third stage, probably simply because it was there. A feature of all of Chapman’s valved AM and AM-FM tuners, going back to the original S4 of 1951, was that the AM section included an EBF80 in an IF side-chain to provide amplified and delayed AGC. This approach avoided differential distortion and would also have minimized modulation rise in the final IF stage.
Cheers,
Steve
The AM side looks to be the regular S6BS circuit, which was Chapman’s piece-de-resistance, the original version dating from early 1953.
When the original S6BS/FM was introduced, c.1958-59, it was said to be a combination of the S6BS and FM91 units. It does look as if with the S6BS/FM Mk II, there was some improvement. (The FM91 itself was discontinued at about the time that the Mk II version was introduced, so probably never received the same improvement.
As best I can determine, Chapman initially used the ECC85 front end in its FM81 tuner of 1953, but switched to pentodes, including a pentode self-oscillating mixer with the FM81 Mk II in 1956, and stayed with them thereafter. Evidently it thought that two pentodes were better than a single ECC85. AFC was used on the FM80 switched tuner of c.1957, and adopted on the variable-tuned model with the FM91 iteration in 1958. This required a triode-pentode in place of the pentode, still self-oscillating, with the triode as reactance modulator. Several British makers retroactively added AFC to their variable FM tuners, probably having discovered that however stable they thought their oscillators were, it wasn’t quite enough. This group included Armstrong, Chapman, Jason, Lowther, Pye and Quad. Leak used AFC from the start with its Troughline series. In some cases, such as Quad, the AFC action was relatively mild, intended simply to correct minor drift, and not to be a substitute for initial correct tuning.
Two-gang front ends, with an aperiodic input to the RF amplifier, were the norm in British valved FM tuners, although there were some exceptions where three gangs with tuned inputs were used. Leak moved up to three gangs (and a cascode RF amplifier) with the Troughline II of 1959. Chapman had hitherto followed the norm, using two-gangs. But then came the previously mentioned improvement to the FM section of the S6BS/FM Mk II. It had three gangs, but the third gang was not used to provide a tuned input, but rather a bandpass interstage. One may only guess at the reasoning behind this. Presumably the aperiodic input was not seen to be a problem; the RF stage was not AGC’d and likely an aerial input attenuator would be used in excessively strong signal conditions. The tuned interstage would provide greater RF selectivity than a tuned input stage anyway, so using the third gang to provide a bandpass interstage would have provided the biggest benefit in terms of pre-mixer RF selectivity.
The two-stage IF strip and ratio detector was characteristic of the Chapman FM tuners throughout, although some of the lesser AM-FM models with combined IF strips did have a third stage, probably simply because it was there. A feature of all of Chapman’s valved AM and AM-FM tuners, going back to the original S4 of 1951, was that the AM section included an EBF80 in an IF side-chain to provide amplified and delayed AGC. This approach avoided differential distortion and would also have minimized modulation rise in the final IF stage.
Cheers,
Steve
