Michael D. Sullivan <userid@camsul.example.invalid> wrote:
> Television, on tho other hand, started out in two discontiguous
> VHF bands, with somewhat variable spacing between channels and
> a need for precise tuning, and tuning in on a single band by
> twiddling an analog variable tuning capacitor to the right
> frequency would have been difficult. This tuning method was
> used on some early TVs; I don't know whether they were tuned
> by numeric frequency or by channel number, but
> it would not have been very convenient. The TV industry
> instead standardized on TV tuners that had 12 discrete fixed
> settings, pre-tuned to channels 2-13, with a fine tuning
> control that allowed one to tune the frequency higher or lower
> to account for offsets....
Whereupon Robert Bonomi (bonomi@host122.r-bonomi.com) wrote:
> Plausable, just 'false to fact'. <wry grin>
> In the early days of TV receivers, they were equipped with
> continuous-tuning knobs/dials, just like an AM radio receiver.
> For the TV band, however the indicator assembly was marked by
> "channel", *not* by frequency.
> I used to have a 1930's Crosley TV that had that kind of
> continuous tuner. *BIG* gap on the dial, between channel 6 and
> 7, It actually tuned across that entire 'midband' space -- with
> all kinds of interesting results. You could "see" aircraft
> band transmissions, and hear stuff on broadcast FM, 2m Ham, and
> business-band.
Sullivan is correct.
As Sullivan acknowledged, some old TV sets did work like Bonomi's
1930s Crosley: they required "tuning in on a single band by twiddling
an analog variable tuning capacitor to the right frequency [which]
would have been difficult."
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.
Sullivan continued:
> Later on, [turret] tuners had separate fine- tuners for each channel
> so one wouldn't need to retune when switching from station to
> station.
On each channel, the fine-tuning control engaged a tuning slug inside
one of the little hand-wound coils.
Here's a link to a picture showing a turret VHF tuner (left) and what
appears to be a continuous-tuning UHF tuner (right). This particular photo
happens to be on a British website, but the basic structure of the turret
mechanism is the same in the USA.
http://www.thevalvepage.com/valvetek/guidgrid/GGRID2.JPG
Back in my cable TV days during the 70s, turret tuners used to drive
us nuts. There was only one VHF TV station in the market (Channel 3),
so if a viewer wasn't hooked to cable, the only exercise the tuner got
was from getting flipped back and forth between UHF and 3. This kept
the contacts on UHF, 2, and 3 clean, but the rest of the contacts got
pretty dusty and/or corroded. If this viewer then connected to cable
(just 12 channels in those days), suddenly, all 12 VHF circuit boards
were needed. We spent a lot of time explaining, "I'm sorry, sir, your
TV set's tuner needs to be cleaned ... please take it to the TV repair
shop of your choice ... no we do not repair television sets ... our
franchise agreement specifically prohibits it."
Things got even worse when we introduced our first pay service (HBO) in
1978. We "hid" it in the midband on cable channel 17, "in the clear" (not
scrambled, not trapped). We provided each HBO sub with a primitive
converter: a little box with a single two-position switch:
- One position converted channel 17 to channel 2 for HBO.
- One position passed the incoming cable signal through
unaltered for channels 2-13.
It wasn't very good security, but the powers-that-were considered it to be
good enough, since turret tuners couldn't tune it.
Well, it wasn't long before local TV shops discovered a new line of
business: retuning one of the lesser-used turret circuit boards (like
public access) to channel 17.
Of course, Bonomi's old Crosley would have tuned to it. But by the
1970s, virtually all TV sets used turret VHF tuners. Varactor tuners
with digital displays were just coming on the market, and the old
continuous-tuning models had just about disappeared.
A few years later, we moved HBO to channel 2 (so we could sell HBO to
hotels and motels), installed negative traps to secure it, and
abandoned all those old two-position-switch converters. But some
people never figured it out: for years thereafter, those old boxes
kept appearing at garage sales and flea markets, sometimes accompanied
by "get HBO free" signs.
As for the folks who paid some TV shop to illicitly retune a circuit
board to channel 17 ... well, they got what they paid for.
Neal McLain
Neal McLain (nmclain@annsgarden.com)