Discussion in 'General Technical Questions and Answers' started by AE4TR, Apr 2, 2004.

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  1. AE4TR

    AE4TR Ham Member QRZ Page

    [​IMG] A BIG HOWDY!!!   I have this Pyramid Gold Series Model
    PS 36KX regulated DC Power supply, which has went south on me.  This is the second time.  First time the SCR shorted and I replaced it, problem fixed.. that was several years ago.  This time I was transmitting and it suddenly went into alarm. This time the SCR is not defective.  I replaced it anyway...  
    I have checked everything except the IC chips.. which I have no way of checking other than replacing them...  I was wondering if someone would have a parts list, or a troubleshooting guide for this power supply?  I have a schematic, but info on it is very limited.  No part numbers etc.
    Or maybe someone has had the same problem with a similar unit?  Any help will certainly be appreceiated.. Thank You.  
     You may use my email to answer or post here.  My email is: ...    [​IMG]
  2. W9GB

    W9GB Ham Member QRZ Page

    Bennie -

    I found many Pyramid linear power supplies on "give-away" tables or in the trash barrels at hamfests.  They are a source for building your own linear power supplies.  They appear to have undersized heat sinks for the ratings advertised. Linear power supplies are straight forward to troubleshoot and repair.  I would suggest the following web sites:

    KBT DC Supplies (troubleshooting - they also do repairs)

    David Metz, WA0AUQ article [Power Supply construction]


  3. AE4TR

    AE4TR Ham Member QRZ Page

    This Power supply is regulated Supply (protected for over voltage and for too much current draw) therin lies the problem.. It is in shut down condition. Taking all load off, it is supposed to reset itself, it does not. So seems to me there is something wrong in the shutdown circuit. There are two 14 pin IC"s in there that I have no information on. Not knowing what is inside makes it extremely difficult to troubleshoot. I have many years experience in the tv and radio repair field, but need help on this. I could of course rebuild it and make it a linear supply, but don't wish to do that if, I can find some way to fix it first. Thanks...
  4. W9GB

    W9GB Ham Member QRZ Page

    B H -

    Your Pyramid power supply uses the popular LM 723 voltage regulator chip (a proven IC and over 25 years since first released).  I am surprised you have not come across this popular IC in the radio/TV repair business
    The KBT DC web site has all of the specification sheets on the LM 723 voltage regulator IC as well as the 2N3055 or 2N3771 pass transistors.  I provided the spec sheets - so that they would be available for easy reference

    Repair tutorial number 1

    Repair tutorial number 2

    Pyramid does not answer e-mails and I am not aware of "on-line" schematic sources - other than this one diagram. The Pyramid PS-52KX manual can be found here.

    The KBT web site has over 30 pages of information on the repair of linear power supplies using common IC regulators and pass transistors (e.g. Pyramid, Astron, TrippLite) -- better than a SAMS PhotoFact.


    KB7KBT has been able to "walk" many amatuer radio operators with little experience through the repair process.  Just drop kb7kbt an eMail, tell him what you have and what you think might be the problem.

    All power supplies need to be sent to KBT postage paid and insured round trip. Repair fees will consist of parts, plus a flat fee of $25.  You can then put their "Life Time Warranty" to work.
  5. W7DJM

    W7DJM Banned QRZ Page

    This does not answer the above question, but I have a couple of damaged supplies (not Astron) which I ran through the dishwasher after a house fire.

    I discovered, and ordered one each regulator board from Astron for their RS20 and RS35 supplies, $20.00 USD each plus a little shipping.
  6. KD7WHQ

    KD7WHQ QRZ Member QRZ Page

    Check the zener in the crowbar circuit.
    If it shorted, it would explain all..
  7. AE4TR

    AE4TR Ham Member QRZ Page

    [​IMG] > I am surprised you have not come across this popular IC in the radio/TV repair business < Truth is I have come across this IC many times, but this one is not marked as such.
    It has a couple of circled plus signs and number 609A on it.
    Many thanks for all the helpful info. I got a schematic from one of the urls that you provided and checked the circuitry looks to be the same as mine. So I ordered the IC and will install it to see what happens. Will post the results later. agn, thanks.
    [​IMG] [​IMG]
  8. W9GB

    W9GB Ham Member QRZ Page

    B H -

    Let us know of your results.  
    Too much "off-brand" labeling in equipment these days.

    WD7WHQ mentioned to check the 15 volt zener diode (a good idea).

    The zener diode monitors the output voltage of the power supply. When the voltage gets to 15 volts, the Zener diode conducts.  This in turn fires the SCR and therefore short circuits the output of the supply. Eventually draws enough current to trip the breaker or "blow" the AC input fuse.

  9. AE4TR

    AE4TR Ham Member QRZ Page

    [​IMG]   To all who replied, many thanks.  Here is what the problem was:  The four output transistors went south.  None were shorted, and this wasn't caused by an overload, They probably got too hot.  
    The problem as I see it is this:  The thermal heatsink compound had dried up, and was not conducting heat from the transistors to the heat sink.  I bet a lot of other users of
    power supplies like this have the same problem.  Anyhow replacing them and applying a liberal amount of new compound
    fixed the problem.  It is now in fine working condition.  P.S. I bet the manufacturers did that on purpose, applying a real small amount of compound and hoping it will last till the item is out of warranty, and then, sell a new one, or charge a lot to repair them?.... HAW!!!   [​IMG]  [​IMG]
  10. W0LPQ

    W0LPQ Ham Member QRZ Page

    TR: It does not take a lot of the conductive grease to make an adequate transfer material. A thin layer on either one, then assemble.

    By the way, this stuff is bad news. Make sure you clean your hands, discarding the paper towel (or whatever you used). Then wash liberally. Dow Heat Sink compound is on the list of "bad things" to be around.

    After the transistors are bolted to the heat sink, take a paper towel and wipe off the edges of the transistors to get rid of the excess.


    Bill, W0LPQ
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