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DC Power Supply Upgrade

By on February 27, 2015
power suppy circuit















power supply

This is my GQ-A305D Variable DC Power Supply that most of you saw my repair video on. It has been working great. I really didn’t think about upgrading the power transistor and op-amps. I expected that I would usually be using it at around 2 Amps or less. When I did the repair on the unit, I didn’t even look to see if there were better components now that I could use to make the power supply more accurate and dependable. But after reading the comments and suggestions from our friend Paris Azis that commented on my repair article, I decided – why not? It would be much better to have it upgraded for when I do need better accuracy. And being more dependable is always a plus.


Thanks Paris for your comments and suggestions for the upgrade of my Power Supply.

 power supplyupgrade

I don’t want to spend much more money on this unit. So I’m just going to upgrade the power transistor and op-amps. I’m not going to spend money on a better heatsink. It would be cheaper to just repair or replace the fan when it quits. I’ve already upgraded the wiring in this unit.

In the above photo of the control board, I have changed the UA741CP op-amp to the TL081CP.


Here on the main board of the power supply I have replaced the UA741CP single op-amp with the TL081CP; and the dual op-amp LM2904D to the TL082CP.

 dc power supplies

And I changed the last single op-amp UA741CP on the main board to the TL081CP. Now all of the op-amps are replaced. While I was changing the op-amps, I also removed a lot of lead-free solder on many components with my Hakko 808 and re-soldered them with some good lead solder. There is not much non-lead solder left in this unit now.

 power supplies

I decided not to spend money on a new heatsink. But I will keep my eyes open for some aluminum strips to add some mass to this one.

 power supply repair

The heatsink already had holes drilled for 2 transistors. So I only had to drill holes for the third transistor. I drilled the holes and chamfered them, and then roughed up the surface with some steel wool. I’m waiting for my electrically conductive thermal grease to get here from The severe winter weather that we in the USA have been having has caused me to wait longer than normal for it.

 atlas dca pro


I’m going to leave the original transistor in the unit, and add the 2 with it that I bought in the first batch because their gain is closer to the original transistor and because I already cut the collector leads, and bent the other leads. The three transistors that I bought in the second batch have gains of 102, 100, and 99. So I’ll keep them as a set. Plus I haven’t cut them or anything, so I can use them for something else if I want to.

 power supply dc fan

Here is a photo of the 24V fan that is in the unit. It is 80X80X25 mm, which thankfully is a common size. They are not very expensive and I can get one just about anywhere. I’m going to buy a new fan to have in case this one quits. I can just replace it real quick and try to fix this one when I get the time.

 power supply fet

I’m going to see how well these 3 transistors work together. If I had bought 3 of the 327 batch, I would have just used them and kept the original transistor in my spare transistor box, but I only bought 2 of them.

 power supply heatsink

My electrically conductive thermal grease finally arrived. I put 3 layers of heat shrink on the base and emitter pins just to make sure they don’t short on the heatsink.

 power supply wire

 power supply resistor

 power supply resistors

The transistors are soldered up and the unit is ready for adjustment, testing, and reassembling.

 power supply repairing

I have the unit adjusted, and it is working very well.

 power supply working

Now my Power Supply is back in its place on my workbench and ready for work. I’m sure it will be much more dependable now, especially at high amps.

robert calk











This article was prepared for you by Robert Calk Jr from Texas, USA. He loves learning electronics and device repair.



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  1. Anthony

    February 27, 2015 at 1:25 pm

    Hi Robert,

    Not a lot I can say other than SUPERB work and I'm sure you would have made a great surgeon if
    you weren't an electronics enthusiast ! Thanks for the fantastic photos too.....really helps tell the story !

    Looking forward to reading your next accomplishment, maybe you can publish a book as well !

    Kind Regards


    • Robert Calk

      February 27, 2015 at 8:24 pm

      Thanks Anthony, you are very kind.

  2. Yogesh Panchal

    February 27, 2015 at 1:48 pm

    Thanks for sharing your experiment, sir just want to know what rating of transformer used in this device?.

    • Robert Calk

      February 27, 2015 at 8:30 pm

      You are welcome, Yogesh. I don't know. I don't remember ever seeing any numbers on the transformer, and I never had reason to remove it from the unit. Does it matter? As long as it will handle 5 Amps like it is supposed to that's all that matters, right?

  3. Mihretu Begasahw

    February 27, 2015 at 5:07 pm

    good Thanks,

    • Robert Calk

      February 28, 2015 at 1:04 am

      You are welcome, Mihretu.

  4. Paris Azis

    February 27, 2015 at 7:52 pm

    Hi Robert

    You did an excellent job! I only want to draw your attention on a detail.
    In the photo that shows the paralleling of the three output transistors, I see clearly the current sharing resistors connected to their respective emitters, the yellow cables which parallel the three bases, but I see no paralleling cables on the three collectors. Of course, I don’t know if this photo was taken in an intermediate stage of the modification procedure.
    Anyway, if it was taken at the final step just before the reassembly, you need to put connection tags on the collectors of the two transistors (left and right of the central one) and connect all the tags together as a common collector.
    As I see this photo, only the central transistor is electrically operating. Just verify this. This is the only remark based on the photo I see.
    As aforementioned, you did an excellent job. I am sure that you have now a bulletproof PSU (…with all remarks about the function of the cooling fan valid).

    Best Regards
    Paris Azis

    • Robert Calk

      February 28, 2015 at 1:02 am

      Hi Paris,
      Thanks. The collectors of the transistors are internally connected to the back of the transistor bodies. That's why I used electrically conductive thermal grease, to make sure the collectors made good electrical contact with the heatsink. It should only need one wire to the heatsink IMHO. That's why the heatsink is electrically isolated from the case of the unit.

      • randy warren

        March 6, 2015 at 5:06 am

        needless to say, this is ONLY in the case where the heatsink is electrically isolated from the chassis, where it is NOT, you must use a non-electrically conductive heatsink compound...

        • Robert Calk

          March 6, 2015 at 4:28 pm

          Hi Randy,

          Transistors that do not conduct the collector through the heatsink will not have the collector pin cut and will have a thermal conducting plastic looking piece between the transistor and the heatsink, or will not have an exposed thermal plate on it's back that could make contact.

    • Robert Calk

      February 28, 2015 at 10:00 pm

      To add: But it is my fault for not pointing that out, sorry. I should have mentioned that especially for people that are new to electronics.
      For those interested, if you look at the 5th & 6th large photos, you will see the plastic feet on the bottom of the heatsink. They keep the heatsink lifted off of the bottom of the case, and electrically isolated from the case and the rest of the unit, except for the black collector wire that bolts onto the middle transistor and the heatsink. I apologize for not explaining that in the article.

      • Paris Azis

        March 1, 2015 at 1:56 am

        Hi Robert

        I have never used any similar chemical compound in mounting power transistors on a heatsink. I also didn’t notice that the heatsink is electrically isolated from chassis. So, everything seems to be clear now.
        I am sure that you won’t regret for the money you spent for the modifications you did!

        Best Regards

        • Robert Calk

          March 1, 2015 at 9:32 pm

          Hi Paris,

          I'm sure that I won't regret it either. Most articles and videos that I have seen say that regular thermal grease is sufficient because the transistor will make enough contact to the heatsink when you tighten it down. But I wanted the best connection possible, so I used electrically conductive thermal grease.

        • Robert Calk

          March 19, 2015 at 11:23 pm

          Hi Paris,
          I'm going to run wires to the other transistors to make sure that I don't have problems with them in the future. No sense in taking unnecessary chances. Thanks.

          • Paris Azis

            March 28, 2015 at 5:22 am

            Hi Robert

            Before doing this, just check out its necessity.
            So, verify first the current share between the transistors. If this verification shows you equal current flow between every transistors' C-E, you don’t have to do anything more for improvement.
            The method is straightforward. Put a reasonable load at the output, say 1A. The voltage is not important for this check. So you can connect a 12V/10W lamp at the output, one of those used for car turn signal light indication, and measure afterwards the voltage drop across every current sharing resistor.
            Under normal conditions you should read almost the same mV value across each resistor, that is, equal current flow in every transistor.
            To make things even more easier, when taking the first voltage depress the REL button of your Fluke. From now on, this value will be stored in its memory as a reference. This will then be compared with the rest of them each time you take a new measurement and give you the difference from the stored reference.
            Now you already have the overall picture of the total current as it is divided in three equal sub-currents within the transistors, which add again at their common emitter connection point.
            In conclusion, if you notice small differences, say around 5-10mV, you don’t need to make any further improvement.
            On the contrary, if you notice great declinations between the measurements (which are not expected because all of the transistors share a common heatsink), only then you should bridge the collectors together with copper wire of proper diameter (and this finally will signal that something is wrong with the compound used)…
            So, apply Ohm’s law, use reasonable percentage tolerance for the differences (if any) and common sense to decide for a more drastic intervention.
            These guidelines will help you save valuable time, otherwise spent in vain…

            Best Regards

          • Robert Calk

            March 28, 2015 at 9:28 am

            Hi Paris,

            Thanks. I meant that in the next few years as the thermal grease ages I might start having issues. I'm not sure how long the grease is good for. And building & installing the wires would not take me long to do. I appreciate your help.

  5. Evan Evans

    February 27, 2015 at 8:33 pm

    No comments this time, just a BIG THANK YOU for another fine, well-documented article!

    • Robert Calk

      February 28, 2015 at 1:06 am

      Thanks Evan, I appreciate your kind words.

  6. Humberto

    February 27, 2015 at 9:46 pm

    Hi Robert, good idea in substituting the UA741CP for TL081CP OAs. I've been seeing in your articles you own good tools, congratulations and keep redacting your experiencies.

    • Robert Calk

      February 28, 2015 at 1:08 am

      Thanks Humberto. It was Paris Azis that gave me the idea to upgrade the unit. I was just so glad to get it fixed I didn't even think about an upgrade. lol

  7. Dan

    February 28, 2015 at 12:05 am

    wondering if you have any experience on, well push button on and push button off equipment? I have a ice fishing camera that will only powers up if you keep the push button push in with your finger??? let me know if you can help. Thanks Dan

    • Robert Calk

      February 28, 2015 at 9:30 pm

      Hi Dan,

      No, I haven't worked on a camera yet. Can you get a new switch and see if that will fix it?

  8. randy warren

    February 28, 2015 at 12:21 am

    Robert, I think I asked you this before, but I cannot find the answer written down. What is the head magnifier you are using? I have a cheap one I got at a hamfest and it is turning into a real pain as the tension knobs continually loosen, driving me nuts! I looked into an Optivisor, but while somewhat expensive, the main issue is that they have no way of easily switching mag lenses, which my cheapo does by flipping down in front of the mounted lens, similar to yours. Thanks, Randy

    • Robert Calk

      February 28, 2015 at 9:45 pm

      Hi Randy,
      You can get it at this link.
      It's kind of expensive but much better than the cheap ones. The Aven has a plastic carry box for the lenses, and they are easy to change.

      • randy warren

        March 6, 2015 at 5:07 am

        thanks, Robert, much appreciated!

        • Robert Calk

          March 6, 2015 at 4:38 pm

          You are very welcome. You will be very happy with the visor. It also uses 3 AAA batteries and better LEDS than the cheap ones.

  9. mahmoud_tajpour

    February 28, 2015 at 2:37 am

    Hi Robert really you a super electronic man I wish u are successful at this way.

    • Robert Calk

      February 28, 2015 at 9:46 pm

      Thanks Mahmoud. I'm glad you liked it.

  10. mahmoud_tajpour

    February 28, 2015 at 2:39 am

    Hi Robert really you are a super electronic man I wish u are successful at this way.

  11. Mustamin

    February 28, 2015 at 3:24 pm

    Thank you sir. Healthy hopefully always to be with you.

    • Robert Calk

      February 28, 2015 at 9:48 pm

      Thanks Mustamin. I'm glad you like it.

  12. dicksy

    February 28, 2015 at 3:28 pm

    what a nice article.thanx

    • Robert Calk

      February 28, 2015 at 9:49 pm

      Thanks Dicksy. It is my pleasure.

  13. Innocent Urum

    March 1, 2015 at 7:51 pm

    Hi Sir,I am really enjoying your articles, is really impressive. keep it up!with reference to the last article you sent to my mail box, what if there was no glow at all in CFL bulb how would someone troubleshoot the problem? Thanks !

    • Robert Calk

      March 1, 2015 at 9:39 pm

      Thanks Urum, I'm glad you like my articles and took the time to comment.
      I would start by checking the PCB for blown components, bad solder joints, and broken traces. If I didn't see anything obvious, I would start with checking the power cord, and work my way through the components and connections.

  14. syed

    March 8, 2015 at 10:27 pm

    I have faulty 24v smps,output is blinking

    • Robert Calk

      March 12, 2015 at 1:50 am

      Hi Syed,

      The SMPS is trying to work. I would trace the blinking circuit back from the output and find the culprit. I hope you are not working on it without an Isolation Transformer.

    • Paris Azis

      March 29, 2015 at 2:33 am

      Hello syed

      Blinking output means, in the most of the failure cases of such power supplies, that a protection circuit is getting energized for some reason (that you are called first to identify and then delete).
      This practically means that the protection circuit “sees” either a tendency for overvoltage at the output or perhaps some kind of overloading as the output voltage tries to build up.
      You can start the easy way, by a visual check of the output electrolytic capacitors.
      In order for me to enter in further technical troubleshooting details, I would like you first of all to describe to me your technical profile. The reason is simple. I would never try to expose you (or anyone else, no matter if he or she likes electronics) to the risk of possible electrocution of any degree. Needless to refer to the worst case scenario...
      So, evaluate please your skills and let me know in turn if you really are capable to “swim in deep waters”.
      If you already feel that this task is beyond your skills, suitable for a properly qualified technician, I would kindly suggest you to quit from the idea of any further intervention no matter how tempted you may be for repairing the unit by your self.
      Always remember that you alone are the undertaker of the full risk against your own life and also responsible for the application of your personal skills for implementing any advice that you may receive from anyone interested to help you. This should be fully understood in advance.

      Best Regards

  15. Taring K Arioka

    April 16, 2015 at 10:51 am

    Thank you for sharing these good articles


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