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High voltage problem in TVs

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kulkell

New Member
Hello friends, one problem I have not been able to solve on my own in TV repairs is in managing high voltages. 1: where do I place my H-voltage probe tip? Is there usually a recommended test point on the :( PCB (I doubt) or directly on the anode cap with it unpluged from the tube? In a nutshell, how do I use it? 2: In a case where the voltage is too high and the X-ray protection is being triggered, how do I lower it? Is there always a preset adjustment for that purpose or does it just require the replacement of either the chopper transistor, SMPS controller IC, transformer or some other thing in the circuit. thanks for your help in advance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
kulkell said:
Hello friends, one problem I have not been able to solve on my own in TV repairs is in managing high voltages. 1: where do I place my H-voltage probe tip? Is there usually a recommended test point on the :( PCB (I doubt) or directly on the anode cap with it unpluged from the tube? In a nutshell, how do I use it? 2: In a case where the voltage is too high and the X-ray protection is being triggered, how do I lower it? Is there always a preset adjustment for that purpose or does it just require the replacement of either the chopper transistor, SMPS controller IC, transformer or some other thing in the circuit. thanks for your help in advance.

You use an EHT probe on the anode cap, they have a long metal spike designed to slide under the rubber.

BTW - it's fairly uncommon to need to do this, I've been repairing TV's professionally for over 32 years - I perhaps do this once or twice a year, that's all.

If the EHT is too high, it's almost certainly because the HT is too high, the most common reason for this is O/C electrolytics in the PSU - mostly the one decoupling the feedback regulation voltage.
 

olly_k

Member
Hi Nigel, seeing as we are on the subject, could you help me understand how EHT compensation works on newer chassis?
I assume video processor (in this case at least!) controls this thru monitoring beam current? I could post an example if it helps you explain?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
olly_k said:
Hi Nigel, seeing as we are on the subject, could you help me understand how EHT compensation works on newer chassis?
I assume video processor (in this case at least!) controls this thru monitoring beam current? I could post an example if it helps you explain?

My experience is that they don't seem to bother much these days! - most modern sets have pathetic EHT regulation!.

For those who don't know what we're talking about, EHT is 'Extra High Tension', the high voltage supplied to the tube.

'Beam current' is the amount of current drawn from this to power the electon beams which scan the picture, the more brightness, the more current is required.

As with any power supply, as you draw more current from it, the voltage will drop - regulation helps to revent this - just as in any regulated power supply.

The effect in a TV is that as the EHT gets lower (with increasing beam current) the picture gets larger - this effect is called 'blooming' - at least in the UK!.

Old TV's used to have circuitry to compensate for this, the old valve sets used a shunt stabilising triode (usually a PD500) directly across the 24,000V EHT supply - a seriously nasty looking idea!. Newer sets don't seem to bother much, usually there is a feed back to the width circuitry, to help to compensate width variations - but mostly they seem to rely on the LOPTX (Line Output Transformer) to be stable enough to be acceptable.
 

olly_k

Member
thanks for prompt reply!
Wow, I am impressed with this telly already! especially considering it is a Wharfedale CTV-850 - AKA Tesco Special! Or maybe the LOPT is so rubbish in this design (as it has failed) it requires EHT comp :?
I don't suppose you have experienced this (E9 16:9) chassis have you nigel? would you be willing to help me decide whether I should attempt to further repair this set (after replacing LOPTx then found blown supply-fuse resistor to tda6107 and reduced 4:3 white-noisy raster :twisted: )
At the moment recent move = lack of workbench add to that new family member has made me rather skint and has left me wandering whether I should bother much further with this set!? Although I do want wide-screen telly!!!!
I will be greatfull for any hints or help you could offer me!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
olly_k said:
thanks for prompt reply!
Wow, I am impressed with this telly already! especially considering it is a Wharfedale CTV-850 - AKA Tesco Special! Or maybe the LOPT is so rubbish in this design (as it has failed) it requires EHT comp :?
I don't suppose you have experienced this (E9 16:9) chassis have you nigel? would you be willing to help me decide whether I should attempt to further repair this set (after replacing LOPTx then found blown supply-fuse resistor to tda6107 and reduced 4:3 white-noisy raster :twisted: )
At the moment recent move = lack of workbench add to that new family member has made me rather skint and has left me wandering whether I should bother much further with this set!? Although I do want wide-screen telly!!!!

As you probably already know, these sets are as cheap and nasty as you can get!. I would expect EHT regulation to be pretty poor anyway, but if you've had to change the transformer it's possible that some resistors (or other components) may have been damaged. Check carefully around the LOPTX and see if you can see and discoloured components.

If the LOPTX you've fitted is a replacement, rather than an original one, it's also quite possible that EHT regulation could suffer.

One thing you might try doing is ringing Tesco's, one particular range of Wharfdale TV's was subject to a safety recall - eventually they decided that they couldn't be made safe - and Tesco's will give a full refund of the original purchase price. If you ring Tesco's they should be able to tell you the affected model numbers.
 

olly_k

Member
If the LOPTX you've fitted is a replacement, rather than an original one, it's also quite possible that EHT regulation could suffer.
I got the transformer from wizzard distr. and it has the original number on so I suspect it is the original - I may ring up tomorrow to find out!
Also I was concerened about the RGB driver resistor that blew cos it looks like a 1 or 2 watt resistor (fairly large) but in the manual it specifies a 0.75W fusible resistor so I am thinking this may have been a possible mod.
As far as the recall set that was a different model but I will call anyhow.
Thanks for your help Nigel I shall report back as to the fate of this set...
If it doesn't work out I may be selling a 28" WS panasonic tube if you are interested LOL!
 

margaret

New Member
HIGH EXTRA TENSION

The comment's made previously regarding using a probe for high eht were applicable but misleading for Kulkell, who say's he is a beginner to the repair of tv's

First of all a cct diagram is needed for the receiver in question or any other for that matter.

Why does Kulkell as a novice decide he has to check the eht on his tv when this is a very dangerous procedure, the first check should be in the p.s.u. (power supply unit) for the correct HT. (high tension) eg, 115v which determines how the rest of the set behaves but not necessarily the fault especially with SMPSU and their are some methods that can isolate sections of the set to try and narrow the search down,... the L.T.B. ( line timebase)... section is prone to breakdown like the psu followed by the F.T.B (field timebase) , the IF (intermediate frequency) sections are less prone to fault, colour decoder's even less especially in todays modern tv's that compared with a solid state set, eg, brc 2000 the first solid state set on the planet bristling with components unlike today's tv that have fewer component count than some VTR's,... having said that they are still potentially lethal to our poor resistance to current be alert.
 

Nigel Goodwin

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Re: HIGH EXTRA TENSION

margaret said:
brc 2000 the first solid state set on the planet bristling with components

The BRC 2000 chassis was actually the worlds first production solid state "COLOUR TV", there were already many fully solid state B/W portables available. From what I've seen the USA were very slow to get rid of valve TV's, they were still selling valve sets a long time after the UK were all transistor.

BRC were actually world leaders in many respects of domestic electronics, first solid state colour TV (2000 series), first SMPS in a domestic product (3000 series), first music centre.

I worked on the 2000, though they were a fairly rare set, but I did a LOT of work on 3000's - superb TV's, I even built a jig for doing panel repairs.
 

margaret

New Member
brc 2000 chassis

Nigel, i have already mentioned who made the planet's first solid state tv, your word's in reply seem to to imply that i forgot to include the word colour,... it was not necessary for me to mention the word colour chassis as brc didn't make a 2000 mono chassis when they introduced the globes first all transistor receiver.

Don't get upset though check your sentencing & my sentence's at the start of your's and near the end of mine. my recall is of the 1400. 1500, 1600, 1700, etc, all singing & dancing black is white portables.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: brc 2000 chassis

margaret said:
Nigel, i have already mentioned who made the planet's first solid state tv, your word's in reply seem to to imply that i forgot to include the word colour,... it was not necessary for me to mention the word colour chassis as brc didn't make a 2000 mono chassis when they introduced the globes first all transistor receiver.

You know that, and I know that, but probably hardly anyone else on these forums does - so it's very misleading unless you mention it was a colour set. Don't forget, most of the members here were probably not even born when the 2000 was about :lol:

Don't get upset though check your sentencing & my sentence's at the start of your's and near the end of mine. my recall is of the 1400. 1500, 1600, 1700, etc, all singing & dancing black is white portables.

I don't remember a 1700, but the 1400 was an all valve dual standard set, the 1500 a 'mostly' valve single standard set, and the 1600 an all solid state set - none were portables, they were 19 inch and upwards.

You don't have a location listed, but I presume you are in the UK (as you are familiar with old UK TV's), I also presume English is not your first language as your writing is 'unusual' :lol:
 

margaret

New Member
brc 2000 chassis

Yes i agree with the point you make regarding the brc 2000 chassis, not everyone know's that i referred to colour, also i misdirected when i added portable's,... i only meant to refer to the Ferguson 1790 a 14in rotary tuner model.

As for my writing as you put it being unusual,... i feel this was just a little dig you added,... your grammar in the previous replies might suggest from a distance your education is a bit thin on the ground, your presumption's in the latter part of your remark's made to me as you well know! were calculating,.. methink's your halo has slipped and you are now called the former god rather than the forum god. was this name self imposed?. :lol:

NB: just part of the incorrect grammar/spelling used was the sentence: "COLOUR TV'S,....well fink you said sumfing like it ennyway, summen upp, capital letter's should not be used for complete word's,... you are probably an old guy who lives in the uk and progressed from cowboy to a valve jockey. :roll:
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: brc 2000 chassis

margaret said:
As for my writing as you put it being unusual,... i feel this was just a little dig you added,... your grammar in the previous replies might suggest from a distance your education is a bit thin on the ground, your presumption's in the latter part of your remark's made to me as you well know! were calculating,.. methink's your halo has slipped and you are now called the former god rather than the forum god. was this name self imposed?.

I'm not getting at you in any way, I merely asked what country you might be in (as you still haven't mentioned it), and if English was your first language?. Your written English is still slightly 'unusual', which leads me to suspect it's not your first language.

The 'God' title is given by the forum software based on the number of posts you have made - there are a number of different titles, which change at specific levels.

NB: just part of the incorrect grammar/spelling used was the sentence: "COLOUR TV'S,....well fink you said sumfing like it ennyway, summen upp, capital letter's should not be used for complete word's,... you are probably an old guy who lives in the uk and progressed from cowboy to a valve jockey. :roll:

Capital letters are used in electronic communications to give emphasis to the particular point, it's a standard technique.

Depends what you mean by old!, I'm 48 years old and have been professionally employed as a TV service engineer since 1971. I'm also twice winner of the Sharp Electronics Engineer of the Year competition, and once winner of Grundig Satellite Engineer of the Year.
 

margaret

New Member
BRC 2K TV.

i have given your replies to me some thought and decided to forego any attempt at punctuation grammar spelling and treat your naievity with the contempt it deserves, forums email are numerous and each individual can decide whether to blow ones trumpet about ones acheeevments world wide talking about the world did i not beat you on the head with a brush in a small village in prague as childrenn ... i never did mention what country i belong to why should i need to reveal this to anyone this is electronic mail comrade that goes worldwide any standard practice for electronic titles in CaPitAls ie COLOUR TV may have been used by yourself before e-mail but is not acceptable to use now on the internet its just like showting.

Ok you have a black belt for jew jitsew, which you have broadcast to the world.......,, in the old country as you well know this is frowened aponn ! why i myself have attributes including a red belt......... the reason i achieved this accolade was that sometimes when i had to use my oriental expertise their nose would bleed after a belt. :D

NB: No reply is needed. : 8)
 

Nigel Goodwin

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Most Helpful Member
I'm not interested in arguing margaret!, I don't even know what you are arguing about?. As for complaining about me listing some of my achievements - that was solely in response to your attack on my abilities.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I'm not interested in arguing margaret!, I don't even know what you are arguing about?. As for complaining about me listing some of my achievements - that was solely in response to your attack on my abilities.
 

margaret

New Member
BRC 2K receiver.

Nigel.
i realise i was obnoxtious, you have progressed where i haven't,.. i do drink too much.

I wish you well.

Margaret.
 

margaret

New Member
colour tv

At the drop of a hat it is human nature to try to supersede one's neighbour by the sudden proculamation of title's, deserved or not, sometimes if word's cannot be found to answer the retort then bring out the achievment's, that should frighten most of us off,...not so!... i also have serviced all kind's of brown good's since 1974 and have gathered many accolades, my recent success being a smoke alarm that can be controlled by the flick of a wall switch, i am content within myself & can reply to self appointed electronic expert's with thought.
 
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