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B&W TV Thorn all solid state large screen B&W chassis

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irob2345
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A friend of mine was given a late B&W Thorn 17" portable with what at first glance seemed to have a 1500 hybrid chassis in it (Thorn built the 1500 in Oz with changes to use locally sourced parts and a mains transformer).

But this one is an all solid state version. No valves!

I have never seen one of these before nor did I know they even existed.

Was this a UK or an Oz design?

It's in rather sad condition but if it's a rare beast I'd like to save it (UK made Ediswan CRT and all). It's copped a hit on the tube socket which is smashed to bits but the CRT is still under vacuum, getter is intact.

I'll take some pics tomorrow and post them.

It looks like it's using a similar circuit topology to what Pye, Kriesler and Philips were making in Australia at the time. 35 volts main rail, linear regulator, mains transformer. Except it looks like a 1500 and even fits into the same plastic hinges.

 
Posted : 27/12/2023 7:42 am
crustytv
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We had a modified version of the 1500 that employed a solid state field stage see here, post #6 shows the modified cct from Thorn, but it still had line stage bottles.

Perhaps Thorn 1600 series? This one I had in my collection was B&W 24" and totally solid state, the last of the big black and whites. Sort of resembled a 1500,does yours look similar?

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Posted : 27/12/2023 9:17 am
irob2345
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Thanks for that!

No, this chassis is totally different. Not even a vague resemblance. Apart from the fact that it's a single swing-out PCB. No huge dropper resistor for a start!

It was obviously designed to drop straight into existing brackets with controls in the same place as the 1500.

I'll post pics tomorrow.

 
Posted : 27/12/2023 9:27 am
Jayceebee
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Please do, should be very interesting like the unknown to me Spanish 3500 from a while back.

John.

 
Posted : 27/12/2023 11:50 am
irob2345
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OK, have a look at these and tell me what you think.

Radio Rentals also operated in Oz and they also had Thorn badge TVs for them. This is one of those sets.

One of the things I noticed was that on this chassis the adjacent channel traps were an optional extra. Sure they weren't needed in all locations, but really! This is penny pinching at its worst!

 
Posted : 28/12/2023 3:26 am
Nuvistor
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@irob2345 It’s not a chassis I recognise although my experience with Thorn sets is limited. Thorn like some other makes had a history of penny pinching in mono sets, the result in performance were usually good enough for most customers. I think it was the 850 chassis that left out a trap, most sets didn’t need it but there was a mod to add one between the tuner and IF strip if required. Then there was the 1400 with all its FM sound problems.

Pye stripped out circuits in one of their sets in the middle 1960’s, I think they changed back to direct line sync among other items. The rep told me due to financial problems they had to save around £2 in build cost for each set. Again in most cases it didn’t matter.

RMB produced a TV135RU mono set, it worked quite well though not as relable or performed as well  has the TV135U set produced at the same time at around £10 more expensive. The R version was produced I was told by the rep to compete in the rental market with Thorn whose main market was rental.

 

Frank

 
Posted : 28/12/2023 8:43 am
Jayceebee
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@irob2345 Thanks for that. Not seen anything that resembles that chassis at all, there was a 17" 1600 but I've never seen one and I think it used an audio IC for field output. The cabinet has similarities with a Ferguson 1400 transportable and a 1500 D.E.R. 5803. I've not come across the use of a TBA440 in any Thorn sets, the screening can around the LOPT is much more substantial than used in production over here, usually none at all with jellypots after the 950. The customer rear controls are better than we are used to see. It does look to me that they took an off the shelf design and did their own layout, just by looking at it there are no Thorn brushstrokes I can see in the circuitry. In a way the construction reminds me a little of the Thorn R2M chassis built downunder. Don't know if it's just me this looks rather untidy and rushed out?

I think you will find the designers may have had a lot of headaches from the bean counters especially due to the use of the mains transformer. It seemed taken to extremes sometimes here.

John.

 
Posted : 28/12/2023 10:43 am
irob2345
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Re the mains transformers - I could never understand the UK reluctance to use them, they were not an expensive item - surely no more than a huge dropper resistor. I guess it was economies of scale made them cheaper. That A&R transformer in that TV sold for about $2 as a spare part, it was used by other makers.

You could not sell a live chassis TV in Oz. Anyone who tried (such as EKCO in the '50s), the word "deathtrap" got attached. Makers (including Ekco) learned quickly.

As I mentioned earlier, I've never seen one of these before and my company used to do warranty service for everyone except Pye in those days. So I should have seen one before!

Some possible explanations:

This chassis had a VERY short run because the B&W TV market vanished overnight when colour launched in '74. You couldn't give a B&W TV away!

There was pressure for Thorn to build something more up to date, probably coming from AWA who were using the R2M chassis in their TVs to replace the long-running 50 series all valve hand wired chassis. The 50 series had benefited from decades of development and was an excellent, reliable performer. The R2M was a step backwards in terms of performance and picture quality. The company I worked for used AWAs in their rental fleet and the R2M chassis was not well received.

But the all solid state chassis just came too late.

And maybe the few that were released simply didn't break down!

It probably was based on the Philips C1, it uses Philips deflection components, just rehashed to fit in the R2M's footprint. Small semis are Philips Hendon (South Australia) made. At least the components don't sit at odd angles like they do in the R2M. And like the Pye T26 it was based on, the C1 was a rock solid reliable design.

Just so I understand, what is the connection between the 1500 and the R2M? Circuit diagrams here are confusing....

 
Posted : 28/12/2023 11:35 am
Nuvistor
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@irob2345 Live chassis sets were just down to costs, but repaired correctly and safety features kept in place they were not dangerous. 
Slap dash servicing could make them a problem but that goes for any piece of equipment.

With the UK being mainly a rental market cost was one of the priorities along with safety. We were taught in college the safety features and not to bypass them. 

Frank

 
Posted : 28/12/2023 3:51 pm
Cathovisor
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@nuvistor In addition to which unlike Australia, we had pockets of DC mains for a long while - I am led to believe the last public DC supply disappeared in 1981 with the closure of the printing presses in Fleet Street. With little need to connect peripheral equipment, DC/AC technique was perfectly safe provided you didn't do anything stupid. What was a pretty loathsome thing to do was break the neutral on a AC/DC set to switch it off - I'm looking at you, Bush! - and considering they were very late to sell DC sets before the war because of safety concerns makes it worse.

It hung around a lot in industrial areas well into the 1950s as often it was the industry that generated the power in the first place.

 
Posted : 28/12/2023 4:46 pm
Jayceebee
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Posted by: @irob2345

 

Just so I understand, what is the connection between the 1500 and the R2M? Circuit diagrams here are confusing....

I see many similarities, although the IF strip is to your spec it has the printed coils seen here, the video, sync and line drive have very few changes. The biggest difference is what appears to be the use of a LOPT from possibly a local supplier rather than shipping jellypots from the UK. Another difference is the low voltage rails for the IF is produced from the cathode of the field output pentode section rather than the heater chain.

More detailed info here.

 

John.

 
Posted : 28/12/2023 7:18 pm
irob2345
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Ah, so the R2 was the version with the transformer and the R2M was the series heater chassis. This was used only in Radio Rentals TVs.

Thank you!

That article you linked to explains some of the background of transformerless TVs in Oz.

 
Posted : 28/12/2023 9:02 pm
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irob2345
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The LOPT in the R2 and R2M was a Philips NT3102 or its Plessey-Rola clone. Yoke was sometimes Philips, sometimes Plessey-Rola, sometimes Pye. All local production, all interchangeable.

 
Posted : 28/12/2023 9:54 pm
irob2345
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I've been told that my mystery TV is a Thorn T series chassis.

 
Posted : 30/12/2023 4:59 am
Nuvistor
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@irob2345 One of the ex Thorn forum members may help shed some light on that chassis. The last chassis I remember was the 1500.

Frank

 
Posted : 30/12/2023 8:59 am
Jayceebee
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I’ve never heard of it. There was a TX250 which was an export version of the 1540 based on the 1500 but much improved. Used a proper IF strip instead of the printed coils, TAA700 in the signal stages and a PCF802 sine wave line osc. The 1540 was only ever released as a schools receiver in the UK.

John.

 
Posted : 30/12/2023 9:53 am
RichardFromMarple
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@jayceebee I assume the isolated chassis, as required for sets in schools would have been more suitable for the Australian market.

 
Posted : 30/12/2023 12:32 pm
Jayceebee
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@richardfrommarple Very true, the isolation was only required because of the AV in/out sockets for VCR etc. The TX250 made do with a conventional dropper.

John.

 
Posted : 30/12/2023 1:00 pm
irob2345
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Well I have a schematic and a page from the service manual!

Quick phone camera pics emailed to me by a local collector. They took some cleaning up but here they are.

Circuit has strong Oz Pye influences. Note that DC restoration has returned!

The intro that says this is "a replacement for the very successful R series" has to be a bit tongue in cheek. The R series was rubbished in the trade. The workshop manager where I worked at the time said "Mate, it is what it is". That's Oz speak for "get used to it, we're stuck with it"

 

No-one else had released a TV with AC coupled video and such poor video resolution for at least 10 years. And the PCB layout was untidy, it just looked like someone didn't care.

As one retailer said after the R series appeared in AWA TVs "they help to sell colour sets!"

Anyway, this chassis looks like someone took notice. It just happened too late.

Still even in this chassis there is nothing there that doesn't need to be. No bootstrapping of the audio and vertical output stages, for example.

 
Posted : 31/12/2023 4:25 am
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Nuvistor
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@irob2345 I find it interesting looking at how  designs from different companies produce the same results, i.e  picture and sound. The 33.5v regulator transistor had me wondering until I realised it was in the negative chassis end, my excuse it’s early in the morning.

”Mate, it is what it is” didn’t realise it was of Australian origin, it’s used here quite a lot in the UK these days among those I know.

How did the public react to the R chassis? I found it surprising how some in the UK would put up with really poor performance and not question it, even after they saw TV with a better picture.

After saying that I can quite happily watch an old poor quality picture if the content is what I like rather than some of the modern super quality pictures of modern output. Last night I watched Sammy Davis in a 1960’s BBC recording, not sure where the BBC obtained the material the video quality was poor but the content was top class. 

Frank

 
Posted : 31/12/2023 8:56 am
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