Thorn 1400 'School's set' :- any info?

[1100 man]

When I started secondary school in 1978, the school set was a huge Thorn 1400 on a tall wheeled stand. I've a feeling it was branded 'Baird'.

It had connections (SO239 and 1/4" jack) for video & audio in/ out. It also, consequently, had a hefty isolation transformer.
When not in use for a class, it was connected to a Sony CV2100 open reel B/W video recorder to record programs for future classes.

When I was 15 or so, I persuaded dad to buy a second hand CV2100 ACE (which I still have!!) I already had a 1400 chassis, so I traced out the in/ out PCB fitted to the school one and made a copy and modified my one accordingly.
No isolation transformer of course. However, I did fit a relay between chassis and earth to disconnect the supply if the chassis became 'live'- quite safety conscious I thought!!

I had never come across RCD's then so connecting neutral and earth together via connected equipment never caused me any problems!
I took it into school for some reason and discovered that when I connected it to the video recorder- the whole science block tripped out!! That was my introduction to RCD's and the difference between earth & neutral!

Does anyone have the circuitry for the school's modifications? I would like the in/ out pcb which from memory used half a dozen or so BC108's. It would be interesting to see if they modified the power supply or if the transformer was just a 240-240 isolation one.

As I still have the Sony open reel VTR, one of my 'future' projects is to fit the same mods to a 1400 set. I now have a rather beaten up 1400 20" set for repair that I bought in the auction at Harpenden. Yes, I know someone had a much better one on a stall, but it was fully working:- where's the fun in that?
This time round I suppose I'll have to find an isolation transformer for it!!

All the best
Nick

[jayceebee]

Don't remember if the 1400 or 1500 schools versions were anything other than a simple 1:1 isolation transformer. The ones used in the 3700 chassis which was the schools version of the 3500 apart from isolation had additional windings feeding the 30V regulator and CRT heaters.

More schools receiver info here.

https://www.vintage-radio.net/forum/...ad.php?t=62293

John.

[1100 man]

Thanks for the link. Reading through it, I found these pictures from 'System A' of a 1400. I think that might well be the model I remember.
So all I need to do is zoom in to the PCB on the back of the chassis and trace out the circuit again like I did 35 years ago!!

https://www.vintage-radio.net/forum/...2&postcount=58

Cheers
Nick

[volte-face]

Originally Posted by 1100 man

Does anyone have the circuitry for the school's modifications?

I think what you are asking for is attached.

Credit to "dazzlevision" for allowing me to scan the original a while ago.

[jayceebee]

Thanks V-F and Dazzlevision.

John.

[1100 man]

That's absolutely brilliant!! Thank's very much for that That is exactly what I wanted and I've just been comparing the circuit with the standard 1400. My 1400 circuit is my original photocopy that I used all those years ago:- dad having gone to the library in his lunch hour to copy it out of the hallowed 'red books'!

I discovered that on my copy, I have marked the video and audio in & out points and crossed through C34 which needs to be removed. That confirms to me that it was definitely a 1400 that I modified!

The transformer is a 230-0- 230 type with a 135V tap for the heaters. I will have to keep my eyes open for something suitable. I may end up having to use two separate transformers though.

I now need to twist System A's arm to take some detailed photos of his set!

Cheers
Nick

[HamishBoxer]

Beat me to it! I used to install and service those sets and the worse thing was the hinges.

In fact I think I remember the 950 chassis one,s.