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Thread: Astec Power Supply Issue... I am stumped!

  1. #1
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    Unhappy Astec Power Supply Issue... I am stumped!

    I have been working on two Astec Kaypro power supplies (AA12450 and AA12350) on and off for the better part of a year or so. I simply cannot get them working! Both units have been totally recapped, resistors checked, and I even replaced all the transistors to include the power transistor. Also replaced were all the diodes and bridge rectifier. I have replaced almost everything except for chokes. All resistors were good and I see no shorts on the common mode, control and power transformer (T1, T2, and T3).

    Here is the issue with both units (they both exhibit the same problem):

    - While slowly increasing voltage on a variac, the crowbar engages at around 40-50VAC. I do have B+. There is a high pitch chirp coming from the capacitors, and I can also hear the crowbar clicking.
    - I see -12vdc when the crow bar engages and, as intended, no output on the 5VDC and 12VDC points.
    - Lowering the variac voltage to just before the crowbar kicks in, I see approximately -12VDC, 6VDC, and 14VDC.

    I am completely stumped and refuse to give up on this! I was careful when replacing components and used only exact replacements and close values.

    Any assistance would be greatly appreciated!

    Thank you,

    Chris
    "Programs that are easy to learn are called *user friendly.* I'm not sure what programs that are not easy to learn are called, but I've run into a few user unfriendly ones, and a couple that have been downright hostile"

    ~ Peter McWilliams, "The Personal Computer Book," page 61

  2. #2
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    What are you using for a load on the +5V (where the crowbar SCR usually is?).

    Small Astec switchers generally use a simply Zener+SCR crowbar on the +5. If you don't have a load on the +5 line, the crowbar will fire as soon as the voltage exceeds VZ. Use a load that's about 10% of the rated +5 rail output current.

  3. #3
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    Thank you for your reply! Thinking it was a load issue, I installed the power supplies in a working Kaypro (swapping out the good power supply). The Kaypro refused to start but rather the power and drive LEDs pulsated with the crowbar kicking in. I know I should have used resistors for a load and tested before installing; but I was sure that I had resolved the problem. There are not many components left to replace. I can check the resistor values again. I have triple checked all replaced components with a known working power supply--all match.

    I find it odd that both power supplies exhibit the same problem. These were pulls and there was damage to some of the diodes. I tested the transformers for shorts but maybe there is a problem with them?

    Did I mention how much I hate working on switched mode power supplies?
    "Programs that are easy to learn are called *user friendly.* I'm not sure what programs that are not easy to learn are called, but I've run into a few user unfriendly ones, and a couple that have been downright hostile"

    ~ Peter McWilliams, "The Personal Computer Book," page 61

  4. #4

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    You need to power it with an external supply on the 5v and usually the +12V. Don't keep connecting it to AC. Do you have a schematic? How does it start the voltage to the regulator circuit?
    Dwight

  5. #5

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    Quote Originally Posted by vanderk View Post
    Thank you for your reply! Thinking it was a load issue, I installed the power supplies in a working Kaypro (swapping out the good power supply). The Kaypro refused to start but rather the power and drive LEDs pulsated with the crowbar kicking in. I know I should have used resistors for a load and tested before installing; but I was sure that I had resolved the problem. There are not many components left to replace. I can check the resistor values again. I have triple checked all replaced components with a known working power supply--all match.

    I find it odd that both power supplies exhibit the same problem. These were pulls and there was damage to some of the diodes. I tested the transformers for shorts but maybe there is a problem with them?

    Did I mention how much I hate working on switched mode power supplies?
    I was about to ask what you were using as a load. I have two thoughts.

    1) Have you resoldered the output connector? These seem to have problems with cracked solder/dry joints.
    2) have you looked for a short on the 5v rail?

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    I don't know precisely which Astec supply your Kaypro uses--or if there are any schematics for it. I do recall a similar problem with a non-Astec power supply that drove me nearly to tears.

    It wasn't the overvoltage crowbar that was kicking in, but the overcurrent protection. Turned out to be a current sense resistor that aged too much and triggered the overcurrent protection on very light loads. Not saying that it's what's going on with yours, but something to investigate.

  7. #7

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    If you can post the schematic indicating what you have replaced I might be able to make some suggestions about a test sequence to get to the bottom of it. If you don't have the schematic, likely you will need to copy it out from what is there anyway to end up with a successful diagnosis and repair. Global parts replacements can often lead to more trouble than good. The repair always needs to be targeted after data is acquired from testing and a theory formulated about what is going wrong. Ideally you would also have an isolated scope for tests in the live side of the psu (I use a Tek powerscout 222ps for this ).

    I'll give an example of a fault in one psu I fixed against all the odds some years back: The unit would shut down on occasion for no apparent cause. It was determined after extensive testing by others that none of the resistors, capacitors, transistors or IC's were faulty, at that point the pcb itself was getting blamed.

    The fault turned out to be in a ferrite cored transformer, driving the output transistor on the primary side. The glue had separated between the two ferrite halves to make a very small air gap and the inductance of the transformer had dropped. This was undetected by more than 5 technicians who had worked on the board before me. I found it because I scoped the drive waveform at the transistor base and noticed an unusual waveform.

    So it is quite interesting what faults can still be present when most of the parts have been replaced, so you have to have a very open mind about what could be wrong and as noted set up the supply with suitable dummy loads.

    One interesting thing is that both supplies are doing it, which suggests the remaining components that have not been replaced on each board, one has exactly the same fault as the other (Seems very unlikely) or of the components that have been replaced, one on each board is for some reason an incorrect or unsuitable part, rendering the same fault to both boards, which seems more likely.
    Last edited by Hugo Holden; March 18th, 2020 at 09:01 PM.

  8. #8

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    You can do any trouble shooting unless you can look at what the regulation circuit is doing. You can't do that unless you can see the values on each stage. That is real hard to do while it is tripping every time you tune it on.
    Use a bench supply. You can put it in and out of the normal operating range with a bench supply and find what is failing. Stop replacing parts that are likely not even the failure.
    If you don't have a schematic, trace it out and draw one.
    Dwight

  9. #9

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    Quote Originally Posted by Hugo Holden View Post
    One interesting thing is that both supplies are doing it, which suggests the remaining components that have not been replaced on each board, one has exactly the same fault as the other (Seems very unlikely) or of the components that have been replaced, one on each board is for some reason an incorrect or unsuitable part, rendering the same fault to both boards, which seems more likely.
    Is the working PSU you have also an Astec? Could you use it as a visual reference to check that components on the faulty boards are in the right place, have the right values and polarity?

  10. #10

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    Automotive light bulbs make good loads for computers. Once stabilized they are almost constant current loads. This means a 12v bulb that draws .15 amps will draw about .15 amps at 5V. They also provide less resistance until they stabilize, similar to a large capacitive load.
    Dwight

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