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Old 19th Jun 2021, 6:20 pm   #1
Heatercathodeshort
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Default Ferguson 3629 11" 900 series. 1964/5

Thorn 900 series Ferguson 11" model 3629 1964/65
The Thorn 900/950 chassis once seen in hundreds of thousands have now virtually disappeared. Strange, you see the odd 850 and quite a number of the smaller screen 1400 but very rarely the 900/950 series, the exception being the Ferguson 3629 11" version. An autotransformer replaces the nasty dropper employed in the 850 chassis considerably reducing the internal temperature of the cabinet. It was known as 'The Cool Chassis' and it's much increased reliability reflected it's use. A 14 ohm resettable fusible resistor is fitted in the HT line that will spring open if there is a short usually caused by a faulty PY800 /PY801 boost diode. I was never very keen on these spring off resistors. You had to take care when remaking the solder connections, holding the springy bit carefully in place or it would open again a few days later for no apparent reason. At least it was a move towards better safety but saying that you often discovered the contacts wrapped with fine gauge wire before soldering...

System switching is carried out via two Bowden cables. One switches the main 405/625 switch and the other connects the UHF tuner to the mixer stage of the VHF tuner [with the oscillator HT disconnected] to provide an additional stage of I.F. amplification on UHF. The later 950 chassis employed a single solenoid to do the job with a physically longer I.F. coupling coil in the UHF position in the VHF tuner. I'm not a great lover of Bowden cables. Philips were the great users of them and managed to incorporate one in practically every chassis produced!
3The main difference between the 19"/ 23" versions of the chassis and the junior 11" 3629 is the insertion of an extra resistor wired in series with the main surge limiter to slightly reduce the HT voltage. Later 950 mk2 large screen models had a much higher EHT provided by a five stick EHT 'tray' but that is another story.

The very early 900 series was fitted with the 850 series transformer with EY86 EHT rectifier but most will be found to contain the revolutionary jelly Pot Transformer, the design of which finally made line output transformers reliable. It was fitted with a bright red half wave clip on unit containing two rectifier sticks in series. The Paxolin sticks contained hundreds of tiny selenium discs that were not so reliable. Failure was very rare unlike the 950 series that upon failure producing a stink that could be smelt a good distance away! I was once accused by an elderly couple of 'ruining their relations' due to this pong but that is another drama for another time!

The little 11" version is typical of 1960's trend with it's Teak veneered cabinet and the 'long low look' that was popular back then. It fitted in well on a simple bookshelf and was much loved by trendy young couples. It had a class look about it and Jules Thorn certainly knew how to read public taste when designing his products. They were always smart and durable. [OK don't mention the painted woodgrain but that was soon given the boot.

I have no idea where this came from..I don't think it came off the tip like so much of my junk collection but it's been around for many years and may have been saved from the crusher. It's possible that I posted some basic information about this model a good while back. I though it about time that the 3629 was given a full check over with a write up covering the full story.
7It was very clean with all the correct back screws in place. It had not been fitted with a UHF tuner from new and had probably been used in a bedroom. Being lucky, it had that thin layer of dry fluff over the long single printed circuit board. You can see how clean it is by the picture. Access for servicing is superb requiring the slackening of two brass nuts to free the chassis from the case.

I think this easy service approach was promoted by Thorn to facilitate in house service. They owned the majority of the big rental companies and it was very costly and time consuming to have complicated chassis removal, awkward service access, receivers that were fussy about the quality of the signal and just generally 'iffy'.

After all these years It was time to plug it in and see if I could tidy up a few faults that long term storage had created. I should have done a more thorough job when I first acquired it but it seemed Ok back then but of course it must have been 20 years younger! With the RF system connected I was not too surprised to discover a fair picture with bad frame linearity and good sound. The video output valve is one half of the very high gain double pentode, video out/sync sep Mullard PFL200, well capable of driving a 23" tube with ease. It had an easy life with the tiny Mazda CME1101 CRT.

Of course the system switch needed a clean together with the valve bases, all routine stuff that would have been carried out as a normal procedure during a service in the 60s/70s.

The capacitors in the frame circuit were a mixture of the usual grey/blue and yellow Dubiliar and TCC of plastic construction. These types have enjoyed a very long life but age eventually catches up and they had to be retired. The two .01uf in the linearity feedback circuit were leaky and the others were suffering a similar condition. All were replaced and a switch on proved the point. The height and linearity controls had to be reset to midway and Test Card C began to look very reasonable.

Another frame fault that can be puzzling is failure of the compensation thermistor in series with the frame coils. It is usually tucked into the coils themselves between the yoke and the windings or mounted on the clamp band of the yoke. When it goes O/C it produces a frame collapse with a wobbly line . It's a dead giveaway. It can be shorted for test but the height will shrink about an inch when the coils warm up and their resistance rises. The reason it is distorted is due to the floating frame coils picking up a bit of line waveform induced from the line scan coils. If you have a wiggly line, check the scan coil wiring on the yoke itself and if a later model such as the 900 [ usually a 110 degree model], the thermistor in the coils. It can be hidden so have a very careful look. It fell to bits when I prodded it with a screwdriver and I guess it was just about to fail. The replacement is a VA 1033 [Radiospares TH5]. If you see a packet at a junk meet I would snap them up because they are hard to find. As mentioned the chassis will work without it but you will need to set the height control when warm that is after approximately 30 mins but it will be over scanned when cold.

The contrast control did not operate correctly. At minimum setting it was still far too high a level but this was soon rectified by replacement of the AGC line decoupling capacitors that like the frame ones had similar incontinence.

It is advisable to replace C106 the 100pf high voltage ceramic capacitor in the line stabilization circuit. Use a better quality component component with a higher voltage rating. It can go S/C burning up the width/set boost control and a couple of resistors. Very many years ago I can remember replacing the the damaged components in a 950 to discover that soot from the destroyed pot had coated the printed circuit of the flywheel panel mounted above it. It gave some very odd line sync faults until cleaned!

It was now looking very encouraging and I decided to deal with the known simple stock faults that would be encountered in many Thorn Chassis from the 850 to the 1400.

First to go was the mains filter followed by the .22uf boost capacitor. The 1uf electro decoupling the screen grid of the PL500/504 was replaced by a .1uf, one of the official mods. When this cap goes O/C you get a pattern across the screen rather like a net curtain. Both S curve correction capacitors were replaced as these often failed cramping the center of the picture but customers never noticed. A quick check proved everything was still happy to function.

A hunt through my junk discovered a Thorn transistor UHF tuner complete with mounting bracket, B7G plug and leads. I was very lucky with the L shaped mounting bracket as it fitted the cabinet perfectly! This method of mounting the tuner in Thorn receivers is very common and this is probably the reason for having one amongst by general bits of mixed treasures.

It was only a case of rigging up a 12v supply, easily cobbled together with a couple of resistors and a zener diode and I had UHF signals. The picture was flashing about with the oscillator stopping at random intervals. Anyone familiar with that otherwise reliable tuner will know that all this mayhem was simply caused by the tuner rotor spindle earthing clips making bad contact with the spindle itself. The grease used to lubricate the clips became hardened and solid adding to the problem. It's a simple matter to unsolder each one in turn, clean them together with the gang spindle with Meths and refit. Keep the tuning gang closed when you carry this out and use a hot iron. You do not want to damage the angles of the outer vanes of the tuning capacitors. Yes they look as if they have been bent out of shape but they are adjusted that way in the factory to create an even spread across the band when aligning the tuner.

With the tuner refitted good UHF gain and stability was restored but there was an annoying intermittent line twitch that after a lot of frustration was caused by C100, a 1uf 400v elctro listed in the parts list as Boost HT Smoothing. It did not effect the frame height or the picture brilliance. It was completely O/C but probably made a low level connection intermittently causing the line to jitter. The flywheel sync unit piggy backed onto the rear of the chassis became a standard feature of all 900 and 950 chassis.

This series proved to be very popular. Sensitivity was excellent on VHF 405 capable of dealing with very low level signals. Considering it was fitted with a valve UHF tuner, providing the PC86/88 valves were up to scratch, the UHF performance was certainly better than average.

Pictures show. 1 Service manual with VA1033/TH5 thermistor 2 Internal chassis. 3 The auto transformer. 4 Off screen picture. 5 Fusible resistor.
Regards, John.
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Old 19th Jun 2021, 7:04 pm   #2
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Throughly enjoyable read as always. At least yours has the metal inserts on the knobs not like mine. . Andy
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Old 19th Jun 2021, 7:23 pm   #3
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Default Re: Ferguson 3629 11" 900 series. 1964/5

This takes me back to my first job at Granada Seedley (Salford) referb unit.

The 1uF and the scan coil thermistor used to impress certain people when you told them confidently what was wrong.
I don't think we had any 3629s out on rental though, I agree they were classy looking sets.
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Old 19th Jun 2021, 7:25 pm   #4
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Default Re: Ferguson 3629 11" 900 series. 1964/5

This takes me back to my first job at Granada Seedley (Salford) referb unit 1971.

The 1uF and the scan coil thermistor used to impress certain people when you told them confidently what was wrong.
I don't think we had any 3629s out on rental though, I agree they were classy looking sets.
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Old 19th Jun 2021, 8:00 pm   #5
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Although I did substantially restore mine some time back, it has the annoying problem
where the line oscillator takes a while from cold to reach the correct frequency, after which lock is totally solid.
I have followed the set up instructions and replaced the line oscillator valve, and all critical r's and c's have been replaced.
My set had a really poor varnish job, I stripped this to the veneer and applied danish oil.
The results are much better. Incredible gain and agc these sets have.
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Old 20th Jun 2021, 7:47 am   #6
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Default Re: Ferguson 3629 11" 900 series. 1964/5

That is an unusual one. The picture on this one is locked by the time the line timebase has 3/4 scanned the tube. It's a quick warm up and it has the usual heater thermistor. It was shorted out on later versions to speed up warm up time but I think the initial surge was the cause of the odd O/C CRT heater.

It may be worth checking the values of the hold controls themselves. They can drift up in value and become unstable. The flywheel sync lock on all Thorn receivers including this one is usually 100%. John.
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Old 27th Jun 2021, 2:01 pm   #7
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Thanks for a brilliant article John. It seems that one of the most common stock faults on these is the gold bits falling off the knobs.
I have two of these- a pristine one with the original valve tuner, and a beaten up one fitted with a thorn push button job from a 1400/1500, which basically works but I will rob for spares.
When I got the pristine one it had full frame scan but terrible frame linearity. The previous owner had gone through the frame timebase with new Cs to no avail. The fault turned out to be that thermsitor which had gone high value. Someone cranking the height up filled the screen but put the o/p valve on the non-linear part of its curve. Needless to say, the thermistor on the spare chassis was also duff. Since then I've been running with it shorted out, with a height diffrence of about 1.5 inches between hot and cold. I never hoped to find a replacement so was very grateful to hear of the TH5 equivalent John. I went to Ebay, and lo and behold, there were three TH5s available so I bought one for a fiver. Many thanks for that piece of information.
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Old 27th Jun 2021, 3:04 pm   #8
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Thanks Steve.
You may well find that yours is mounted on a little clip as in the picture. Tuck it well into the yoke assembly between the Ferrite core and the coils. I wonder how many guys got caught by that poor linearity display when the thermistor goes high resistance? It puzzled me when first encountered as they usually go O/C. Regards, John.
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Old 27th Jun 2021, 3:35 pm   #9
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Here are some scans of the Ferguson sales leaflet for this set.
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Old 27th Jun 2021, 4:09 pm   #10
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Default Re: Ferguson 3629 11" 900 series. 1964/5

A very informative write up I really enjoyed reading that ! I saw a fair few 900s & 950s as a lad. They were being disposed of in their droves in the mid 70s. Probably because of the colour boon.
I never saw one of these personal sets though, I didn't know they made them until the internet. I have a couple of 950s in my collection and an 850 but all are 19" . The only smaller screen portable sets I have are a 1400, that funny folded around chassis. 980? and a couple of the 405 only sets with the controls on the top the chassis number escapes me at the moment.
I didn't know about the thermistor fault either it sounds as if it was fairly common. ISTR repairing a set with a "wiggery" frame collapse. I probably just swapped the coils. I had loads of parts from scrapped sets I rarely threw anything away. When I moved out of home the sheds were cleared and the whole lot went in the dustcart. I never thought I would want it again!
Rich.
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Old 27th Jun 2021, 8:22 pm   #11
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Lovely brochure scans! I've not seen those before.

Out of interest, did any other set on the UK market use this same 11 inch crt?

Steve
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Old 27th Jun 2021, 8:49 pm   #12
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Default Re: Ferguson 3629 11" 900 series. 1964/5

The tube is a GE (Canada) TSD259, which I think was also used in the KB VC11, or
Featherlite and similar RGD model.
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Old 27th Jun 2021, 9:35 pm   #13
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Default Re: Ferguson 3629 11" 900 series. 1964/5

Ah, I did wonder if the KB/RGD Featherlight portables may have used it. There's actually a Which? consumer test which compares those with the Ferguson Personal set.

Steve
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