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Thread: Mac Classic Analog Board - Low Voltage Problems

  1. #1
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    Default Mac Classic Analog Board - Low Voltage Problems

    Hello,

    Recently, I acquired a broken Macintosh Classic from my university for free. Sadly, it does not work. Upon inspection, I found that the voltages out of the power supply were too low - around 9V and 4V for the 12V and 5V rails. I replaced all of the capacitors in the big clump by the low voltage output, but it only helped a little. Now, I can get 11.5V, 4.6V out when I turn the pot up all the way.

    The high voltage side of the board is running fine; the screen is nice and bright. So whatever the issue is, it seems to be only affecting the low voltage side of the board. Any ideas what to try next? I inspected all the parts and physically, they look fine.

  2. #2
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    Start checking the other components like diodes, resistors, transistors, etc. They can be bad as well and cause issues.

    If you have the motherboard plugged in, try unpluging it and see if the voltages come up. Something on the motherboard could be failing.

  3. #3
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    Did you scrub the board while you had everything out? One of mine had leaked all of the fluid from the caps all over the board, scrub with isopropyl alcohol and then white vinegar. The residue is conductive.


    Later,
    dabone

  4. #4
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    Quote Originally Posted by GiGaBiTe View Post
    Start checking the other components like diodes, resistors, transistors, etc. They can be bad as well and cause issues.

    If you have the motherboard plugged in, try unpluging it and see if the voltages come up. Something on the motherboard could be failing.
    I tried without the motherboard but with a dummy load, and the voltage came up a bit, but not much. As far as I can tell, even a tiny current draw causes the voltage to drop significantly. I'm not sure if that points to any part in particular.

    I tested a bunch of diodes and resistors at random and didn't find any problems. I will keep looking.

    Quote Originally Posted by dabone View Post
    Did you scrub the board while you had everything out? One of mine had leaked all of the fluid from the caps all over the board, scrub with isopropyl alcohol and then white vinegar. The residue is conductive.


    Later,
    dabone

    I scrubbed where the caps were because there was a lot of capacitor fluid. I inspected the rest of the board and didn't find any corrosion.

  5. #5
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    Got some more concrete info.

    Pot all the way one way - ~10.3V on the 12V rail, no load

    Pot all the way the other way - ~11.0V on the 12V rail w/o load, drops to 10.4V with a 1A dummy load

    On power on, the voltage starts at around 10.9 volts, and creeps up slowly until it gets to around 11.05 volts.

    I also tried replacing the solder holding the transformer on. It didn't help.

    With the logic board connected and no dummy load, the voltage stabilizes @ 10.1V. Occasionally I hear a click from the analog board(this happens without the logic board attached too). Whenever there is a click, an artifact appears on the screen. The screen is a checkerboard w/ a vertical stretch. It also has a wider section that moves down the screen slowly. It looks like someone takes a lens and slowly moves it down the screen, distorting the image under it, basically.

    When I remove mains power, the voltage rises and I can hear the fan speed up. The checkboard stops distorting, and is no longer elongated. This lasts until the screen shuts off.
    Last edited by steve314; February 16th, 2019 at 05:22 PM.

  6. #6
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    Clicking/popping is usually arcing in the flyback transformer. This can be caused by the flyback failing or a problem on the PCB.

    Just remember when testing to remove the legs of the component from the board so other nearby components don't interfere with the test.

  7. #7
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    Quote Originally Posted by GiGaBiTe View Post
    Clicking/popping is usually arcing in the flyback transformer. This can be caused by the flyback failing or a problem on the PCB.

    Just remember when testing to remove the legs of the component from the board so other nearby components don't interfere with the test.
    I believe is it something PCB related. When there's a click, the voltage drops.

    I hooked up my oscilliscope and found more interesting info.

    The 12V line has a sawtooth ripple - 250Hz, 0.8V peak to peak. I tried adding another capacitor, and it helped a little, but not much. Keep in mind this ripple remains whether the logic board is connected or not.

  8. #8

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    Quote Originally Posted by steve314 View Post
    I believe is it something PCB related. When there's a click, the voltage drops.

    I hooked up my oscilliscope and found more interesting info.

    The 12V line has a sawtooth ripple - 250Hz, 0.8V peak to peak. I tried adding another capacitor, and it helped a little, but not much. Keep in mind this ripple remains whether the logic board is connected or not.
    There are two possible sources of oscillation. One is the current foldback. In other words, something draws the current enough to trip the current foldback. The voltage drops enough for the current load to back off. The voltage goes up and it trips the current foldback again.
    There is also possible oscillation in the voltage regulation. There is usually some non-electrolytic capacitors involved in the regulator circuit that dampen this.
    It sound more like the first one. With the logic board disconnected, it is possible to connect an external bench supply, at about 4.7v or so and watch the voltages at a steady state. Pay especial attention to the correct forward bias on the regulator transistors, base to emitter. Move the bench supply up and down, slowly to see how the regulator responds.
    Dwight

  9. #9
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    Quote Originally Posted by Dwight Elvey View Post
    There are two possible sources of oscillation. One is the current foldback. In other words, something draws the current enough to trip the current foldback. The voltage drops enough for the current load to back off. The voltage goes up and it trips the current foldback again.
    There is also possible oscillation in the voltage regulation. There is usually some non-electrolytic capacitors involved in the regulator circuit that dampen this.
    It sound more like the first one. With the logic board disconnected, it is possible to connect an external bench supply, at about 4.7v or so and watch the voltages at a steady state. Pay especial attention to the correct forward bias on the regulator transistors, base to emitter. Move the bench supply up and down, slowly to see how the regulator responds.
    Dwight
    I'm sad to say I don't have a bench top power supply. (Moved from Halifax for uni, didn't bring my supply with me)

    I'm reading 36 ohms between 12V and ground with nothing plugged in. Could this be a problem? That's only 330mA, but I suppose it's possible that the resistance changes when the board is actually powered up.

    Connecting a 16V, 2200uf cap between 12V and GND reads 100mA AC rippling to and from the cap. Correct me if I'm wrong, but that's quite a lot, since that current only deals with the 0.5V ripple(less cause of the new cap) and not the rest of the 11V, which would be coming directly from the PSU.

    Connecting the same cap between 5V and GND instead reads 0mA AC.

  10. #10

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    Two diodes are known to be bad on those AB boards, and cannot be detected (they are not shorted, just leaky) DP3 and DP4 (located near the IC : TDA4605) pretty common diodes 1N4148.

    You can replace those as a matter of fact, also all small capacitors in the neighborhood of the IC.

    The optoisolator can be faulty as well (CNY17 or equivalent)

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