Help fixing a CRT terminal

[rea5245]

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

[drussell]

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?

[perieanuo]

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?

[rea5245]

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.

[rea5245]

Q202?  Do you mean Q303?

Oops. Yes, Q303. I've fixed it in the original post.

[Renate]

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.

[helius]

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.

[Renate]

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.

[rea5245]

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?

[rea5245]

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?

[helius]

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

[rea5245]

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?

[Renate]

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.

[Renate]

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.

[BrokenYugo]

 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.

[rea5245]

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

[Renate]

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.

[rea5245]

(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.

[Renate]

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?

[rea5245]

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. :-(

[Renate]

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.

[rea5245]

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.

[Renate]

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?

[rea5245]

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.

[Renate]

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 +?