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Electronics => Repair => Topic started by: rea5245 on March 27, 2021, 01:43:00 pm

Title: Help fixing a CRT terminal
Post by: rea5245 on March 27, 2021, 01:43:00 pm
Hi,

I have a Heathkit H9 CRT computer terminal. When I turn it on, it's supposed to display a screen full of question marks. Instead, it displays a single line of dashes in the vertical center of the screen.

I've confirmed that the character generator is producing the bits for question mark characters. I believe the problem is there's no vertical deflection, so the screenful of question marks looks like a line of dashes.

I've attached a schematic of the vertical video circuit. I've confirmed that at point A (the positive side of C303), there is a sawtooth wave. At point B (the base of Q303), however, it's just a constant zero volts.

I've checked continuity from C303 to R307, then to C304, then to the base of Q303. I've also replaced those four components with new ones. I've confirmed that R309, R313, and R318 are all at their appropriate values and that the base of Q303 is not shorted to ground. C304 is a 0.1uF Mylar cap and when I measure it in-circuit, I get 70nF. I don't know if I should be worried about that.

I'm out of ideas. Can anyone offer advice on what to do next?

Thank you,
   Bob
Title: Re: Help fixing a CRT terminal
Post by: drussell on March 27, 2021, 01:59:52 pm
First, whenever you have no vertical deflection be sure to keep the brightness way, way down to prevent burn-in damage to the CRT along that collapsed line!

I've confirmed that R309, R313, and R318 are all at their appropriate values and that the base of Q202 is not shorted to ground.

Q202?  Do you mean Q303?

Is there voltage at the collector of Q303?  Like could R312 or R311 be open so just no power getting to Q303?  If there is power there, have you checked that C306 is not shorted (although you would still see some sawtooth there due to the 1M R309 unless it was shorted too.)

Basically you will need to probe and see where your signal disappears.  If you have signal at A and R307 and C304 are good, you should have signal at B.  Do you see signal at B with Q303 out of circuit?  What is the voltage at the collector pad on the PCB with it in circuit vs. out of circuit?

Title: Re: Help fixing a CRT terminal
Post by: perieanuo on March 27, 2021, 04:17:48 pm
i don't get the defect, can you post screen pictures?
you suspect character deformation if i understood correctly?
that's why you suspect vert/horizontal deflection?
vertical center for me means  hor deflection problem
the image on screen stretches on H and V or not?
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 27, 2021, 04:22:57 pm
i don't get the defect, can you post screen pictures?
you suspect character deformation if i understood correctly?
that's why you suspect vert/horizontal deflection?
vertical center for me means  hor deflection problem
the image on screen stretches on H and V or not?

The horizontal deflection is working, so there are dashes at the left of the screen and the right of the screen.

Vertical deflection is not working: the top half of the screen is blank, the bottom half is blank, and running through the middle is a line of dashes, stretching from the left edge to the right.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 27, 2021, 04:23:37 pm
Q202?  Do you mean Q303?

Oops. Yes, Q303. I've fixed it in the original post.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 27, 2021, 07:56:03 pm
At point B (the base of Q303), however, it's just a constant zero volts.
Do you really mean 0.000 V or 1.4 V DC?
There should be no perceivable sawtooth at B because it gets the current feedback.
Title: Re: Help fixing a CRT terminal
Post by: helius on March 27, 2021, 08:25:04 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.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 27, 2021, 08:29:50 pm
Maybe this is too obvious, but have you tested the continuity of the yoke itself? It could be open, or have shorted turns.
I was thinking shorted, if in fact that Q303 B had 1.4 VDC and no (i.e. a balanced signal).
An open yolk, er yoke would give you an unbalanced signal at Q303 B.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 27, 2021, 08:39:45 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?
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 27, 2021, 09:36:23 pm
Should I be able to feed a constant voltage into the vertical deflection coil, and cause the line of dashes to move up and down on the screen? If so, wouldn't that be an easy way to test the yoke?
Title: Re: Help fixing a CRT terminal
Post by: helius on March 27, 2021, 10:08:23 pm
The simplest test of a yoke is whether it's open (a broken wire). A DMM set on resistance or continuity will do.
A shorted turn is when the insulation varnish on the wires is broken and the copper of two turns is touching. An LCR meter set to measure Q should show this, but you need to know the expected Q of the coil. Ringing testers count the number of rings in the RLC tank circuit and report it as an integer. Less than 8 rings indicates shorted turns.

Should I be able to feed a constant voltage into the vertical deflection coil, and cause the line of dashes to move up and down on the screen? If so, wouldn't that be an easy way to test the yoke?
Only in case of an open, which you can test unpowered with a multimeter anyway
A shorted turn doesn't remove the ability of the yoke to deflect, but it changes its ability to respond to a dynamic signal from the sweep circuit
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 27, 2021, 10:21:38 pm
Well, right now there's no deflection at all, so if the problem is the coil, I guess it would be an open, not a shorted turn.

If it's the yoke, is there any hope of finding a replacement? I have a part number for the CRT (310GJB4) but nothing for the yoke. How do I find a compatible yoke?
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 27, 2021, 10:55:06 pm
I think that it's premature to run around looking for a yoke.
Have you verified that there is no sawtooth on the emitters Q305, Q306?
Have you checked if R323 is reading 3.9 Ω ?
The vertical yoke is not particularly resonate and half the windings shorted would just mean a half height raster.
The collector of Q303, base, emitter of Q304, bases of Q305, Q306 should be measuring ~18-20 VDC on a DVM.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 27, 2021, 11:04:04 pm
Should I be able to feed a constant voltage into the vertical deflection coil, and cause the line of dashes to move up and down on the screen? If so, wouldn't that be an easy way to test the yoke?
Yes, that would.
But the vertical and horizontal are on the same plug and you need the horizontal yoke connected (for it to work and not to blow up anything).
If you can safely/easily disconnect just the vertical yoke a few volts will get you some wiggle.
Title: Re: Help fixing a CRT terminal
Post by: BrokenYugo on March 27, 2021, 11:07:22 pm
 Trace the signal path as suggested, work from both ends, keep the brightness turned down. This hopefully goes without saying but also keep in mind there's fairly high voltages on the neck board of a CRT, probe carefully.

Did you test the components you replaced?

My wild ass guess is C309 dried out.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 28, 2021, 12:55:36 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
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 02:29:53 pm
So is it now time to run around looking for a new yoke?
Maybe, maybe not.
(We would have gotten here a lot quicker if you had just checked the output first.)

Look at the yoke very carefully. Are the connections good?
Is there a thermistor in series with the vertical yoke?
They do that sometimes to compensate for the the thermal coefficient of the windings.
If you find one, check the resistance of that.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 28, 2021, 02:59:07 pm
(We would have gotten here a lot quicker if you had just checked the output first.)

I did, even before I posted. The signal across the vertical coil makes it look like the video circuit is broken. I've attached a screenshot.

Look at the yoke very carefully. Are the connections good?
Is there a thermistor in series with the vertical yoke?

They appear to be good, and I don't see any thermistor.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 03:21:48 pm
I've attached a screenshot.
Sorry, I don't know what you're looking at there.
What point are you measuring?
If the yoke is open you should see a large signal going from 0 to 38? V on the emitters of Q305, Q306.
Even without a sweep signal it should be sitting in the middle somewhere, 18-20 V

Sure, the vertical winding of the yoke could be open.
It's just that arcing over and burning is more typical of the horizontal winding.

Are you sure that you have good contact, because you say that the horizontal ohmmeter readings fluctuates.
Are you measuring at the terminal board on the yoke or on the plug?
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 28, 2021, 03:37:34 pm
Sorry, I don't know what you're looking at there.
What point are you measuring?

That's measured across the vertical deflection yoke. The scope lead is on P301-1 and the ground is on P301-4. I even tried unplugging the yoke from the video board, and the wave changed a little, but was basically the same.

So it's possible that both the board and the yoke are bad. Or maybe I'm measuring something wrong (I'm not very experienced with this) and everything will "magically" start working if I get a new yoke.

One thing I am certain about, though: the resistance across the vertical coil, measured by a multimeter while the yoke is disconnected from the board, is infinite.

Another thing I am certain about: I overpaid when I bought this on eBay. :-(

Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 03:51:50 pm
The scope lead is on P301-1 and the ground is on P301-4.
OUCH! Please do not do this, even if the device under test is floating!
You are going to burn out your probes or the ground path in your scope.
Just keep your ground on the ground.
If you want differential, your scope has a Ch1-Ch2 somewhere.
Use two probes.

I don't know if you disconnected (while running) all four wires of the yoke, but don't do that!
Besides burning the CRT, without the horizontal windings the voltage on the horizontal output transistor could go to who knows where and blow it up.

Just set the scope to 10 V/div and look at those emitters (either one) Q305, Q306.
If you have a signal, check on the other side of Q309.
You're going to have to get a signal there even if you do need and get a new yoke.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 28, 2021, 05:16:22 pm
OK. I guess I'm lucky I haven't fried anything. Thanks.

I'm now using the negative lead of C305 for the ground. The emitters of Q305 and Q306 have a constant  500mV on them. The emitter of Q304 has a constant 300mV on it. The base of Q304 has a constant 600mV. All of these are slightly noisy (high frequency noise; nothing like 60 Hz).

All these transistors are new from Digi-Key, as are the caps.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 05:29:48 pm
I'm now using the negative lead of C305 for the ground.
No, no, no!
Use ground as ground. No part of C305 is ground.


We're not looking for millivolts here, we're looking for a signal that is 30-something volts peak-to-peak.
Ground clip on ground, probe on emitter of Q305 or Q306, please.
What is the voltage in a round number of volts?
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 28, 2021, 05:34:49 pm
I'm sorry if I'm being slow, but I'm confused. The schematic shows that C305 is tied to ground. Why isn't that a good place to clip onto?

I chose it because it has a nice, long lead that is easy to clip to. I'm just trying to find a good place on this board to use as ground, and it looked like a good choice.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 05:46:13 pm
Oops, sorry, my bad. :-[
C305 is fine, I was getting it confused with C309.

So, the entire vertical output is dead.
Is there voltage on collector Q304?
Is there voltage on collector Q305?
Is there voltage on C305 +?
Title: Re: Help fixing a CRT terminal
Post by: floobydust 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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 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
Title: Re: Help fixing a CRT terminal
Post by: james_s 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.
Title: Re: Help fixing a CRT terminal
Post by: Renate 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.
Title: Re: Help fixing a CRT terminal
Post by: james_s 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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 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).
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 06:53:59 pm
So, remove Q303 and that should make the emitters Q305, Q306 go to 30 V
Title: Re: Help fixing a CRT terminal
Post by: james_s 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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 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.
Title: Re: Help fixing a CRT terminal
Post by: floobydust 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.
Title: Re: Help fixing a CRT terminal
Post by: Renate 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.
Title: Re: Help fixing a CRT terminal
Post by: xavier60 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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 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)?
Title: Re: Help fixing a CRT terminal
Post by: Renate 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.
Title: Re: Help fixing a CRT terminal
Post by: xavier60 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.
Title: Re: Help fixing a CRT terminal
Post by: james_s 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.
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 28, 2021, 08:39:22 pm
Good advice, but a lot of components have been replaced already.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 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?
Title: Re: Help fixing a CRT terminal
Post by: Renate 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.
Title: Re: Help fixing a CRT terminal
Post by: BrokenYugo 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.
Title: Re: Help fixing a CRT terminal
Post by: james_s 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.
Title: Re: Help fixing a CRT terminal
Post by: floobydust 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.
Title: Re: Help fixing a CRT terminal
Post by: james_s 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.
Title: Re: Help fixing a CRT terminal
Post by: BrokenYugo 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.
Title: Re: Help fixing a CRT terminal
Post by: helius 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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 29, 2021, 06:24:45 pm
Assume the glue between the base and the tube itself is fairly weak.

Fortunately, this is a Heathkit. There ain't no glue.
Title: Re: Help fixing a CRT terminal
Post by: floobydust on March 29, 2021, 07:02:23 pm
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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 29, 2021, 07:05:17 pm
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.)
Title: Re: Help fixing a CRT terminal
Post by: floobydust on March 29, 2021, 07:08:49 pm
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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 29, 2021, 07:34:07 pm
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?
Title: Re: Help fixing a CRT terminal
Post by: floobydust on March 29, 2021, 07:49:41 pm
I can see the break? DO SURGERY  :-/O
Although, the wire should not have fused like that so I'm wondering what happened.
Title: Re: Help fixing a CRT terminal
Post by: james_s on March 29, 2021, 08:13:47 pm
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.
Title: Re: Help fixing a CRT terminal
Post by: james_s on March 29, 2021, 08:16:14 pm
I can see the break? DO SURGERY  :-/O
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. 
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 29, 2021, 08:24:15 pm
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?
Title: Re: Help fixing a CRT terminal
Post by: Renate on March 30, 2021, 01:36:07 am
I can see the break? DO SURGERY  :-/O
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.
Title: Re: Help fixing a CRT terminal
Post by: floobydust on March 30, 2021, 02:36:48 am
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.
Title: Re: Help fixing a CRT terminal
Post by: james_s on March 30, 2021, 07:20:42 am
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.

Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 30, 2021, 12:51:44 pm
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

Title: Re: Help fixing a CRT terminal
Post by: Renate on March 30, 2021, 01:20:15 pm
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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on March 30, 2021, 01:34:07 pm
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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 07, 2021, 11:20:50 pm
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 \$\Omega\$.

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:
[attach=1]

I put my scope's ground on point G and probe point A and get this:
[attach=2]

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:
[attach=3]

How can that be?

Thank you,
   Bob
Title: Re: Help fixing a CRT terminal
Post by: JayArr on May 08, 2021, 02:41:43 am
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.
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 08, 2021, 05:06:29 am
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.
Title: Re: Help fixing a CRT terminal
Post by: Renate on May 08, 2021, 09:28:06 am
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.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 09, 2021, 07:28:53 pm
Thank you Renate and xavier60. You were right: there's a decent sawtooth wave at C303:

(https://www.eevblog.com/forum/repair/help-fixing-a-crt-terminal/?action=dlattach;attach=1217909;image)

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

(https://www.eevblog.com/forum/repair/help-fixing-a-crt-terminal/?action=dlattach;attach=1217911;image)

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:

(https://www.eevblog.com/forum/repair/help-fixing-a-crt-terminal/?action=dlattach;attach=1217913;image)

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
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 09, 2021, 10:57:55 pm
Measure the voltages on   Q303, Q304, Q305 and Q306 to ground.
Title: Re: Help fixing a CRT terminal
Post by: Renate on May 09, 2021, 11:06:49 pm
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.
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 09, 2021, 11:11:53 pm
When waveforms are abnormal or not present, DC voltage measurements become useful to us.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 09, 2021, 11:12:36 pm
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.

[attachimg=1]
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 09, 2021, 11:16:54 pm
You need to measure the DC voltages of all 3 pins of  Q303, Q304, Q305 and Q306 to ground with a DMM.
Title: Re: Help fixing a CRT terminal
Post by: Renate on May 09, 2021, 11:25:49 pm
When you're hunting for bear and you don't see any, you don't use a magnifying glass to see if you missed any.

Keep the sweep on V rate. We're not interested in anything happening in H.
Keep the gain on 5V/div for a single trace.

Double check that you actually have +36 (+38? I can't read it) supply.

Q303 is not getting its bias. Check voltage on C305.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 09, 2021, 11:26:36 pm
Renate, thanks to you, too, for your help.

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.

In this display, yellow is Q303's collector, cyan is Q306's emitter, and magenta is C309's minus end. I guess the ~0V on Q303's collector is a real problem. That means there's no vertical deflection signal, and we're just picking up noise from the horizontal?

- Bob


Title: Re: Help fixing a CRT terminal
Post by: Renate on May 09, 2021, 11:35:02 pm
You've got a flat line.
Your patient is dead.
You don't zoom in and say, "Hey, we've got a pulse! The heart is going 10,000 beats per minute."
"He's dead, Jim."

Seriously, leave the sweep at 2 mS/div

You've got a massive lack of interesting voltages there.
Check +36 supply and C305.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 09, 2021, 11:43:00 pm
Seriously, leave the sweep at 2 mS/div

You've got a massive lack of interesting voltages there.

I was just hitting the Auto button on the scope, and it chose the sweep and voltage ranges. I did check, before posting the images here, whether there was anything in the 60Hz range and there isn't.

So yeah, I guess it is, as you say, flatlined. I need to hunt down what's going on with the 36V supply (which is coming is from  another board). Thanks for your help, and I'll be back either with good news, or more pleas for help.

Thanks,
   Bob
Title: Re: Help fixing a CRT terminal
Post by: Renate on May 09, 2021, 11:58:31 pm
"Otto is not your friend, Luke. Use the force!"
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 03:13:32 pm
It turns out the 36V supply is fine.

I've measured voltages around the transistors using a voltmeter, not a scope. Here's what I found:
[attach=1]
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 04:07:39 pm
Try removing Q303. The effect to check for is the output voltage going high. If it does go high, don't leave it powered for long.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 04:12:44 pm
Try removing Q303. The effect to check for is the output voltage going high. If it does go high, don't leave it powered for long.

The output voltage where? What point should I measure at?

Instead of desoldering Q303, could I just jumper the base to ground?
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 04:15:44 pm
The output of the vertical stage, the + end of C309.
There is no voltage measured on the Base of Q303, so shorting it to ground should do nothing, try anyway.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 05:19:42 pm
With Q303 removed, the plus end of C309 reads 29V.
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 05:25:27 pm
With Q303 removed, the plus end of C309 reads 29V.
If you replace Q303 with something common like a BC548, the vertical stage should work in a fashion. 
Q305 might run at a higher temperature, so don't leave it powered for long.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 05:41:53 pm
Q303 is an MPSA13. Are you thinking that it's defective?

The BC548 does not appear to be a Darlington. MPSA13s are still being made (https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSA13/11649904 (https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSA13/11649904)). Should I get one of those?
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 05:48:32 pm
Q303 is an MPSA13. Are you thinking that it's defective?

The BC548 does not appear to be a Darlington. MPSA13s are still being made (https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSA13/11649904 (https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSA13/11649904)). Should I get one of those?
Yes, get the original. Seeing it work with a BC548 will help to confirm that Q303 is really faulty.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 05:57:37 pm
Sitting in my parts box are some BC547s and MPSA42s. Looking at the data sheets, I'm thinking the MPSA42 might be the better one to try. Do you agree?
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 06:06:21 pm
Sitting in my parts box are some BC547s and MPSA42s. Looking at the data sheets, I'm thinking the MPSA42 might be the better one to try. Do you agree?
The BC547 has much higher current gain so is likely to work better. Or you could make a Darlington out of both with the Bc547 driving the MPSA42 with a 47K between B-E of the MPSA42.
What ever you do, I would like to see the waveform at the Collector of Q303. I suspect that it spikes to a high voltage during retrace.
The MPSA13 is rated for 30V.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 06:40:57 pm
OMG, it works! Thank you!!

Here's Q303's collector:
(https://www.eevblog.com/forum/repair/help-fixing-a-crt-terminal/?action=dlattach;attach=1218182;image)

And here's the Heathkit H9:
(https://www.eevblog.com/forum/repair/help-fixing-a-crt-terminal/?action=dlattach;attach=1218184;image)
Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 06:48:05 pm
Good to see. Is that with a made up Darlington?
The raster appears to be rhomboid shaped. The vertical yoke core might need to be rotated.
Title: Re: Help fixing a CRT terminal
Post by: rea5245 on May 10, 2021, 07:12:37 pm
That's with just the BC547.

Rotating the yoke helped. I need to center the image horizontally and vertically - it's getting smooshed at the bottom.

Title: Re: Help fixing a CRT terminal
Post by: xavier60 on May 10, 2021, 07:36:32 pm
 I wouldn't expect it to work properly without Q303 being a Darlington.
Title: Re: Help fixing a CRT terminal
Post by: james_s on May 13, 2021, 11:07:17 pm
You're about 98% of the way there, install the correct replacement part and then get everything dialed in. Looks like the tube is in good shape and so is the HV transformer and the yoke is at least in reasonable condition. Pretty much everything else is easily replaced.