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Old 10th Oct 2017, 10:17 pm   #41
jayceebee
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

There looks to be a fusible resistor R70 in the collector circuit of the E-W transistor TR12. After this kind of failure there's a good chance it could be o/c.

Appears to be a 0.1 ohm.

John.
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Old 17th Oct 2017, 10:24 pm   #42
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Originally Posted by Clydeuk View Post
Looking at the picture, aside from the E/W fault, the corners of the picture do not appear to reach the edges of the screen, also there is a black band top and bottom. Is this due to the source material being displayed or could this indicate the EHT is still too high?
You can blame me on that, i was messing around with the builtin service mode that you can access with a special key combination
These "modern" crt TVs don't have trimpots at all, so that's the only way i had to mess around a bit with vertical deflection and other things.


Quote:
Originally Posted by Argus25 View Post
...I could have added if you look at the waveforms around the mosfet TR12 as noted on the schematic, you will see they are Vertical rate waveforms with a basically parabolic shape, this is the modulation that corrects the scan width over the vertical period as the raster is scanned from top to bottom, and the shape of them is designed to correct the parabolic looking defect you are seeing on the scan currently...
You're right, but unfourtanely i don't own a scope at the moment. It's harder to go through these faults without one i know, but at the moment a multimeter and one of those ebay component testers is all i have.


I've checked the BUK474 transistor as recommended and it seems to be alright. I've also tested all the resistors around it and they were fine (some were harder to find because they were smd parts under the PCB). Also the fusible resistor R70 is ok.
Since the E/W signal is fed to the BUK474 gate from the TDA8844 (which some people seem to call the jungle IC), could this IC be bad?

Oh, and removing the BUK474 from the circuit makes absolutely no difference on the image shape.
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Old 17th Oct 2017, 11:15 pm   #43
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Yes, it is possible the jungle IC could have been damaged by the high voltage event and possibly the BUK474 is getting no drive to its gate, but before concluding that and removing it, it would be good to look at the parabolic drive voltage coming out of it to the BUK474 fet gate, to see if it was abnormal or not there.

Also to check that the IC wasn't being upset by a failed component or an abnormal voltage on one of its other pins.

There is nothing worse than removing & replacing a multi-pin IC to find out that it is not the cause of the fault. This is where a scope, even a really cheap low bandwidth one would help you here, it might be time for you to get one.

If I was stuck on a desert island with minimal tools, I would grab set of small headphones (but not put them directly in my ears in case I overloaded them) or small speaker and put a 1uF capacitor in series with them and put that across the BUK474's gate and source and see if there is a 50Hz buzzing sound, which would indicate that the parabolic gate drive signal was "probably" coming out of the IC. A meter set on AC volts might also help.
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Old 17th Oct 2017, 11:43 pm   #44
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

One quick way of determining "which side" the fault is on, by that I mean whether it is on the line scan proper side or on the parabola drive small signal side is to temporarily short the drain and source of the EW driver transistor.

If the width fully fills the screen now albeit still with EW bowing then the fault lies either with the Mosfet itself (in this case) or in the small signal drive side.

If the width doesn't change then the fault is in the line output "power" stages, and you need to start looking around the diode modulator parts, high voltage capacitors, and any inductors associated with that stage,
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Old 18th Oct 2017, 12:02 am   #45
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Originally Posted by Red to black View Post
One quick way of determining "which side" the fault is on, by that I mean whether it is on the line scan proper side or on the parabola drive small signal side is to temporarily short the drain and source of the EW driver transistor.
That is one way to deactivate the mosfet, but the op already said the problem remains whether the mosfet is physically in the circuit, or not. But it would show if the D & S were shorted if that was affecting the scan width.

Last edited by Argus25; 18th Oct 2017 at 12:07 am.
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Old 18th Oct 2017, 12:10 am   #46
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Yes, but if the mosfet is either faulty (O/C ) or is not driven then removing it in this (and similar chassis) doesn't help determining which side the fault lies as the width is still at minimum with no correction (which is the current fault the OP has).
If the linescan side is ok otherwise then by temporarily shorting across the mosfet would give maximum width albeit with the picture still bowed as it would not have the parabola correction still.

This was standard fault finding techniques to see which side is faulty in the absence of a 'scope.
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Last edited by Red to black; 18th Oct 2017 at 12:18 am.
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Old 18th Oct 2017, 12:38 am   #47
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

I'm really thankful for your tips (they make perfect sense to me) and your patience. I'm going to try them both this weekend when i'll get access to this tv set again.

I've already tried something similar to shorting the drain and source of a mosfet on a low voltage device but never directly, i've always used a resistor with a really small resistor value just in case, but i'll try a direct short as advised.

Also out of curiosity, back in the old days when people actually took their stuff to a repair shop, would this TV be worth to repair?
I already replaced a few critical components and now if this big chip (TDA8844) turns out to be bad, it seems that this would be a lost cause.
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Old 18th Oct 2017, 12:55 am   #48
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Originally Posted by Red to black View Post
This was standard fault finding techniques to see which side is faulty in the absence of a 'scope.
Yes, I think this is a very good suggestion too.

Since the AC parabolic drive voltage at the gate is supposed to be about 1V according to the schematic, it should be easy to detect that with a small speaker and series capacitor. If the mosfet is ok and all the components around it ok on the ouput side, likely its not receiving any drive voltage from the IC.

One other trick is to use a smaller coupling capacitor, say 0.01uF and couple that 1V fet drive signal into the set's audio amplifier circuit, if you can find a handy point.

So it will be interesting to see what the short of D & S test reveals.
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Old 18th Oct 2017, 1:55 am   #49
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Originally Posted by Rick62008 View Post
I already replaced a few critical components and now if this big chip (TDA8844) turns out to be bad, it seems that this would be a lost cause.
I don't thick that is true at all, it looks like a DIL IC that would be fairly easy to remove with a solder sucker, and the IC looks like its available on ebay too and not particularly rare. So if it turned out to be that IC, it would be a shame to trash the set over it. But if its a small outline package one it could be more difficult to remove.
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