Author Topic: Help fixing a CRT terminal  (Read 3851 times)

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Offline floobydust

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Re: Help fixing a CRT terminal
« Reply #25 on: March 28, 2021, 06:16:13 pm »
I would first check C309 500uF, it is a high failure rate part in TV's and monitors due to the high ripple current it sees. I outright replace them.
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #26 on: March 28, 2021, 06:27:12 pm »
Oops, sorry, my bad. :-[
C305 is fine, I was getting it confused with C309.

'Salright. I'm grateful for the help you're giving me.

So, the entire vertical output is dead.
Is there voltage on collector Q304?
Is there voltage on collector Q305?
Is there voltage on C305 +?

Q304's collector is 28V
Q305's collector is 28V
C305+ is 30V
 

Offline james_s

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Re: Help fixing a CRT terminal
« Reply #27 on: March 28, 2021, 06:31:27 pm »
Maybe this is too obvious, but have you tested the continuity of the yoke itself? It could be open, or have shorted turns.
Testing shorted turns requires a "ringer" that connects a capacitor to the yoke to make an RLC tank, then injects an impulse and counts the rings (in other words, a Q measurement). Shorted turns will give the yoke a low Q and make it hard to drive.

It did, indeed, occur to me that the yoke might be bad. It sounds like I need some special equipment to test it?

I've *never* seen a bad yoke. It can happen but it's unlikely, and checking the DC resistance with a simple multimeter will be enough to tell whether it's in a condition that should result in at least a recognizable picture.
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #28 on: March 28, 2021, 06:35:30 pm »
So you said that the the emitters Q305, Q306 were reading a DC of basically zero volts?
Ground your "Point B", (base of Q303), the emitters Q305, Q306 should go high to 30 V.

This is basically an audio amp and the vertical yoke is like a speaker coil.
There's not a lot of dynamics or resonance or anything.
There's just a bit of current feedback to make the sweep linear.
 

Offline james_s

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Re: Help fixing a CRT terminal
« Reply #29 on: March 28, 2021, 06:41:13 pm »
I think that it's premature to run around looking for a yoke.

OK, I just measured the resistance on the horizontal coil and the vertical coil. The horizontal fluctuated between 30 and 40 ohms. The vertical is infinite resistance.

So is it now time to run around looking for a new yoke?

- Bob

You won't find an exact replacement, unless it's from another similar monitor. Yokes are custom parts made for a specific monitor chassis. Fortunately there is not much to them and the vertical portion can be rewound fairly easily but I've never found that to be necessary in a repair. The winding is mostly right out in the open and visible, the only place I've ever seen one go open circuit is right at the terminal strip on the yoke so check that carefully. There is also a splice between the two halves of the winding but I haven't ever had that fail. Look closely for physical damage, like if something whacked it and broke a wire. If you get really desperate, try connecting one of those little stun gun HV modules to the winding (UNPLUG IT FIRST!!) in a dark room and see where it arcs. This ought to be a very simple repair but be careful, CRTs have not been made in years and if you crack the exhaust pip in the middle of the neck socket it's game over. You have horizontal scan, HV and the video amplifier is doing something so all the hard stuff is ok, you're 90% of the way to a working monitor.
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #30 on: March 28, 2021, 06:49:26 pm »
Yes, the emitters of Q305 and Q306 are essentially 0V.

I grounded the base of Q303, but there was no change to Q305's or Q306's emitter voltages. As I wrote in my OP, even without forcing Q303's base to ground, it was reading 0V (but I still went ahead and jumpered it to ground for this test).
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #31 on: March 28, 2021, 06:53:59 pm »
So, remove Q303 and that should make the emitters Q305, Q306 go to 30 V
 

Offline james_s

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Re: Help fixing a CRT terminal
« Reply #32 on: March 28, 2021, 07:30:46 pm »
I'm a little confused, is the yoke open or not? Can we establish that the yoke is not an open circuit or a dead short before we start poking around in the circuit? I feel like this thread is jumping all over the place and it would help to be a little more organized and methodical.
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #33 on: March 28, 2021, 07:33:06 pm »
is the yoke open or not?

It appears to be open. But fixing or replacing the yoke will take a while, if it's possible at all. So I also want to figure out if the circuit is working right.
 

Offline floobydust

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Re: Help fixing a CRT terminal
« Reply #34 on: March 28, 2021, 07:39:19 pm »
I would not pull Q303 and run it for long, it may roast Q305, Q306 they will be full on, assuming the yoke and its connectors/wiring has continuity.
Very rare for a yoke to fail, they are just copper wire. I think I've seen one fail with shorted turns, out of 100's. Check the connectors and soldering at the winding ends, they can corrode with old flux.

P.S. check R323 is not open and causing a goose chase.
« Last Edit: March 28, 2021, 07:41:17 pm by floobydust »
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #35 on: March 28, 2021, 07:43:44 pm »
I'm a little confused...
I'm more than confused. :-//
That's ok.
Apparently??? the vertical winding on the yoke is open.
I dunno, it would get the 5th degree from me.
I'd put it under a bright light, right in its eyes.
I'd say, "Yeah, punk, are you really as open as you pretend to be?"

But we still don't have an vert out? Why not?
Did shotgunning parts throw a wrench in things?
Was there an original problem that burned out the winding? (mmm, kind of doubt it).

@floobydust There's 10 Ω + 3.9 Ω doing current limiting and C309 would have to be shorted anyway.
 

Online xavier60

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Re: Help fixing a CRT terminal
« Reply #36 on: March 28, 2021, 08:06:45 pm »
With the vertical yoke winding open, the amplifier wont work properly because there is no ground return path for R318. The output capacitor is used for bootstrapping the driver.
A high wattage resistor could be used as a test load.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #37 on: March 28, 2021, 08:12:25 pm »
With the vertical yoke winding open, the amplifier wont work properly because there is no ground return path for R318. The output capacitor is used for bootstrapping the driver.
A high wattage resistor could be used as a test load.

I have some 10 watt resistors. Either 10 ohms or 50 ohms. Which do you like (if either)?
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #38 on: March 28, 2021, 08:17:39 pm »
The feedback is AC coupled so the amplifier would just bounce off the walls without any load anyway.
 

Online xavier60

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Re: Help fixing a CRT terminal
« Reply #39 on: March 28, 2021, 08:19:32 pm »
With the vertical yoke winding open, the amplifier wont work properly because there is no ground return path for R318. The output capacitor is used for bootstrapping the driver.
A high wattage resistor could be used as a test load.

I have some 10 watt resistors. Either 10 ohms or 50 ohms. Which do you like (if either)?
Use 100 ohms or higher. I just realized that the open yoke should not cause the low voltage at the output transistors.
What are the voltages on Q303.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline james_s

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Re: Help fixing a CRT terminal
« Reply #40 on: March 28, 2021, 08:34:20 pm »
is the yoke open or not?

It appears to be open. But fixing or replacing the yoke will take a while, if it's possible at all. So I also want to figure out if the circuit is working right.

Figure out the yoke before you worry about anything else. What you're doing is the equivalent of twiddling every adjustment you can find, every action you take risks breaking something that was working and at some point you're not going to have any clue what you've fixed and what you've broken. The yoke is trivially simple, it's just two windings, each terminating to a pair of wires. The horizontal is usually a few ohms, the vertical usually a few tens of ohms. Don't touch anything else until you figure out what's up with the yoke, and make sure you are measuring it correctly.
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #41 on: March 28, 2021, 08:39:22 pm »
Good advice, but a lot of components have been replaced already.
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #42 on: March 28, 2021, 08:46:11 pm »
Figure out the yoke before you worry about anything else.

OK, but I'm not sure what I can do. My multimeter reads infinite resistance on the vertical coil. I haven't removed the yoke from the CRT yet, but I've looked underneath it with a mirror. I don't see anything broken. And I don't know where I could buy another. Xavier suggests I put a 100 ohm, 10W resistor in parallel with the vertical coil. Do you agree that this would be a good way to verify that the video board works?
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #43 on: March 28, 2021, 08:50:04 pm »
That's fine, you can stuff some sort of resistor in there.
But since the totem pole is reading low, adding a load won't make it higher.
 

Offline BrokenYugo

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Re: Help fixing a CRT terminal
« Reply #44 on: March 28, 2021, 09:31:27 pm »
You need to get out a magnifying glass and bright light and inspect the yoke, it's wiring, any relevant solder joints, etc. I assume this was kit built and between the builder and and any repairs and modification in the interim this Heathkit stuff is often full of well below par soldering, assembly errors and questionable workmanship in general just waiting to cause some intermittent annoyance. Bad connection is more likely than open winding, are you probing right at the yoke?

I also find it's easy to fool myself measuring ohms/continuity if I'm not extremely careful. E.G. if something looks open I make a habit of putting both probes on the same terminal to ensure I'm making contact and not impeded by flux/corossion/etc. If you're not getting steady readings you have bad contact or maybe a bad meter. Sharp probes probably help.

And as always, pictures can help a lot.
 
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Offline james_s

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Re: Help fixing a CRT terminal
« Reply #45 on: March 28, 2021, 11:30:59 pm »
Figure out the yoke before you worry about anything else.

OK, but I'm not sure what I can do. My multimeter reads infinite resistance on the vertical coil. I haven't removed the yoke from the CRT yet, but I've looked underneath it with a mirror. I don't see anything broken. And I don't know where I could buy another. Xavier suggests I put a 100 ohm, 10W resistor in parallel with the vertical coil. Do you agree that this would be a good way to verify that the video board works?

Well it's easy to remove the yoke, there's no convergence or purity adjustments to worry about like there is with color monitors. Inspect it carefully, especially where the magnet wire connects to the terminals and make sure you're measuring across the right terminals. I wouldn't bother trying to hook up a resistor, if the yoke is open it doesn't matter if there's a fault with the circuit, it won't work until the yoke is fixed. Since the yoke is easy to test then that's the obvious place to start. Once you have that showing a reasonable resistance then plug it in and see if the circuit requires work, it may not. You can't buy another, they don't make CRTs or yokes anymore and haven't in years now. You might get lucky and find something on ebay eventually but there's no guarantee that the characteristics will match closely enough to work. Fortunately you shouldn't have to get a new one. I have rewound yokes to build vector monitors, the vertical winding is not hard to work with.
 

Offline floobydust

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Re: Help fixing a CRT terminal
« Reply #46 on: March 29, 2021, 05:00:18 am »
I would discharge the CRT, pull off the socket and unscrew the yoke clamps and pull it off for surgery.
But because I'm lazy, I would first try to just resolder the ends of the windings at the terminals.
With age something seems to etch copper at those ends, I've seen in old magnetics.
 

Offline james_s

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Re: Help fixing a CRT terminal
« Reply #47 on: March 29, 2021, 05:54:34 am »
No need to discharge the CRT unless you disconnect the anode, and there's no need to mess with that to remove the yoke.
 

Offline BrokenYugo

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Re: Help fixing a CRT terminal
« Reply #48 on: March 29, 2021, 04:23:57 pm »
The real gotcha on the socket/neck board is you don't want to just yank it off. Very gently rock it back and forth to get it off, or carefully pry between the socket and plastic tube base, again working your way around. Assume the glue holding the tube's side of the connector to the tube itself is fairly weak.

I'd also leave the anode alone, no need to mess with it and discharging is a shock risk in and of itself, especially with improvised equipment.
« Last Edit: March 29, 2021, 10:24:33 pm by BrokenYugo »
 

Offline helius

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Re: Help fixing a CRT terminal
« Reply #49 on: March 29, 2021, 06:21:31 pm »
I have found that if Silastic is used to hold the socket to the neck, a scalpel works well to slice through it, cutting directly alongside the socket. Not much force is required and the pins aren't damaged.
 


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