NARC med res tube swap

dethfactor

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So I picked up a Narc to help RenegadeFred out and I knew going in the monitor (K7000) was an issue (no green). Threw the tube on the rejuvenator and no dice (no green emission at all) so I started looking through a few Discarded TVs I've collected over the past year. I pulled one down and the yolk resistances were off but not by much.

The yolk that was on the tube in it had
horiz res - .8 ohms
vert res - 8.8 ohms

The tv I had -
horiz res - 3 ohms
vert res - 9 ohms

So I decided to be better safe than sorry and that I'd need to transplant yolks. Unfortunately the thing was so dry rotted it crumbled as I was taking it off of the tube and now makes a great paper weight.

Since I read that sometimes the resistances can be close I decided to use the yolk from the TV and it's pretty close.

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There's some horizontal curl over on the right side of the screen. I've tried adjusting the width coil but was wondering if there's something you monitor gurus can tell me. Maybe by swapping out some components on the horiz deflection circuit or changing the width coil. Not sure. If not does anyone have a comparable yolk to sell?

Cheers,
-M
 
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So I went ahead and ordered Bobs width kit and remove the width bag cap on my k7000. It was pretty corroded so I put it on my meter and read 409k nf so the next smaller size was 227k nf and I put that in and there's no real difference to what I had and the width coil can't be adjusted now.

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Should I go smaller/bigger or am I barking up the wrong tree and need to buy another yolk?
 
I thought you needed to have a medium res yoke (different windings) to work with a medium res chassis. how did you luck out with a tv yoke?
 
*shrug* I pulled a zenith tube.
Tube 'a48acb02x', manufactured in 1989 the horiz resistance is about 2 ohms off which is probably why the screen is horizontally short.
 
Sorry not any good input on my end obviously, but it's nice to actually see some green on that screen. Hopefully someone can chime in and help you out, I'd like to see it once everything is back in order.

Hell I admit, I already miss it, there's still a hole along the wall where it was. I need something new now to fill that spot. Best of luck to ya on that monitor!
 
Tube 'a48acb02x', manufactured in 1989 the horiz resistance is about 2 ohms off which is probably why the screen is horizontally short.

I think you have it backwards. The higher resistance of your horizontal winding coil implies it has more turns in the coil than the original yoke. More turns make a higher magnetic field for a given current; more magnetic field makes the beam deflect more. A yoke with an overwound horizontal coil would give a picture that was stretched, not compressed, horizontally.

Also, an over- or under-wound coil would stretch or compress the whole picture along the horizontal axis, instead of displaying correctly near the center but stopping very suddenly at the edge.

For some reason your horizontal deflection circuit isn't making it to the peak positive and negative voltages it's supposed to reach -- the waveform is flattening out short of the peak voltages, so the beam can't deflect all the way to the left or right edge of the screen. The problem could lie in any of the components in the horizontal deflection circuit, or, since that circuit is driven by the B+ voltage, you may have a fault in the power supply circuit that's causing B+ to be too low.
 
I think you have it backwards. The higher resistance of your horizontal winding coil implies it has more turns in the coil than the original yoke. More turns make a higher magnetic field for a given current; more magnetic field makes the beam deflect more. A yoke with an overwound horizontal coil would give a picture that was stretched, not compressed, horizontally.

Also, an over- or under-wound coil would stretch or compress the whole picture along the horizontal axis, instead of displaying correctly near the center but stopping very suddenly at the edge.

For some reason your horizontal deflection circuit isn't making it to the peak positive and negative voltages it's supposed to reach -- the waveform is flattening out short of the peak voltages, so the beam can't deflect all the way to the left or right edge of the screen. The problem could lie in any of the components in the horizontal deflection circuit, or, since that circuit is driven by the B+ voltage, you may have a fault in the power supply circuit that's causing B+ to be too low.

Thanks for the info/knowledge. What I'm not getting is that the old tube didn't have a width issue. Hmmm.. I know this sounds unlikely is there a polarity of the winding? I had to splice a connector, I doubt it but could the two horizontal windings be switched?

http://forums.arcade-museum.com/showthread.php?t=239973
 
Thanks for the info/knowledge. What I'm not getting is that the old tube didn't have a width issue. Hmmm.. I know this sounds unlikely is there a polarity of the winding? I had to splice a connector, I doubt it but could the two horizontal windings be switched?
The picture would be back-to-front if that was the case.

Shupac said:
For some reason your horizontal deflection circuit isn't making it to the peak positive and negative voltages it's supposed to reach
The higher impedance of the coil means more voltage is needed to reach full deflection. A 3 ohm winding is absolutely NOT a suitable replacement for 0.8 ohms.
 
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The picture would be back-to-front if that was the case.


The higher impedance of the coil means more voltage is needed to reach full deflection. A 3 ohm winding is absolutely NOT a suitable replacement for 0.8 ohms.

Hmm.. So is there a way to increase the voltage of the horizontal deflection circuit?
 
The higher impedance of the coil means more voltage is needed to reach full deflection. A 3 ohm winding is absolutely NOT a suitable replacement for 0.8 ohms.

Slight correction: it's current, not voltage, that creates the magnetic field that deflects the beam. The voltage helps you push current through the coil, but the electrons in the beam don't know and don't care what voltage is driving the current. All they "see" is the magnetic field generated by current flowing through the coil.

I agree that the problem lies in using a 3-ohm coil in a circuit designed for a 0.8-ohm coil. Consider Ohm's law, V=IR. If you want the same current to flow through a coil with 3x the resistance, you have to supply 3x the drive voltage. Because the horizontal output transistor (HOT) can't drive the coil with a voltage higher than the B+ supply voltage, it's reasonable to infer that the higher resistance of the 3-ohm coil is pushing the HOT to the limit of its operation, thereby limiting the current that the deflection circuit can drive through the coil.

That theory matches the screen you're seeing. At a certain amount of X-axis deflection, the horizontal output transistor is maxing out its ability to supply current to the coil, placing a hard limit on the horizontal beam displacement. So any graphics at that X coordinate, or greater, are getting squashed into a vertical line.

If you're feeling brave, and don't mind tampering with the yoke and potentially destroying it, the solution would be to remove windings from the horizontal deflection coils. Believe it or not, this can be done, and if you have some degree of skill and confidence, it's easier than you might think. I've done it.
 
Slight correction: it's current, not voltage, that creates the magnetic field that deflects the beam. The voltage helps you push current through the coil, but the electrons in the beam don't know and don't care what voltage is driving the current. All they "see" is the magnetic field generated by current flowing through the coil.

I agree that the problem lies in using a 3-ohm coil in a circuit designed for a 0.8-ohm coil. Consider Ohm's law, V=IR. If you want the same current to flow through a coil with 3x the resistance, you have to supply 3x the drive voltage. Because the horizontal output transistor (HOT) can't drive the coil with a voltage higher than the B+ supply voltage, it's reasonable to infer that the higher resistance of the 3-ohm coil is pushing the HOT to the limit of its operation, thereby limiting the current that the deflection circuit can drive through the coil.

That theory matches the screen you're seeing. At a certain amount of X-axis deflection, the horizontal output transistor is maxing out its ability to supply current to the coil, placing a hard limit on the horizontal beam displacement. So any graphics at that X coordinate, or greater, are getting squashed into a vertical line.

If you're feeling brave, and don't mind tampering with the yoke and potentially destroying it, the solution would be to remove windings from the horizontal deflection coils. Believe it or not, this can be done, and if you have some degree of skill and confidence, it's easier than you might think. I've done it.

Whoa... Thanks for spelling it out for me. The whole while unwinding was my hunch but my lack of knowledge of deflection and its circuitry led me to thinking there'd be other options. The truth of the matter is the original yolk is cracked up, the clips that hold the convergence rings/rest of the yolk to the neck are separated. I could possibly glue it all together though, I just figured using a yolk that was close might be a cleaner/easier alternative. At this point I have 3 possible yolks; the first being the original yolk i can *rig* together on the new tube, the second is a very similar yolk to the one currently connected (exact same horiz/vert resistance on the coils) that I could attempt/practice unwinding and lastly the one I currently have in the game that I can also unwind.

Any tips on unwinding? I imagine going from 3ohms to .8 is going to require a significant amount of removal.

Also thanks everyone for the input/help. This has been an interesting learning experience into the forays of tube swapping.
 
Any tips on unwinding? I imagine going from 3ohms to .8 is going to require a significant amount of removal.

Be warned, this is tricky business. A lot of things can go wrong, and if they do, the yoke is trashed. Take notes and/or pictures along the way and don't rush. Understand that this is a hack, and if you're a perfectionist, you will probably decide the final result doesn't look quite as good or converge quite as well as a yoke built by robots. But I think you may find the results of this operation good enough.

If you want to try it, here's what to do:

(1) Remove the plastic cover to expose the ends of the windings. You'll see that the ends of the vertical and horizontal coils are attached to metal posts. Since the vertical coils are on the outside of the yoke, it should be easy to visually follow the wires and see what posts those wires connect to. The horizontal coils connect to the OTHER set of posts. Make a note of how the horizontal deflection coils are connected to these posts, then clip the wires to disconnect the coils.

(2) We want to get the two horizontal saddle coils out of the yoke. To do this, you'll have to disassemble the yoke. (a) Remove the glue holding the vertical coils in place. (See first pic.) (b) Using a small flathead screwdriver, pry loose the clips that hold the two halves of the toroid together. (See second pic. Don't lose the clips!) Be careful from here on, since the heavy toroids will be hanging from the posts by a couple skinny wires. If the wires break, it's not a catastrophe, because you can reattach them, but it's easier if you don't break them in the first place. (c) Pry apart the two plastic halves of the "bell" of the yoke. This may require undoing, or even breaking, the plastic locking tabs on either side of the housing. With the horizontal saddle coils disconnected from the posts, they should fall out on their own, or you can wiggle them out manually.

(3) Each of your horizontal coils probably looks like the one in picture no. 3. You'll see that the windings are glued to each other to keep them conformed to the saddle shape. Grab the loose wire on the OUTSIDE of the coil and start unwinding by carefully, but firmly, peeling the wire away from the other wires. Do NOT force it -- pull too hard, and you'll break the wire, and you will have major problems trying to retrieve the broken end that's now buried within the coil.

(4) If the wire gets stuck, or you find yourself using too much force trying to peel it away from the winding, use a lighter to heat the area of the coil where the wire is stuck. Usually a little heat will soften the glue enough that the wire will then pull away without any problem. Don't let the flame set fire to the glue or the enamel coating on the wire.

(5) Sometimes you'll find that the wire takes a detour into the "inside" of the coil; that is, that instead of running along the outside edge of the coil, it dives underneath a bunch of other windings. This can be a problem, but using a lighter and a small, sharp tool like a dental pick, you should be able to gently pry the wire loose and continue unwinding.

(6) At this point you may be wondering how much wire you should unwind. Better to unwind too little than too much, because you can always unwind more, but you can't put back wire you've already peeled off. Also, the magnetic field generated by a coil is proportional to the number of windings in the coil, so if you take off too much wire, you may not have enough turns left in the coil to get full deflection even at max current. The resistance of the coil is proportional to the length of wire in the coil, so if you want to decrease the coil resistance by half, unwind half the wire. (In your case, I'll venture to guess you'll be OK at around 2 ohms resistance, so aim to remove about 1/3 of the original 3-ohm winding.)

(7) The amount you unwind is not critical; you only have to end up in the ballpark of your target resistance. What *is* critical is that you unwind the two saddle coils by the same amount. Any asymmetry between the two coils will mess up your convergence and distort the display since one coil will have more turns (and therefore more magnetic influence) than the other. I have two ideas about how to keep the coils symmetric: (a) To measure how much wire you've peeled off, wind the unwound portion around a spool -- or even your hand -- and count the number of turns. E.g., if you unwind a length of wire that loops around your hand 40 times, make sure you take 40 hand-loops of wire off of the other saddle coil. That's the way I did it, and it was good enough. (b) A better way would be to use a small electronic scale to weigh the coils as you unwind them. That way you can determine how much wire you've removed by how much lighter the remainder of the coil becomes.

(8) When you've removed enough wire, cut off the part you've unwound, making sure to leave enough length to reconnect the coil to the solder posts. Remount the saddle coils in the yoke housing. Although the coils are now noticeably smaller, they're still the same shape as the original coils, so they should fit back into the housing just fine.

(9) Snap the two halves of the yoke housing back together. If the locking tabs and slots are mangled, no worries; a little hot glue will stick those puppies together real good.

(10) Before you solder the ends of the coils back onto the posts, you have to remove the enamel coating from the coil wire. I like to burn off about a half-inch of coating using a lighter -- it will actually produce a small, brief flame. When the flame burns out, scrape away the sooty residue by running the end of the wire over a file, or an emery board, or, best of all, some 400-grit sandpaper. You will be rewarded with a shiny bit of uninsulated wire, ready for soldering.

(11) Re-solder the coil ends to the posts. You did take notes on which wires go to which posts, didn't you?

(12) Replace the vertical coils on the yoke housing and snap the metal retaining clips back into place.

(13) At this point the coils are electrically reconnected and the yoke is mechanically reassembled. You may notice the horizontal saddles and vertical toroids are a little jiggly since you removed the glue that was holding them in place. Don't worry about that yet.

(14) Place the yoke on the CRT and plug it into the monitor PCB. Don't bother with the purity rings at this point; we just want to verify that we have proper horizontal deflection.

(15) Put the neck board back on and fire up the monitor. If everything went well, you should now see full horizontal deflection. You can adjust the width by using a hex key to twist the core inside the width coil. (Note that if you turn the core with a metal tool while the monitor is turned on, the tool will get super-hot. This is definitely not recommended. If you don't have a plastic tool for adjusting the width coil, do your width-coil adjustments when the monitor is powered down.)

Finish up by hot-gluing the saddles and toroids in place, replacing the purity rings on the CRT neck, and adjusting the convergence.

Good luck. Let me know if it works.
 

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Shupac thanks for the GREAT post about unwinding the deflection coils. I think someone should definitely sticky it somewhere.

So after trying to disassemble another yolk and destroying it, I decided that unwinding is not for me. After a long discussion with VonGoosewink I decided to try to salvage the old yolk. Luckily the parts of the old yolk that broke were just enough to set the purity rings in place and with a little bit of fidgeting and a piece of 1'1/4" plastic drain pipe, some pressure and a clamp. I was able to get the yolk/rings in place.

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Before convergence:
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Great job, that's going to look great once fully converged! Glad you saved the monitor, those med-res are getting hard to find!
 
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