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Old 3rd Feb 2018, 9:56 pm   #1
FERNSEH
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Default Gec bt1091.

Brought this GEC BT1091 into the workshop from storage upstairs. The first fault to sort out is the failure of the frame timebase caused by the faulty valveholder of the KT61 output valve.
The under chassis picture shows that many components have already been replaced. The capacitors will be recognised as the type sold by Maplins.
The fourth and fifth pictures show a small single valve unit fitted above the deflection components. No mention of this assembly in the service manual, at least it's not in the issue No. 120.

The GEC BT1091 was possibly the companies first TV to employ flyback EHT.

DFWB.
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Old 4th Feb 2018, 12:16 am   #2
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Default Re: Gec bt1091.

Quote:
Originally Posted by FERNSEH View Post
No mention of this assembly in the service manual, at least it's not in the issue No. 120.
It gets a mention in Trader sheet 1019.
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Old 4th Feb 2018, 5:52 am   #3
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Default Re: Gec bt1091.

A am in admiration of anyone taking on a restoration of a TV in this old rusted and altered condition.
A radio in this state I would consider, its a fairly simple easy to work on item, but a TV with so many components and age related problems is a giant leap of faith.
Having worked on high vacuum devices long ago I am continually amazed that the CRTs still hold a working vacuum after 70+ years, does not helium infiltration destroy them at least in part?
I will watch with interest.
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Old 4th Feb 2018, 8:07 am   #4
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Default Re: Gec bt1091.

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Originally Posted by Boater Sam View Post
I am continually amazed that the CRTs still hold a working vacuum after 70+ years, does not helium infiltration destroy them at least in part?
I think you may well be right about this.

I have found that the worst affected CRT's are the relatively thin walled pyrex ones. The most notorious for this are the pre-war 5BP4 and 5AP4, new old stock, often rendered useless. The VCR97 has a thicker wall than the 5AP4 and 5BP4 and these are a little better, on the average. However, the thicker walled (basically soda glass) CRT's of the post war period, especially NOS ones, seem to do much better with respect to gas infiltration. Used ones can suffer from aging with loss in cathode emission & phosphor degradation & ion burns, but new ones are usually just fine.
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Old 4th Feb 2018, 12:21 pm   #5
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Default Re: Gec bt1091.

"A am in admiration of anyone taking on a restoration of a TV in this old rusted and altered condition."
Hi Sam, It's truly a horrible rusty set and in fact if it hadn't such a rare and interesting model it would have been subject to the shop clearout. I'll do my best to tidy the thing up and make it more presentable. Most of those Maplin capacitors will be replaced with restuffed waxies. I've found some mid-fifties Dubilier capacitors which will look a lot better than those modern components. "It gets a mention in Trader sheet 1019."
Hi Colin,
thanks for the information. So in fact it's a proper white spot inverter rather than the more simpler peak white clipper.
The L63 valve is adjusted to conduct only on the white interference spots, the inverted spots are supplied to the grid of the CRT. Cancellation of the interference takes place in the CRT.

DFWB.

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Old 4th Feb 2018, 6:16 pm   #6
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Default Re: Gec bt1091.

I have my own thoughts on tube life. It is my view that the massive gun employed with electrostatic tubes is partly to blame for their short life.

It must have been difficult to heat the assembly up to a sufficiently high temperature during evacuation to expel all the air molecules from the gun assembly itself. The slow release of these molecules eventually poisoned the cathode assembly.

Large screen electrostatic tubes had a very short life and due to the cost of manufacture were not available post WW2. During 1946/47 manufacturers who were overhauling receivers returned to them by customers and dealers direct in time for the restart of the Television Service, were forced to scrap the entire chassis and fit their latest magnetic chassis and tube in it's entirety.

Murphy and RGD come to mind but fortunately electrostatic receivers were manufactured in very small numbers making the makers task a little easier. J.
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Old 4th Feb 2018, 6:39 pm   #7
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Default Re: Gec bt1091.

Close up of the faulty valveholder, the rivet heads have been filed off so it's now ready to be replaced. A good quality Ampenol socket will used as the replacement.

DFWB.
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Old 4th Feb 2018, 8:41 pm   #8
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Default Re: Gec bt1091.

Quote:
Originally Posted by FERNSEH View Post
......it's a proper white spot inverter rather than the more simpler peak white clipper....
As you say the preferred approach which gives black dots rather than peak white plus ones.
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Old 4th Feb 2018, 10:34 pm   #9
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Default Re: Gec bt1091.

A electrically similar add-on white spot inverter was available for the first generation EMI televisions, models 900, 901 etc. Like the GEC BT151 unit a triode valve was employed, an MH4 which was the general purpose triode of those times.
With this type of circuit it is essential the video DC component from the video amplifier is maintained.

DFWB.
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Old 6th Feb 2018, 9:12 pm   #10
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Default Re: Gec bt1091.

Gerry always said GEC valve holders were rubbish and I think he was right. I had to change 3 out of 5 octal holders in a GEC 1947 radio recently. J.
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Old 6th Feb 2018, 9:49 pm   #11
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Default Re: Gec bt1091.

"Gerry always said GEC valve holders were rubbish and I think he was right. I had to change 3 out of 5 octal holders in a GEC 1947 radio recently. J."
Hi John,
experienced the same problem with the GEC BT8161 pre-war TV set.

The BT1091. The frame output valve octal socket has been replaced with a good quality Ampenol type. Also, those Maplins capacitors replaced with re-stuffed Dubilier waxies. Dubilier make capacitors were more likely to be the originals to be fitted in this set rather than those made by TCC.

DFWB.
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Old 14th Feb 2018, 10:38 pm   #12
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Default Re: Gec bt1091.

Removed more of those Maplins capacitors and have replaced them with restuffed Dubilier and TCC capacitors. All the capacitors have the correct working voltages in accordance to the manufactures component list.
Of course the new capacitors inside the old casings have higher voltage ratings than the originals.

DFWB.
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Old 15th Feb 2018, 12:19 am   #13
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Default Re: Gec bt1091.

That looks nice Are those old capacitor casings or did you have to re- create new ones?
Are those blue resistors & the blue & black electrolytic on the hit list as well?
It's coming on well though.
All the best
Nick
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Old 15th Feb 2018, 8:55 pm   #14
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Default Re: Gec bt1091.

Hi Nick, The restuffed capacitors were removed from scrap radio and TV chassis.
I've never like the look of hi-viz capacitors in vintage equipment so while there is next no modern stuff coming in for repair the freed up time can be devoted to redoing the restorations on as many of my vintage sets as possible.
Those blue and black electrolytics will be replaced with rebuilt period capacitors.
I'll try to replace as many of those four value band resistors as possible. There's no doubt those new resistors are excellent but like the hi-viz capacitors the modern resistors just don't look right in this set.

DFWB.
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Old 16th Feb 2018, 12:06 am   #15
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Default Re: Gec bt1091.

Quote:
I'll try to replace as many of those four value band resistors as possible. There's no doubt those new resistors are excellent but like the hi-viz capacitors the modern resistors just don't look right in this set.
Hi David,
Four band colour code resistors are just plain wrong, especially small ones on a blue body A 10K should look like a 10K, not a 100 Ohm with extra bits!!

I found the CFR100 range from RS look similar (at least in spirit) to the old Erie ones. The body size is almost the same and they have a proper colour code. Unfortunately the range is limited so for the other values I use the RS Pro 2W range. These are the same size but have a pinkish body but look acceptable once installed.
RS will also send free delivery next day even for 10 resistors!!
Pictures here in post 13 https://www.vintage-radio.net/forum/...d.php?t=134572

I'm sure you probably know that already though!!
All the best
Nick
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Old 16th Feb 2018, 11:47 pm   #16
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Default Re: Gec bt1091.

Not much work done on the GEC BT1091 chassis today. A replacement HT reservoir capacitor clip fitted and a salvaged 25mfd 25volt electrolytic from a scrap Cossor 916 chassis replaces one of those blue capacitors.

DFWB.
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Old 16th Mar 2018, 2:11 am   #17
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Default Re: Gec bt1091.

Just about all the Maplins sourced components have been replaced.
Tested the receiver today. A few faults have appeared. A coil on one of the vision IF transformers was found to be open circuit. The lead out wire to a coil former tag had become detached, possibly when I was replacing the decoupling capacitors.
The local oscillator had packed in. Another faulty GEC valveholder. Rock the L63 valve in it's holder and the oscillator will come back to life. When the picture appears it is apparent that the CRT is getting brighter with use. There is some smearing on the picture which will be most likely be a fault in the video amplifier stage.

DFWB.
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Old 16th Mar 2018, 10:02 am   #18
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Default Re: Gec bt1091.

Good work David. Why do pre war and early post war GEC chassis look such a mess? Their radio chassis were no better. It was much improved by the time of the 1949 BT2147.
During the following decade their construction was first class. John.
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Old 17th Mar 2018, 12:06 am   #19
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Default Re: Gec bt1091.

The first attachment shows the smeary picture and the second the distorted video waveform at the cathode pin of the CRT.
The video waveform across the diode detector load resistor is perfect.
A fault in the white spot inverter was considered but it's working OK, simply remove the L63/6J5 valve to disable the circuit.
The CRT has no graphite coating, hence no EHT smoothing capacitor. Try fitting a 1000pF 20KV capacitor inside the EHT box. Still no improvement.
No faults could be found in the video amplifier.
The service manual for later production sets fitted with the L63 spot inverter triode informs us that the original diode white spot clipper should be disabled by removing the wire link between the anode of the video amplifier and the cathode of the diode.
In fact the wire was still in place. Clipping the wire out cured the fault.
So it would seem that this set has always displayed a very poor quality picture.
Until now that is, the picture is bright and well contrasted but the CRT focus is poor.

DFWB.
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Old 17th Mar 2018, 12:29 am   #20
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Default Re: Gec bt1091.

Quote:
Originally Posted by Boater Sam View Post
does not helium infiltration destroy them at least in part?
I will watch with interest.
Don't think the concentration of helium in the atmosphere would make that a significant factor. Any such would have to have been produced by radioactive decay. Any produced would head up fast in any case. Different story if the CRT was put in a closed vessel with helium gas in it.

Which reminds me of a Sci Fi story I read when I was young, about a space ship (which used thermionic valves) that landed on a planet with an atmosphere with a lot of helium in it. The helium diffused into the valves and poisoned them all, including all the spare ones of course, so the space ship was stranded....
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