Mac Color Classic CRT/Analog board problem

[techknight]

I dont know if the problem is in the CRT, or the analog board. but the horizontal convergence is gone. vertical convergence is A OK.

the ALPS white pot on the CRT neck/video board does nothing to the picture. has no effect. the pots on the top of the yoke change the vertical and horizontal convergence a little bit. there is a E/W coil with a ferrite adjustment screw off to the right and i broke the ferrite into many peices to try to adjust it. (didnt affect the convergence hardly at all).

i tried using the convergence rings on the back of the neck, and if i play witht hose i can bring the picture in ALMOST, but not enough where it wouldnt drive me nuts. so something is wrong somewhere, so i returned the adjustment rings back to thier original positions.

i ESR tested all the electrolytics and they are fine.

any ideas? i dont have another machine to swap yoke, CRT, analog boards to figure out where the problem lies.

 

[tomlee59]

First, it's not the crt. Or, more precisely, I can't think of a plausible crt failure mechanism that could possibly cause what you see. But I can think of (and have seen) quite a few electronic failures that produce the symptoms you describe. So let's start with that as a working assumption for now.

I assume that you have the service manual, and have followed the procedure with no luck.

As you'll be working on a powered machine with its internals exposed, exercise due caution. Lethal harm can come to you if you are careless.

First, accurately mark the present settings of any control you intend to tweak. Convergence can be a tricky art, and having no known initial condition makes it unnecessarily hard.

Next, degauss the unit. Yes, there is an internal degaussing system, but it is relatively feeble, and takes many off/on/off cycles to do the job. Get a real demagnetizer and make sure that the unit is properly degaussed before attempting any convergence. It is also a good idea to maintain the mac's orientation wrt the earth while you work on it (to prevent spurious remagnetization). Needless to say, make sure that your adjustment tools are also nonmagnetic!

The fact that the E/W amp adjustment coil did nothing suggests a starting point. For example, if the coil has developed an open (either internally, or through a bad solder joint), it could very well produce what you're seeing. This theory is strengthened if you observe an ability to achieve reasonably good convergence on one half of the screen (by adjusting the E/W control on the crt neck assembly), but not both.

It's also important to note that the yoke's position along the neck affects convergence (so does the HV, for that matter, but if you don't have a HV meter, you'll be forced to consider other factors first). If the unit has been abused, the yoke can shift enough to induce a wide variety of ills. As a last resort, sliding the yoke backwards or forwards can often bring a Trinitron into convergence when all else fails. Before doing so, again mark the initial position carefully!

If investigation absolves the E/W amp coil, trace the circuit it connects to. Test the components that do so along the way. Inspect the traces and solder joints. Look for fried or discolored components. There's no substitute for a good, careful visual inspection.

 

[techknight]

ive already played with the yoke position. the yoke can only be in one position to maintain proper purity. and it has very little affect on convergence where the yoke is placed.

there is a coil on the right side of the yoke. not the left, but it dont do anything to the picture. i shattered the adjustment ferrite core trying. so that yoke is either bad, or the analog board is bad.

 

[techknight]

as far as the yoke, theres only one plug that goes from it to the analog board. the horiz/horiz return, and vert/vert return wires. 4 wire plug. thats it. no extra wires. so all the controls on the yoke affect whats within the yoke.

 

[tomlee59]

Yes, the main convergence corrections are all synchronous with the deflection signals, and therefore the former are derived from the latter.

If you have followed the procedure outlined in Apple's service manual, and still cannot achieve convergence, then it's time to perform component-level troubleshooting, using my previous post as a possible starting point. Moving the yoke is a correction of last resort, so I hope that you were able to restore it to its original position.

 

[techknight]

yea i got it all back to its original position. ive done this before, but i dont see anywhere on the circuit that would have anything to do with convergence aside from the yoke itself??

all the capacitors ESR test comes out good. so i can rule out caps.

only thing left is diodes and resistors and transistors. i have full deflection, no pincushion distortion, or insufficient vertical sweep. all that is fine.

would be nice to have a schematic diagram of this damn thing.

 

[techknight]

well i just found out it IS the analog board.

i have an LC/performa 550 model with the SAME board in it, but its wider.

there is a pot on the back of the video board if i adjust it, it causes the horizontal convergence to adjust left to right ;-)

but on the classic analog board, that pot does NOTHING. the only thign i can figure, is because im missing a grid voltage somewhere..

but anyway, i took the 13" 550 CRT out, and stuck my 9" CC CRT in place of it, and boom, perfect convergence after adjusting the pot.

i put the 13" CRT back in, and stuck my color classic analog board in and bam my 13" cRT is out of convergence and pot does nothing.

so something is wrong right there... either im missing an anode voltage, or something.

 

[tomlee59]

well i just found out it IS the analog board.
i have an LC/performa 550 model with the SAME board in it, but its wider.

there is a pot on the back of the video board if i adjust it, it causes the horizontal convergence to adjust left to right ;-)

but on the classic analog board, that pot does NOTHING. the only thign i can figure, is because im missing a grid voltage somewhere..

but anyway, i took the 13" 550 CRT out, and stuck my 9" CC CRT in place of it, and boom, perfect convergence after adjusting the pot.

i put the 13" CRT back in, and stuck my color classic analog board in and bam my 13" cRT is out of convergence and pot does nothing.

so something is wrong right there... either im missing an anode voltage, or something.

Well, yes, of course it's the analog board.

It sounds as if you did not actually look at the manual. The manual clearly shows all of the relevant controls, along with a description of what they do.

You are not missing "an anode voltage." There is no anode voltage that affects convergence of only one region of the display.

Please carefully reread what I have written. It'll save both of us time. The relevant advice: Now that you have identified a non-functioning control, you have a starting point for further troubleshooting. Test the control itself. Look at the associated connections. DO A VISUAL INSPECTION of the circuits connected to the control. Then follow up with as many electrical tests on those same components as you can. The most common faults are single-point/component failures, so your chances of finding the problem are pretty good, even without a schematic. Convergence circuits, particularly for Trinitrons, are actually not all that complex. There aren't zillions of transistors along the way, for example, so it is a well-bounded problem.

 

[techknight]

yes i read the manual. so don't mis-judge me would ya please? i hate that...

anyway, the convergence control pot on the back of the video CRT neck board (assuming you know what that is, remember the part about mis-judging) does not function.

this pot is electrically connected to one of the CRTs grids to ground. the pot reads the same as the pot on the LC 550 analog board. so it isnt the pot. only thing i can figure its a low or missing CRT grid voltage somewhere. has to be.

there are NO other bad parts on the analog board itself. i tested all the resistors, they are within tolerance, caps are good, even the ceramics and mylars. Diodes statically check fine, but that dont mean they arnt breaking down under load. there was 2 buldged looking electrolytic capacitors that were under the shield near the video output IC, they ESR and LSR check fine, but i replaced them anyway. no difference.

so the only possible thing that can be wrong in the analog board is a bad fly, or a bad IC somewhere since i cant physically test those without a USEFUL service manual...

 

[tomlee59]

yes i read the manual. so don't mis-judge me would ya please? i hate that...

Hey, I'm just trying to help. The manual shows the location and function of all of the convergence controls. Your post seemed to express surprise at the function/discovery of one ("there is a pot on the back of the video board if i adjust it, it causes the horizontal convergence to adjust left to right"), so it was completely natural to think that you had not read the manual, ok? Plus, you gave much too serious consideration to the "crt is at fault" hypothesis, which told me that you are relatively inexperienced in these matters. Don't get defensive when someone's trying to help. After all, you were the one asking for assistance.

Sheesh!

Now on to more technical topics. Abandon the mysterious "grid voltage" fault theory. A trinitron has a very simple crt structure. Its convergence circuitry is also very simple. All grids are common to all guns, so you cannot have a failure in a grid that selectively affects convergence of only one color.

There are, however, trinitron crt elements called deflectors. Convergence is achieved in part by applying sweep-synchronous ac voltages to these (electrostatic) deflectors.

When you say that the pot "reads the same" as the pot on the 550, are you referring to a pair of resistance readings (you need to measure a pair for one pot), or to a measurement of the tap voltage? If the latter, does it vary as you rotate the pot? And if so, how does the voltage control range compare with that on the 550?

I agree that a schematic would be nice to have, but you do have the 550 to use for comparison, which is almost as good. Just hang in there, work logically, and you will crack this nut.

 

[techknight]

yes i read the manual. so don't mis-judge me would ya please? i hate that...

Hey, I'm just trying to help. The manual shows the location and function of all of the convergence controls. Your post seemed to express surprise at the function/discovery of one ("there is a pot on the back of the video board if i adjust it, it causes the horizontal convergence to adjust left to right"), so it was completely natural to think that you had not read the manual, ok? Plus, you gave much too serious consideration to the "crt is at fault" hypothesis, which told me that you are relatively inexperienced in these matters. Don't get defensive when someone's trying to help. After all, you were the one asking for assistance.

Sheesh!

Now on to more technical topics. Abandon the mysterious "grid voltage" fault theory. A trinitron has a very simple crt structure. Its convergence circuitry is also very simple. All grids are common to all guns, so you cannot have a failure in a grid that selectively affects convergence of only one color.

There are, however, trinitron crt elements called deflectors. Convergence is achieved in part by applying sweep-synchronous ac voltages to these (electrostatic) deflectors.

When you say that the pot "reads the same" as the pot on the 550, are you referring to a pair of resistance readings (you need to measure a pair for one pot), or to a measurement of the tap voltage? If the latter, does it vary as you rotate the pot? And if so, how does the voltage control range compare with that on the 550?

I agree that a schematic would be nice to have, but you do have the 550 to use for comparison, which is almost as good. Just hang in there, work logically, and you will crack this nut.

only "one" color? all colors are affected bro.

 

[techknight]

do you happen to have a pinout of the CRT? so i can trace the electrostatic convergence driver circuit?

 

[tomlee59]

What I meant by "one color" was that the convergence control for all of the colors would be affected, not just for one color. Your problem is that only one of the controls fails to do anything. Hence, it's extremely unlikely for it to be a "grid voltage" problem, "bro."

Regrettably, I don't have any schematics handy. But again, you have a 550 that works. I recommend again the steps I mentioned in the previous post (measuring the voltage adjustment range of the working vs. nonworking convergence control). Also, perform a careful comparison of the crt pin voltages of your working 550 and not-quite-working CC. That should be sufficient to localize the problem. From that point on, you should be able to pinpoint the defective component. And again, the value of a careful visual inspection is not to be underestimated. This includes a look at the crt socket pins and connections.

Good luck. I'm sure you'll figure it out.

 

[techknight]

I know this thread seems a little old now, But i never did get this thing working.

I got down to replacing the entire neck board, and went through all the resistors in the main circuit for any out of tolerance values or opens. Nothing.

Checked all the transistors for opens and shorts, all seem good. I tested all the electrolytic capacitors with an ESR/DCR check, and found a couple of weak ones in the video circuit, but the rest are good. Swapped the two in the video circuit, it made the image a little bit more crisp, but the convergence issue still resides.

So that eliminated the convergence adj pot, and the neck socket, becuase i swapped the CRT boards between the LC550 and the CC. and no difference.

I'm at a standstill. I need to figure out what in that little analog circuit generates the dynamic convergence waveform that the tube uses for that adjustment to even work. thats where the fault is going to be.

 

[techknight]

You also mention the plates in the trinitron CRT that has an AC sweep-synced signal that goes to the CRT. that might be whats missing, But im unsure of to what pins these are. Do you know? Thanks.

 

[techknight]

Since you brought up convergence deflector plates that i seemed to have overlooked in december, the thing that comes to mind is the old 40s electrostatic TV sets.

if the design is anything like that, the V and H sweep signals are coupled to the boost voltage to the convergence deflectors via a high voltage mylar cap.

or something like that i think. Those caps could be open....