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GEC BT5147

 
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GEC BT5147

Post by PYE625 » Tue May 03, 2016 5:39 pm

Hello all,
This is a GEC BT5147 from 1953 and is BBC channel 1 to 5 only.
Well after collecting the said set from near to Leicester today, I could hardly wait to get her on the bench.
It looks to be complete and un-molested and, believe it or not, an intact barretter valve which I know is prone to failure.
The set is so dirty inside that I thought it best to remove the tube to a safe place whilst I clean and investigate the electronics.
There are a number of difficult to get to waxies that will need replacing and I would rather do it with the tube out of the way.
It all comes apart very nicely and here are a few pics to show you.....
Attachments
rsz_1001.jpg
rsz_002.jpg
rsz_004.jpg
rsz_005.jpg
rsz_006.jpg
rsz_007.jpg
rsz_1008.jpg
rsz_003.jpg

 
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Re: GEC BT5147

Post by marc » Tue May 03, 2016 6:29 pm

Hi Andrew,

So the little beasty has landed ! :)
Relatively speaking your set looks a darn sight cleaner than my last one (BT1746 band III) I felt and looked like I'd been down a mine for a week after cleaning it but it came up well in the end.

The barretter on your set looks different to both my sets it's a lot fatter at the top, is it a 305 and does it have a metal cage slid over it when it's in position ? :aaq

Looking forward to watching it's progress. :)

Marc.

 
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Re: GEC BT5147

Post by davidh1041 » Tue May 03, 2016 6:31 pm

That's an interesting set you have there Andrew, look forward to follow your progress.
David

 
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Re: GEC BT5147

Post by PYE625 » Tue May 03, 2016 7:11 pm

Thanks guys,

Here is the barretter back in its position, but there is no metal cage over it. 305 is the type.
Attachments
rsz_001.jpg

 
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Re: GEC BT5147

Post by marc » Tue May 03, 2016 8:53 pm

Hi Andrew,

Seeing the barretter in position it does look right now where as on the first image it just looked too big, bit of an optical illusion I think :aaq
It should have a cage over it, a round ventilated metal tube, but obviously it's got lost over the years. If I get the chance I'll dig out a photo of one.

Marc.

 
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Re: GEC BT5147

Post by Till Eulenspiegel » Tue May 03, 2016 9:48 pm

In the early 1960s as a replacement for the often unreliable 305 barretter the GEC service department supplied a resistor fitted into the ES cap. The barretter itself was a clever device, it had an iron filament and the bulb was filled with hydrogen. The heater chain current of 0.3amp was maintained over a wide range of voltages.
Many pre-war radios employed barretters, for 0.2amp heater chains Mullard supplied the types C1 and C1C.

Till Eulespiegel.

 
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Re: GEC BT5147

Post by PYE625 » Tue May 03, 2016 10:07 pm

Thanks Marc and Till, it would be interesting to see what the cage looks like.

It's interesting to read the Practical Television issue of July 1959 where G.J. King discusses the premature failure of the 305 barretter along with other fault symptoms.
They mention about intermittent heater to cathode shorts in the valves immediately after the barretter. They suggest that the U329 boost diode is the most likely suspect.
They also suggest to temporarily fit a normal 150w lamp during a test, the lamp glowing indicating a short.
The U329 in this set looks slightly tired, worse than the photo suggests.

Also, insulation breakdown is mentioned in C41 connected between cathode of U329 and ground causing white spots on vision and hiss on sound.
However, if you look very closely at the connections of the cap in my set, it could go dead short with no effect !! :bba
Attachments
rsz_1001.jpg
rsz_001_2.jpg

 
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Re: GEC BT5147

Post by Till Eulenspiegel » Tue May 03, 2016 10:29 pm

From the Radiomuseum, details of the Osram U329: http://www.radiomuseum.org/tubes/tube_u329.html
The Mazda U251 is similar. If you looking for reliable replacement consider the PY88, 30volt heater.

Till Eulenspiegel.

 
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Re: GEC BT5147

Post by PYE625 » Tue May 03, 2016 10:41 pm

Till Eulenspiegel wrote:From the Radiomuseum, details of the Osram U329: http://www.radiomuseum.org/tubes/tube_u329.html
The Mazda U251 is similar. If you looking for reliable replacement consider the PY88, 30volt heater.

Till Eulenspiegel.


Cheers, a PY88 would reduce the volts drop across the barretter a bit too. :qq1

 
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Re: GEC BT5147

Post by marc » Wed May 04, 2016 12:13 am

Hi Andrew,

Here's a photo of the barretter cage on my BT5147.....
DSCF3674 (Small).JPG
The cage has a split from top to bottom to allow it to stretch around the barretter.

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 9:14 am

Thanks Marc, your set looks nice with the barretter guard in place.

Well, I have replaced all of the Hunts and wax capacitors, not one of them was useable due to being very leaky so no real surprise there.
All of the electrolytics reformed nicely including the low voltage ones, a 100uf 25v in the sound stage, and a 500uf in the frame.
I cleaned and re-fitted the tube and associated parts including the focus ring assy.
For the time being, I have left off all the valve spring retainers.
Attachments
rsz_img_2081.jpg
rsz_img_2082.jpg
rsz_img_2083.jpg

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 9:36 am

So....
The time for first light !!
Rather dark I'm afraid....
Dim heaters, R78 a 470 ohm dropper section getting rather hot.
If it worked, it would be boring !!

Turns out the thermistor is O/C although mechanically sound. Strange, but anyway....replaced it and we have proper heater volts now.
Set warms up, but low sound and no EHT although line stage is partly running.
HT is approx. 50% and the metal rectifier is pretty dud.
Fitted a 1N4007 and HT comes up to around normal value.

Louder sound and a raster appears !!
Then all goes dark again :aai
Noticed the tube heater had gone out along with most of the halve heaters on the RF deck.
It then comes to life again and heaters alight for a few moments, only for the cycle to repeat.
After some checking around, it turns out that V12 on the RF deck has an intermittent heater to cathode short.
Replaced it and normal operation results.
Feed in a signal and tune to CH1, and hey presto !!!
Attachments
rsz_img_2084.jpg
rsz_img_2085.jpg

 
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Re: GEC BT5147

Post by Michael Watterson » Thu May 05, 2016 9:37 am

Till Eulenspiegel wrote:In the early 1960s as a replacement for the often unreliable 305 barretter the GEC service department supplied a resistor fitted into the ES cap. The barretter itself was a clever device, it had an iron filament and the bulb was filled with hydrogen. The heater chain current of 0.3amp was maintained over a wide range of voltages.
Many pre-war radios employed barretters, for 0.2amp heater chains Mullard supplied the types C1 and C1C.

Till Eulespiegel.

Zenith Transoceanic and some other USA battery/mains sets used Barretters up till 1959 as 110V to 220V adaptors.

The testcard looks good :)

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 9:58 am

It took a fair bit of knob twiddling to achieve the test card as shown, I do admit lol :bba
Verticals are a bit ragged and if I tune for best picture, the sound is almost absent.
Will now go through the I.F. Alignment table on the trader sheet.

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 11:35 am

Before I check the I.F, I thought I'd look at the metal rectifier.
Sadly, there is no way of repairing these that I know of, so a bit of silicon will have to be mounted as a replacement.
Attachments
rsz_006.jpg
rsz_1007.jpg

 
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Re: GEC BT5147

Post by Michael Watterson » Thu May 05, 2016 12:07 pm

No way to repair them and NOS ones quickly fail.
I leave them in place (disconnecting one end and use the other as a tag, as long as no HV insulation breakdown issues to chassis) and put the 1N4007 or 1N5407 etc or whatever behind them. I usually add a series wire wound resistor to simulate the original voltage drop. I don't like replacing valve rectifiers with silicon unless the smoothing caps are upgraded as it's instant on vs gradual. Usually MR PSUs have sufficient working voltage on the caps.

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 3:38 pm

I fitted the IN4007 in a yellow sleeve and it feeds a 25ohm resistor I mounted it on a tag strip just behind the main dropper.
The HT is now correct (was slightly too high with just the diode) and I re-fitted the metal rectifier in its place, but out of circuit.
The blue wire goes down to C66 where the old metal rectifier was connected.

Not too sure of the power rating of the resistor I fitted as its getting a bit hot, perhaps ought to find a beefier one.

The picture has become brighter since running for a while, but at least the tube is ok.
I don't know if it is aluminised as it is quite early.
Attachments
rsz_001.jpg
rsz_002.jpg

 
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Re: GEC BT5147

Post by Cathovisor » Thu May 05, 2016 4:30 pm

PYE625 wrote:I don't know if it is aluminised as it is quite early.

According to this useful page, no...

 
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Re: GEC BT5147

Post by marc » Thu May 05, 2016 4:45 pm

Hi Andrew,

Looks like you've landed a good CRT with this set, I normally seem to be short of luck with these type of screens (except my 2 GECs) either by zero emission or O/C heater.
Looking forward to seeing it all going back together again.

Marc.

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 4:51 pm

Possible data error?
I have the data in a Wireless World 1954 issue and it gives the heater voltage of 6.3V and current of 0.3A for the Mullard MW36 24.
Now in my set, the heater voltage on the tube is 3.15v and the current is 0.3A.
All other heater volts are correct through the set.

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 4:52 pm

Cheers Marc,

Will be together soon lol :bba

 
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Re: GEC BT5147

Post by crustytv » Thu May 05, 2016 5:29 pm

Andrew you may already be aware but if not, the GEC manufacturers service manual is in the data library.

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 5:50 pm

Thanks Chris......have to admit I didn't look :ccg

 
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Re: GEC BT5147

Post by PYE625 » Thu May 05, 2016 10:43 pm

Well I've now got the ion trap magnet the right way up and adjusted it for best bright picture. :ccb

 
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Re: GEC BT5147

Post by PYE625 » Fri May 06, 2016 3:51 pm

Here are just a few more pics, I have re-aligned the I.F and sound is now louder and picture is great, I'm really pleased with the set.
Attachments
rsz_007_2.jpg
rsz_1003.jpg
rsz_2003.jpg
rsz_11005.jpg

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