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Old 3rd Jan 2017, 7:33 pm   #1
FERNSEH
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Default 14" BBC only Sobell TV set.

Just arrived today, this 14" Sobell TV possibly of 1953 vintage. The label on the chassis indicates it is a Sutton Coldfield receiver but I'm sure it will tune any of the other four BBC channels.
What's the chances it could be made to work again? These were very reliable sets and I do remember repairing a few survivors in the early sixties.
The circuit diagram will be in one of the Radio and Television servicing books.

DFWB.
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Old 3rd Jan 2017, 8:30 pm   #2
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Default Re: 14" BBC only Sobell TV set.

I saw that one! A bargain methinks, glad it's gone to a good home. I was tempted, but I talked myself out of it- I find them less tempting once the tubes start turning square. Looked eminently redeemable to me, I'm sure you'll have no problem.
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Old 3rd Jan 2017, 9:40 pm   #3
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Default Re: 14" BBC only Sobell TV set.

The back is missing to find out model type but looking at all the available Sobell information in the 1953/54 R & T servicing book it's safe to suggest the set is most likely to be the model T143.

DFWB.
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Old 3rd Jan 2017, 10:23 pm   #4
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Default Re: 14" BBC only Sobell TV set.

I think my mum would have made me leave that one outside in the coal shed David..

Seriously a great simple reliable model like many others of the original Sobell company.
Should be an easy one to get some sort of results. I note by the label that the MW36-24 is the original. A raster by tomorrow? Good luck with it. John.
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Old 4th Jan 2017, 2:22 am   #5
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Default Re: 14" BBC only Sobell TV set.

Hi John,
The first task was to remove the chassis from the cabinet so that it's condition can be assessed. The PL81 and PY81 valves were missing in the line output stage compartment. All other valves present and correct. Valves found and fitted. Before supplying the set with any mains power a few tests were performed. No HT shorts, using the Simpson 260 multimeter the HT line to chassis measured 20Kohms gradually increasing as the HT capacitors charged up. Next, the heater chain, this was found to be open circuit at the top of the chain. Working from the bottom of the heater chain it was found many of the valveholders had high resistance contacts. A whole hour was spent on the heater chain, the worst valveholder was the ECL80 frequency changer valve.
So in due course the heater chain was repaired, next item to test was the on-off switch, this was found to be OC on the mains neutral side and the live side was measuring 10 ohms, too high. I've found that old rotary switches can be revived by squirting switch cleaner into the switch and heating up the tags with the soldering iron. The trick worked and the contact resistance was reduced down to less than 1 ohm.
Time to give the set a taste of electricity, start off with fifty volts from the variac and over an extended time turn up voltage in stages. When 200 volts was reached a faint line whistle could be heard and even a small spark at the anode of the EY51 EHT rectifier. Just go for it and turn the output from the variac to max volts. Not before too long a nice flame like spark at the EY51 anode and a good spark at the anode of the CRT. no illumination on the screen though. Moving the ion trap magnet on the CRT neck gave no results, a faulty tube? Well maybe not because when the set is switched off a bright horizontal line appeared briefly on the screen.
Perhaps the CRT will be OK after all.

DFWB
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Old 4th Jan 2017, 12:13 pm   #6
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Default Re: 14" BBC only Sobell TV set.

Good luck with this. I never knew an ECL80 could be used as a frequency changer before.
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Old 4th Jan 2017, 12:37 pm   #7
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Default Re: 14" BBC only Sobell TV set.

Hi Edward,
The ECL80 was employed as the frequency changer in the Pye FV1 series. Very versatile valve.

DFWB.
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Old 4th Jan 2017, 4:09 pm   #8
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Default Re: 14" BBC only Sobell TV set.

More progress. The line timebase speed was too slow. Replacement of the 150Kohm resistor in series with the line hold control solved that fault condition.
CRT socket voltages were: first anode 323 volts, cathode 140 volts, both these figures are correct. The grid measured 83 max volts, that's too low and the CRT is cut-off. Replaced a leaky 0.5mfd CRT grid decoupling capacitor. Grid volts are now 0 to 150V depending on the setting of the brightness control. The result of all this work is a bright raster. Frame Timebase speed is too fast. More waxies to replace.

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Old 4th Jan 2017, 10:14 pm   #9
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Default Re: 14" BBC only Sobell TV set.

The frame timebase can now be adjusted for the correct speed, that is 20mS. The fault was traced to the frame linearity control having gone open circuit, it's P6 in the circuit diagram, 50Kohms.
The ramp forming capacitors C52 (0.01mfd mica) and C53 (0.05mfd paper)have surprisingly low values for a frame timebase. C53 was replaced as a matter of course and the C52 checked out to be perfect.
So the next stage of the restoration my attention will turned to the signal circuits.
Will a bright Test Card C appear on the screen?

DFWB.
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Old 5th Jan 2017, 2:19 am   #10
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Default Re: 14" BBC only Sobell TV set.

The set is now displaying pictures. The CRT is excellent, you could even say it's as good as new, sharp focus, good definition.
In order that line and frame timebase sync was possible the coupling capacitor between the video amplifier and the grid of the sync separator needed replacement. C22 in the circuit diagram. It has the unusually low value of 0.01microfarads, 0.1mfd is the more typical value.
More picture height is required, the control is at the max position.
The HT supply is 173volts, a bit on the low side.
Although the label on the chassis indicates this a Sutton Coldfield receiver. Sometime during the lifetime of the set it was retuned to Channel 2.

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Old 5th Jan 2017, 10:05 am   #11
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Default Re: 14" BBC only Sobell TV set.

I like your service method David. No mass cleaning and component replacement that leads to frustration and disaster. Just the minimal of replacements to allow you to see the condition of the CRT and LOPT.
By the look of it I guess you have been 'tinkering' with television receivers for a few years and learnt a few tricks.
Regards, John.
PS. Is that a real test card or a cardboard cut out? Fantastic!
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Old 5th Jan 2017, 4:12 pm   #12
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Default Re: 14" BBC only Sobell TV set.

Quote:
Originally Posted by Heatercathodeshort View Post
By the look of it I guess you have been 'tinkering' with television receivers for a few years and learnt a few tricks.
Regards, John.
Hi John,
Been fixing TVs for more than fifty-six years, should really give it up and get a proper job but there was no vacancies at the local brick yard.
Seriously though folks, the last time I repaired a similar set was about Easter 1961 and the set was the Banner branded version, model B114.
Sobell went on to make very popular derivatives of the T143, two well known models were the TS17 and the 14" T346.

DFWB.
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Old 5th Jan 2017, 8:24 pm   #13
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Default Re: 14" BBC only Sobell TV set.

What a fantastic result David! Just goes to show you can't judge a book by it's tatty cover. I'm sure many would have passed that set by, put off by the flaky case.

Was there an earlier 12" version of this set? I'm sure I've seen one with the same cabinet with it's distinctive curved profile at the bottom. The 14" screen surround looks like a bolt-on job for the larger crt. Just like one of your famous 'adaptor plates'!

Steve
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Old 5th Jan 2017, 9:25 pm   #14
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Default Re: 14" BBC only Sobell TV set.

Hi Steve,
There was indeed a 12" version of the T143, the T224. It's likely Sobell wanted to follow the success Ferguson had with the 988T, later versions were called the twelve plus model with a modified mask.
On the subject of adaptor plates, such a device with a special purpose will be fitted in the Sobell. Watch this space.

DFWB.

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Old 9th Jan 2017, 2:47 pm   #15
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Default Re: 14" BBC only Sobell TV set.

Substituting the MW36-24 with an AW36-20.
In fact there is no need to make a new CRT rear support for the electrostatic focus CRT. It should be a simple task to remove the focus magnets from the assembly shown in the attachment. The picture shift ring can be still used.
The parts will cleaned up and the original appearance retained.

DFWB.
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Old 1st Feb 2017, 5:52 pm   #16
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Default Re: 14" BBC only Sobell TV set.

Have just fitted the AW36-20 and picture brightness is absolutely excellent.
Before fitting the new CRT the magnets were removed from the focusing assembly, unfortunately the picture shift ring doesn't work anymore so the usual electrostatic CRT shift magnet will be required. In order that component can be fitted a new rear CRT rear support bracket will have to be made. This job hasn't proved to easy after all! So do I continue with the CRT changeover job or keep the AW36-20 for something better?
So if I do continue working on the Sobell there's a chance it could be made into something really special.

DFWB.
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Old 2nd Feb 2017, 10:50 am   #17
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Default Re: 14" BBC only Sobell TV set.

Oh dear David,
You must stop unscrewing parts from a television receiver chassis and throwing them in the dustbin..

The AW36-20 looks really bright. I must admit I would have thought that the beam shift plates would have worked at least to some extent but of course their positioning in relation to the gun assembly is far closer to the CRT base than the electrostatic shifter would have been.

I wonder what the effect would be if you just substituted the AW for the old MW tube leaving the old focus assembly in place. I think I tried this when I was still in short trousers but I can't remember what the effect was.

Dig the old bits out of the bin [unless of course the dustmen have been] and give it a go. The gun assemblies are similar up to the A1 but the spacing between the 1st anode to make way for the focus electrode may be the critical factor.

It has just occurred to me. We are doing all the silly experiments we did the best part of 60 years ago. They say you start in nappies and end your days in them. What a thought!

Regards, John.
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Old 2nd Feb 2017, 11:42 am   #18
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Default Re: 14" BBC only Sobell TV set.

Brilliant result, the rectangular tubes seem to have a generally longer life than the round ones - some of them anyway.

Can't quite see the 3Mhz bars yet.

Peter
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Old 2nd Feb 2017, 8:53 pm   #19
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Default Re: 14" BBC only Sobell TV set.

Quote:
Originally Posted by Peter.N. View Post
Can't quite see the 3Mhz bars yet.
Hi Peter, Something to attain for and I think it's most likely that the 3meg bars will be seen on the screen of this set.
Hi John, all the removed focus magnet components can be reassembled together again and refitted into the set.
Who knows what the results will be like when one considers the AW36-20 has a self focussing gun?

DFWB.
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Old 8th Feb 2017, 10:53 pm   #20
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Default Re: 14" BBC only Sobell TV set.

What was wrong with the original tube?
You said it produced a good display? It looked good from the photo.
Did it die?
I wonder why it was replaced with an electrostatic focused tube?

Andy
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