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Old 10th Feb 2018, 10:52 am   #21
Heatercathodeshort
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Default Re: GEC BT336 possible tube change

Stain.. That is a weird one usually associated with early Mazda tubes only.

My thoughts drift to the CRT but like so many of these faults a 'hands on' approach would probably solve the problem. If the focus drops to such an extent, the only fault that can cause this is dropping EHT voltage but this must drop a considerable amount to have such a dramatic effect.
The second reason would be a slightly gassy tube and this is backed up by 'The Stain'. [That could be a title for a 1950's horror movie]

110 degree tubes are pre focus and require very little in the way of focus potential if any at all. I think you must measure the EHT to narrow this down.

The fact that the raster does not increase in size with reduction of EHT points to the saturation problem with the LOPT. A rough visual test is to observe the brightness of the EHT rectifier heater. If the brightness drops with the reduction in focus, it's a good bet that the LOPT is to blame.

These are good transformers and should dry out given time. The dampness is deep set and has taken decades to creep into the pitch coating and windings. It was never a problem when these chassis were in service. The fault did not exist in the 60's/70's/80's. John.
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Old 11th Feb 2018, 4:06 am   #22
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Default Re: GEC BT336 possible tube change

John I did check the brightness of the EHT valve and it's not easy to see but you could be right it's dimmer when I get the problems with the focus . Also I didn't realise that the hight of the picture expands so much again over maybe half hour.
The snaking of the pictures gets very bad up to 15 mins it then gets slightly better as time goes on but never goes away.
Stephan I think all but a couple valves are original but I don't have spare the same type to try but you could be right the could be warn out.
I think I will leave the set on a good amount of time tomorrow maybe a hour on then a hour off bases see if things improve. Thanks Andy
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Old 11th Feb 2018, 10:19 am   #23
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Default Re: GEC BT336 possible tube change

If the height increases in keeping with the reduction in focus the EHT must be falling off.
Certainly sounds like a damp overwind. The whole transformer is dipped in pitch but it is the overwind that is almost certainly causing your problem. J.
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Old 12th Feb 2018, 3:32 am   #24
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Default Re: GEC BT336 possible tube change

John . I have had the tv on and off for a long time today .
The width has gets better if the set is left on for over an hour.
Can I ask the best way to try and dry out the lopt I think you said to feed a current in to the overwind of 30 ma .Can I just double check this as I don't want to wreck anything Andy
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Old 12th Feb 2018, 7:43 pm   #25
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Default Re: GEC BT336 possible tube change

You really need a variable power supply typically 0-30v at maybe an amp. Connect it to the EHT overwind, that is the line output valve anode [top cap] to the anode of the EHT rectifier.

Switch on the power supply and increase the voltage until around 30m/a passes through the winding. Test it with your finger after about 30 mins, it needs to be just warm. You can vary the voltage to obtain the best overall result.

Once the temperature is stable you need to leave it connected for a couple of days. DO NOT hurry this procedure and let it 'cook'. Test after two days and if there is an improvement, do the same thing for another two days. Remember it took nearly 40 years to suck in the dampness so patience must be observed.

I would not suggest you attempt to dry it out by operating the chassis for extended periods. The transformer may break down requiring a rewind.

I would suggest you obtain a few essential tools if you want to expand your vintage interests. A variable bench power supply, an EHT meter 0-30kv or 0-20kv if you are sticking to monochrome, a signal generator [nothing fussy] and a simple scope.

All of these items appear at BVWS auctions at very low prices or of course you can visit on line auctions but at the BVWS meets you can actually get hands on before purchase.

These few items will make life a lot easier. I presume you have a reasonable meter of course. Regards, John.
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Old 14th Feb 2018, 1:19 am   #26
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Default Re: GEC BT336 possible tube change

John
Thanks for all your advise . I got everything set up with a variable power supply and a avo in line to check the ma load. I'm not going to try it just now as I want to make sure I'm around a bit to check it's not over heating . Thanks as well for your suggestions on test gear. I will look out as I do go to some BVWS auctions . I will let you know how it goes here in a few days time Andy
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Old 14th Feb 2018, 9:25 pm   #27
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Default Re: GEC BT336 possible tube change

Hi Andy,
After seeing this method discussed by HCS & Fernseh on this forum, I have used it on two transformers, neither of which had seen power for probably 40 years! I wanted to try and prevent any problems before powering them.

I left them cooking for about three days I think, running at just slightly hotter than 'mildly warm'. I did the primary winding as well as the EHT overwind. I used a variable power supply and Avo to monitor the current as you describe.
Certainly one LOPT had been stored in very wet (way beyond damp!) conditions.
After the treatment, both transformers worked fine. Of course, I don't know if they would have worked anyway, but it seemed a very good idea to dry them out first. I will do the same for future projects.

Have you changed capacitors around the line stage? Particularly the boost cap?
I don't have the circuit to hand, so can't tell you which one that is. Maybe someone could post the relevant part of the circuit?

All the best
Nick

Last edited by 1100 man; 14th Feb 2018 at 9:26 pm. Reason: extra text
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Old 14th Feb 2018, 9:56 pm   #28
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Default Re: GEC BT336 possible tube change

When using the bench power supply method to drive moisture out from line output transformer the windings are not under any high voltage stress. No corona discharge. Using this method the heat dissipation through the windings is only 1 to 2watts.

DFWB.
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Old 15th Feb 2018, 12:51 am   #29
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Default Re: GEC BT336 possible tube change

Hi Andy,
You will need to measure the resistance of the overwind before advice can be given on the value of the current that will need to be passed through it to generate about 1Watt of heat.
There seems to be a very large variation of resistance values between different transformers.
Cheers
Nick
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Old 15th Feb 2018, 3:33 am   #30
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Default Re: GEC BT336 possible tube change

I have changed all the black capacitors as I been going along I don't think there are any I missed.suprisingly even though a good few had cracked cases on replacing they were Almost spot on. the original value.having said that almost all the lower value ones were very leaky and way off value.
Going back to the lopt I started off with 30ma of power going through it as John suggested but after half hour not even warm .I been slightly upping the current by 10ma every half hour checking so far it only minutely warm . I was going to keep slowly going but I will measure the resistance it that can help me .
Thanks everyone Andy
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Old 15th Feb 2018, 9:25 am   #31
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Default Re: GEC BT336 possible tube change

Just a thought.
You say you checked the caps and they were spot on. what are you testing them with?
To be sure they need testing at the working voltage.
A Chinese magic box will read high with most old caps due to the leakage and the 9v battery is nowhere near enough.
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Old 15th Feb 2018, 11:06 am   #32
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Default Re: GEC BT336 possible tube change

The overwind may be wound with Eureka resistance wire and will require a higher voltage to produce the warmth needed. It's trial and error.

The GEC service manual quotes a DC resistance of 67 ohms between tags 6/7, the overwind + or - 10 percent.
In practice, resistance readings of flyback transformers can vary widely due to changes in production during the life of the design. J.

Last edited by Heatercathodeshort; 15th Feb 2018 at 11:14 am.
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Old 16th Feb 2018, 2:05 am   #33
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Default Re: GEC BT336 possible tube change

Sam
I test the capacitors normally with a Amtron Capacitor tester that I got for a Christmas present as a boy in the early 70s. I also have a Peak tester.
John the resistance is measuring 74ohm. I think I have now got it about right.
I'm feeding about 11.5 volts at about 190ma .just checked it now and it's warm but not hot.
Andy
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Old 16th Feb 2018, 10:55 am   #34
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Default Re: GEC BT336 possible tube change

You will need to actually replace the original caps usually black moldseal types. They leak when they get warm. Capacitor testers tell lies.. That setting sounds fine. Hope it works. John.
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Old 27th Feb 2018, 2:32 am   #35
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Default Re: GEC BT336 possible tube change

Well as the LOPT was drying out I got down to some decorating so it's had a good few days drying out .

I now have full width and a fantastic picture when first switched on.
But as time goes on the width stays the same but the height increases over half hour with some loss definition but not as bad as before
After about 45 mins the snaking is realy bad even making the picture break up . Then it settles down a little .
So what do I do now
I don't want to give up on this one . I could try and show some pictures if that helps . Andy
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Old 27th Feb 2018, 10:11 am   #36
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Default Re: GEC BT336 possible tube change

Its good the width is not changing, lets see some pictures of the underside of the chassis, and one of the fault onscreen.
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Old 27th Feb 2018, 12:45 pm   #37
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Default Re: GEC BT336 possible tube change

Quote:
Originally Posted by Heatercathodeshort View Post
You will need to actually replace the original caps usually black moldseal types. They leak when they get warm. Capacitor testers tell lies.. That setting sounds fine. Hope it works. John.
Sorry to come into this discussion, but I agree. For me it's obvious to have ALL old capacitors, those glass filled with black stuff, replaced with modern ones. That apply also for all electrolytic (bias and HV). in very old sets.
Good luck John;
Roger
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Old 27th Feb 2018, 3:26 pm   #38
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Default Re: GEC BT336 possible tube change

That is good news. Incredible as it may seem the LOPT is probably still 'damp' but extended intermittent use will probably clear the fault. If not it's back to the warming process for another couple of days. It's inset moisture and can be a devil to drive out.

As for your hum problem it could be a heater cathode leak in the line oscillator or timebase valves. You really need a scope for this to trace where it is coming from.
First suspects are the line oscillator valve, sync seperator and the line output valve itself. Scope the HT line and see if it has a 'wiggle', the line drive to the control grid N389. You must be able to see it on the scope. Almost there. John.
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Old 28th Feb 2018, 2:05 am   #39
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Default Re: GEC BT336 possible tube change

Thanks all.
I have not been in the workshop tonight even though I have a heater ticking over .
I'm back to Drying the lopt for a couple more days.
Just to recap I have changed all the capacitors.
I will try and put more pictures .
I don't have a scope and some spare valves would be useful so maybe I might have to put it on hold to I get to a auction or radio rally.
Thanks again for all the help I'm getting Andy
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