Hugo Holden
Veteran Member
Most likely, by the look of it, that inductor has an adjustable core and is a width coil that will be wired in series with the horizontal coils, but to be sure I would have to look at the schematic again.
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Apple IIc Monitor Tube displaying strange behavior
- Thread starter VERAULT
- Start date
Most likely, by the look of it, that inductor has an adjustable core and is a width coil that will be wired in series with the horizontal coils, but to be sure I would have to look at the schematic again.
VERAULT
Veteran Member
i can see inside it has a hex bit slot so it looks adjustable.. What does it adjust exactly?
Horizontal scan width, most likely.
Hugo Holden
Veteran Member
Yes, and this likely width coil is unrelated to your current problem which is likely with the vertical yoke coils.
Also , do both the yokes and these coils look exactly the same on both CRT's ?
Also , do both the yokes and these coils look exactly the same on both CRT's ?
VERAULT
Veteran Member
Yes they are both identical on both Crt's.
Precisely.
And I've seen bad CRTs in a couple of occasion (on two different oscilloscopes though, not monitors) that would produce a badly distorted image, more likely because of distorted/damaged anode grid.
So if I'd need to remove the yoke, I would first try rotating it to exclude a bad CRT.
Missing connection on a parallel coil arrangement is still my first suspicion though.
Frank
VERAULT
Veteran Member
So If I turn the yoke and the distorted image remains intact but turns as well you suspect a bad crt?
Hugo Holden
Veteran Member
The abnormal deflection that you are seeing is coming about because of an abnormal magnetic deflection field produced by the deflection yoke's vertical coils.Also looking at the images posted, this kind of very distorted reduced amplitude vertical scan likely would only come about if the vertical output stage/amplifier was badly overloaded due to a large drop in inductance of the vertical yoke coils, meaning, likely shorted turns.
In oscilloscope CRT's (quite different) the deflection field is electrostatic and is normally produced buy the CRT's gun deflection plates. These can get damaged if the CRT gets dropped etc. But a magnetic deflection CRT like you have does not have these, just a very simple gun structure to produce a focussed electron stream.
When you rotate the yoke, if you do, the image you see on the screen will simply rotate too.
Many scopes have round CRT's and to rotate the image you simply rotate the whole CRT which of course rotates the deflection plates. For square CRT scopes, it was necessary to include a rotation coil around the neck as you cannot rotate the tube.
Often in TV/monitors etc, where the yoke is clamped to the neck, the plastic tongues (like a split sleeve)that project from the yoke under the metal clamp can stick to the neck, even after the metal clamp is loosened. Don't put too much force on it, try to free each of the plastic tongues off the neck first if you attempt rotation , but I cannot see there is any point in removing the yoke, from the CRT yet, unless you are replacing it. I think your CRT itself is probably fine.
Have you compared the DC resistance of the vertical coils of the two yokes on the meter yet?
In oscilloscope CRT's (quite different) the deflection field is electrostatic and is normally produced buy the CRT's gun deflection plates. These can get damaged if the CRT gets dropped etc. But a magnetic deflection CRT like you have does not have these, just a very simple gun structure to produce a focussed electron stream.
When you rotate the yoke, if you do, the image you see on the screen will simply rotate too.
Many scopes have round CRT's and to rotate the image you simply rotate the whole CRT which of course rotates the deflection plates. For square CRT scopes, it was necessary to include a rotation coil around the neck as you cannot rotate the tube.
Often in TV/monitors etc, where the yoke is clamped to the neck, the plastic tongues (like a split sleeve)that project from the yoke under the metal clamp can stick to the neck, even after the metal clamp is loosened. Don't put too much force on it, try to free each of the plastic tongues off the neck first if you attempt rotation , but I cannot see there is any point in removing the yoke, from the CRT yet, unless you are replacing it. I think your CRT itself is probably fine.
Have you compared the DC resistance of the vertical coils of the two yokes on the meter yet?
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It is the contrary: if the image turns with the yoke, it's a damaged yoke, if the image doesn't turn with the yoke, it's a bad CRT.
Frank
anode grid collapsed can make bad distortion on all or parts of a CRT. Sometimes also the focusing grid can. I've seen a few cases in the last 30 years.
I agree however that it's more likely the yoke, and in that case, it's a lucky situation for the vertical driver(s) to be still functioning.
Frank
David_M
Veteran Member
I'm surprised that people are blaming the CRT and the yoke when in post #2 the OP mentions a missing diode and a broken diode. The diodes are used to suppress ringing in the deflection coils.
Why blame the unlikely instead of the obvious?
Why blame the unlikely instead of the obvious?
VERAULT
Veteran Member
You must have missed the post where I mentioned I have several monitor boards. The board missing the diodes gives me some roster but that is all. I only posted that info in the hope someone could get me a schematic or parts list. The board in question with the collapsed bad image IS NOT missing diodes.
VERAULT
Veteran Member
Got it and understood. Leaving to Virginia for a week, will try turning the yoke when I get back.
And to the post by KC9UDX, didnt swap yokes yet will when I return.
Hugo Holden
Veteran Member
One of the interesting things about faults on CRT based monitors (and TV's) is that if everything is working well enough, to at least see some kind of image on the CRT face, then you can deduce a very large amount of data about the fault condition.
If the beam from the gun is in focus, at some point at least, you will see a trace that is reasonably well focused and that tells you the electron gun and focus electrode voltages are at least roughly correct and that the CRT's grid and cathode voltages are about right.
Then with the beam deflection by the yoke's coils there is a lot of information to be gained from looking at the geometry of the raster scan. For example looking at the photos the OP posted, the CRT's beam mirrors the exact magnetic field generated by the yoke coils. The Horizontal deflection (width) is about the right width (but notice it is not a stable width....see below for the reason). The vertical amplitude of the scan is reduced and distorted, giving great insight into the fault.
We know that the drive circuitry for the yoke is ok on the other CRT and yoke assembly. Therefore the faulty vertical yoke assembly is loading down the vertical amplifier's output stage (likely shorted turns in one of the vertical coils). Due to this high loading, it causes ripple on the power supply voltage and that is why the width of the horizontal scan is altering during the abnormal vertical scan period, from top to bottom, of the scanning raster.
So, even without the good yoke/crt assembly to compare this with, most likely you will find if you measure the DC resistance of the vertical coils separately (they would normally have about the same DC resistance) you will find that one of those vertical coils will have a significantly lower DC resistance than the other due to shorted turns. Looking at the schematic, for the IIc monitor, as is commonly done, the two vertical coils are wired in series, and the two horizontal coils are in parallel.
So a simple check, with the ohm meter, should confirm, that one of the vertical coil assemblies on the faulty yoke has a substantially lower resistance than the other.
If the beam from the gun is in focus, at some point at least, you will see a trace that is reasonably well focused and that tells you the electron gun and focus electrode voltages are at least roughly correct and that the CRT's grid and cathode voltages are about right.
Then with the beam deflection by the yoke's coils there is a lot of information to be gained from looking at the geometry of the raster scan. For example looking at the photos the OP posted, the CRT's beam mirrors the exact magnetic field generated by the yoke coils. The Horizontal deflection (width) is about the right width (but notice it is not a stable width....see below for the reason). The vertical amplitude of the scan is reduced and distorted, giving great insight into the fault.
We know that the drive circuitry for the yoke is ok on the other CRT and yoke assembly. Therefore the faulty vertical yoke assembly is loading down the vertical amplifier's output stage (likely shorted turns in one of the vertical coils). Due to this high loading, it causes ripple on the power supply voltage and that is why the width of the horizontal scan is altering during the abnormal vertical scan period, from top to bottom, of the scanning raster.
So, even without the good yoke/crt assembly to compare this with, most likely you will find if you measure the DC resistance of the vertical coils separately (they would normally have about the same DC resistance) you will find that one of those vertical coils will have a significantly lower DC resistance than the other due to shorted turns. Looking at the schematic, for the IIc monitor, as is commonly done, the two vertical coils are wired in series, and the two horizontal coils are in parallel.
So a simple check, with the ohm meter, should confirm, that one of the vertical coil assemblies on the faulty yoke has a substantially lower resistance than the other.
T-Squared
Veteran Member
You're not the only one who had trouble with deteriorating diodes on their system :D: https://www.classic-computers.org.nz/blog/2008-8-23-repairing an IBM monitor.htm
VERAULT
Veteran Member
Thanks T, those are the exact same diodes I am having the problems with. Unfortunately I still have no way of knowing thier( there are a few different varieties not just one) values.
also I tried clicking on Terry's link to the forum to see what value they gave him but that link seems to be not working.
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Hugo Holden
Veteran Member
You can replace these glass passivated diodes with the 4A rated BY448. If you strike one with particularly thick lead wires, use a BY228 that has a 5A rating. Both of these are over 1600V piv rated and have fast switching times, they are probably far superior to the diode that was in there in the first place and they are easy to get.
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The diode pin in your case corroded because of the leaked electrolyte from the adjacent capacitor as far as I can judge from your pictures but you failed to properly analyze this despite having direct view of the PCB. The capacitor itself was not replaced...