Thorn 2000 CTV: Worlds First all Transistor Colour Television: The prototype

Even though I was never into Thorn sets I was bought up on ITT /KB  I sill admire the technical advances they made like the first all transistor colour set .i can't really comment on the reliability of the sets as I only had a limited nexperience with a few models ie 3500 & 8500 but didn't like them that much 

the 2000 series must have been outstanding in its day & I wouldn't mind owning one if one should come up , the technical advances this set bought about must of inspired other manufacturers to follow in there footsteps 

i have to admit the quality of Thorns pcb are one of the best I've come across but having said that every time there was a house fire caused by a tv it always seemed to be a 3000 series that caused it with the power supply & it's 15 ohm resistor burning up & setting light to soft furnishings , I remember this happening on numerous occasions & even had to go to a set that had set fire to a house the set & the room was gutted so couldn't prove a lot but the customer said the set caught fire & caused the damage .

A few days ago I had the pleasure of meeting a guy who had several 2000 series sets 19 & 25 inch amongst other dual standard colour sets & found it most interesting that these still survive & this forum will help them survive even longer .

Chris 

I don't remember the 3000 and fires, but we did not sell them. The CTV25 was one I do know I fitted the Davy lamp mod but never had a LOPTX problem.

The last of the Pye 691 chassis could burn out the mains TX but apart from some melted wax on the carpet I never had a fire. The replacement TX had an inbuilt fuse and I modded any of the older style with fuses on the DC side so a S/C would blow the fuse not damage the older style TX. 

The later 697 chassis had PCB burnup problems but again I never saw a fire with them.

Frank

nuvistor said
I don't remember the 3000 and fires
 

First time I've ever heard of that too

First saw them at Rediffusion, the field engineers had a large wooden box which had slots inside to slide all the panels for the set in.

Enabled a quick service call and a happy customer.  We repaired the panels back at the workshop and tried to make sure each field engineer had full working set of panels.

I still remember the day our first 2000 arrived we in the workshop stood around to see it switched on and get quick sound an vision with no valves to warm up.

Earlier Rediffusion  used Bush and GEC dual standard colour sets.  Later Decca and Phillips single standard, and Rediffusion own make televisions. Mk1 etc.

I only worked on aerial sets at Rediffusion.

The 2000 25" set was a heavy beast!

John

Let's discuss the circuits employed in the BRC 2000.  Kick off with the frame timebase, the designers got away easy with that one because the circuit is based upon a Mullard design, the Pye CT70 is similar.

The power supplies employ series regulator transistors as the voltage control device, nothing radical there. 

The six button integrated tuner was used a year earlier in the monochrome 970 series. Good reliable unit.

The line output stage employs two transistors in a stacked arrangement, so did the 1960 Pye TT1 portable TV set. Of course there is more voltages and currents in the 2000 line output stage.  The problem was finding transistors for the task, Texas instruments helped out here.

The separate EHT generator is a very good idea, just like high quality monitors. That's why the BBC used specially modified BRC 2000 receivers as a cheapo monitor.  Also the chassis is fully isolated from the mains, you can earth the chassis.

Till Eulenspiegel.

raditechman said
First saw them at Rediffusion, the field engineers had a large wooden box which had slots inside to slide all the panels for the set in.

John  

Not seen the BRC 2000 engineers case but I do have the 3000 one, they sound very similar. Mine came from an engineer who closed his shop down years ago, kept a lot of his kit until he decided to move it all on.

raditechman said

The 2000 25" set was a heavy beast!

I've a couple of 25" 2000, they are dwarfed in size and weight compared to the 26" Aphelion, which can be seen here sitting atop a 1968 Ferguson 3700

Chris said

"Not seen the BRC 2000 engineers case but I do have the 3000 one".

We had those blue 3000 panel boxes later. The 2000 box was similar but was black!

John

Till Eulenspiegel said
Also the chassis is fully isolated from the mains, you can earth the chassis.

Unlike the 3000 series which as I've said elsewhere was a step backwards in that respect.

BRC 2000 as a baseband monitor.   As mentioned earlier the BBC modified the 2000 CTV as a monitor.  Reference to the IF panel circuit diagram we see that the chroma signal is picked off from the 405 video demodulator diode W2 and the 625 luma from demodulator diode W1.  The 6Mhz intercarrier sound comes from W2. The circuit description informs us that both demodulator diodes remain in the secondary circuit of the IF transformer to maintain a constant load in order that the response curve is unaltered. It should be possible to introduce a CVBS input to the BRC2000 and still allow "off air" reception as well.   An additional amplifier will be required to raise the 1 volt input video luma signal to the level required by the video board.  The same amplifier can extract the 4.433Mhz chroma signal which can be buffered by an emitter follower and routed to the chroma board.  So it follows the inputs of the luma and chroma boards can receive switched input signals either from either the baseband input or the IF panel and tuner.  The audio signals will also require "off air" or baseband switching.  

A viable project?

Till Eulenspiegel.           

Well my experience with the 2000 began when I was still an apprentice. A colleague had a frame board that was faulty (a private job I guess) and was going to send it back for exchange/repair. 

You have to remember that many "engineers" had limited transistor experience back then, in fact many were just "bottle-changers".

Anyway, I desoldered each transistor and tested it with my AVO 8 and found just one was faulty, which I replaced with a near equivalent.

He was doubtful that the board was repaired but willing to try it out - and it worked! My recompense was meagre, but a weeks apprentice wage.

My experience in "adult life" with the 2000 was less easy. We had to fit a re-gunned tube to one. These were the early days of re-gunning colour CRTs, and they would often flash-over. Well the earthing system of the 2000 chassis is not built with this in mind at all. Any flashover will kill at least one semiconductor. I think we eventually gave the customer a nice reliable 3000 TV and scrapped the 2000.

As a postscript, when working in Japan at a TV factory, I happened across a discharge test station. Prototype TVs were tested for thousands of flashovers before the earthing scheme was approved for production.

ジョン