Astron 35A power supply tripping out at 20 amps.

Discussion in 'General Technical Questions and Answers' started by KT1F, Nov 10, 2019.

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  1. KT1F

    KT1F Ham Member QRZ Page

    I'm looking for some advice on what you do with my Astron 35A power supply.

    It sometimes trips out at about 20 amps. Here's the exact experience:

    It's probably about ten years old. It powers my Elecraft K3. I don't really use it much but when I do, it's usually on a digital mode which at about 40 watts, draws about 10 amps so I wouldn't normally notice the problem anyway.

    My club uses it and the K3 once a year for field day so I'm mostly concerned with getting it fixed before next field day.

    In the last few months, I think since last field day, I've been operating and the radio has instantly switched off. I don't have much test equipment but I think I've properly concluded that it's the power supply tripping out, not the radio. I powered my little QCX radio from the same power supply and I can see a very brief flicker on its display at the instance the K3 goes off.

    Last week I was able to cause the fault to happen repeatedly. I could tune up the K3 and then key down at about 90 watts which is about 19 amps and all was good. Then increase the power. Every time when I got to 95 watts or about 20 amps it would trip out.

    I read this https://www.ecse.rpi.edu/courses/CStudio/ham_radio_docs/astron-repair-index.htm . I've looked inside and there doesn't seem to be any obvious mechanical issue. The connections to the pass transistors seem good. I gave up, put the top back on, put it under the desk and then it was working fine at 23 amps which is about the most I can make the K3 draw.

    I haven't used it all week until yesterday I tried it and it tripped out once but now it seems to be okay again.

    Is this an obvious mechanical problem / bad solder joint that I just haven't found or is it possible that a transistor or the 723 could be intermittent?

    Should I go for the sledgehammer approach and replace all the transistors and the 723? That's easy enough to do and won't cost a lot. The transistors are 2N3771 and the 723 is in a socket.

    I really need a better dummy load. Some people suggest a car headlamp. Using a $1500 radio to as a test load might not be the smartest thing to do. :) I discovered that if I tweak my manual antenna tuner a little, I can make the radio draw a little more current. I think that's why sometimes, even if the fault exists, I can run full power without problem. Full power is just on the threshold and the exact current drawn by the radio depends on the exact tuner settings / SWR.

    Thanks for any advice.
     
    Last edited: Nov 10, 2019
  2. WA9SVD

    WA9SVD Ham Member QRZ Page

    I'm not quite sure what a n "Astron 35 A" is, but I believe you are referring to an Astron RS-35 power supply. Be aware, it is ONLY rated 35 Amperes on peaks; 25 Amperes in continuous operation.

    I suspect you may have a loose connection somewhere. Open the case again, and especially check that ALL nuts, bolts, screws, etc. are tight, AFTER unplugging the supply and discharging the BIG electrolytic filter capacitor. Be sure the capacitor connections are tight; if are computer grade, they usually have screw terminals, those can come loose or intermittent.
     
  3. WR2E

    WR2E Ham Member QRZ Page

    When you say it 'trips out', what is it actually doing?

    Is it tripping the OVP?

    Do you have a way to measure ac input current?

    Yeah, dont test with the radio!
     
  4. AG6QR

    AG6QR Premium Subscriber QRZ Page

    What do you mean by the phrase "trip out"? Does it blow a fuse or trip a breaker? Does the voltage sag? How much and for how long? Does it recover when the load is removed or does the supply need to be turned off and back on?

    If you don't have a voltmeter, get one, and monitor the output of the supply.

    How hot are the pass transistors? Does the problem happen immediately after the cold power supply is started and placed under load, or does it only happen after things are warmed up?

    It's possible that issues are caused by RF getting into the supply. Does the problem happen when transmitting into a dummy load?

    Using 12V lights is one way to separate the issue of RF from simple overload.
     
  5. KE5MC

    KE5MC Premium Subscriber QRZ Page

    I'm not sure my problem with a new Astron 35A with meters will help, but might be worth sharing.

    The original symptom was intermittent current readings without effecting output voltage/current. It was strictly a metering issue at this point as I was not pushing the power supply to it limits. It turns out they only monitor the current in one leg of the pass transistors block and assume everything is balanced and upscale the reading to represent the total output. It turned out that the socket for one of the legs they were monitoring current on had a loose connection to one of the pass transistor emitter or base contacts and don't recall which one. I carefully removed the transistor, pinched the contacts together and reinstalled. None of the other transistors had a connection problem and power supply has been working for a number of years other than the lamps in one meter burning out. Currently working on converting to LEDs for the meters.

    I'm sure if I was pushing the power supply I would have had similar symptoms, but for me it was just metering issues. With the top off its easy to check connection for the pass transistors by wiggling the wires at each E and B connection. If the socket connection move away from the transistor pin you have a suspect connection.

    Good Luck,
    Mike
     
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    The over-current sensing circuit could be faulty. Could be a bad solder joint on the circuit board. Also, RF getting in the power supply could shut it down.
     
    Last edited: Nov 10, 2019
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    ...As well as being one way to allow the world to listen to your tests. ;)

    I've made QRP contact on a 15W lightbulb. The stories are true.
     
  8. WR2E

    WR2E Ham Member QRZ Page

    But I think the idea is to use the bulbs as a dummy load for the power supply...
     
    AG6QR likes this.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Which model, exactly?
    If the power supply doesn't have meters, just use a voltmeter to monitor the power supply output voltage. Does it drop to zero? Or what does it drop to? If you stop transmitting, does it recover and put out 13-14V again, or do you have to turn it off (AC power) and then back on to make it start working?
    Anything's possible but sounds like it's either the overcurrent protection that drifted or RF getting into the power supply and making it shut off, which is a common problem with some antenna systems.
    I sure wouldn't do that.
    A car headlamp won't draw anywhere near 20A. The old filament type headlamps are about 5-6A, so if you use those you'd need about four of them in parallel to create a ~20A load. Some of the new-gen lamps draw less than that, even though they're brighter.
     
  10. KT1F

    KT1F Ham Member QRZ Page

    Thanks all. I think I'll pull the top off again and check joints etc. But ... I'll answer individually below.
     

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