Demise of School TVs... - Page 5 - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 17th May 2015, 10:13 pm

In my thread about the BRC 980 cable TV receiver I mentioned that I have acquired a comprehensive Baird 620 series service manual.
A special schools receiver was made by Baird television, the 638, a 23" screen model.
Service information can be found on page 25 of the manual.
The general notes inform us that minimum modifications were carried out to the basic dual standard 620/640 chassis, this will permit easy replacement of the chassis. The schools authority insist on a three pin plug and socket for mains connections. Another requirement is mains isolation by a double wound mains transformer. All metalwork must be earthed, and that includes metal emblems and trim strips.
The mains transformer supplies two BY100 silicon diodes in a full wave rectifier arrangement. This effectively supplies the chassis with DC mains. It follows that the mains dropper resistor is adjusted as if the receiver was operating on DC mains. An added bonus is that the HT ripple is 100Hz instead of 50Hz. Improved HT smoothing.
An optional extra for the schools set is a special audio amplifier which employs two PCL82s in a push-pull output circuit. The optional amplifier simply plugs into the valveholder of the audio output valve.
Because the schools receiver has an isolated chassis extension loudspeaker sockets can be provided.
There are no pictures of the 638 set in the manual.

DFWB.

dseymo1 · 18th May 2015, 6:02 pm

Not 'School TV' as such, but when I was in the sixth form, we had a common room, which we were allowed to decorate and furnish ourselves. Of course, TV was not allowed, but a bit of ingenuity saw an ex-auction set (17 " Bush of some sort, I think) built into the cupboard under the sink, and extension speakers into the armchairs, Houses of Parliament style, live chassis notwithstanding!

I later became a school technician, and concur that teachers must be amongst the least technically-competent groups around.

17th May 2015, 10:13 pm	#81
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Demise of School TVs... In my thread about the BRC 980 cable TV receiver I mentioned that I have acquired a comprehensive Baird 620 series service manual. A special schools receiver was made by Baird television, the 638, a 23" screen model. Service information can be found on page 25 of the manual. The general notes inform us that minimum modifications were carried out to the basic dual standard 620/640 chassis, this will permit easy replacement of the chassis. The schools authority insist on a three pin plug and socket for mains connections. Another requirement is mains isolation by a double wound mains transformer. All metalwork must be earthed, and that includes metal emblems and trim strips. The mains transformer supplies two BY100 silicon diodes in a full wave rectifier arrangement. This effectively supplies the chassis with DC mains. It follows that the mains dropper resistor is adjusted as if the receiver was operating on DC mains. An added bonus is that the HT ripple is 100Hz instead of 50Hz. Improved HT smoothing. An optional extra for the schools set is a special audio amplifier which employs two PCL82s in a push-pull output circuit. The optional amplifier simply plugs into the valveholder of the audio output valve. Because the schools receiver has an isolated chassis extension loudspeaker sockets can be provided. There are no pictures of the 638 set in the manual. DFWB.

18th May 2015, 6:02 pm	#82
dseymo1 Retired Dormant Member Join Date: Dec 2007 Location: Shropshire, UK. Posts: 3,051	Re: Demise of School TVs... Not 'School TV' as such, but when I was in the sixth form, we had a common room, which we were allowed to decorate and furnish ourselves. Of course, TV was not allowed, but a bit of ingenuity saw an ex-auction set (17 " Bush of some sort, I think) built into the cupboard under the sink, and extension speakers into the armchairs, Houses of Parliament style, live chassis notwithstanding! I later became a school technician, and concur that teachers must be amongst the least technically-competent groups around.