power supply problem

Discussion in 'General Technical Questions and Answers' started by KI4UOF, Nov 4, 2007.

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  1. KI4UOF

    KI4UOF Ham Member QRZ Page

    I have a pyramid ps 36 kx and it had some bad transistors i replaced them now i have 24 volts ?? i cant figure out where everything went wrong. [​IMG]
  2. K9STH

    K9STH Ham Member QRZ Page

    The voltage regulator circuit is not working or else one, or more, of your "pass transistors' is shorted. 24 volts is pretty common for the unregulated voltage. This is reduced by the "pass transistors" to around 13.8 volts.

    Glen, K9STH
  3. W9GB

    W9GB Ham Member QRZ Page


    Let's start with the Pyramid documents you SHOULD HAVE:

    Pyramid PS-36KX web page

    Pyramid PS-36KX Manual and schematic (4 pages)

    You have NO voltage regulation.

    The pass transistors (2N3055, 2N3771, 2N2772) do not control the "regulation" of the raw rectified DC voltage -- that function is handled by the voltage regulator IC, in this case the well known and documented LM723 (DIP-14 package).

    See the LM-723 in the schematic diagram in the PS-36KX manual?

    LM-723 datasheet

    Did you actually test (VOM or DVM) the NPN transistors "out of circuit" to see if they were bad?  or was this a "shotgun" replacement?

    Pyramid does not normally use a 14-pin IC socket for this part.  You can check, maybe you are lucky and your newer supply has a socket.
    This requires that you desolder the part or "cut the part out" and properly clean the PC board with a good soldering iron and solder wick (or desoldering system).

    The KBT DC Supplies web site exists for DIY repairs of linear DC power supplies by radio amateurs.  
    You can "step through" the proper steps in the menu (gray bar on left side of web page)

    Repair tutorial starts here:

    David Metz, WA0AUQ wrote an article in November 1996 for SEITS -
    that has been widely circulated -- to teach amateurs the basics of linear power supplies.
    Now on the Repeater Builders web site -- for these users as well.
    Adobe Acrobat (PDF) verison of article:

    ARRL "Hands-On" article from December 2005 QST

    Repeater Builders Power Supply (Astron) page
  4. AG3Y

    AG3Y Guest

    It may be pointed out in the "how-to" pages, but I would just mention that if the P.S regulator is not mounted in a socket, you should make sure that you DO install a socket when you repair the supply. It will make the job SOOO much easier if you ever have to replace the regulator, again!

    Best luck. 73, Jim
  5. KI4UOF

    KI4UOF Ham Member QRZ Page

    ok imma working on it thanks guys ill get back to u if i have another stump [​IMG] 73's
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