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Thread: ASTEC AA11040C repair

  1. #1

    Default ASTEC AA11040C repair

    Hi All,
    I'm in a apple II repair month
    I have this Astec AA11040C supply that had the usual smoked rifa X2 capacitor. Other than that, it's outputting about 5.5V on the +5V line and 13V on the +12V line. However, with a 7 ohm load on +5V, the rail goes to 5.0V. I'll check the ESR of all electrolytics, but I was curious to know what's the minimum load (if any) to start regulating correctly the +5V rail?
    Is there a schematic of this power supply floating around?
    Thanks in advance.
    Frank IZ8DWF

  2. #2
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    Quote Originally Posted by iz8dwf View Post
    Is there a schematic of this power supply floating around?
    http://bitsavers.org/pdf/apple/power...lies_Aug82.pdf

  3. #3

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    Hi Al!
    Of course I had already downloaded that, but it covers the AA11040 and AA11040-B only. I don't know yet if the -C that I'm repairing is too much different from -B (so far it looks almost identical), that's why I asked if this 1983 version is documented separately maybe.
    Did anyone try that minimum regulation load? By a quick look at the schematic, it shouldn't have regulation problems with light loads, but I may be wrong on this.

    Frank IZ8DWF

  4. Default

    But you'd expect it to drop to the regulated level once you put some sort of load on it wouldn't you? So long as it doesn't suddenly start putting out 3.75 and 10 instead of 5 and 12 it should be pretty happy I would've thought.
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  5. #5

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    Quote Originally Posted by Anthony Adverse View Post
    But you'd expect it to drop to the regulated level once you put some sort of load on it wouldn't you? So long as it doesn't suddenly start putting out 3.75 and 10 instead of 5 and 12 it should be pretty happy I would've thought.
    I don't expect it to regulate that "poorly". With about 10mA load, its output voltage is 5.5V, with 160mA load, it's 5.2V, with 730mA load, it goes at 5.0V.
    My question since the beginning is: is this supply supposed to need any sort of minimum loading on the 5V rail, and if yes, what is that load, in everyone elses experience?
    It is my first encounter with Apple II SMPS, so maybe it's just poor design from 1980's era, I just don't know.
    Thanks

    Frank IZ8DWF

  6. Default

    I don't have a direct answer to the question at hand. In my experience with IBM "AT" power supplies, the +5VDC rail required a minimum of 2 to 4 amps. And I anticipate the same goes with the Apple II power supplies, since both it and the AT switching power supplies are around the same era (i.e. pre-ATX.)

    You might be complicating things when the answer could be staring right at you. I'd simply connect the PS to the Apple II logic board, measure the current draw, and the result should be your answer. Run the test with no cards and keyboard. This may not be the exact calculated minimum you maybe shooting for. But, in the words of the great ozzie bloak Dave Jones, physical measurements are better than theoretical.

  7. #7

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    Quote Originally Posted by groink View Post
    I don't have a direct answer to the question at hand. In my experience with IBM "AT" power supplies, the +5VDC rail required a minimum of 2 to 4 amps. And I anticipate the same goes with the Apple II power supplies, since both it and the AT switching power supplies are around the same era (i.e. pre-ATX.)

    You might be complicating things when the answer could be staring right at you. I'd simply connect the PS to the Apple II logic board, measure the current draw, and the result should be your answer. Run the test with no cards and keyboard. This may not be the exact calculated minimum you maybe shooting for. But, in the words of the great ozzie bloak Dave Jones, physical measurements are better than theoretical.
    Well, surely I'll try the "real" load, once I make some more tests.
    However in my experience, "AT" type SMPS have a fairly good regulation circuit. I've never seen a 10% variation of the +5V rail even at zero load on all the ones I've repaired and used in many years.

    Frank

  8. #8
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    May 2018
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    I know that some claim the PSU will switch off without sufficient load, but mine runs fine no load to full load, but yes, the regulation is quite 'loose'. Regulation from 5.5 down to 5.0 seems fine to me. There are no IC comparitors in this PSU, its an analog switching circuit.

  9. #9

  10. #10

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    Quote Originally Posted by Gary C View Post
    I know that some claim the PSU will switch off without sufficient load, but mine runs fine no load to full load, but yes, the regulation is quite 'loose'. Regulation from 5.5 down to 5.0 seems fine to me. There are no IC comparitors in this PSU, its an analog switching circuit.
    Well, unfortunately 5.5V is a bit on the edge. This particular PSU was sent to me because it seems the only thing that could have damaged two "FloppyEMU" boards. Unfortunately I haven't yet found a schematic of this FloppyEMU, but it uses a Xilinx XC9572XL CPLD directly connected to 5V I/O. This is usually fine since this part is rated up to 5.5V
    You see the problem here?
    Both FloppyEMU boards have a shorted CPLD

    I think I better tweak somehow the voltage regulation of this PSU if I can... I still can't believe they made a thing that regulate so poorly, even with all analog regulation, it should be better than 5% easily.
    Frank

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