@RSB, I noticed before that you had an interest in old junk. Are you on the UK Vintage repair and restoration forum? If you mention “sticking a Pi in”, you almost get drummed off!
@SGB, the 21-69 are the TV channel numbers within the UHF band (bands IV and V). It may well have band I, which is the frequency band where the original BBC TV pictures were found in the period before 1964 (actually, until 405 lines finished) Band II is used for FM radio. It may also have Band III which was used once “commercial” TV started for 405 line transmission, but was also used on the continent for 625 line TV.
That “white spot in the middle of the screen” was a feature of very early TV sets. When switched off, the horizontal and vertical scans collapse to a point in the middle, whilst the HT voltage and the still hot vision valves continued to produce a stream of electrons to the screen centre. This caused a burn mark with a loss of picture in the centre spot, but eventually there was going to be a spot even if no electron flow, as the CRT also produced heavier ions that destroyed the centre spot. By the mid 1950s, the CRTs were produced with the guns pointing to the side of the tube, but a magnet (an ion trap magnet) was positioned to pull the electron beam back to the centre, leaving the ions on the side of the tube’s neck. Blanking circuits were later developed that stopped the central afterglow.
Most of you here should be too young to know, but I did warn Guglielmo about all this.
Les
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