> But by the 1950s, TV set manufacturers were installing "turret tuners"
> to simplify VHF tuning. A single knob rotated a cylindrical mechanism
> fitted with twelve little hand-wired circuit boards, one for each
> channel. Each circuit board had a bunch of capacitors, some
> hand-wound coils, and a row of metal contacts that mated with metal
> springs. As each circuit board was brought into position by the
> rotating mechanism, the springs mated with the contacts on the board,
> placing that board in the circuit.
> After the introduction of UHF, turret tuners were manufactured with 13
> circuit boards, one for each VHF channel one that switched to a
> separate UHF tuner. The UHF tuner was tuned in one continuous-tuning
> dial.
As you linked to a picture of typical British VHF and UHF tuners,
allow me to add a few more comments from this side of the pond.
In early times, many British sets had no outside channel selection,
just a fine tuning adjustment. As we had only one TV service then,
this wasn't much of a problem, unless somebody moved to a different
area, in which case he would need to have the set retuned. Only
channels 1 through 5 -- our low VHF band I -- were in use.
By 1955 Independent Television arrived and the turret tuner had became
standard. All ITV transmissions were on high VHF band III, channels 6
through 12, later 13. External converter units were also sold to
frequency convert band III signals down to band I for those using
older sets.
There were also some sets in the later 1950s and 1960s which included
an FM radio option (FM radio itself being a relatively new
introduction, having started in Britain in 1955). There were only
three BBC networks in those days, and some sets added three extra
turret positions to tune the "band II" FM broadcast range (a separate
cam and switch arrangement on the turret mechanism was generally
employed to blank the screen during radio reception). These positions
were sometimes labeled H, L, and T on the selector for the names of
the BBC services (Home, Light, and Third).
The plan for UHF was ready by the early 1960s, so sets incorporating a
UHF tuner started to appear. The complication was that existing VHF
transmissions were still 405-line (system A) but the new UHF
transmitters were to be 625-line (system I). The dual-standard sets
had a *monster* changeover switch which had to not only switch from
VHF to UHF but also reroute signals to different I.F. stages, switch
the horizontal scan from 10.125 to 15.625kHz, change from AM to FM
sound, invert video polarity, and so on. The switch was often a
custom-made unit for the set which ran the full length of the chassis
so that each pole was close to the required section of the circuitry.
Two switching approaches were used: One left the VHF turret positions
as normal and used a completely separate rotary or push-button/rocker
control to operate the changeover switch.
The other put a "U" position on the turret tuner to select UHF, with a
cam and link rod on the mechanism to operate the changeover. The VHF
tuner was then used as an "high I.F." for UHF reception. There were
even some designs which used an auxiliary contact on the "U" position
to apply power to a solenoid to operate the switch, presumably on the
basis that a wire between tuner and main board was easier to maintain
than a mechanical link which would need to be removed, reconnected,
and adjusted if anything was taken out for servicing.
The UHF tuners came in two flavors: One was the continuously variable
type as described, the other being a mechanical preset arrangement.
The UHF band-plan was designed to allow an eventual four networks
(BBC1, BBC2, ITV, plus a yet-to-be-determined fourth station), so many
of these UHF tuners had four buttons.
These mechanical UHF tuners operated with permeability tuning (i.e. the
slugs were moved in and out of the coils), with each button being turnable
to set the preset to any channel across the UHF bands (bands IV and V,
channels 21 through 69). This is similar to the arrangement which was
employed on the mechanical presets of car radios at the time.
For several years, people needed both VHF and UHF in order to receive
the full range of programs. The second BBC service, BBC2 went on air
on 625/UHF in 1964. BBC1 and ITV started their 625/UHF broadcasts
around 1968/69, but it would be several more years before UHF coverage
was extended to most of the country. The fourth button on mechanical
UHF tuners (sometimes marked "*" or "ITV2" in anticipation of a second
independent network) was often used by those living on the boundaries
of service areas to select an ITV broadcast from an adjacent region
(BBC was networked most of the time, while in those days the ITV
regions were far more autonomous and often had alternate programming).
Gradually, UHF covered most areas, and in the 1970s we started to see
single-standard sets once again, only this time they were 625-line and
came with only a UHF tuner. Small portables of the era tended to come
with a continuously variable UHF tuner, while by the end of the 1970s
varicap tuning and 6 or more preset positions was becoming the norm
for larger sets.
Just to finish the story, that planned-for fourth network finally went
on-air in 1982, and the 405/VHF transmitters -- still radiating just
BBC1 and ITV in monochrome only -- were eventually closed down in
1985.
- Paul.
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Paul Coxwell (paulcoxwell@tiscali.co.uk)