01-11-2016, 07:48 PM
That looks like the Mk II version of the Chapman S6BS/FM. As best I can determine, the Mk II variant was introduced during 1963, and was mostly a restyling to match the then-new transistorized FM1000 and FM1005 models. I am not sure, but I think that there might have been an update to the FM circuitry, namely a change from a two-gang to a three-gang front end, still with an untuned input but with a bandpass rather than a single-tuned interstage. The initial S6BS/FM model dated from circa 1958, and was essentially the combination of the S6BS AM tuner and the FM91 FM tuner on one chassis and with an on-board power supply.
The S6BS itself probably dated from late 1952. The earliest mention that I can trace is an advertisement in Wireless World (WW) 1953 January. By the late 1950s, it had gone through several iterations , some cosmetic and dimensionally, a change from Rimlock to noval-based valves, and a change in the bandspread ranges from the original 49 through 13 metre bands to 41 through 11 metre bands. (I haven’t checked, but I wonder if it was introduced at a time in the sunspot when the 11-metre band was not usable, but later changed when that band opened up again.) It was originally unpowered, but a self-powered option was introduced by 1957. (Chapman also offered an external power supply unit.)
Chapman had entered the hi-fi tuner market with its S4 model, announced in WW 1951 September. That was a relatively simple model with a two-gang front end, but with the refinement of amplified (and delayed) agc obtained by a side-chain IF amplifier using an EBF80. This was claimed to eliminate differential distortion. It would also have minimized modulation-rise distortion without materially reducing the agc range. This feature was found in all subsequent Chapman valved AM and FM-AM tuners. The S5, in place by 1952 August if not earlier, added an RF stage to make a three-gang front-end. The S6BS added bandspread shortwave tuning, a second IF stage, and a 3rd, narrower selectivity position. By the time it was introduced, bandspread SW tuning was well established on the top-of-the-line British export radio receivers, such as the Murphy TA160, and so was probably expected in export markets. One might infer that the S6BS was intended to provide both high quality, wideband reception of local MF stations, and also reception of more distant MF stations and overseas HF stations at a quality level suitable for reasonably relaxed listening to the programme content.
The last Hi Fi Yearbook (HFYB) listing for both the S6BS Mk II and S6BS/FM Mk II was 1968-69, so the basic S6BS circuit had a long production life. I think it pretty much outlasted all other British valved bandspread tuners and receivers by a few years.
The Chapman S6(E), introduced during 1953, was a non-bandspread model using the S6BS circuit. In one of its later iterations, the S6 was also used by the BBC as the basis for its RC1/1 receiver. In the BBC support data, it was noted that the circuit used a low-distortion demodulator.
The S6BS was also used as a shipboard broadcast receiver, and also I think in some radio relay head-end equipments. I have never seen a full review of any of these models, but there was a brief (positive) commentary about the S6BS/FM in HFYB 1962.
Cheers,
The S6BS itself probably dated from late 1952. The earliest mention that I can trace is an advertisement in Wireless World (WW) 1953 January. By the late 1950s, it had gone through several iterations , some cosmetic and dimensionally, a change from Rimlock to noval-based valves, and a change in the bandspread ranges from the original 49 through 13 metre bands to 41 through 11 metre bands. (I haven’t checked, but I wonder if it was introduced at a time in the sunspot when the 11-metre band was not usable, but later changed when that band opened up again.) It was originally unpowered, but a self-powered option was introduced by 1957. (Chapman also offered an external power supply unit.)
Chapman had entered the hi-fi tuner market with its S4 model, announced in WW 1951 September. That was a relatively simple model with a two-gang front end, but with the refinement of amplified (and delayed) agc obtained by a side-chain IF amplifier using an EBF80. This was claimed to eliminate differential distortion. It would also have minimized modulation-rise distortion without materially reducing the agc range. This feature was found in all subsequent Chapman valved AM and FM-AM tuners. The S5, in place by 1952 August if not earlier, added an RF stage to make a three-gang front-end. The S6BS added bandspread shortwave tuning, a second IF stage, and a 3rd, narrower selectivity position. By the time it was introduced, bandspread SW tuning was well established on the top-of-the-line British export radio receivers, such as the Murphy TA160, and so was probably expected in export markets. One might infer that the S6BS was intended to provide both high quality, wideband reception of local MF stations, and also reception of more distant MF stations and overseas HF stations at a quality level suitable for reasonably relaxed listening to the programme content.
The last Hi Fi Yearbook (HFYB) listing for both the S6BS Mk II and S6BS/FM Mk II was 1968-69, so the basic S6BS circuit had a long production life. I think it pretty much outlasted all other British valved bandspread tuners and receivers by a few years.
The Chapman S6(E), introduced during 1953, was a non-bandspread model using the S6BS circuit. In one of its later iterations, the S6 was also used by the BBC as the basis for its RC1/1 receiver. In the BBC support data, it was noted that the circuit used a low-distortion demodulator.
The S6BS was also used as a shipboard broadcast receiver, and also I think in some radio relay head-end equipments. I have never seen a full review of any of these models, but there was a brief (positive) commentary about the S6BS/FM in HFYB 1962.
Cheers,
