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Astron RS-7A/10A not regulating output voltage low

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    #1

    Astron RS-7A/10A not regulating output voltage low

    Either I am getting old or dumb, probably both.
    This “simple” linear power supply is driving me insane. Should be easy, right?

    Here is the story. Got that Astron RS-7A or 10A as stone dead. I mean turned on and got 0.00V not even an mV on the output. Why dat? Darn electrolytic output capacitor shorted at the binding posts. No, problem… new one in stock and mounted.
    Turned the PSU on again and now I got 0.1V on the binding posts. Everything clear like sunny day and replaced the LM723 regulator IC in the socket. Had it made, I thought.
    Well, not so fast! Now I get something like 1 to 1.5VDC on the output, which is climbing up from zero really slow. Tried a second LM723, same thing!!!
    O.k. Must be something wrong with the transformer voltages. Astron made a little separate supply on the board for the LM723. Guess what? The voltages coming from the supply are bang on. No trouble there. So I checked the current draw on the AC side with no load on the PSU and got about 0.3A. Means the SCR isnt firing either. I even disabled the Firing mechanism of the SCR and same thing.
    Well, the diodes… another dead end. I tested every forking diode in that thing and that piece of shit isn’t regulating worth a damn.
    The socket is good, the LM723 is good (known good and tested), ceramic disk and electrolytic caps are good, the diodes are good, TIP 29 is good, 2N3771 pass transistor is good, SCR a is good and everything is making a connection.
    The only difference I can see is that the case of this one isn’t grounded.
    Checking the pins on the LM723 I get pins: 5,6,7,11 and 12 having correct voltages. Pins: 2,3,4,10 and 13 do not (something like 2VDC, pin 13 about 3.3VDC).
    The 1k pot is good, I checked some resistors which also are good, so WTF am I not seeing in this picture?
    Attached Files
    Last edited by CapLeaker; 09-17-2023, 08:35 PM.
    Tags: None
    #2
    Re: Astron RS-7A/10A not regulating output voltage low

    It looks pretty straightforward.
    The only trap I can see is the current-limiting is done by looking at the pass transistor base drive circuit plus emitter resistor voltage drop.
    So pin 3,2 on the 723 get enough voltage to turn on the CL transistor if the voltage drop across all that gets too high. It might be falsely on.
    What do you measure between pins 2-3? If pin 2 is above pin 3 by 0.7V then that is a problem.
    For the main 723 op-amp, pin 4 needs to be less than pin 5 for the output at pin 10 to come up. Only the current-limiter can override this.

    TIP29 is pretty old, I would expect it to croak lol.

    Careful the main filter caps and output cap can hold a charge if it's not working, so discharge them both before going in. Otherwise the 723 can get zapped.

    Astron info Repeater Builder.com

    edi: oh there is a trap if the 2N3771 does not work on start up, the 723 CL transistor will turn on and it could sit there.
    So TIP-29 E went up in voltage but no main output at 2N3771 E so the 723 CL transistor gets turned on. I'm not sure if it latches...
    Attached Files
    Last edited by redwire; 09-17-2023, 10:34 PM.

    Comment

      #3
      Re: Astron RS-7A/10A not regulating output voltage low

      A quick reply before I go to work…
      Pin 10 base drive for TIP29 and it the base drive for the single 2N3771 transistor never go above like 2.5V. You are right, the difference between pin 2 and 3 is 0.7V.
      Pin 2= 1.725V
      Pin 3= 1.25V
      Pin 4= 0.56V
      Pin 5= 7.23V

      Comment

        #4
        Re: Astron RS-7A/10A not regulating output voltage low

        It might still have a legit short at the output.
        Was it the big 2,200uF 16V output cap that shorted?
        There is also the C4 100uF 25V and 0.05uF parts. I wonder if it saw more than 16V. For sure the SCR is disconnected?
        You could check across the output with power off, on Diode-test or Ohms. Or try force say 5V into the output but that could make smoke.

        If there ever is the case the pass transistor can't keep up - the 2N3771's output is less than the TIP29 driver's voltage - the 723's current-limit transistor gets biased on. So the 2N3771 circuit might have an issue.
        I can't see a way for CL to latch on as it does in some PSU's.

        Comment

          #5
          Re: Astron RS-7A/10A not regulating output voltage low

          Yes, that 16V 2200uF cap was shorted. I replaced it and tested that little 0.1 uF cap across it. The short is gone on that end. I changed C4, because it was out of spec…
          First I disabled the gate drive of the SCR by lifting an end from couple of resistors and rejoined them not using the trace that goes to the gate. I also took the SCR out and that didn’t change anything either. I fired about 13.8V into the output of the PSU with my bench PSU and it only drew like 0.2A. I am dam sure that SCR isn’t doing anything at all.
          I did check the pass transistor and the TIP29 and they show good on the peak tester and also measure correctly if I check them with my Fluke.

          That’s what I am saying… this stupid little linear Astron, which I repaired tons off making me look really dumb. I am having a fresh shipment of UA723’s and a few other things coming. The only thing I don’t have is a spare TO-3 holder. Plan is to replace the TIP29, the 2N3771, etc. basically the whole base drive, pass transistor and few other parts, regardless of what they measure and see what happens next.

          Oh btw, when I back fed power, the Voltages that are off, showed more or less what they should be.

          Comment

            #6
            Re: Astron RS-7A/10A not regulating output voltage low

            Just thinking out loud, I better check the hFE from the 3771. I am sure the TIP29 had an hFE of 40. I wonder if the hFE at the 3771 is something like one or two. Lol

            Comment

              #7
              Re: Astron RS-7A/10A not regulating output voltage low

              I tossed some 1973 TIP29's in the garbage, their hFE was lucky to be 20 lol.

              Maybe check the 2N3771's emitter resistor or base-drive diode/resistor is OK.
              Astron also have last-minute values for Rx etc. to tweak the current-limit.

              You could override the current-limiting by shorting 723 pin 2-3 together. But that is risky, maybe use a variac. Then you can take voltage readings at the TIP29 and 2N3771 to see what the problem is. Use a small load like a light bulb at 100mA or LED.

              Comment

                #8
                Re: Astron RS-7A/10A not regulating output voltage low

                Reason I am saying this is I had worked years ago on a Motorola Micor squelch cards that didn’t work anymore. I tested every transistor with the DMM a in diode mode and they all were fine as kind. Once I took them out I found while the transistor read fine, they had very low gain hFE gain. I replaced the transistors with the low gain and the squelch card was working o.k.
                I am going to put a new 3771 in and see what happens. I’ve repaired truck loads full of linear power supplies, specifically Astron’s, but never had an issue like that on any of them. I suspect if the hFE aims indeed very low, the output of the 3771 would never reach 13.8V triggering a shutdown. Very interesting failure of a 3771. In an afterthought I suppose a 40 year old 3771 may have an issue like this due to old age. First I need to get the parts.

                Comment

                  #9
                  Re: Astron RS-7A/10A not regulating output voltage low

                  Exactly as I said. I did replace the 2n3771 temporarily with a 2N3055 that I had kicking around and the power supply does work now!

                  Sure enough, the old 2N3771 tests fine as a NPN, but it’s got very low hFE of 3! It should have something like 70 or 80?

                  Going to wait for the new parts.

                  Problem solved! Damn power supply still not smarter than me.

                  Comment

                    #10
                    Re: Astron RS-7A/10A not regulating output voltage low

                    Ahhh a good find.
                    I would say the transistor is damaged - three pins so actually 12 measurements I do if I really want to be thorough testing a transistor, like my life depends on it.
                    I do it with big audio power amps where a damaged transistor can look OK yet blow up all the new ones.

                    Diode-test all 3 polarities on 3 pins so 6 readings.
                    Ohms all 3 polarities on 3 pins so 6 readings.

                    I would bet the E-B junction is pooched.
                    With no load on the PSU even a very low hFE part will still work, output 13.8VDC.

                    I also add a power on LED to these and backfeed diode if somebody decides to charge a car battery, in an emergency say.

                    Comment

                      #11
                      Re: Astron RS-7A/10A not regulating output voltage low

                      These are used at local Fire Departments or other commercial outfits for base radios.
                      And no, this old transistor does not work, even so it tests as an NPN. I read it again, swapped the ole 2n3771 back in and no go. It is definitely the transistor, my tester now said an hFE of 2.
                      Last edited by CapLeaker; 09-18-2023, 08:24 PM.

                      Comment

                        #12
                        Re: Astron RS-7A/10A not regulating output voltage low

                        we used to use 723 based psu's in the arcades a lot - i still have tubes of them.

                        i found a modern TIPL762 works well with them even though the hfe is only 60.
                        the original was a 2N6057 darlington with a 3500 gain

                        Comment

                          #13
                          Re: Astron RS-7A/10A not regulating output voltage low

                          Sounds unbelievable, but today I tested the old pass transistor again. Tests as NPN fine, but extremely low on gain. So I want and installed the ole 3771 again. Turned the PSU on and of course nothing. Then I retested the old 3771 and now she is done for. I verified this with my Fluke meter and same thing. Never seen anything like this. Given this PSU is old and ragged looking, rusty screws, pitted TO-3 3771…. Maybe it has something to do with that?

                          Comment

                            #14
                            Re: Astron RS-7A/10A not regulating output voltage low

                            What's a modern TIPL762?
                            Can't be obsolete, right?

                            Comment

                              #15
                              Re: Astron RS-7A/10A not regulating output voltage low

                              TIPL762 is a TO218 packaged 6A transistor.
                              Attached Files

                              Comment

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