fdt-5bx5 Violently Exploding Capacitor

[vol.2]

So, I got my little Watchman back with a raster, and I went about trying to set it up only to notice that the horizontal was quite shifted to the left.

I remembered that it was like this before I mothballed it a couple years ago, and there was some kind of an issue.

So I didn't know exactly what to do because there's no HOR Shift on this set, so I just set it up the best I could and kind of stretched the screen a little bit.

After it had been on for awhile, I was tweaking the purity and all of a sudden the image collapsed horizontally and a capacitor blew up inside the set.

It was C516, a 3.3uF bipolar cap, which was directly in line with the horizontal linearity coil leading to the yoke.

The service manual says the HOR yoke windings should be 0.5ohms, and I read 1ohm, so I think it's fine. I measured it and moved around the yoke and fiddled with it and the reading did not change at all, so I don't think there's a dodgy short.

Here's a picture of the cap and it's location, the part of the service manual showing the blown up cap, and a picture of the raster from before the cap blew up.

If you look at the picture, you can clearly see that the raster was pretty messed up as the circles on the right side are much smaller than the ones on the left; the horizontal linearity was all messed up, and no amount of moving the yoke helped it.



I don't think it was the capacitor's fault because I'm pretty sure it was already replaced by me. It's possible this happened before and I just forgot about it. It's been a long time since I tried to work on it.

Any thoughts?

[Alex KL-1]

This cap in fact carries some quite current, so it can dry or explode ('nuff power for self heating). Can be simply a cap failure. Is worth simply to sub it for a new one. If it have some space, you can put a poly cap in place of it. Carefully watiching the image when CRT heats, and hearing if not make some strange whines.

[Alex KL-1]

In fact, before it explodes, maybe it's ESR are high, affecting linearity.

[vol.2]

Originally Posted by Alex KL-1

This cap in fact carries some quite current, so it can dry or explode ('nuff power for self heating). Can be simply a cap failure. Is worth simply to sub it for a new one. If it have some space, you can put a poly cap in place of it. Carefully watiching the image when CRT heats, and hearing if not make some strange whines.

Well, I did actually replace this cap in the past, so it was a pretty fresh cap that blew up. I compared it to the ones I still have in stock and it's a direct match for them.

I used Panasonic BP caps. 3.3uF, 50V, same as the one that originally came out of there.

I tested the HOR output transistor, and it seems to test like a normal transistor. There aren't any other electrolytic caps in the area that look bad either.

However, perhaps the capacitor got damaged by whatever took out the 24V MOSFET? Maybe I should just try another cap

[jhalphen]

Hi to all,
Hi Vol.2,

could you please tell us where you found the schematic, i also have this model.
Elektrotanya doesn't have it.

Many Thanks! in advance,

Best Regards
jhalphen
Paris/France

[vol.2]

Originally Posted by jhalphen

Hi to all,
Hi Vol.2,

could you please tell us where you found the schematic, i also have this model.
Elektrotanya doesn't have it.

Many Thanks! in advance,

Best Regards
jhalphen
Paris/France

I purchased it from Sam's a long time ago. At this point, I only have a paper copy I printed out of it, I'm not sure what happened to the pdf.

https://www.samswebsite.com/

[Alex KL-1]

Originally Posted by vol.2

Well, I did actually replace this cap in the past, so it was a pretty fresh cap that blew up. I compared it to the ones I still have in stock and it's a direct match for them.

I used Panasonic BP caps. 3.3uF, 50V, same as the one that originally came out of there.

I tested the HOR output transistor, and it seems to test like a normal transistor. There aren't any other electrolytic caps in the area that look bad either.

However, perhaps the capacitor got damaged by whatever took out the 24V MOSFET? Maybe I should just try another cap

Not all BP caps support continuour current flowing, so is interesting to confirm the model used; perhaps it don't support the current.
A poly cap will support it for sure.

[vol.2]

Originally Posted by Alex KL-1

Not all BP caps support continuour current flowing, so is interesting to confirm the model used; perhaps it don't support the current.
A poly cap will support it for sure.

Oh! I didn't realize that. I'm not sure how I'd be able to figure it out. The exact cap I used was this one ECE-A1HN3R3UB

https://industrial.panasonic.com/cdb...A0000C1053.pdf

[Alex KL-1]

For the 3.3V/50V one from this catalog, the maximum ripple is 25mA for 60Hz (120Hz). Certainly is higher for 15kHz, but is too far away...
Is good to take one with higher current classification, or to find the >10kHz ripple current. It needs to be higher than the current voltage product of the H circuit; for example, a small 5" BW draining perhaps 600mA from the 12V PSU, is good to use a cap rated at least for this current to be sure.
Some cap types have high frequency ripple specified.

[vol.2]

Okay. I got a beefy film cap and replaced it and it fired right up.

After running it for awhile and doing some setup, the Vertical started to go, and I found that the capacitor between the pump-up pin and the output power supply on the vertical IC had blown.

This would not be a surprise to me, except that I already replaced that capacitor with a good Panasonic cap. This time I also replaced the 7830 vert IC because I have one in parts bin.

I was able to more or less dial in the purity and convergence, but I'm having some weird issues on the screen still:

1) I'm getting this effect where the top and bottom of the screen are kind of pinched inward.

Usually, if either the top or bottom is pinched in, the other side is bowed outward, indicating that the yoke is tilted. In this case though, both the top and bottom are pinched inward, so I don't know exactly what that means.

2) Horizontal linearity is still messed up. If I display a row of circles across the raster, the circles on the left side of the screen are correct, but the circles on the right side of the screen are horizontally squished. This is especially obvious in test patterns. This isn't a new issue, it was like this before the bipolar cap on the horizontal yoke blew up.



Anyone have an idea as to these two issues might be caused?

[ChrisW6ATV]

That type of capacitor was quite common in monitors that I repaired in the mid-1980s to the mid-2000s or so. In fact, common enough that local parts stores (when those still existed!) had them in stock, in different values.

They are/were called "high frequency non-polarized electrolytic" capacitors.

Because they are explicitly intended for operation in a horizontal sweep circuit, they are much physically larger than a "standard" non-polarized electrolytic capacitor of the same value and voltage.

[vol.2]

Originally Posted by ChrisW6ATV

That type of capacitor was quite common in monitors that I repaired in the mid-1980s to the mid-2000s or so. In fact, common enough that local parts stores (when those still existed!) had them in stock, in different values.

They are/were called "high frequency non-polarized electrolytic" capacitors.

Because they are explicitly intended for operation in a horizontal sweep circuit, they are much physically larger than a "standard" non-polarized electrolytic capacitor of the same value and voltage.

Interesting. I replaced it with a beefy 3.3uf film cap. Do you think this could be causing the weird horizontal linearity issue?

For reference, it was like that when I had a bipolar electrolytic in there, before it blew up.

[ChrisW6ATV]

Based on your comment that it was the same, I doubt that the film capacitor would cause it. At lest one other monitor model I used to repair (Setchell Carlson/Audiotronics/Dotronix 23M922/23VM922) had a big film capacitor in a similar circuit location if I remember right. That one was 4.7uF/100 volts, I think, almost two inches or over 40mm long.

[vol.2]

Originally Posted by ChrisW6ATV

Based on your comment that it was the same, I doubt that the film capacitor would cause it. At lest one other monitor model I used to repair (Setchell Carlson/Audiotronics/Dotronix 23M922/23VM922) had a big film capacitor in a similar circuit location if I remember right. That one was 4.7uF/100 volts, I think, almost two inches or over 40mm long.

Okay thanks. I didn't think it would be an issue, though I don't know what could be causing it.

I replaced the electrolytics in the horizontal area already. There's also the linearity coil which I guess could be a bad value for the circuit and maybe it was always like this, but I can't imagine them thinking it's okay.

I wonder if I can improve the linearity by replacing the coil with something adjustable, or there's some other part of the circuit that I can tweak to improve it?

Maybe adding some capacitance to the big film cap?

[vol.2]

I wonder if this issue I'm seeing is caused by the magnetic core of the horizontal linearity coil losing it's magnetism over time? If that's possible, it would probably show this kind of a symptom I think.

If the reason for the magnetic core is to give the coil a non-linear response to correct for the asymmetrical energy in the flyback pulse, then losing it's magnet would nullify that correction, right?

I don't think it's feasible to replace this thing as they don't seem to make them anymore (there's a couple places that do custom jobs, but that's too expensive). I wonder if there's some other compensation I can do?