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Old 15th Aug 2018, 7:08 pm   #1
Heatercathodeshort
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Default GEC 9" table model BT2147 1950.

This GEC BT2147 Bakelite cased receiver has been lurking around for a very long time.
I must confess I have completely forgotten where it came from but one thing is for sure, it has been stored in a room in a domestic situation. It is entirely complete with a very clean chassis, cabinet, back, bottom and Bakelite feet.

The original makers manual is a great help when working on any vintage receiver. GEC manuals are excellent!

I wasted no time in snipping the mains filter capacitor, better known as a rifle bullet and plugging it in. The valves lit and after a while the line timebase could be heard whimpering into frail life. There was a complete lack of sound from the speaker, not even a trace of hum.

A check with the EHT meter showed around 4kv but there was absolutely nothing on the face of the CRT a GEC manufactured 6505A aluminised triode. Quickly shorting the cathode to earth produced nothing!

I won't bore you with wax capacitor replacement but a few tickles with the soldering iron in the line timebase raised the EHT to around 6KV. This should have produced something and it did... Turning the workshop lights out a very faint distorted raster could be seen but that was in total darkness..

The CRT was completely useless and due to it's flat face was not an easy one to locate a suitable replacement. I laid the chassis down for a couple of months until I had the time to hunt through my tubes. I discovered a Ferranti T9/5, almost identical with a flat face, the only problem was that the Ferranti supported a 4v 1amp heater that would not suit the .3amp series chain of the GEC.

A hunt through my CRT transformers [Radiospares] produced 2v, 6.3v, and 12.6v examples but no 4v! To cut a long story short I rewound the secondary of one of the 6.3v types to give exactly 4v on load. That solved the heater problem but the GEC EHT connection is only a tiny 'pip'. This was quickly replaced by a nice Bakelite Bulgin type, fully shrouded.
The octal base matched the original GEC so no problems there. The transformer was mounted on the front chassis strut and looks as if it has always been there.

Powered up it was an amazing improvement. The CRT lit up brightly and with the workshop RF signal applied a tangled up test card could just be resolved.
It was time to replace all wax caps. Testing a couple showed they were 500k resistors and there was no way the chassis could work correctly in this mess. The originals were brown wax of GEC manufacture with the details printed in white on the surface. Restuffing was not really an option.

With suitable caps fitted the improvement was dramatic. With the slider presets adjusted a very reasonale but pale test card was displayed. Only slight adjustment was required to the focus unit and I reckon I got away with this one far too easily. Even the slider presets all perform normally.

Interlace watched my progress from the end of the bench and looked puzzled. He loves to fiddle with ion traps twisting them around the CRT neck and giving a little grunt when the screen fills with light. To his disappointment GEC and Ferranti aluminised triode tubes do not require an ion trap. He soon cheered up when I gave him a Thorn 3500 to converge. He just twiddled all the presets until the screen was a mass of mixed colours. To him that is colour television...

There was still a lot of work to do. The receiver unit slides into the bottom of the chassis rather like the BUSH TV22. It is a very stable TRF unit using Z77 [EF91] valves, good quality decoupling capacitors and only a couple of brown torpedoes. A few voltage checks led me to the anode load resistor of the video amplifier Z77. It is two 10K 1watt resistors wired in parrallel. It read 19K and was soon replaced by a couple of shiny high stab 1 watts. This brought the contrast back to life giving a bright picture of similar quality to the HMV 1805 and that takes some beating believe me.

The complete lack of sound was then investigated. The dinky little N37 output valve was hot and it's screen grid was not aglow. From this I quickly deduced that the output transformer was probably OK. It was. A check across the speaker terminals gave an O/C reading and to be honest I had to double check this as O/C speech coils are a rarity.

The Speaker is a 5" round design of GEC manufacture and well made. Scraping the black paint/glue from the wires from the speak coil proved this to be good. Replacement of the pigtail braides cured the problem

We now had sound but very distorted, a sure sign that the coupling capacitor had a heavy leak. It was so leaky that turning the volume up caused the picture to reduce in size a sure sign that the sound output valve was drawing very heavy current. Replacing the cap discovered in the RF unit brought forth good sound and no picture variations.

After 45 mins the dreaded EHT began to drop from just over 7kv to a little under 5kv. A current of 150m/a was passed through the overwind for a couple of days. This dried any dampness within the winding resulting in only a slight drop of around 350v after 5 hours. It does not effect focus or picture size and I guess most very early flyback transformers with laminations rather than Ferrite cores would have behaved this way from new.

Surprisingly all the electrolytic capacitors were up to spec, another credit to the GEC. Valve bases, controls, [the brightness 100K had to be replaced. It was practically O/C at the earthy end] presets were all cleaned together with the cabinet that only required a wash down with bathroom cleaner and a light polish with Pledge.

SPEC: 15 valve TV receiver for AC or DC mains. [GEC's first] 9" aluminised tube.
Interchangeable TRF receiver unit for Alexandra Palace or Sutton Coldfield. Later TRF units were available for Holme Moss Kirk O shotts and Wenvoe. A tunable superhet deck was needed and it came along the following year with the introduction of model BT5146.

As with all GEC receivers the quality was very high and like Ekco, new designs were introduced when new components and tubes became available but the original concept remained for over 10 years ending with the BT337 of 1961.

A very satisfactory overhaul with excellent results. A very attractive little receiver that once again must have given an immense amount of pleasure to it's original owner.

Pictures show: Early results with the tube heater powered from the bench power supply, the underside of the RF deck. Various views of the chassis.

Regards, John.
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Old 15th Aug 2018, 7:10 pm   #2
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Default Re: GEC 9" table model BT2147 1950.

The final finished result. J.
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Old 15th Aug 2018, 7:22 pm   #3
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Default Re: GEC 9" table model BT2147 1950.

Very lucky finding that replacement tube, mine had an open cathode which I managed to short to the heater deliberately, then fed it with a suitable transformer. Picture is nowhere near as good as yours though.
Whats going on with the EHT connector ?
Another fine writeup John
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Old 15th Aug 2018, 7:30 pm   #4
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Default Re: GEC 9" table model BT2147 1950.

Very lucky finding that replacement tube, mine had an open cathode which I managed to short to the heater deliberately, then fed it with a suitable transformer. Picture is nowhere near as good as yours though.
Another fine writeup John
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Old 15th Aug 2018, 8:28 pm   #5
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Default Re: GEC 9" table model BT2147 1950.

Quote:
Originally Posted by Heatercathodeshort View Post
The receiver unit slides into the bottom of the chassis rather like the BUSH TV22. It is a very stable TRF unit using Z77 [EF91] valves, good quality decoupling capacitors and only a couple of brown torpedoes.
Regards, John.
To a first approximation the TV22 looks very similar. Who got there first?

Peter
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Old 15th Aug 2018, 8:42 pm   #6
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Default Re: GEC 9" table model BT2147 1950.

With Interlace put to bed I spent a little time this evening adjusting the focus and picture position knurled adjusters. I took the pictures with a small amount of back lighting to prevent over exposure. Other than that they are straight off the screen.

Great care must be taken when aligning the sound rejectors T1 and T2 the Philips beehive trimmers mounted on top of the RF unit. The null point is extremely critical. They state this in the manual but once correctly set and sealed rejection is 100%.

I forgot to mention that the old finned selenium rectifier had been replaced in the past with a BY100 silicon diode and 50 ohm 10 watt surge limiter. This had probably been carried out in the early 1960's.

It may have been only 405 lines but who is counting! John.
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Old 15th Aug 2018, 8:47 pm   #7
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Default Re: GEC 9" table model BT2147 1950.

Hello Peter,
The Superhet TV22 arrived in 1950 replacing the TRF TV12. I think the Bush was the first to be capable of tuning to all 5 channels. J.

PS The standard EHT connection on the Ferranti is at the end of a vulnerable glass 'horn' The GEC has a tiny glass 'pip'. I removed the pip connector and replaced it with the Bulgin connector. The photo shows it hooked up for testing with the old GEC connector still on the end of the EHT lead.

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Old 15th Aug 2018, 9:07 pm   #8
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Default Re: GEC 9" table model BT2147 1950.

Thanks John. The screen shots are stunning!

Peter
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Old 15th Aug 2018, 11:45 pm   #9
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Default Re: GEC 9" table model BT2147 1950.

That's amazing John.

Paul.
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Old 16th Aug 2018, 6:45 am   #10
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Default Re: GEC 9" table model BT2147 1950.

That little GEC now has a cracking picture, just as it would have had when new!
A really attractive little set, GEC probably based the chassis design on the TV12.
Quote:
I think the Bush was the first to be capable of tuning to all 5 channels.
You are indeed correct, my early 1950 TV22 has that luxury, while other manufacturers continued to produce different RF chassis for different areas for some time to come.

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Old 16th Aug 2018, 7:46 am   #11
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Default Re: GEC 9" table model BT2147 1950.

The PYE FV1 [Five vision one] was a close contender. I think this arrived about 9 months after the BT2147. It was a very popular model with good gain and worked well in 'difficult' areas. The LV30/51 series was PYE's last TRF series.

This link will give the details of conversion to Band 3 of GEC TRF receivers. This avoided the 'aerial' converter with all it's problems. John

https://www.vintage-radio.net/forum/...ght=GEC+Bt2147

and this one will be of interest if you discover one and want to have a go.

https://www.vintage-radio.net/forum/...ght=GEC+BT5144

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Old 16th Aug 2018, 11:34 am   #12
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Default Re: GEC 9" table model BT2147 1950.

A very enjoyable write up John, thank you. Lucky that you had a suitable tube to hand and what excellent pictures that set is capable of.

John Joe.
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Old 16th Aug 2018, 10:09 pm   #13
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Default Re: GEC 9" table model BT2147 1950.

Those are really good pictures on your GEC john , another excellent restoration.
Nice to see you bringing another tv back to life.

Robin
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Old 16th Aug 2018, 10:46 pm   #14
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Default Re: GEC 9" table model BT2147 1950.

Wonder if the little telly showed The Human Jungle first time around?
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Old 16th Aug 2018, 11:46 pm   #15
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Default Re: GEC 9" table model BT2147 1950.

A superb job and write-up Sir well up to the usual standards!
What an outstanding picture these things can produce. I'm surprised that all those pre-set sliders were good. My own BT2147 which we rebuilt last year is still running beautifully.
I had exactly the same lopt saturation problem soon after the rebuild and the eht would drop 2kv after about 30mins. This appears to have sorted itself out on its own over time and (now) a nice warm place to live!
Great to see another one alive John!
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Old 18th Aug 2018, 8:01 am   #16
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Default Re: GEC 9" table model BT2147 1950.

Well done John. A tip top restoration and write-up as usual.

Kind regards.

From Mike.
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