Daiwa RS-300A power supply issue

Discussion in 'Radio Circuits, Repair & Performance' started by N2BSS, Jan 28, 2016.

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  1. N2BSS

    N2BSS Ham Member QRZ Page

    This gets even more interesting. I noticed all the voltages at pins 1, 2, and 3 of the op amp were oscillating with a regular period. My scope isn't working so I couldn't take any measurements on frequency and voltage swings. Anyway I was probing a few other voltages when I accidentally shorted either pins 5 and 6 or 4 and 5 on the op amp. There was a release of magic smoke and the scent of burnt electronics. I gave a fast glance and didn't see anything fry so near as I figured the smoke release came from the LM324. I was about to cuss it out and call it a night when I noticed the supply was putting out 13.8v.

    Voltage measurements at Q1 and 3 of the 4 op amps were completely normal. I threw increasing resistive loads on it. I got to a 5A draw when I noticed the voltage dropped to 13.4v. Pins 5,6,7 look to be part of the metering circuit so I can't quite understand why an accident short and release of smoke there would do anything.

    So this is perplexing. Why did an accidental short "fix" the output voltage, what did I really fry, and how well is it regulating with a .4v drop at a 5a draw?
     
  2. W9GB

    W9GB Ham Member QRZ Page

  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Not sure. I would not trust it.

    It may have smoked the protection electronics.
     
  4. KW4EK

    KW4EK Premium Subscriber QRZ Page

    Pin 4 is presumably the +30VDC internal rail, 5 and 6 are the inputs to the OpAmp, none of those combinations are likely to have harmed the OpAmp itself, however there are other candidates that might have been harmed, but the reason for you now having normal output voltage will be clear in a moment, though not necessarily a great thing without extensive checking as to precisely what was damaged.

    You will want to ensure that the 16V Zener diode and then either 1nF or 1uF (I can't make out the symbol) bypass capacitor across pins 5 and 6 were unharmed by the short as the capacitor permits brief overcurrent conditions without tripping. This portion of the OpAmp is part of the current foldback circuit that essentially kills the output, save for the 0.5V you previously measured, when an overload condition is detected. Any damage in this part of the circuit will mean that over current protection may either not be available or may not function properly the next time a fault condition occurs, so it is important to ensure that this protection has not been compromised (and that there is an appropriately sized fuse installed before the transformer).

    What was crippling your power supply previously was a short in the output somewhere forcing the supply into current foldback protection mode to prevent damage to the supply's attached load. When you shorted pins 4,5 or 5,6 you either disabled or damaged the current foldback circuitry, forcing full power to be output regardless of the detected fault and, as a consequence, burned out whatever component was shorting the output. The most likely candidate will be a capacitor or one of the series pass transistors (and, possibly a resistor along the way, so all should be verified). You really need to figure out what component failed before using the supply as the loss of the damaged component will have altered the power supply's specifications and that can have consequences either down the road or when exposed to RF.

    Also inspect all electrolytic capacitors as they can easily be damaged by both over voltage conditions and age alone as their electrolytic dries out over time and they lose their filtering capacity and/or short out or develop high ESR. Odds are good that a failed electrolytic capacitor may be what forced your supply into foldback protection and was the cause of your original fault.

    If you do press this power supply back into service again, and especially if you do not find and repair what was damaged, you will want to add a suitably rated Crowbar Protection circuit (essentially a voltage activated SCR circuit that will short the output) to protect your attached electronics should the output voltage ever rise above 15-15.5VDC. It does so by immediately shorting the output causing either the power supply to go into current foldback protection or the input fuse to blow. This greatly reduces the risk of damage to your electronics should the maximum rail voltage of 18-25V find its way to your electronics by way of a failed series pass transistor (the one possible failure mode in linear supplies that can damage your electronics). After installing make sure that the input protection fuse is of reasonable size for your power supply and fast-acting as it must be possible for a dead short to blow this fuse quickly if the power supply does not automatically go into current foldback protection itself.
     
    KB0MNM likes this.
  5. W9GB

    W9GB Ham Member QRZ Page

    W2AEW YouTube video
    How to test transistors ...
     
    KA9JLM likes this.
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    12 minutes to learn electronics 101.

    That was fast.

    That supply may have those self healing caps.
     
    Last edited: Feb 17, 2016
  7. N2BSS

    N2BSS Ham Member QRZ Page

    Thanks all for the feedback. Since he has better test equipment than I have I dropped off the supply at a fellow ham's qth to check out the rest of the circuit. Due to the supply's age we'll be replacing the filter caps. The saga continues!
     
  8. N2BSS

    N2BSS Ham Member QRZ Page

    Latest update: It was only the op amp that had failed again, probably from the original short in Q1. I burned out the section that controlled the protection circuit. We replaced the chip and everything checked out OK. Overcurrent protection is verified working. For good measure I am replacing all the filter caps as they are looking sick and likely to fail at any moment. Case closed. Thanks for all the help!
     
    KA9JLM likes this.
  9. W9GB

    W9GB Ham Member QRZ Page

    The original Daiwa build used 25 V electrolytic capacitors (8) in the DC filtering section.
    It is good practice to replace with 35 VDC units, better overhead from rectified DC.

    greg
     
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Great Idea. :)
     

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