"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
Early TVs often had a faint hum bar in the vertical. By being locked
to the line frequency, it was fixed to one location, and most people
never saw it.

I'm not sure when the UK came off mains lock - somewhere like the late
'50s. And there must have been older TVs still in use when this happened,
as even some very early single channel ones were converted when ITV
started in the mid '50s. And I can't remember rolling hum bars being
common.


The US TVs had sync, that matched the line frequency. Some early
sync generators simply multiplied the line frequency by 525 then divided
by 2 for the horizontal frequency.

Even when they built crystal controlled generators, the line
frequency remained close enough that it would take minutes or hours to
roll though a frame. Also, by the '50s the power supplies were better
filtered. The hum bars were faint, but visible on older TVs, and one of
the first signs of trouble when they became more pronounced. A lot of
US monochrome TVs were transformerless, and used a voltage doubler in
the power supply. pairs of 300 uF 160 volt electrolytics, where some
early TVs had 8 or 16 uF filtering.


--
You can't fix stupid. You can't even put a band-aid on it, because it's
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