fdt-5bx5 Violently Exploding Capacitor

[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?

[old_tv_nut]

I presume you're talking about L503 and the label "MAGNET?"

This is the first time I've seen a magnetic core labeled "MAGNET," but presuming it's not a misnomer, maybe this is meant to be a saturable reactor?

I'm having trouble seeing how this sweep circuit works - what is connected off to the right?

[vol.2]

Originally Posted by old_tv_nut

I presume you're talking about L503 and the label "MAGNET?"

This is the first time I've seen a magnetic core labeled "MAGNET," but presuming it's not a misnomer, maybe this is meant to be a saturable reactor?

I'm having trouble seeing how this sweep circuit works - what is connected off to the right?

I looked up saturable reactor because I've never heard that term before. Honestly not sure what the deal is. If what you are suggesting is true, perhaps the core is saturating and that is the reason for the distortion. If so, maybe there's some way to limit the current to the coil?

I did a bunch more tests and checked voltages, and the only thing I found that wasn't as per the schematic is the 170Vpk sine wave on the horizontal is actually about 208Vpk as I measure it on my scope. I didn't think that was any issue though as I'm sure I've noticed that voltage be similarly high in other sets.

This is what's to the right:

[old_tv_nut]

OK, the flyback is to the right, powered from 22 volts, and the yoke is across a capacitive voltage divider. Makes some sense now.

A saturable reactor is meant to saturate. A transformer wound (two separate windings) saturable reactor was used in some motor speed controls. A DC control winding would change the core saturation and thus control the transfer of AC power on the controlled windings.I can't remember seeing a single coil meant to saturate, just trying to understand what could be going on here.

I was making a wild guess that a magnet could bias the core flux so that the inductance could decrease with heavy current flow one way and not the other, thus expanding one side of the image. Never saw a circuit like that, and really don't know if that could be what's supposed to happen here.

Is there any kind of mechanical adjustment screw on the magnet of L503?

[vol.2]

Originally Posted by old_tv_nut

Is there any kind of mechanical adjustment screw on the magnet of L503?

Nothing like that. The only adjustment I get is horizontal size.

I found that there is a "Horizontal Width Adjust" transistor, Q380, that goes between the luma and some of the horizontal width stuff. I've never seen that in a circuit either. This set is filled with things that are a first for me.

It's a Japanese made Sony Watchman, but the tube is not Trinitron and was sourced from Samsung.

One thing I already tried was putting an extra .22uf capacitor across the S correction cap, but that didn't seem to fix it. I didn't notice much difference in the linearity, but maybe it was a bit more stretched out horizontally.

[old_tv_nut]

"Horizontal Width Width Adjust" sounds like a bad translation of something. I can't see anything adjustable connected to it, so maybe it's a blanking pulse shaper that "adjusts" (changes) the pulse width to the desired waveform (?).

Edit: I note that RV504 is labeled HORIZ SIZE, not "WIDTH."

Next thing I would try is shorting L503 to see what happens.

[old_tv_nut]

Also, have you checked D505 and D506 and the three caps connected to them?

[vol.2]

Originally Posted by old_tv_nut

Also, have you checked D505 and D506 and the three caps connected to them?

I pulled both damper diodes (I think that's what they are) and they both seemed good. Test OL in one direction and .4V in the other direction on diode test.

The caps seem to be fine. They measured okay and are high quality panasonic film caps.

I note that RV504 is labeled HORIZ SIZE, not "WIDTH."

Is there a difference? When I change that, it changes the width of the screen.

Next thing I would try is shorting L503 to see what happens.

L503 is rather large chunky magnetic core inductor with thick copper wire that is quite buried into it's structure. I don't see any way that I could shorten the wire without completely destroying it. I guess I could cut it somewhere in the middle and try to bodge it back together, but I feel somewhat wary to do that as I don't know how I'd keep it from shorting out.

Here's what it looks like:


There's also the small inductor, L502, but it's between the pincushion controls and not in the direct path of the yoke like L503 is.

[old_tv_nut]

1) Ok, diodes and caps are checked.

2) No difference between horizontal size and horizontal width in English. That's why I think the purpose of the transistor Q380 is a bad translation and has nothing to do with the horizontal sweep but probably the horizontal blanking pulse width.

3) L503 -- I meant shorting, not shortening, in other words, temporarily connecting a plain wire across the terminals of L503. HOWEVER, NOW THAT I SEE ITS SIZE, I WOULD NOT DO THAT AS THAT MAY ALLOW TOO MUCH HORIZONTAL SWEEP CURRENT AND INCREASE THE PULSE VOLTAGE BEYOND SAFE COMPONENT LIMITS IF YOU DO THAT. BETTER TO SEE IF YOU CAN FIND A REPLACEMENT.

[vol.2]

Originally Posted by old_tv_nut

BETTER TO SEE IF YOU CAN FIND A REPLACEMENT.

The only source I know of for magnetic core inductors is a custom shop that will wind them for you, but they cost way more than the TV would ever be worth.

Not to mention that I guess at this point I can't even be sure that the coil is at fault. I'm just guess about it losing it's magnetism. In all other ways, the coil appears to be fine from the outside, so other than demagnetizing over time (which I don't even know if that could happen), I don't really have much to go on with it.

If you know of some way I could cheaply buy a replacement (ideally something adjustable) that I could try out, that would be great.

Or if there's any other obvious things to try of course.

[old_tv_nut]

Out of curiosity, could you test with a piece of steel to see if L503 actually is magnetized?
Again, I have never seen a permanently magnetized inductor, and that is why the notation "MAGNET" on the schematic is mysterious to me.

[old_tv_nut]

I found a paper on permanent magnet inductor design. The abstract says they are useful in DC applications, but L503 is being used for AC.

https://ieeexplore.ieee.org/document/5770892

Curiouser and curiouser.