fdt-5bx5 Violently Exploding Capacitor
 

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?


https://i.imgur.com/o5EWdtE.jpg
https://i.imgur.com/T7pWf5S.jpg
https://i.imgur.com/YNLrG79.jpg

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.

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

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

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

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.

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/

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

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.

Okay. Thanks for the explanation, this is new stuff for me.


I found a film capacitor, but I don't totally know what math I need to do in some cases. For example, it has a 3.3 at 100V, and it has no ripple rating. All it has is 20V/microsecond, given as a dv/dt rating.

https://i.imgur.com/NhHt0rR.png

The pulse rating if if it are absorbing pulses directly. Here is working as coupling.
Anyway, unless is a too much compact unit, poly caps in general will support the typical H sector TV current load.

Too bad that is difficult to find a nonpolar cap with full specs....

That cap was a common fail especially in Sony & Zenith B&W sets.
Wrong one will explode sooner or later. In the olden days we just
used the OEM, NP caps were hard to come by.

Zeno

Okay, that makes me feel better about it then. I am going to order up a poly cap that can take a lot more abuse, something on the order of 8A, and see if it can budge that.

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.

https://i.imgur.com/YNLrG79.jpeg

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

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.