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Old 4th Sep 2017, 10:19 pm   #21
Maarten
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

@Argus25: nice modification. Vertical deflection IC's have been troublesome for sure, but not in all cases. TDA3653 seems to live forever in appropriate designs. The IBM 5154 monitor also fried its PCB around the discrete video output stages. Salora (at least that was the ODM for European models, probably Australia models as well) generally made nice sets, but the reliability could often be better.
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Old 7th Sep 2017, 12:41 pm   #22
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Hi, i have plenty of new BU2520D, BU2508D, BU4508D, BU2520D, BU2725D.
If you need a couple, let me know and i will pop them in the post for you, Malc.
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Old 8th Sep 2017, 1:37 am   #23
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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The IBM 5154 monitor also fried its PCB around the discrete video output stages.
Yes, if only they had ventilated it better, both the metalwork around the video output board, the cabinet and if they had used fibreglass pcb's too. But phenolic pcb's were the fashion in most computer VDU's at the time.
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Old 8th Sep 2017, 9:28 am   #24
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Change C67 base coupling cap as that will blow the lop transistor. 4.7uf normally.
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Old 11th Sep 2017, 11:35 pm   #25
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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 MALC SCOTT View Post
Hi, i have plenty of new BU2520D, BU2508D, BU4508D, BU2520D, BU2725D.
If you need a couple, let me know and i will pop them in the post for you, Malc.

Thank you a lot that's very kind. Although i already have that transistor, i'm having a hard time trusting local stores when it comes getting that critical 15nF \ 2000V "tuning" transistors as you called it.
I can find a few very cheap on ebay but i trust them even less when it comes to quality. Although these little guys caught my attention http://www.ebay.com/itm/2CS-GENUINE-...UAAOSw5cRZJgNj

They're not cheap but they seem to be of some quality. I'm not sure if they're suited for the job even though they share the exact same spec.

Would you advise me to get this capacitor from somewere else? Taking into account i'm from Europe.
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Old 12th Sep 2017, 12:06 am   #26
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

WIMA FKP1 is perfectly suitable. The price and shipping are not too far off if you have to order by mail. Local repair shops or what remains of them might be a bit cheaper but not much.
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Old 12th Sep 2017, 12:50 am   #27
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Quote:
Originally Posted by MALC SCOTT View Post
Hi, i have plenty of new BU2520D, BU2508D, BU4508D, BU2520D, BU2725D.
If you need a couple, let me know and i will pop them in the post for you, Malc.

Although these little guys caught my attention http://www.ebay.com/itm/2CS-GENUINE-...UAAOSw5cRZJgNj
Gulp !

I would be extremely reluctant to use a capacitor of the same basic design as the ones that fail, it will do the same thing if you wait.

Over the years I have experimented with tuning capacitors in this H deflection application to find ones that are stable & reliable. Regardless of the fact this type is axial and won't look quite right on the pcb it is by far a superior bet:

http://www.ebay.com/itm/1lot-4PCS-Vi...IAAOSwjKFZOKPX

It will solve the problem.

But if you really wanted to trust your life with it, there is another step you could take, as there is another option:

One other trick that works very well, if you want to totally guarantee longevity of the capacitors, is to use two units of about twice the nF value in series, still both 2kV rated (no balancing resistors are needed), this reduces the dielectric stress as they are only exposed to about 1/2 the peak voltage, and they don't have to be "exactly" the same value either.

Then often it is possible to physically fit them each end up in the existing pcb holes and link them at the tops. I promise you that if you did this, the capacitors would outlast every other part of the set:

http://www.ebay.com/itm/1lot-2PCS-ER...IAAOSwGYVXC6J7
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Old 12th Sep 2017, 7:46 am   #28
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

You mean two with twice the nF value in series, but each? Like 2x33nF as shown in your link? Would there be any visible side effect on the set after performing this?

It's not a easy choice as the set by itself is hardly worth more money than what i already spent (it's a 2001 CRT TV but a really big one at that, forgot how many " ).

I'll have to consider my choices now.
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Old 12th Sep 2017, 10:01 am   #29
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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You mean two with twice the nF value in series, but each? Like 2x33nF as shown in your link? Would there be any visible side effect on the set after performing this?
Yes , if you have the pcb space, just fit two 33nF caps in series and that is close enough to 15nF. With the ERO axial caps, you can just put them end up on the pcb and join the two free leads together which puts them in series and halves the total value to 33nF/2.

Those Ero caps make particularly good tuning caps for line output stage use. They were chosen as the cap of choice by the famous video monitor makers, Conrac in the USA for some of their monitors for this particular application.

If the pcb is a bit crowded, just use one of the ERO 2kV rated 15nF caps.

Last edited by Argus25; 12th Sep 2017 at 10:15 am.
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Old 12th Sep 2017, 12:39 pm   #30
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Alright, i guess i'll do that, and even if the TV doesn't work afterwards it's always good to have those in stock.
Within a few weeks i'll keep this thread updated with the outcome along with photos if possible.
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Old 12th Sep 2017, 2:45 pm   #31
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Hi.

I guess that with flyback tuning capacitors in a conventional transistor line output stage it's best to aim for the correct value or slighly higher. Slightly higher will result in increased deflection width due to a reduced EHT.

Something I've noticed with flyback tuning capacitors is the intended frequency of operation is often quoted on the capacitor namely 15.625kHz for the 625 line system. I assume these capacitors have been tailored for best efficiency at or around this frequency. I've not found any explanation for the actual reason for quoting the frequency on the capacitor, perhaps a more experienced forum member might be able to enlighten me on this point.

Regards
Symon.

Last edited by Philips210; 12th Sep 2017 at 2:48 pm. Reason: typo
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Old 13th Sep 2017, 1:40 pm   #32
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Something I've noticed with flyback tuning capacitors is the intended frequency of operation is often quoted on the capacitor namely 15.625kHz for the 625 line system. I assume these capacitors have been tailored for best efficiency at or around this frequency.
Regards
Symon.
The only reason I can think of for the frequency marking is to indicate the application of the capacitor, in that it has suitable electrical properties for the line output stage tuning application, it wouldn't matter if it was actually used for 15,625 or 15,750 Hz or any other value in that range, that is not its operating frequency. The capacitor operates (tuning the yoke and line opt transformer inductance) at about 1/(twice the flyback time) ,maybe around 50kHz to 70kHz. This frequency is interrupted at the horizontal rate and you only get to see 1/2 a cycle of the oscillation due to the damper diode.

The important point about this sort of capacitor, aside from high voltage ratings are a low ESR, low dielectric losses and low self inductance. Any losses get converted to heat. Also, ideally it would be in a flame retardant casing. To get the low ESR it requires that the lead in wires are physically welded to the foils. If resistance crops up there they heat up and this can be seen in the poorer quality types that fail. In other words, the capacitor requires similar properties to those used in "RF" circuits, its almost RF if you believe RF starts at 100kHz.

Last edited by Argus25; 13th Sep 2017 at 1:46 pm.
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Old 13th Sep 2017, 7:45 pm   #33
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Thank you Argus25 for your detailed reply, most helpful.

Regards
Symon.
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Old 15th Sep 2017, 12:26 pm   #34
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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Gulp !

I would be extremely reluctant to use a capacitor of the same basic design as the ones that fail, it will do the same thing if you wait.

Over the years I have experimented with tuning capacitors in this H deflection application to find ones that are stable & reliable. Regardless of the fact this type is axial and won't look quite right on the pcb it is by far a superior bet:
I too have very good experience with Axial KP capacitors from Roederstein. Radial FKP1 capacitors from Wima should be allright as well, as will KP/MKP from Vishay/BC. What do you mean by 'the same basic design that fail'? It wasn't fitted with an FKP1 or am I missing something? I have found them to be quite reliable.

I consider it overkill to put 2 in series, and even a little dangerous since the chance it will fail from non-voltage related issues is now twice as big. The only advantage would be the slightly higher capacity which could make the tube last a bit longer (or cause cathode poisoning - a two edged sword).

I do, however, routinely replace 1600V types with 2kV since that ought to be enough for most designs with 1600V being the 'economic' choice.

Last edited by Maarten; 15th Sep 2017 at 12:32 pm.
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Old 15th Sep 2017, 2:14 pm   #35
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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What do you mean by 'the same basic design that fail'? It wasn't fitted with an FKP1 or am I missing something? I have found them to be quite reliable....I consider it overkill to put 2 in series
I think the benefits of higher voltage rated caps or using two relates to the physical size of the capacitor. Since after the end of scan plus half way into the flyback, the capacitor is expected to be able to store the entire electromagnetic energy value of the yoke and output transformer. Therefore for a physically smaller part the dielectric stress is higher and the ESR will always be higher for a smaller part than a bigger one, due to the smaller plate to foil weld cross sectional areas.

So by using either a higher voltage rated part (which is physically larger), or spreading the energy between two capacitors (which is effectively one physically larger component) the stress and the temperature is also lower. For example if you look at the capacitor which failed, you can see heating effects around one of the pins discoloring the resin.

It really comes down to the simple fact that a larger component is more robust for the application. You would find of course that something like a 4kV rated capacitor of some value, will be about the same physical volume as two 2kV units where each of those has twice the capacity of the 4kv unit.

Since the better quality caps are axial rather than radial (a lot of the time for this application, but not always) it is therefore convenient to use two as I suggested, if there is room on the pcb. It may well be overkill, but that is preferable to the type of failure seen if the cap goes low value, as you know the damage includes possible failure of the voltage multiplier, failure of the lopt, failure of circuits running of the lopt, failure of the HOT, possible arcing to the crt neck, so it is a total disaster better avoided. In my article on modifying the IBM color monitor, I likened this event to putting a drop of Captain Nero's Red Matter into the line output stage.
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Old 9th Oct 2017, 9:33 pm   #36
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Small update..the caps have arrived and even though they look out of place and a bit overkill, they've worked perfectly.

Click image for larger version

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I decided to add a bit of electrical tape on the top just as a safety measure.

The TV works finally. I had to replace that old tuning cap, the horizontal transistor, and the vertical deflection IC. Also i replaced a 1uF capacitor in that area that had its platic shroud wrinkled and shrunken.


But not everything is perfect still..the image is still not quite there as you can see here.
Click image for larger version

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Googling this fault it seems to be address as "pincushion horizontal fault", and my bet would be some eletrolytic capacitor in the horizontal deflection circuitry.

Any guesses?

Again, the schematic if it helps:

https://www.vintage-radio.net/forum/...2&d=1504395924

I'm almost there, i can't give up now
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Old 10th Oct 2017, 5:51 am   #37
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

The third harmonic tuning capacitor is VERY critical and must be replaced by a special component designed for this circuit position.

Failure to address this will result in high EHT, destroyed line output transistors, CRT flash over causing failure of the CRT, frame output I/C [at least] and other faults.

The original A823 RBM colour chassis originally employed polystyrene types but these soon failed [weeks] and were replaced by special capacitors manufactured by Plessey and marked 16kc/s. I was surprised at the time that a company such as RBM could make such a mistake. You must use the EXACT values. You do not get a second chance with transistor line output stages. John
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Old 10th Oct 2017, 7:24 am   #38
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Hi again,

The reason why the value of the tuning capacitor value is NOT too critical is this:

At the end of the scan for a fixed amount of H deflection/width, there is some fixed amount of stored magnetic energy E. After this field has collapsed and exactly halfway through the flyback time, this energy is all transferred to the tuning capacitor, the usual equation applies which is E = CV(squared)/2.

If you rearrange this it is easy to see that the peak voltage on the tuning capacitor is then V = Square root (2E/c) in other words the peak voltage on the capacitor (and on the HOT's collector) is proportional to the inverse of the square root of the capacitor's value.

So lets say the capacitor is 1.2 times (or 20% bigger than the original value) the peak voltage will be 1/root 1.2 or 91% of what it was and likewise if the capacitor is 20% smaller then the peak voltage will only be roughly about 1.1 times higher.

Ideally though it is replaced with a similar value to what the designers had in there (and as noted on previous posts, with a capacitor physical design suited to the application).

Therefore, it is unlikely there is any damage even if you used say a +/-20% value part or value range. If the value is a bit bigger the flyback will be longer with a lower peak. And as you'd expect the flyback shorter if the cap is smaller with a higher peak.

Deviating in the direction of a slightly higher value tuning cap, if one does go away a little from the original value, is always the safer option as the peak collector voltage on the HOT is a tad lower.

The replacement tuning caps you put in are excellent and won't let you down.

So what has happened here is that when the original tuning cap went way low in value, and the voltages peaked very high, as well as damaging all the parts you have replaced so far, it has taken out something in the pin-cushion correction or East -West modulator circuit. It is just a matter of finding what is damaged. You are almost there.

Look around TR12 with the scope, this Fet has possibly failed or a component associated with it.

Last edited by Argus25; 10th Oct 2017 at 7:32 am. Reason: add suggestion.
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Old 10th Oct 2017, 7:53 am   #39
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

...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...
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Old 10th Oct 2017, 4:24 pm   #40
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Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

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?

I don't have access to the circuit diagram, but the failure of the E/W correction circuit was very common especially if there's been a high EHT or flashover problems.
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