TV Prisma 91 - Belgian 625/819 automatic hybrid

[LENINGRAD T2]

A few weeks ago, an unattractive 21-inch TV came to me. The seller informed me that this is a Swedish product. This intrigued me a bit. Swedish TVs are not very popular. I have attached his photo below. The receiver is missing the speaker cover and the tube neck cup. I have already designed these elements for 3D printing. It took me a long time to find out that the TV was made in Belgium and has the name Prisma 91. Year of manufacture probably 1956. There is almost zero information on the Internet about this device. One description is very interesting. The Prisma 91 is a 625/819 auto-switching model with no mechanical switch.
Does anyone know anything more about this TV?
Does anyone know other TV sets with automatic standard switching, e.g. 625/405?

   

[Murphyv310]

Hi Adam
Any chance of putting up some internal photos. I know David Boynes has contacts in France who may be able to shed some light on the chassis.

[LENINGRAD T2]

Thank you Trevor, thank you David. In a few days I will remove the chassis and take pictures. I attach the text that initiated the creation of the post.

[ppppenguin]

I've never heard of a TV with auto 625/405 switching. I can't see there was ever any need for it since 405 and 625 were never broadcast on the same channel.

Some professional video monitors had auto 625/405 switching. The all-transistor Prowest PM14/1A, the little Peto Scott TB1204A 8.5" all-valve monitor that ran very hot. Probably others too.

[Mike Watterson]

Belgium spawned the Barco. In the 1970s a Barco did everything except 405 lines.

There was a market in Belgium for TVs to receive also French, Dutch and German TV. Belgian and French 819 were different. Also Dutch or German 625 might have been on a similar frequency to Belgian or French 819.

Ireland "solved" this issue by using 405 lines (1962-1985) in areas that could pickup BBC/ITV 405 from NI or Wales and 625 in the rest of the country. Most of Ireland was "fringe" reception then for minimal number of TX sites, so I doubt that dual standard was common until after BBC2 started in NI and Wales, and then would have been sets purely for UK market.
625 Only sets for Ireland had Band I and Band III as that was used in the west, mid west & south west.

[Geordie McBoyne]

A made for Belgium multi-standard TV receiver will need to operate on 819 lines systems E and F, the latter being the narrow vision bandwidth system with the same vision-sound spacing as as system B but with AM sound. As far as the IF amplifier stages are concerned narrow bandwidth 819 and 625 CCIR system B are no more difficult to implement as 405/625 switching arrangements, in fact probably simpler. The real difficulties begin with the requirement to receive full bandwidth 819 transmissions. Vision-sound spacing is 11.15Mhz compared with CCIR 5.5Mhz. Sound IF amplifier might employ double-conversion in the same manner as certain Murphy TV sets equipped with FM radio.

Attachment shows a Peto-Scott TV monitor which has automatic switching between 405 and 625 lines.

Geordie McBoyne.

[ppppenguin]

Attachment shows a Peto-Scott TV monitor which has automatic switching between 405 and 625 lines. 

That's the Peto Scott TB1204A 8.5" monitor that I mentiioned in a previous post. I used to look after several of these but they are long gone. Gave good pictures but got horribly hot. I remember stripping out all the dual standard bits to try and reduce dissipation a little. Probably not much, as the standards sensing was the only solid state part.

One of these monitors also gave me a hideous fault that took me a long time to fix. This was probably back in the 1980s. One of these monitors would occasionally suffer field collapse. I changed every part in the field timebase, including the FOPT and scan coils by swapping it with another monitor. A wire to the FOP valve heater had never been soldered. Worked 99%+ of the time.

I'm sure we have a few of those in the BECG stores.

[LENINGRAD T2]

In the border areas of Belgium, the Netherlands and France, it was possible to receive a signal from the national transmitter operating in one system and from the neighbor's transmitter operating in a different standard. This undoubtedly enriched the program offer. Many Philips receivers from the 1950s were equipped with the 625/819 switch. It was probably heavily exploited. The development of a TV set with automatic standard switching was supposed to improve the "ergonomy" of viewing. Unfortunately, the Prisma company, we are talking about was a small concern. Only rudimentary documentation about it and its products has survived. It is known that it was located in Antwerp. It operated in the 50s. It produced only a few TV models. Prisma 91 was available in tabletop and console versions. The receiver has a transparent plexiglass cover on the front. The case was covered with veneer or leather. I have attached a photo with the type and layout of the valves. Unfortunately, I did not get to the wiring diagram of the receiver. I even made contact with Belgian lovers of old electronics. They also do not have such documentation. The internet also has its limitations.
I was supposed to put the Prisma 91 TV on the shelf. However, I think that without a schematic, you can also try to run it. But he has to wait in line. In the electrical system, the presence of the 85A2 voltage regulator draws attention. This is a rarely used solution in television receivers.

I wonder what happened to the millions of 405 line receivers in the UK when the 625 standard became widespread. Has a national recycling program for old receivers been established?

I think a little Peto scott would look good in my collection .

   

[Mike Watterson]

I wonder what happened to the millions of 405 line receivers in the UK when the 625 standard became widespread. Has a national recycling program for old receivers been established?

The 4th UK channel didn't start till November 1982.
The 5th UK channel was in 1997 and coverage was such an issue that it was FTA on Analogue Satellite with a "fake" flag claiming encryption.

The BBC 2 launched in 1964, the first channel not on 405. It was part-time and niche, so the 405 sets often used as 2nd sets for BBC1 and ITV, still the most popular channels even in 1985 when the 405 System closed down.

There was no recycling scheme for any electronics from 1960s to 1980s.

Unwanted TVs were dumped.

The average life of a 405 set might have been 10 years, or maybe less. In 1971 and 1972 most of the sets I repaired with 405 were dual-standard and often the standard switch link to knob would be removed. Some would be soldered in 625 position due to switch failure.

Most of the 405 only sets had smaller screens (more than 17" rare) and getting dim. Few would have been operational by 1985. I don't remember seeing any 1975 to February 1983 when I left the UK.

Ireland also turned off 405 in 1985, but it had never been nationwide. Cable TV (later Analogue Satellite) as well as pick up of UK BBC 2 and C4 would have resulted in quicker demise of 405 only sets in the Irish areas that used it. In areas that couldn't get BBC with an aerial there had only ever been 625 (started 1962, though officially December 1961).

[Murphyv310]

Hi
Contrary to common perception 405 line sets lasted longer than you'd believe.
I started with British Relay in 1970 and the amount of 405 line only sets out on rental was staggering. The piped cable line system had switched to 625 lines in 1969 only, off air 405 line sets on rental was still a mainstay of their business. The sets were often 12 to 15 years old including Murphy V310-510, Ekco T344/368/377. By the end of 1974 though very few remained mostly those in no UHF reception areas which there was a few pockets in Ayrshire, some sets were approaching 20 years old.
The sets when scrapped were not recycled, something unknown in the 70s, smashed up and went to landfill.