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Thread: A2M2010 Apple II monitor all of a sudden started showing double image! Help

  1. #21

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    Quote Originally Posted by VERAULT View Post
    I assumed the pot was bad but tested it as you said,. Surprisingly the pot is fine.
    So lets assume the Capacitor is in fact bad now. Since its a film resistor can I replace it with an electrolytic? I cant seem to find its value on the schematic PDF.
    No, an electrolytic is not suitable for a timing application like this. Generally they use a polystyrene type capacitor, but any non electrolyic polyester/mylar cap will be good enough. The schematic suggests its a 0.15uF, or sometimes a 0.22uF and 100V rated, its probably like that not that it needs to be but for a bigger sized more stable part.

    Before doing anything else though I would:

    Make sure the CRT brightness is turned right down to avoid a phosphor burn from the collapsed raster

    Check all the voltages on the IC pins, especially the pin that sources the charge current for C302.

  2. #22
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    Good to know. I looked up the marking on the cap 2A 224K and found these : https://www.goldmine-elec-products.c...?number=G23642 but they are rated for 50v
    If I look on ebay I find these: https://www.ebay.com/itm/25-S-M-B325...4AAOSwSqtc1JCO

  3. #23
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    So while deciding on what caps to buy I decided to try the cap from a known working spare CRT board. I soldered it in and no change. Still a horizontal line.
    As far as testing the IC its really hard to do it as its under the stem of the CRT tube and there is a tall shield around the IC. Could we be following a red herring?

    Should I just replace all the electrolytics? I dont usually do that, I like to replace them IF they go bad not before.

  4. #24

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    Quote Originally Posted by VERAULT View Post
    So while deciding on what caps to buy I decided to try the cap from a known working spare CRT board. I soldered it in and no change. Still a horizontal line.
    As far as testing the IC its really hard to do it as its under the stem of the CRT tube and there is a tall shield around the IC. Could we be following a red herring?

    Should I just replace all the electrolytics? I dont usually do that, I like to replace them IF they go bad not before.
    Well, I guess so far we know that the vertical timebase (which is part of the IC) was running at half the normal frequency and now it appears to have stopped altogether, and the V hold pot and the timing capacitor appear ok. Aside from the external components around the IC, assuming they are ok, that just leaves the IC and its power supply, though there is always the possibility of a poor mechanical contact like a deflection yoke plug, that could explain the scan collapse, but not the slow scan frequency & double image. Check the vertical yoke coils have continuity.

    The next move is to check the power supply to the IC, the main supply in the monitor is probably basically ok as the is H scan is ok, albeit with reduced width a little.

    I wouldn't just jump in and replace the electrolytics or that vertical IC right away without a diagnosis & some tests, ideally you would take some measurements of the voltages on the IC pins, with a scope.

    Access can sometimes be difficult. A small test clip and lead can be attached to the IC pin where it enters the IC body, while the monitor is off. Then power the monitor to make the test. I'm not sure if you have any IC pin grabbing test clips that do this. It pays not to try to connect them with the circuit running as you could short out adjacent IC pins.

    Sometimes its necessary to disassemble things enough to get access to the trackside of the pcb to make the test there.

  5. #25

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    ...everything you have seen so far though is still consistent with a fracture to the connections of the V hold potentiometer on the solder side of the PCB, have you been able to inspect these connections, or did you test the potentiometer from the component side of the PCB ?

  6. #26
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    I took the pot out of the circuit to test it. So resoldering would have eliminated any bad solder joint on the pot. But that obviously didn't fix it.

  7. #27

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    Quote Originally Posted by VERAULT View Post
    I took the pot out of the circuit to test it. So resoldering would have eliminated any bad solder joint on the pot. But that obviously didn't fix it.
    The vertical oscillator has stopped and/or the vertical output stage is not working or driving any current via the yoke. Time to check that IC & its voltages. For the oscillator, the IC pin provides the charging current for C302. That point should be checked on the scope to see if the correct sawtooth voltage is there.

  8. #28

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    check for solder cracks on IC301 TCA1170. It's the one with the U shaped metal heatsink surrounding it. The vertical drive circuit is totally feedback driven.
    Any open in the circuit causes the oscillation to quit and you get a horizontal line. Also look very carefully for cracked traces in the area. Taking the back cover off
    and putting it on flexes the board ALOT. Also hammered a lot during shipping or just transporting.

  9. #29

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    Quote Originally Posted by retrogear View Post
    check for solder cracks on IC301 TCA1170. It's the one with the U shaped metal heatsink surrounding it. The vertical drive circuit is totally feedback driven.
    Any open in the circuit causes the oscillation to quit and you get a horizontal line. Also look very carefully for cracked traces in the area. Taking the back cover off
    and putting it on flexes the board ALOT. Also hammered a lot during shipping or just transporting.
    IC301 a TDA1170, maybe the TCA1170 is a typo. The feedback around it is only for linearity control, not to sustain oscillation. The oscillator section of it is independent of the feedback. It would be worthwhile checking the electrolytic cap between pin 5 & 3. But the voltages need to be checked.

    To some extent it was ambitious of Philips to make an IC with the expectation of it being a vertical osc and output amplifier device, to the extent that the output stage in it must function as a class AB amplifier to get good linearity with no cross over distortion. Meaning its going to get hot, hence the heat flags on the IC body. It is not that uncommon for IC's like this to fail. In larger sized monitors (or color monitors with higher EHT that have a higher deflection current demand) they usually add a power transistor output stage to drive the V yoke coils, but in a smaller monochrome monitor they can just, most of the time get away with it, with just the IC. Probably the longevity of the IC could be greatly increased if it had better cooling.
    Last edited by Hugo Holden; October 14th, 2019 at 06:28 AM.

  10. #30
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    Are there any tests I can perform on the IC with it out of the circuit?

    I did the continuity check on the yoke, I am getting 7.85 Ohms on the vertical and 0.32 Ohms on the horizontal; I checked another working CRT and that is within the same specs.
    Last edited by VERAULT; October 14th, 2019 at 11:02 AM.

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