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

  1. #11
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    Angry

    Thank you everyone for your guidance. Unfortunately, I do not have an external PS for low DC voltages. I have just about everything on my bench, but this is something I will need to build or purchase. I may have some spare car bulbs I can use for loads. I presently don't have the correct value resistors for dummy loads. The working Kaypro power supply does not go into protection mode without a load.

    I did verify all components with a working Astec PS from a Kaypro; all new components match up. I conducted an exhaustive search for schematics for the A12350 and AA12450, but they are unobtainium; however, I did find some information on the Astec AA12090 and it has similar components but a different layout. There was some troubleshooting information that I followed to no avail. With no schematic, I became desperate and followed the "shot gun" approach and started replacing (carefully) suspect components.

    To an earlier question, I did install both units in a working Kaypro but the crowbar kicked in (click, click, click, click...)

    Despite seeing voltages on the secondaries, perhaps there is a hidden issue with the transformer as was suggested by Hugo? I have been tempted to pull the transformer from a spare, working PS and swap it out.

    I have already put so much time into these power supplies, but I have contemplated tossing them in the trash and cutting my losses. I've worked on countless pieces of equipment from HV tube gear to modern HF transceivers, and these SMPS have proven to be the most challenging items that have been on my bench! Go figure!
    "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. #12
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    Default

    Are you sure it's the overvoltage crowbar? What happens if you lift a leg of the crowbar SCR? Have you checked out the zener connected to the gate? A shorted zener there would certainly create the effect you're describing.

  3. #13
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    Quote Originally Posted by Chuck(G) View Post
    Are you sure it's the overvoltage crowbar? What happens if you lift a leg of the crowbar SCR? Have you checked out the zener connected to the gate? A shorted zener there would certainly create the effect you're describing.
    Hi, Chuck, Yes, I did check the Zener. It was fine. Out of curiosity, I did pull the leg of the crowbar. Considering I am totally fed up with these supplies, I applied power, stepped away, and pop! One of the 16V caps on the 12V rail railed.

    EVERY component has been either checked or replaced. There must be an issue with the transformer. Unless someone wants these free, I am going to toss them. I could have built a power supply from scratch with all the time I put into both of these supplies. I certainly don't want to waste anyone's time on this forum as well.

    I am extremely thankful for the quick responses!
    "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. #14
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    Not every component, I suspect. A shorted transformer primary winding (not completely unknown) would certainly drive the output voltage up too high. I don't know how you would repair this, short of rewinding the thing. Since you've got a good supply, compare the bad supply transformer winding resistance with the good one.

    A fully-loaded PSU should never have to pop the overvoltage crowbar.

  5. #15

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    Quote Originally Posted by vanderk View Post
    Hi, Chuck, Yes, I did check the Zener. It was fine. Out of curiosity, I did pull the leg of the crowbar. Considering I am totally fed up with these supplies, I applied power, stepped away, and pop! One of the 16V caps on the 12V rail railed.

    EVERY component has been either checked or replaced. There must be an issue with the transformer. Unless someone wants these free, I am going to toss them. I could have built a power supply from scratch with all the time I put into both of these supplies. I certainly don't want to waste anyone's time on this forum as well.

    I am extremely thankful for the quick responses!
    Where these earmarked for a project Kaypro or are they spares? If it's for a project the BBC Micro community have had a lot of success switching out their failing Astec supplies for modern MeanWell triple output PSUs.

  6. #16
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    We had a difficult time repairing a DEC PDP-12 switching power supply. It turned out that the layout of the E-B-C in the replacement transistors was different than in the original transistors. The replacement transistors had the same part number as the original transistors. Once we determined which replacement transistors were in backwards, and replaced them turned 180 degrees, the power supply worked OK.

    On another linear power supply that was a difficult repair, we connected a lab power supply through a high value resistor voltage divider to the base of the output transistor. We verified that making changes to the lab supply voltage changed the voltage output when connected to a fixed load. We then worked our way back through the regulator circuitry until we found part of the circuit that didn't behave correctly. This only works on linear power supplies.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  7. #17
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    One of the 16V caps on the 12V rail railed.
    After that 16 Volt cap blew, did you reconnect the Crowbar and see if the short was cleared? It would be
    an interesting test. I'd check the same cap on the other supply.

    Larry

  8. #18

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    [QUOTE=vanderk;612254 I am going to toss them. I could have built a power supply from scratch with all the time I put into both of these supplies. I certainly don't want to waste anyone's time on this forum as well.

    I am extremely thankful for the quick responses![/QUOTE]

    I think that it is important to follow the repair task through to completion. If you give up on them, then the input you have put in so far will be wasted and on top of that it will always bug you that you failed at the task. I have a motto or policy of never giving up on repairs, sometimes that does cost me though.

    It really is not a hard task to document the schematic. I have done this before for many types of apparatus, switch-mode psu's and even entire video monitors where the schematic was unobtainium. Once you get into the swing of it, it is quicker than you think. Start with pencil and paper and once the schematic is known, draw it out neatly with a computer drawing program. I use the vintage program Microsoft Picture It.

    The schematic of course holds all the clues as to how it is supposed to be working and where tests should be made. Without it you are flying blind.

    The main reason why you have not succeeded repairing the supplies so far is you don't have the schematic. Without that for any more than a very simple fault, it is not possible to make many tests, acquire meaningful data and formulate a theory on what could be wrong. So to succeed without the schematic (unless you are lucky with a simple fault) it is a low probability event.

    Possibly, if you had copied it out in the first place, you would not be in the frustrating predicament you are in now and feeling like giving up. So I really would encourage you to copy it out and study the particular design before moving on to more testing/fault finding. Also, if you copied it out you could post it, then many others could help with suggestions.

  9. #19

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    I agree with Hugo. You should not give up. Not having the needed bench supply is maybe a good reason to get one. You may be surprised at the amount of uses they have. I consider it to be one of my most important pieces of test gear. I consider it only second after my scope ( actually scopes ). My meter is third. Even a cheap kit supply can be useful for this use.
    Drawing a schematic and making a layout drawing will allow you to do a much better troubleshooting. What you have tried so far is the typical easter egg hunt. The fact that you have a similar schematic to start from is a great starting point. Posting the schematic, even a pencil picture, here will allow people like Hugo and others that understand such things to show you where to look for things that can go wrong. It will help you to fix your problem and help others to understand the mysteries of switching power supplies. It will not only benefit you and others.
    This group is a great asset. You first come with I'll lost. Maybe you were looking for the simple answer. We usually have none to supply you with. We do supply a great resource in trouble shooting abilities but it is a two way street. Doing with a message board is slow. Most only have one or two times a day to respond. Still, the value in learning for you and others is great.
    Dwight

  10. #20
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    there is a collection of astec schematics here to get you started, but as Hugo and Dwight have said, you should try to draw out your boards
    http://bitsavers.org/pdf/astec/

    many of their designs are similar
    if you do come up with a drawing, i'd like to archive it to help others in the future

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