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Thorn 4000 chassis sightings  

 
EmleyMoor
(@emleymoor)
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I'm sure followers of this site are well aware of how rarely a Thorn 4000 chassis TV is seen - even in their day. I am not expecting to find one now, but if I did I know what I would do with it (if I found two, it might be different).

However, I believe I did once see such a set - near neighbours of mine rented one from MultiBroadcast, until at least 1983. I base this upon the remote control - I never saw inside the set. I did, on a few occasions, reset the controls after the toddler disturbed them.

"Yes, a bit of wet string may get you a good TV signal here on four channels, but you'll have to dry it out to get Channel 5!"

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Posted : 14/03/2020 11:41 pm
crustytv
(@crustytv)
Vrat Founder Admin

Ah..4K The holly grail as far as I'm concerned. I've all but given up on the hope of ever finding one.

The only thing that gives me hope is it took me 10 years to find the family, exact first colour TV. That was a challenge as it came in at least 4 other guises and the one I wanted was the rarest, but as I say, I did eventually find it, so I live in hope. 

Until then, I drool on the brochure examples.

HMV 2726

Ferguson 3726

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Posted : 16/03/2020 7:47 pm
RichardFromMarple
(@richardfrommarple)
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I've wondered why the 4000 is such a rare set, were they just a transitional set between the 3500 & 8000 chassis?

I know most were exported with not many sold, which makes me wonder if the 4000 chassis was designed to be a system B/G set, & the home versions were just to help boost set numbers during the colour boom.

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Posted : 16/03/2020 10:45 pm
EmleyMoor
(@emleymoor)
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I'm quite curious more generally as to whether the 3000 and later made it to Ireland equipped for VHF - but of course a system B/G set would be a similar proposition.

"Yes, a bit of wet string may get you a good TV signal here on four channels, but you'll have to dry it out to get Channel 5!"

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Posted : 17/03/2020 1:04 pm
crustytv
(@crustytv)
Vrat Founder Admin

I've encountered very few folk who worked on the 4000 series and those that did give mostly casual observations of how awful they were, you can almost see the trauma on their brow! As you may or may not be aware, very few if any survive to this day (from the tales I think we can see why) mostly these TVs were destined to be export sets. This led me to search further afield finally coming upon a 2005 article in Silicon Chip.

Below you will find a report by an Australian TV engineer, he gives an interesting account of the '4KA' as the 4000 series was known over there. Hopefully you will find it as interesting as I did, it's the most I've ever heard retold of the 4000 chassis. Interesting to note they found it so unreliable AWA Thorn started importing the much older (and my all-time favourite) 3500 series chassis known over there as the 3504.


The Thorn 4ΚΑ

This chassis was almost as unreliable as the C210 but at least there was a reasonable possibility of fixing the 4ΚΑ and having it keep working long enough to get it out the door!

I think the 4000 chassis would have to be a leading contender for the most over-designed set in television history! Admittedly, the Κ9 was a pretty complex beast but at least they mostly used common parts, and they didn't break down all that often. In fact, while there are quite a few Κ9s still working even to this day, I don't know of anybody who had a working 4ΚΑ past 1990!

The 4ΚΑ was the "Ocker-ised" version of the English Thorn 4000 chassis. The UK version had a live chassis and used a full-wave rectifier (ie, it was "hot" which ever way round the mains Active and Neutral leads were connected). For Australia, they simply fitted it with an isolation transformer, a move which was adopted by a number of European manufacturers as the easiest way to make their sets meet local safety standards.

It's interesting to ponder just what went through the designers' heads when they came up with the 4000. Like most of the locally-made sets, it came with a choice of 56cm or 67cm 110° picture tubes but unlike most of the other manufacturers' offerings, these were a special RCA narrow-neck delta gun tube (which didn't seem to work any better than the standard wide-neck Philips tubes). The matching deflection yoke was also from RCA and was originally designed for use with a Thyristor line output stage. That plus the fact that the 4000 uniquely had separate horizontal output and EHT generating transistors strongly suggests that it was originally meant to use SCRs in the horizontal deflection section.

The 4000 also had an incredibly comprehensive set of convergence controls, all brought out via a monstrous cable to a paperback-book-sized hand-held control box that could be unclipped and brought round to the front of the set. Instead of the usual conglomeration of variable inductors and wire-wound pots, the controls were all thumb wheels similar to those on a pocket radio. They were clearly marked with their functions and were a delight to use when the thing was working properly which sadly, wasn't all that often!

Thorn had developed an unfortunate fixation with thick-film modules, which still live on today in the form of the ubiquitous "Sanken" audio amplifier modules. The notion was fine in theory: a resistor network could be formed onto an insulating ceramic substrate, trimmed with a laser, connecting wires, transistors and other components soldered on, and then the whole assembly dipped in epoxy. The idea was that complete circuit modules could be built this way and the heat-conductive ceramic substrate would ensure that all the components were kept at the same temperature and so avoid thermal drift problems.

There were several of these in the 4000 chassis, and they were all hopelessly unreliable. Towards the end of the 4ΚΑ's production life, AWA-Thorn started substituting small circuit boards which were far more reliable but suffered horrendous thermal drift problems. The static convergence would often drift 5mm during warmup! The 4ΚΑ also had tremendous problems with its chroma decoding circuitry. This must have started fairly early in the piece because the sub carrier oscillator and chroma processing circuitry were all located on a small plug-in board and several boards were used, none being particularly reliable.

I think the original idea was to have an elegant state-of-the-art two-chip colour processing system: a ΤΒΑ395 for the chroma processing and sub carrier oscillator and an MC1327 for the decoder and output, but Thorn just couldn't get it to work properly.

The 4ΚΑ was so unreliable that, in 1975, following HMV's lead, AWA Thorn started importing British-made 56cm and 67cm sets using the older Thorn 3500 chassis. Although this seemed like a huge step backwards for many, at least these sets with their antediluvian 90° delta gun tubes and strange transistor types were reliable and properly set up, they gave an excellent picture. The Australian version of the 3500, (dubbed the 3504) was fitted with an isolation transformer and a standard 13-channel VHF tuner.

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Posted : 18/03/2020 9:47 pm
Nuvistor
(@nuvistor)
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The author think didn’t much of them ? 

I never saw one although I can understand the use of RCA CRT’s, the Skelmersdale Thorn CRT factory was part owned by RCA and by all accounts could only produce 90deg wide neck CRT’s. Presume they chose RCA over Mullard to provide the new type CRT’s, or perhaps Mullard didn’t have the capacity.

It’s a really good write up, written with feeling. Was this the same time that the ill fated 1600 chassis was designed?

It makes me wonder if these are the same designers that brought out the later sets, which I understand were quite decent or did Thorn bring in a new team?

 

Frank

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Posted : 19/03/2020 9:08 am
Till Eulenspiegel
(@till)
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Used to look after two HMV 2726 console sets. One fault kept coming up with one of the sets, failure of the line output transistor. By reducing the HT to the line output stage it was found by oscilloscope analysis to be an intermittent flyback capacitor. The flyback pulse would shoot up to a very high amplitude. The HMV cabinet was just a fake wood effect.

Did have problems with the thick film modules.

Also a Ferguson 3C05, a compact 22" table model.

Excellent picture quality and good convergence.

Nevertheless, I was glad when the sets were withdrawn from service.

There's got be a 4K survivor somewhere. 

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Posted : 19/03/2020 5:30 pm
mfd70
(@mfd70)
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There is a servicing article on the 4000 in Television from March '81, so I guess they must have sold in reasonable numbers in the UK.

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Posted : 19/03/2020 8:53 pm
crustytv
(@crustytv)
Vrat Founder Admin

For those without data library access, the extracted 4K article (along with many others) is available via the Free-to-view section of "Service Dept", "Servicing the sets", up top, shortcut here ?

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Posted : 19/03/2020 9:20 pm
PaulGoggo1
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During my time with Multibroadcast in the Mid 80s, I saw only a couple of these sets. Many were meant for export, but we had a couple of customers who had them. The design used a lot of Thick film units if I remember. Think they were fairly reliable at the time. Thing the ones we rented out had a VHF tuner too as well as UHF. So definitely an export set.

Super rare now I guess, not seen one since 1987!

 

Paul

 

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Posted : 19/03/2020 10:54 pm
EmleyMoor
(@emleymoor)
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@paulgoggo1 The only one I ever saw was from Multibroadcast, but had a UHF only tuner.

 

"Yes, a bit of wet string may get you a good TV signal here on four channels, but you'll have to dry it out to get Channel 5!"

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Posted : 20/03/2020 10:16 am