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Old 19th May 2010, 10:43 pm   #61
PJL
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Default Re: Oh no - another V4!

Hi Dom, sorry to ask another stupid question but it's the only way to learn - what's still wrong? I was under the impression that damp in the lopt tended to result in a gradual EHT collapse as the heat increased the humidity. Your boost rail doesn't seem too bad either?
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Old 19th May 2010, 11:02 pm   #62
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Default Re: Oh no - another V4!

I'm glad someone else is confused and asked this I was hesitant. I have been following this with great interest too for obvious reasons, I too do not understand as the testcard picture you had up was almost there. Reading Kevin19's PYE VT2 thread, which was suffering from damp in the LOPT you did not seem to be getting any of that.
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Old 19th May 2010, 11:27 pm   #63
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Default Re: Oh no - another V4!

The EHT and Boost Voltage are lower than they should be. This is the problem, although the set does seem to work.

If he's checked and changed everything else, then it must be the LOPT. When a LOPT gets a little bit of moisture in it, then it loses efficiency. This one still works, but when it has dried out properly, these voltages may be higher - more like what they should be.

This is why I said to dry out the LOPT fully, then try it.

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Old 20th May 2010, 12:00 am   #64
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Default Re: Oh no - another V4!

OK, the bit I don't understand is the width being low. The width is determined by the peak current and this is only related to inductance and not voltage so I would have thought it would not change with a very slightly low boost rail (perhaps 5%). But a low EHT I thought was supposed to make the picture bloom, i.e. width increase.

Sorry about all the theory and lack of practical experience...I expect what I am missing is the focus magnet has been moved to refocus with the reduced EHT?
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Old 20th May 2010, 12:05 am   #65
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Default Re: Oh no - another V4!

I have to say I'm a bit confused too. The boost rail goes up to about 450V immmediately after switch on then settles to about 420V when the EHT rectifier has warmed up - at this point the picture looks pretty good. I should know I managed to make a schoolboy error when measuring the boost tonight - one hand on chassis other hand reached for screwdriver which was somehow touching boost cap. Didn't half make me swear!

EHT goes up to about 9kV by my measuring probe. I've no idea how accurate that is though it measured my Decca Bradford to be spot on when I checked it. But that's probably quite a bit lower impedance.

EHT and boost voltages drop to being hopeless after about half an hour. Not measured how low but the picture is very soft. The LOPT itself doesn't get any warmer than the surrounding chassis.

One thing I will double check before putting away to dry out the LOPT is that the "Line Drive" adjustment doesn't seem to do a whole lot. The only components I've not touched are all the mica caps (such as the 27p feedback to LOP valve). I see on the Service Notes that John posted earlier that these low value caps aren't above suspicion. Plus I'll double check all the various coils. I'm not sure what the line drive should do but it looks as if it should sharpen up the flyback and therefore adjust the EHT?

I'm not sure how best to check the micas - would a cold resistance reading be good enough or do I need to get out a HV supply and check for leakage that way?

One thing I have noticed is that the soldering on this set is a bit below par, I've had several wires drop off when moving things about and all look to be due to dry joints.

I'd really like to get this set fettled and so I'm not sure how best to dry out the LOPT - the backside over radiator solution is out, if I'm not to end up single. So I might try directing a fan heater at it. However, I'd be more inclined to have a go at a more immediate solution.

Dom
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Old 20th May 2010, 12:12 am   #66
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Default Re: Oh no - another V4!

Sorry PJL, we crossed over there.

That is what is making my brain ache! The picture starts out "bloomed" when the set first comes on during the time the EHT rect is warming up. i.e. scan coils getting full juice but low EHT.

After that EHT and scan drop off gradually together losing a small amount of width but not more than about 1/2", however the picture goes quite blurred.

Dom
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Old 20th May 2010, 3:39 am   #67
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Default Re: Oh no - another V4!

The Line Drive trimmer is worth a check, along with C99, R124, R125 and C102. This does go to the LOPT by the way, so in a way points back to the LOPT.

Also, if you haven't already, check R116, R115, R117 and R120.

I recall, deep in my er.... mind..., another case like this with a V4. How about taking off the EY51 and fitting a diode instead.... If nothing else works including the drying out of the LOPT, you could try this...

Cheers,

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Old 20th May 2010, 10:01 am   #68
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Default Re: Oh no - another V4!

Hi
Lopt saturation without a doubt! from cold EHT width and boost will be just about correct perhaps just a little low, after half an hour or so the EHT may well have lost 2kv and your boost will drop 50-80v width will drop and be more noticeable on the left with a distorted circle. You are correct with low EHT the picture will be bigger due to the slower path of electrons from the gun which will be easier to deflect but scan power dramatically drops when this fault appears so overall doesn't seem that noticeable.
Steady heat on the tranny for a week or two works and can be a complete cure, or a couple of hours in the oven at about 75c will work too.
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Old 20th May 2010, 10:21 am   #69
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Default Re: Oh no - another V4!

Hi Dom.

Yes that sounds just like my Pye VT2 LOPT! I took drastic action with a hot air gun which is scary and I would maybe not recommend or do it again myslef on another set! As I think there is a very fine line in doing this of wrecking the LOPT. But it worked for me I have since had the set on for very long periods with no loss now to the EHT and picutre focus before it was hopless! I have another PYE set from the same damp barn location a FV1 and I'm going to try the LOPT in the hot aring cupboard for a very long while and see if that does work by long time I mean maybe 6 to 9 months or even a year! It almost seems they need the damp boiling out off the pitch so as Trevor does say the oven may be a quick safer option.

It is very frustrating to find this on a set after you have got it going. Hope you get it dried out and sorted.

Kev.
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Old 20th May 2010, 10:39 am   #70
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Default Re: Oh no - another V4!

Re the oven Trevor,whats that in gas mark ?

Cheers David
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Old 20th May 2010, 11:18 am   #71
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Default Re: Oh no - another V4!

Quote:
Originally Posted by Steve_P View Post
How about taking off the EY51 and fitting a diode instead....
Steve,

can you expand on this solution for the benefit of us new to TV repair,

What this entails?
What diode you use, IN4007?
How it's incorporated into the circuit, is it just a direct removal of the EY51 and putting in a diode? .
How does this negate the saturated LOPT?

Sounds like a good fall back solution if all else fails, Dom may not wish to do this but at least it's an option.

Chris

Last edited by oldticktock; 20th May 2010 at 11:48 am.
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Old 20th May 2010, 11:53 am   #72
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Default Re: Oh no - another V4!

Not a 1N4007! It's got to take EHT voltage and a 1N4007 is only rated at 1000V. Also the 1N4007 is too slow. By the time it has thought about turning off it's way past the time to turn on again. BY140, BY176, BY182 are all suitable. Also a rectifier salvaged from a TV EHT tripler. Not sure about modern types and availability.

Direct sub fof the valve except you don't need a heater winding. Hence a faulty heater winding doesn't matter. Also even if the LOP is running badly you don't suffer from low EHT due to inadequate heating of the rect.
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Old 20th May 2010, 11:55 am   #73
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Default Re: Oh no - another V4!

Yes you could use an eht stick reccy not a 1N4007 as it is a 1000v PIV diode, there are some silicon diodes rated at 10kv and above, in reality the EY51 is fine here and you have the advantage of seeing the heaters which gives you an idea that the line stage is working.

Regarding Lopt Saturation I have one here thats been cooking onm the boiler for 9 months so that should be ready to use, I also have another somewhere to be done so perhaps I should do an exchange service!

David. I dont know the gas mark settings, But I go for a medium rare appearance on the transformer!!!
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Old 20th May 2010, 12:01 pm   #74
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Default Re: Oh no - another V4!

Interesting info chaps, not wishing to hijack Dom's thread but I have the same scenario as Dom with my V4. I shall take the diode path though.
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Old 20th May 2010, 12:02 pm   #75
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Default Re: Oh no - another V4!

Quote:
Originally Posted by ppppenguin View Post
Direct sub fof the valve except you don't need a heater winding. Hence a faulty heater winding doesn't matter. Also even if the LOP is running badly you don't suffer from low EHT due to inadequate heating of the rect.

Yes you will not suffer from decayed EHT from a poor heater but the o/p from the overwind also drops so the EHT will still drop anyway.
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Old 20th May 2010, 12:07 pm   #76
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Default Re: Oh no - another V4!

"David. I dont know the gas mark settings, But I go for a medium rare appearance on the transformer!!!" Turning it every half hour

Seriously, wouldnt the humidity in a gas oven running at 75 be quite high?
An electric may be a better option.
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Old 20th May 2010, 12:09 pm   #77
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Default Re: Oh no - another V4!

Quote:
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Interesting info chaps, not wishing to hijack Dom's thread but I have the same scenario as Dom with my V4. I shall take the diode path though.

Fitting a diode doesn't cure saturation problems due to damp, it will give you some EHT where the the EY51 has stopped conducting. the only cure for Damp related saturation is to "boil" off the moisture. Do some reading up on saturated transformers then you may understand that some have that built into the design for good reasons this should not be confused with the saturation caused by damp ingress although they are on similar lines.
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Old 20th May 2010, 12:13 pm   #78
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Default Re: Oh no - another V4!

Quote:
Originally Posted by Alistair D View Post
Seriously, wouldnt the humidity in a gas oven running at 75 be quite high?
An electric may be a better option.
Good point,
Electric heat is dry not so in a gas oven!
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Old 20th May 2010, 12:13 pm   #79
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Default Re: Oh no - another V4!

Ok my mistake I assumed from #67 & #72 it was an option to the issue in hand.
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Old 20th May 2010, 12:51 pm   #80
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Default Re: Oh no - another V4!

Trevor, the bit that I didn't have in the equation was the gradual loss of EHT which Dom hadn't mentioned previously. It is interesting it impacts width but from the distortion you describe, it sounds like the additional loading from the damp EHT overwind can pull the line output valve out of saturation and I guess it is no longer a purely inductive load so anything could happen.

Removing damp really requires a desiccator - trying to dry with heat is a very slow process. You can buy tablets designed for hearing aids which you throw in to a sealed container with the item to be dessicated. Means removing the LOPT unless you could get a reasonable seal with a plastic bag and tape. If you have access to a vacuum pump you can do the job very quickly.
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