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Old 31st Jan 2016, 3:56 am   #1
rbm1973
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Default GEC BT320 width decreasing and balooning

after repairing this set its been running for several hours now and i noticed the width had decreased
and also now whenever the brightness or contrast are adjusted the picture balloons and will at high settings fade out the set was fine earlier with proper regulation brightness and contrast working as they should but this fault has developed after a few hours use i wondered if anyone here had any ideas as i am not familiar with this model and do not have a circuit diagram.

also how do i remove the front glass as there is a foam strip that has slipped down and is visible and annoying.

Regards Simon
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Old 31st Jan 2016, 4:32 am   #2
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Default Re: gec bt320 width decreasing and balooning

Looks like the EHT is dropping off.
We need to know you level of ability with TV repairs to give you any meaningful help that you could safely follow.


Have a look through the threads on the site under reduced EHT, picture ballooning, small picture, and the model # of your set etc. and you may find the answer.

Could be simply a failing line output valve or rectifier, assuming that this is a valve set! I have no knowledge of the model.

It is NOT the " picture valve "!
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Old 31st Jan 2016, 4:57 am   #3
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Default Re: gec bt320 width decreasing and balooning

i have high ability with tv repairs but not familiar with this chassis i was suspecting poss boost capacitor ??
when i did switch off the line output transformer was hot hotter than it should be!
but after the set was off for 30 minutes or so while i removed the cover over the line output txfmr i switched back on and the ballooning is not present but no doubt will reapear if the set is left on long enough i am just hoping its a faulty boost or tuning capacitor as they are all original capacitors or line output or line output boost valve i was hoping someone may know this chassis like i said it has been running for about 4 hours with no problem but then i noticed a slight reduction in width only 1/4 of an inch each side if that but then it would balloon with brightness and contrast adjustment to the point where the picture would disappear indicating that there was no eht regulation and as you say the eht is dropping off.
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Old 31st Jan 2016, 9:43 am   #4
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Default Re: gec bt320 width decreasing and balooning

Hi Simon,

The boost capacitor does not usually give this fault, when it fails it normally goes short circuit and stays that way giving a "no picture" fault.
It could be the EHT rectifier of worst case it could be a failing lopt EHT overwind.
As you said that the transformer was hotter than normal when the fault was present yet worked fine again when cool then in my opinion it looks like it could be the lopt.

I would remove the lopt and put it in the airing cupboard or boiler cupboard for a month, it may be damp, if you can't do this then stick the whole set somewhere very warm and dry for the same period.

Good luck!

Cheers
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Old 31st Jan 2016, 9:46 am   #5
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Default Re: gec bt320 width decreasing and balooning

Running a vintage set with it's original caps is asking for trouble, I would certainly have changed the boost cap before running the set for any length of time.

It does not sound good that the LOPT is overheating, it may have shorted turns.
I would advise that you recap the line stage before powering up again, fingers crossed that the LOPT has survived.

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Old 31st Jan 2016, 11:29 am   #6
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Default Re: gec bt320 width decreasing and balooning

i have now replaced the awfull hunts caps with polyester film ones from scrap plasma ysus pcbs temporary and the set has been running fine since 4 oclock this morning
fingers crossed the loptx has survived it runs warm but not over heating anymore and no more ballooning
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Old 31st Jan 2016, 12:09 pm   #7
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Default Re: gec bt320 width decreasing and balooning

Hi Simon

That's good news

Re the foam I think from memory you can't remove the glass from the outside you need to take the Chassis out of the cabinet.

If you look at the video they make fitting the chassis into the cabinet look easy!

Its been 40+ year since I worked on one of these though

Cheers

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Old 6th Feb 2016, 1:16 am   #8
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Default Gec Bt320 lack of width

after my episode of ballooning and width reduction and lopt running hot i recapped lop stage and got full picture back no ballooning it ran ok for over 24 hrs but then i looked at it and the width has gone in by about 2/3 of an inch each side on normal setting when link put to full width the side castelations do not go right to the edge there is a few mm gap still so the center circle looks like an egg
it is like this all the time i also had frame cramping and frame trying to open up this was the 4 ohm ntc varite resistor on frame scan coils i have sorted that problem out

but have not yet traced the very slight lack of width ?

ps the lopt runs at normal temperature now and the set does produce a very good picture

i'm thinking maybe lop valves but unsure of equivelents the service sheet marks them as v12 lop valve n308 and boost valve v13 u339
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Old 6th Feb 2016, 2:10 am   #9
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Default Re: GEC BT320 width decreasing and balooning

Have you replaced the 0.01microfarad coupling capacitor between the line oscillator and the grid of the line output valve? There should be no positive going component present at the grid of this valve. The entire line drive waveform is negative going.
Check the condition of the screen feed resistor of the line output valve. A carbon resistor can go low in value.

DFWB.
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Old 6th Feb 2016, 7:22 am   #10
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Default Re: GEC BT320 width decreasing and balooning

havent checked c118 control grid feed resistor yet but will replace on spec
one thing i did notice is that the manual states 125v at screen grid pin of lop valve via r94 100 ohm when the set and oscillator is running this voltage is only 114/115 volts ?
it comes from the ht line via 2.7k wirewound resistor r93 i have yet to check both of these
as the timebase has to be removed to check them as they are both down the bottom and the printed side of the pcbs on the 320 is not easy in fact you cant get to anything without removing the timebase panel
i will check all 3 and report back

the other thing is the manual does not state the ht voltage and from memory it was about 170v

i checked both parts of smoothing cap and got 170 and 191 respectivley
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Old 6th Feb 2016, 8:08 am   #11
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Default Re: GEC BT320 width decreasing and balooning

well update
replaced the little Hunt ! c118 and now have full width on normal setting

maybe if i had scoped the line drive would have prob seen dc and realised oh well its ok now
mind you as soon as i looked at c118 and saw it was a tiny Hunt ! i should have known
i'm going to remove all the little ba***rds and change them !
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Old 6th Feb 2016, 9:16 am   #12
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Default Re: GEC BT320 width decreasing and balooning

For future ref.
http://www.r-type.org/exhib/abh0032.htm

http://www.r-type.org/exhib/aas0027.htm
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Old 6th Feb 2016, 10:21 am   #13
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Default Re: GEC BT320 width decreasing and balooning

The wax cap on the CRT base has also been replaced
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Old 6th Feb 2016, 10:42 am   #14
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Default Re: GEC BT320 width decreasing and balooning

There is no point in running these old sets with original wax caps in place. For initial testing after a long period it's fine to replace just a few crucial ones to get it going and assess the CRT/LOPT etc...boost cap, line osc coupling to line o/p valve, possibly any obvious HT decouplers etc. Changing just those for an initial assessment will often protect the LOPT and line o/p stage from anything nasty happening, then change the other waxies to clear all the other faults.
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Old 6th Feb 2016, 11:23 am   #15
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Default Re: GEC BT320 width decreasing and balooning

Good advice indeed, I always change all waxies and those nasty Hunts mouldseal caps on sight. They will all leak like a sieve and cause all sorts of faults, they can put unwanted strain on other components if left in circuit.

I usually find that the main smoothing caps are fine, as are the ceramic and mica caps.

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