Help fixing a CRT terminal

[rea5245]

Assume the glue between the base and the tube itself is fairly weak.

Fortunately, this is a Heathkit. There ain't no glue.

[floobydust]

No need to discharge the CRT unless you disconnect the anode, and there's no need to mess with that to remove the yoke.

Is that from experience or theory?
In the shop it was standard procedure to discharge a CRT before working with it, even the electron gun end removing the neck board to get the yoke off. In service manuals it's also advised, so I see no problem using a jumper and screwdriver and doing it. As a multi-plate capacitor (with partial discharges on the envelope) CRT's do give surprises. It's more important with colour TV's and monitors.

I thought maybe this terminal came as a kit, you know like the Heathkit TVs? but it looks like not.

[rea5245]

I thought maybe this terminal came as a kit, you know like the Heathkit TVs? but it looks like not.

Yes, it was a kit, but the previous owner assembled it; not me.

(Although I owned and assembled one of these back in 1980, so I bought this for nostalgia. Alas, so far, I've just gotten agita.)

[floobydust]

I was looking for the instructions how to install the yoke, or if the CRT came with it installed.
Try to have fun with it, repairing old stuff is always an adventure. Heathkit is worth it.

[rea5245]

OK, I've removed the yoke and tested it for continuity. One of the coils is fine; the other one is open. Even touching the bare wires doesn't show continuity, so it's not just the solder connections.

Video, or it didn't happen: https://youtu.be/VmHmMATU98k

Am I well and truly screwed?

[floobydust]

I can see the break? DO SURGERY 
Although, the wire should not have fused like that so I'm wondering what happened.

[james_s]

Is that from experience or theory?
In the shop it was standard procedure to discharge a CRT before working with it, even the electron gun end removing the neck board to get the yoke off. In service manuals it's also advised, so I see no problem using a jumper and screwdriver and doing it. As a multi-plate capacitor (with partial discharges on the envelope) CRT's do give surprises. It's more important with colour TV's and monitors.

~25 years of experience repairing CRT displays, only as a hobbyist but I've fixed hundreds of them over the years, especially back in the early 2000's. Won't hurt anything to discharge the anode, but I don't bother to do it unless I'm disconnecting it.

[james_s]

I can see the break? DO SURGERY 
Although, the wire should not have fused like that so I'm wondering what happened.

That's the horizontal winding, not the vertical. Are you sure it's actually a break and not just some dirt or something?

There is a splice between the two halves of each winding. I'd start there. 

[rea5245]

That's the horizontal winding, not the vertical.

I'm impressed. How can you tell? All windings look the same to me.

There is a splice between the two halves of each winding. I'd start there.

There is? Where? I don't see it.

The yoke is wrapped in electrical tape. Is further inspection going to require me to remove that? What are the implications of doing so? Are the windings going to fall out of place?

[Renate]

I can see the break? DO SURGERY 

From here it looks like an arc-over in the horizontal windings.
That's the kind of failure I'd more likely expect in a yoke.
But that's just a video, you need a real closeup.
It could be a dust bunny.

In any case, that's not vertical (unless they are being weird and the vertical is on the inside).
You've got to get some sharp pointy probes.
Also, I don't know at what resistance your meter beeps, is it really measuring infinite on the vertical?
Probing the wires may not tell you anything, they're shellaced.

[floobydust]

OP can you post a pic of the blackened section, or look at it up close with a magnifier. Hopefully it's just a dead bug.
To me it looks like trouble and we don't know if "someone else has been in there".
I'm can't remember if a breakdown can form between H and V coils, it's not making sense if the H coil has an issue.

[james_s]

That's the horizontal winding, not the vertical.

I'm impressed. How can you tell? All windings look the same to me.

There is a splice between the two halves of each winding. I'd start there.

There is? Where? I don't see it.

The yoke is wrapped in electrical tape. Is further inspection going to require me to remove that? What are the implications of doing so? Are the windings going to fall out of place?

The horizontal winding is on the inside and it's an air-core winding, this is due to the much higher frequency, 15.7kHz for NTSC and somewhere in that neighborhood for this compared to the vertical which is driven at around 60Hz. The vertical winding has a ferrite core, and at least in every yoke I can recall messing with it consists of two halves which are clipped and usually glued together. They are wound separately and there's a splice to connect them both in series normally. The horizontal winding doubles as an inductor and is part of a resonant tank that also includes the HV "flyback" transformer. The vertical winding is just an electromagnet that is driven by a sawtooth waveform resembling a crude audio amplifier. The horizontal winding is critical and has to be just right or the magic smoke is released. The vertical winding has a lot more wiggle room and will usually work to some degree even if something is way off without anything blowing up.

It's unusual for it to be wrapped in electrical tape, that shouldn't be necessary to hold anything together, it's possible somebody applied it later to cover something up, such as a repair to a damaged winding.

[rea5245]

Here's a new video. I started peeling the tape off and found that someone had, indeed, repaired (or tried to repair) this yoke.

The tape goes between the layer of the outer coil and the inner coil. It looks like it might be tough to get it off. I'm wondering how someone got it on in the first place.

I also tried to show the connections better, where one of them is blackened. But despite its appearance, I verified that it's a good connection.

https://youtu.be/rYXsFPLKnSE

[Renate]

https://youtu.be/rYXsFPLKnSE

At 1:59 you rotate the yoke into view and at the 2 o'clock position there is that black spot again.
I'm not saying that that could be an open, I'm thinking that it's a short between a few windings?

They clearly separated the H & V sections to wrap the tape in there.

Although it doesn't seem to be your problem, in other cases I would worry about the hobbyist crimps on the plug.
I've found that plier crimped, then soldered connections are very unreliable.
It's too easy to get solder on the outside and not the inside.
The best is to use a real crimper.
If not, definitely tin the wire before plier crimping, then soldering.

[rea5245]

At 1:59 you rotate the yoke into view and at the 2 o'clock position there is that black spot again.
I'm not saying that that could be an open, I'm thinking that it's a short between a few windings?

Oh, that black spot.

It just seems to be dirt. I got a little of it off with my fingernail, but not all. Then I tried with a pick and I scraped a little insulation off. So now I've made it worse. I guess I need to somehow re-insulate that before plugging it in again.

[rea5245]

I'm back, still working on fixing the vertical deflection in my Heathkit H9 terminal.

I have rewound the vertical yoke coils. I unwound the old ones, counting the number of layers and the turns in each layer. And I did, indeed, find multiple breaks in the wire. Then I rewound it, and the vertical coil now measures about 6 .

But my vertical deflection still does not work. So I'm trying to trace through the circuit, and I quickly find something I don't understand.

Here's the circuit for the vertical deflection:


I put my scope's ground on point G and probe point A and get this:


That's good. That's a digital vertical sync pulse. It's supposed to feed into Q301 and drain capacitor C303. (C303 will later be charged through Q302 to produce a ramp.)

But when I move my scope probe to point B, I see this:


How can that be?

Thank you,
   Bob

[JayArr]

I know I'm late to this thread but back in the 90's and early 00s I repaired thousands of CRT monitors (4-6 per day) and we found the yokes pretty interchangeable. Once in a while we'd get a model with a strange value yoke but for the most part the CRTs were all so similar that the yokes were as well. We would often grab a CRT out of a monitor that had a bad flyback and use it in monitor that had a fuzzy or dim CRT, we wouldn't bother to change the yokes, we just left them on the CRT we were swapping. A yoke off a Samsung would work on an Innovison etc. The pinouts were usually all the same as well so the little CRT board would just push onto the new CRT and with a little adjustment it would work.

If you can get a yoke off a similar sized CRT you could give it a try.

[xavier60]

The waveforms indicate C302 has some ESR or there is a series resistor not shown.
The waveform at the Base of Q301 looks like enough anyway. What's happening at its Collector?
What is the DCV at the output of the Vertical stage? The + of C309.

[Renate]

That's what I'd expect on the right side of C302.
The pulse is being clamped at Vbe of Q301, so you get a large negative swing.
Look at C303 and see if you get a nice sawtooth.
I'll bet it's there.

[rea5245]

Thank you Renate and xavier60. You were right: there's a decent sawtooth wave at C303:



C303 feeds into R307, and on the other end of R307, the waveform doesn't look so good:



Jumping ahead to the signal going into the vertical yoke, the yellow trace is one side of the yoke, the cyan trace is the other side, and the purple trace is the first minus the second:



The good news is, it's sort of a sawtooth. The bad news is, it's at 15.5kHz. It's the horizontal sweep! But I'm quite certain I'm probing the vertical coil: if I were probing the wrong coil, the voltages would be far higher.

So it seems that something goes wrong at R307 and then the horizontal pulse gets into the vertical amplifier somehow. I have not found any solder bridges or shorts that would explain it, and the H and V components are physically separated from each other on the board.

Any advice?

Thank you,
   Bob

[xavier60]

Measure the voltages on   Q303, Q304, Q305 and Q306 to ground.

[Renate]

The right side of R307 is basically a summing point.
It gets the negative current feedback from the yoke.
So I wouldn't expect to see anything resembling a sawtooth.

A-B really only makes any sense if both channels are on the same voltage scale.

You're just seeing the H bleed through.
For after Q303 keep your scope on V rate and the amplitude at 5 V/division.

What's collector of Q303 doing?
It should have a nice big (20V?) sawtooth on it.

Go to emitter Q306, it should have basically the same thing.
Then the minus end of Q309, basically the same thing but centered on zero volts.

[xavier60]

When waveforms are abnormal or not present, DC voltage measurements become useful to us.

[rea5245]

xavier, I know you're basically holding my hand here, and I just wanted to say thank you for your continued help and patience. I can do some digital electronics, but I'm way out of my comfort zone with this stuff.

I assume you meant to measure the bases of those transistors. Here they are. Yellow is Q303, cyan is Q304, magenta is Q305, and blue is Q306.

[xavier60]

You need to measure the DC voltages of all 3 pins of  Q303, Q304, Q305 and Q306 to ground with a DMM.