Path: not-for-spool
From: Jeff Layman <jmlayman@invalid.invalid>
Newsgroups: uk.tech.digital-tv
Subject: SOT - Old Rediffusion cable converter box
Date: Sat, 7 Mar 2020 08:45:05 +0000
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Whilst clearing out some electronic junk, I came across a Rediffusion 
converter box used at Thamesmead in SE London in the early 80s (the 
system had been in use since 1970). According to 
<http://www.rediffusion.info/London/>:
"Rediffusion London had a Community Service contract to provide an h.f. 
system to the first 9,000 dwellings. Incorporated in the Thamesmead 
project was to be all the latest developments of Rediffusion 
engineering. With 12-pair cable being used throughout, there was no 
anticipated problem with channel capacity. Two 405 and three 625 line 
television programmes and Radios 1, 2, 3 and 4 were being relayed at 
that time."

My parents were there in 1972. TV aerials were not allowed (maybe they 
were thought to soften the "brutalist" architecture too much!), and TV 
was available only through the Rediffusion cable service. There was a 
box on the lounge wall with a circular multiway switch that was used to 
select the TV (or radio) programme you wanted (photo at post#12 and more 
info at <https://www.vintage-radio.net/forum/showthread.php?t=63338>). A 
cable went from the selector switch to the box, which was mains-powered, 
and the output from the box fed through a coax cable to the aerial input 
of a normal UHF TV.  At 
<http://offog.org/articles/rediffusion/switch.html> there is a picture 
inside the selector box and a comment that "The yellow cable with a knot 
in it is the output from the switch; it contains just a single pair, and 
would have run from here to the receiver". The cable on mine is grey, 
about 7 mm in diameter, and has a twisted pair inside. The twisted pair 
wires have quite thick insulation on them, and are somewhat inflexible.

There are some photos here:
<https://ibb.co/hY8HsWS>
This shows the box itself - about 15x10x3cm. Top left is the twisted 
pair input, top right mains input, and bottom right coax output.

<https://ibb.co/CwznSH7>
This is the device with the box top removed (I've also removed the top 
of the screened RF converter section at middle bottom). This uses a 
BF480 transistor, which appears to be the only active device in the 
whole circuit. The power supply is just a capacitor dropper feeding a 
full-wave rectifier, and maybe 12v zener (It has PH 79C 12 on it - 
Philips BZX79 series?). The two capacitors at top left in series with 
the input are marked "1500 2% 8KV"!

<https://ibb.co/F7SWpZm>
This is the underside of the PCB. I suppose one thing of interest is the 
printed inductors towards the top right next to the RF converter. The 
other thing of note is the wording near the bottom left which has 1983 
as the date. My guess is that in the early 80s, when Channel 4 started 
broadcasting, the original converter had to be replaced as it was 
limited to three 625-line channels.

Anyone here remember the Rediffusion system and perhaps work on it? 
Could this system be described as a sort of early broadband, using low 
MHz frequencies on twisted pairs?

There don't seem to be any pictures of this box on the internet, so 
would the box be of interest to any tv museum? I'd be happy to post it 
to them (FoC of course) before it goes for recycling.

-- 

Jeff