Thorn / BRC850 repair

[Niechcial,Steve]

PS It's not hard to fabricate that cover over the LOPT. Mine is missing too. Perhaps some one who has the original with the valve line up label on it would be kind enough to copy it for us? Thanks

[Heatercathodeshort]

The Thorn 850 was the first 'plug and play' dual standard chassis. The 800 series did have the 405/625 switch but it did practically nothing. It did not even change the line speed.

A clever but awkward conversion kit was available to mount the UHF I.F. panel on the back of the chassis. This involved a new back assembly with a rectangular hump to cater for the extended chassis. It worked quite well but very few were actually converted. Most of the conversion units were sold off by Manor Supplies a few years later.

Rambo mentions the 700 series and I too have never seen a converted one. Unlike the arrival of the ITA in 1955 BBC2 was very poorly received in two ways! Expensive conversion and new aerials were simply of no interest.

With a new pair of PC88/86 valves in the UHF tuner they do work fairly well on UHF. Took this pic this morning and it's quite watchable.

I hope you get yours up to standard Graham. They are quite a rare chassis today. The landfill must be full of them! John.

[Niechcial,Steve]

Re the convertible 700- was the systems switch just a ploy for marketing to be able to claim the set as 'convertible'? Several manufacturers did this. The most extreme example I can think of is the Perdio Portarama Mk 2 where the only 625 bit is a four pin power socket labelled '625' or something similar! In fairness to Perdio though they did actually manufacture a conversion kit and I know someone on the forum has got one. Considering there is no line switching or extra taps on the LOPT, and certainly no 625 signal stages, converting the Mk2 must have been an expensive job- which probably explains why so few converted ones exist.

[Richard_FM]

I think someone mentioned on here that they were working at a TV shop when BBC2 started and they had a list of customers wanting to have their converted, only for many to cancel when they realised the programming on BBC2 was too niche for them!

[dazzlevision]

The Ferguson 700 series models were originally 405 only, with no provision for conversion to dual standard working. However, a “Phase 2” chassis was later used which could be converted in the way described in post 22 by HCS.

The 850 chassis wasn’t the first Thorn chassis to be fully dual standard - there was a Thorn 800 chassis, which was very similar to the 850 d/std chassis. The main difference was that the IF output of the UHF tuner went straight to the IF strip, via a B7G socket mounted on a bracket on the main chassis, rather than the VHF tuner’s mixer valve, which provided additional IF gain.

There were also “convertible to dual standard” (in a similar fashion to the 700 series) versions of both the 800 and 850 chassis - Thorn published separate full service manuals for all four.

[ms660]

The 900 was the real game changer for dual standard TV sales where I worked.

Lawrence.

[Heatercathodeshort]

There was a Thorn 800 chassis, which was very similar to the 850 d/std chassis. The main difference was that the IF output of the UHF tuner went straight to the IF strip, via a B7G socket mounted on a bracket on the main chassis, rather than the VHF tuner’s mixer valve, which provided additional IF gain.

Agh yes! The 800 series dual std. The UHF performance was very bad even in Wimbledon that was 'swamped' with UHF signal. Thorn later supplied a small transistor I.F. stage that coupled the UHF tuner to the B7G socket.
They were very rare in London. I think I only encountered a couple of them. John.

[Iontrap]

Here you go Steve
850 LOPT cover
Simon

[Niechcial,Steve]

Fantastic. Thanks!

[Graham_W]

Excellent, thanks. Shouldn't be difficult to make one.
Just ordered more capacitors!
Regards
Graham

[Niechcial,Steve]

I've got the tube back and the picture is very good. I've lashed up a temporary transistor UHF tuner and the thing is working on 625 for the first time in its life. Next job is to find and rig up a permanent valve UHF tuner.

[Niechcial,Steve]

I am very impressed with the results on this. The 3 mhz bars are well resolved on 405. I managed with much chopping around to get in a valve uhf tuner from an old KB. The only problem is that the mechanism runs the wrong way round, so when the knob says channel 69, you're actually on 21! The 625 picture is good, but the contrast is only just adequate -as is to be expected with a valve tuner. I haven't made a replacement LOPT can yet. The patterning and bars on the pic are camera artefacts.