Astron RS 50M power supply problem

Discussion in 'Radio Circuits, Repair & Performance' started by N2MIR, Jan 17, 2012.

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  1. N2MIR

    N2MIR Ham Member QRZ Page

    Hello; I have had a short circuit accident with this power supply. A wire had touched the lower 1N1184A diode to ground. The 1N1184A tested ok. So far I found a blown power transistor 2n3771 on the back. That one was easy because it unplugged for testing. Can someone look up the schematic and walk through the circuit and tell me what other components may have blown? It would save time from me un-soldering parts just to test them. I am not that good with switching power supply theory. Thank you.
  2. W9GB

    W9GB Ham Member QRZ Page

    The Astron RS-50M is NOT a Switch-Mode Power Supply (SMPS) ... it is a LINEAR Design Power Supply (heavy EI transformer).

    PRIMER - Regulated Linear Power Supply Construction, What's inside your Astron®?
    David Metz, WA0AUQ

    Repeater Builders Techncial Informaiton Page (RBTIP) has EVRYTHING you need to troubleshoot and repair this specific Astron model.

    Introductory Information on Astron™ power supplies

    Schematics and service information on Astron™ power supplies

  3. N2MIR

    N2MIR Ham Member QRZ Page

    When I bought this power supply 22 yrs ago I was told in the catalog that it was a switching supply. There are 4 each 2n3771 with a 5th mounted on the back. I have attached the schematic. The positive side 1n1184A was the one that got shorted out. I just need to know what might have blown elsewhere from the 2n3771 mounted on the back?

    Attached Files:

  4. N2MIR

    N2MIR Ham Member QRZ Page

    Ok, I looked at the back label. It says 37amp continous and 50 amp interminten cycle. I am reading the articles you put on here. Thank you so much!
  5. KD8DEY

    KD8DEY Subscriber QRZ Page

    If you can afford it There is a ham selling an "Upgrade regulator board".

    Over the years the Astron was "Revised" numerous times to correct for minor problems such as RF blowing the regulator IC etc and it gets hard to find the correct schematic.

    Since you blew one of the 2n3771 it is recommended/best to replace all 4 since they are in parallel.....
  6. W9GB

    W9GB Ham Member QRZ Page

    Bubba -

    That is Skipp May. He is no longer selling these boards (and does not answer e-mail queries for them)
    He can be found on the Repeater Builders mail reflector -- addition of a few key components can accomplish same results as his board.
    This is outlined at RBTIP (Information there) and can be DIY performed cheaply.

    It is NOT a Switch Mode Power Supply (SMPS) which are lightweight = 10 lbs,
    it is a Linear Supply (EI transformer) which are heavy = 46 lbs, and use the LM723 regulator and 2N3711 NPN pass transistors.

    Most likely a misprint in that catalog 22 years ago =OR= a mis-understanding on your part.
    BTW, Astron does NOT make or sell SMPS rated at 50 Amps !!

    HERE is the ASTRON Product Page for Linear Power Supplies

    CORRECT, these 4, 2N3771 on heat sinks are the NPN pass transistors to handle the high current ratings (50 amps) for this supply.
    The 5th one (2N3771) is Q100 a driver transistor that can handle the switching current (LM723 regulator is only rated to 100 mA).
    These NPN transistors can FAIL, either as OPEN or SHORTED.

    They have to be TESTED individually with a VOM (analog meter) or a DVM (digital readout).
    The testing procedurtes are DIFFERENT, depending on the meter that you are using !!

    YES, the 1N1184A is the Power Silicon Rectifier Diode (there are 2 of them), rated at 40 Amps each in a DO-5 stud case.
    These serve as the MAIN rectifier for the output voltage.

    The 2N3771 transistors mounted (on the heatsinks) are the pass transistors in direct line to the stud rectifier (1N1184A) you shorted.

    The 2N3771 on the rear panel is Q100 .. it is used as a DRIVER from the LM723 regulator.
    IF you were lucky .. it is OK. IF Q100 is BAD (Shorted or Open) ... then you may also have a BAD LM723 regulator.

    Last edited: Jan 17, 2012
  7. KD8DEY

    KD8DEY Subscriber QRZ Page

    Wish I was close enough to take a look. Would be a fun repair project.
  8. WA9SVD

    WA9SVD Ham Member QRZ Page

    Actually, the "5th" 2N3771 drives the pass transistors (four 2N3771's) but there's also a TIP29 transistor that provides drive from the 723 regulator IC to the single 2N3771. (A 2N3771 alone doesn't have sufficient gain to be able provide sufficient drive to push four 2N3771's to pass up to 50 Amperes!) That TIP29 transistor should also be checked.
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