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Mac Plus screen adjustment
- Thread starter marcelv
- Start date
techknight
Well-known member
thats the horizontal linearity but I dont think there is an adjustment for that?
What did you replace C1 with?
What did you replace C1 with?
R1 and L3 are the components responsable for the horizontal linearity.
Measured R1, 220 ohm seems ok, L3 also ok.
Noticed L3 has been reversed on the board (the glue was on the R1 side) by someone previously repairing the board?
After reversing L3 back the image improved somewhat
Measured R1, 220 ohm seems ok, L3 also ok.
Noticed L3 has been reversed on the board (the glue was on the R1 side) by someone previously repairing the board?
After reversing L3 back the image improved somewhat
Last edited by a moderator:
tomlee59
Well-known member
Reversing the polarity on L3 cannot have any effect if C1 is truly healthy, since the job of C1 is to block dc. If there's no dc, then L3 behaves the same regardless of which way it's inserted. Since you replaced C1 with a high-voltage cap, I'm inclined to absolve the capacitor of any role in producing nonlinearity, but that also makes me doubt that anything changed at all when you reversed the connections on L3.
It's hard to tell from a nearly blank display field whether anything changes, and by how much. You might want to put up a page of text filled with characters. I wonder how much of what you think you see is actually there. You will want to know if the linearity gradually shifts in going from left to right, or if there is a more abrupt change (i.e., linearity is good within both halves, but the two halves don't match). Details like that are useful clues as to what could be wrong.
Putting magnets on L3 will certainly have an effect, as you've noticed, but that's not a cure. You need to find what's causing the nonlinearity in the first place. And the first step before that is to characterize it more carefully so that you can quantify any fixes that you implement.
Standard things to try first include touching up all the solder joints on J1, the flyback transformer, and components associated with those. Check CR1 and CR5 and their connections. Resistances in all of those connections can cause nonlinearity. There's also a cap from flyback pin 4 to +12V; check it, too [pin numbers are from my Compact Mac Repair doc; I don't think there are any numbers silkscreened on the PC board]. In other words, check those obvious things first before applying band-aids.
Thanks for your reaction,
The L3 is a coil so reversing it should have no effect, I thought also. It does have a little effect as you can see the left margin of the screen got bigger.
Addes a new photo with some lines. The difference between the left and right side of the screen is >10%, it gradually changes from left to right.
The magnets I put on L3 was a test following some blog about the horizontal linearity problem. The magnets are removed now.
The standard things I do after opening a mac is replacing the solder on J1, J2 and J3 and inspect for further solder cracks.
The flyback solder did look very good but replaced it as for ruling this out.
have to check CR1 and CR5
the capacitor you mention is C3, will test it.
The L3 is a coil so reversing it should have no effect, I thought also. It does have a little effect as you can see the left margin of the screen got bigger.
Addes a new photo with some lines. The difference between the left and right side of the screen is >10%, it gradually changes from left to right.
The magnets I put on L3 was a test following some blog about the horizontal linearity problem. The magnets are removed now.
The standard things I do after opening a mac is replacing the solder on J1, J2 and J3 and inspect for further solder cracks.
The flyback solder did look very good but replaced it as for ruling this out.
have to check CR1 and CR5
the capacitor you mention is C3, will test it.
Last edited by a moderator:
tomlee59
Well-known member
If you look carefully, you will see that both vertical and horizontal dimensions are affected, not just horizontal, and only slightly (10% is not a huge variation). That's an important clue. Your problem may be no more complex than the yoke not being mounted flush against the crt. Take a careful look at it and remount if necessary.
If the yoke is mounted fine, then your only remaining option may be the geometry adjustment magnets arrayed around the yoke. I recommend first noting their positions before randomly rotating them, otherwise you may end up making things worse than they started.
techknight
Well-known member
The other thing you could do is pick up a ring tester and check the yoke for shorts. Same thing with the flyback.
apm
Well-known member
It could be the magnets around the yoke, or maybe even the yoke itself. It's interesting that the horizontal yoke coils contain a diode and other components inside the case. See the thread below. I don't know exactly what the diode is for, or what the effect might be if it failed, but almost all the other electrical parts of the circuits should be more or less symmetrical so it might be worth a voltage-drop test (which might require taking it out of circuit).
