Re: Earliest BBC2 Sets?
Hello,
The first TV set my parent's bought that was "convertible to dual standard operation" was a Decca model DR41 (in 1963). It was made by Ferguson (Thorn) for Decca Radio & Television and used the Thorn 850 convertible chassis.
The Thorn 800 and 850 convertible chassis are factory fitted with:
a 405-625 line timebase,
switching provision for the 405/625 video drive to CRT and
the 405/625 AM/FM sound.
The factory fitted 405-625 switching consists of a rotary "wafer" type switch and a linear switch (as used in most dual standard TV receivers). These two switches are mechanically linked and operated by a rotary knob on the left hand side of the cabinet (when viewed from the front).
The IF amp/sound detector and audio output/video detector and amp and sync separator circuitry is 405 only and contained on a single large printed panel, mounted above the CRT neck and tilted to face rearwards at an angle of approximately 45 degrees. All connections to this PCB are via wire wrap pins. The chassis design employs direct line sync.
A B9A socket is fitted on the main chassis and this can be used for two purposes:
adding a line flywheel sync unit (for weak signal areas) or
connecting a 625 converter IF/video/sound detector unit (hand wired chassis).
Towards the end of the 1960s, after I had developed an interest in radio and TV electronics, I did write to Decca R&TV spares department (located at Ingate Place, Queenstown Road, London), asking for a quote for the conversion kit. They replied that they were priced at fifteen Guineas (i.e. £15, 15 shillings) - rather a lot in those days, so I didn't order one.
Dual standard conversion of the Thorn 850 convertible chassis results in a particularly cumbersome set-up. The 625 IF amplifier sits behind the existing 405 IF/sound output/video amp/sync separator printed panel and thus requires a modified back cover to be fitted. The cabinet is already pre-drilled to accommodate a UHF tuner and UHF aerial input socket (side mounted in the case of the DR41). All connections to the main chassis are by a B9A plug, push on connectors. Several factory fitted wire links (mainly on the 405-625 changeover switch) have to be cut.
The converted receiver wasn't a good performer on UHF/625, due to the high noise figure and low gain of the valve UHF tuner and the relatively low gain of the (EF183/EF184) 625 IF amplifier (as the UHF tuner's IF output wasn't first routed via the VHF tuner's mixer stage, in order to provide additional IF gain on 625). However, Ferguson did offer a small 625 single transistor IF preamplifier unit that could be clipped onto the 625 converter chassis, in order to increase sensitivity.
A couple of years later, when BBC1, BBC2 and ITV were all available on UHF/625, I acquired a nice 19" Ferguson 3618, which used the Thorn 850 dual standard chassis. I fitted a transistorised UHF tuner, in order to improve reliability, signal to noise ratio and sensitivity. This was then the main family viewing set until the arrival of our first colour TV (a 22" Philips G8, early "520" series models version).
I then proceeded to modify the Decca DR41 for UHF/625 only operation, by fitting a Pye 40F series integrated transistor UHF/VHF pushbutton tuner (with a Pye model 40F escutcheon, to make it look neat) where the front facing loudspeaker was. I also fitted the correct Thorn 625 IF unit. Both of these units were obtained from that Aladdin's Cave - Manor Supplied of West Hampstead, London).
I modified the 405 IF PCB to leave only the sync separator (EF80) and sound output (PCL82) circuitry. The other areas of the PCB were cut away, leaving sufficient room to fit the Thorn 625 IF chassis where the 405 IF PCB used to be. This meant the original back cover could still be used. I had to ensure correct heater chain current and fit a smaller loudspeaker, plus provide a low voltage dc supply for the Pye tuner. The modified set worked quite well and was my bedroom TV for a few years until I bought an ex-rental 19" Thorn 2000 CTV for £11 in 1976.
Regards,
Dazzlevision
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