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Paco – DER-2000 Automatic Voltage Regulator Repair

By on May 8, 2015









There is a place in the city I live, where one can find used electronic equipment, either domestic or industrial, at really very low prices. Sometimes these devices, if one is lucky, work perfectly and need only a good cleaning. I never understood why they were thrown away as garbage!! Anyway in most cases they need to be repaired.

Last week I, along with my close friend George, visited the place for “hunting down” anything interesting in order to repair it and then put it in regular use.Well, I found a 2KVA  A.C voltage regulator of the so called “tap changer” type. These below are its front and back views (after I cleaned and repaired it).

automatic voltage regulator repair

I took the unit at home and tested it to see its operational condition. When powered, the display was normally on. It was not as bright as I would like it to be, but this was due to the chemical changes within the plastic red cover because of aging. When troubleshooting it later on I didn’t find any electrical cause for it. Measuring the voltages across the current dropping resistors and applying the Ohms Law I found the current flows through the LEDs to be at reasonable levels for a normal brightness and within their limits. I also checked the brightness after removing the board from the front cover, without the filtering action of the red cover and it was fine. So, when I saw that the display was normal, I connected the unit to my variac in order to check its switching points (i.e. the tap changing according to the input voltage threshold variation). The first indication was as below.

automatic voltage regulator           

When varying the input voltage within the operational range of 100-245V of this regulator using my little variac, I noticed that the output voltage was not changing normally and also I could not hear clearly the closing click of one of the four relays of its controller board.

My next step was the installation of a 2mm LED annunciator across each coil of every relay, in order for me to have a visual indication of their activation. So, I measured the coils’ supply voltage which I found to be at 19,5V D.C. This voltage is on purpose higher than the nominal coils’ voltage, rated at 12V D.C. The purpose is to drive satisfactorily the relay moving contact at low input voltages. However voltage/current dropping resistors were also provided, connected in series with the coils, in order to absorb their excess currents when working near or above the nominal input line voltage.

Below you can see these annunciator circuits I installed on the solder side of the controller PCB.

 avr repair

 I always install circuits like these when working with relay circuits just in order “to make my life easy”. It is very difficult to tell which relay has a stuck contact and produces weak clicking sound when activated or deactivated. Moreover, the contacts distribute line voltages at the output of the device, one of them being really high around 290V A.C, depending on the input voltage, rendering any related measurements at this level very dangerous. So, it is always better to have a simple visual indication of each relay’s activity and measure the result safely, directly at the output outlet terminals of the regulator.

With the aid of the LEDs, I spotted the stuck relay quickly and replaced it. The new one is the blue colored, as shown below.

automatic voltage regulator repairing

 After a new test, I noticed that there was inconsistency between the voltage display of the regulator, when measuring its input voltage by pressing the DISPLAY button, and my multimeter’s display. In normal operation the regulator’s display shows its output voltage. When this button is pressed, the display toggles momentarily to its input voltage, and then back to its output voltage again.

I tried to readjust the switching thresholds (or investigate for any change in behavior) varying the values of the two trimmers shown above, one of them each time and observing the resulting changes. As you can see in the above picture, I already  replaced the left trimmer (both had the same shape and same value of 5KOhm) which was open and there was no change when I tried to adjust it. After the replacement and the following adjustment by varying the input voltage, the circuit had healthy behavior.

I checked the device throughout its entire dynamic range and its output voltage was within acceptable limits. The tap changing sequence is shown in the table below.

 automatic voltage regulator repair5

In order for you to decode the information of the above table, please note that the names of the relays are arbitrarily given by me. As regards their location, you are kindly requested to see the photo of the solder side view of the controller PCB (either above, with LEDs “off”, or  below, with LEDs “on”). The LEDs are connected in parallel with the relevant relay coils, one per each coil.

D is the one located at the leftmost side. C is at its right side, next follows B and finally A is the replaced one,  at the rightmost side of the PCB, the blue one as seen from the components’ side view.

Observing this table, one might ask: “O.K, but what is the use of the D relay as it is always activated” ?

The answer is simple. This relay serves the “DELAY” function, that is, it’s always activated, but only after the delay time expiration. Electrically, it keeps the phase output conductor (L) disconnected during the delay count down time. Therefore the “game” is practically played with the rest three relays.

I also noticed that the displayed values are tricky! When the input voltage is far below the nominal one, the input shows a flashing “L” meaning “low input”. This happens below 160V input level. The trick is when the input voltage approaches 180V. From this level up to 245V input, the display shows permanently 220V output, while in reality this changes starting from 245V (at 180V input) to end up at around 230V at maximum input (above 245V). Note that all the tests to identify the switching points were performed with no load at the regulator’s output because my variac is a very small unit of only 150VA, which I use exclusively for adjustments. Not for load tests. For full load tests of this regulator I would need a 5KVA variac, but I don’t need to own such a “monster” because these tests are not my every day professional tasks and what’s more, this kind of equipment apart from being very specialized, is very expensive as well.

I also tested the DELAY function switch for proper operation and it was functioning properly. In general, when first time powered, the unit counts 6 seconds before connecting the load to its output. Within this time interval, the operator can press the DELAY switch and the unit counts now 120 seconds (or 2 minutes) before connecting the load to its output.

This function is very important for (simple types or, in other words, for non microcontroller operated) refrigerators possibly connected to the regulator’s output. The reason is that if the refrigerator is subject to a power failure when normally running and the power is restored after a few seconds, there is always a possibility for its compressor not to be able to reach its maximum speed due to high hydraulic pressure developed within the relevant network and this creates a domino-like phenomenon to the current draw of the compressor in relation to its temperature, that is, the current gets higher due to the lower than normal speed and as the speed is not increasing this results in increase of its windings’ temperature. The temperature increases in turn their resistance, decreasing even more the compressor’s speed (it may also stall) and it heats up even more as well, increasing more its power consumption…and the story goes on until it finally either develops inter-winding shorts or its windings open completely in their weakest point, rendering it destroyed because of that domino-like acting thermal stress.

The long time delay serves the purpose of letting the hydraulic pressure to drop in order to make things easy for the compressor, helping it to restart normally without being overloaded. And my reference about non microcontroller operating devices has to do with those old and “primitive” designs. Most modern refrigerators and air-conditioners are equipped with sophisticated  microcontrollers and relevant protection systems which avert such perils.

So, when I finished with the functional tests, I installed on it two protection circuits. First was the installation of input overvoltage protection circuit consisting of three varistors connected as follows; ground to phase, ground to neutral and phase to neutral. They are all rated at break-over voltage of 300V.

avr repairs

I connected the protection circuit right after the 15A input fuse. So, whatever happens to the device, the fuse will immediately blow and protect the main winding of the (auto)-transformer and the load itself. I also installed an additional R-C snubber circuit right at the output side of the main power switch contacts which feed the transformer in order to protect both the switch contacts from arcing and the output load from overvoltage spikes. You can see this circuit in the photo below. The component values are typical: 100nF/630V for the capacitor and 100Ohm/1W for the resistor.

automatic voltage regulator repair7

And this below is the overall solder side of both the display and the switch boards.


Below you can see all the LED annunciators glowing, while the input voltage was adjusted to 245V level.

 repair avr

And these below are the component sides of both the display and the switch boards.

automatic voltage regulator repair10                                 

Finally I thought to take a look at the fuse holder. There was a surprise waiting for me there. Well, this drives me crazy whenever I see it and my eyes start glazing! The fuse was blown and somebody had put a (fortunately thin but nevertheless dangerous enough) copper strand of soft multi-stranded low power cable around the glass body of the fuse, eliminating thus any chance of the device’s survival under either an unacceptable loading or overvoltage condition which could possibly occur during its operational life!!…

Anyway, the three defective parts I found are shown below.

bad avr relay

 My friend George had bought the same device two years ago. A few days ago when I visited him, he had the unit staying in his own laboratory at home without even having tested it. As I saw it, I asked him to put it on the bench and test it. George was a colleague of mine in the past and he has the same interest with me in repairs. So I used his bench to check the device and make comparisons with what I had seen with mine. Unfortunately George was not lucky at all! When I powered the unit, there was no display at all, no output and instead there was strong 50Hz hum noise coming from the transformer, clearly heard from a 50cm distance away from the device!

I checked its fuse and the result was really disappointing. The fuse had a thick wire soldered across its ends…Unfortunately I threw it away without taking any picture of it in order to post it, but I guess you can imagine about it…

Then I opened its upper cover to see the rest of the disaster…

You can see below a side view of the autotransformer. Please take a good look at the color of its core…You will clearly see there the remains of the enamel varnish, in light brown color. I cannot imagine what temperature level had this core reached before shorting the coils around it as I finally found.

 avr repairings

This below is a top view of the same transformer…No difference from the side view…


Later on I removed the controller board and after that all the relays on it and tested them separately. The version of this PCB was not the same with mine. It uses relays of different shape which can withstand heavier currents than the relays installed in my version. Nevertheless one of them was completely stuck again.

These are shown below. Note again the color of the PCB itself. When I touched any component on it, immediately its solder point pads on the other side were lifted!! Such a toasted PCB I have never seen before in my life!

avr relay

George was completely disappointed with the sad end of his voltage regulator…But he kept the entire empty case of it in order to enclose some other home-made electronic construction as he usually does. Well, this is also a good habit. Whatever escapes from scrap keeps its value as it can be reused as useful material again, keeping the garbage pile at low level as well…

I hope you enjoyed this repair description as I enjoyed the repair itself!

paris aziz

This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician.  He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.

Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!

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  1. Albert

    May 8, 2015 at 10:36 pm

    Hi Paris,

    Nice article and good thinking on the LED indicators part !

    Just something one needs to do in such circumstances.
    Led's are really clever indicators and also cheap and almost use no energy.

    In this regard even the IR led types are very handy!
    They can be used to transmit IR light. But also can be used, for instance on the BNC input channel of an oscilloscope, to detect IR signals from any remote control. And this way you can easily check your IR tv controller at no costs!


    • Paris Azis

      May 9, 2015 at 11:37 pm

      Hi Albert,

      Thank you for your good words.
      The idea about the oscilloscope and the IR diode is good. I used a test jig in the past regarding the IR LEDs, consisting from a super alpha transistor connection (i.e. a very high gain Darlington pair, driven by an IR detector diode, with its collector circuit driving a normal 5mm red LED. In this way I had directly a "light-to-light" indicator.
      I did't like (at that time and still do so) to use my expensive oscilloscope for such a purpose!!


  2. Evan Evans

    May 9, 2015 at 12:38 am


    Thank you for the well-researched and well-written article!
    It is quite refreshing to read a description of a thoroughly researched procedure of basic, safety-based and common-sense steps to parse the problem from one level to the next until the final resolution (not always pleasing) is reached. While current economics forces many to the "quick & dirty" shortcuts and half-measure troubleshooting steps, there is still no replacement for a well thought out & purposeful attitude toward troubleshooting (especially where dangerous voltages are concerned).
    While the language determines that we sometimes makes contextual interpretations whilst reading such articles, as we all speak the universal the language of the laws of nature, the procedures always ring TRUE.

    Again, many thanks for the article and here's hoping to hear much more from you in the future.

    • Paris Azis

      May 9, 2015 at 11:41 pm

      Hi Evan,

      This is what I call mutual understanding or (in its Latin expression) consensus...
      Thank you too for your good words and your comment.


  3. Fotis

    May 9, 2015 at 1:22 am

    Hello Paris,
    Repairing an electronic device for the fun of it - and putting it in good use afterwards, if possible - is something I also like to do. You seem to be an experienced technician, with a good writing style and I would love to see many more repair articles from you in the future! As I also live in Athens-Greece, I wonder if the place you mention is the Sunday flea market near Eleonas Subway station, around Agias Annis street, in Votanikos.
    I frequent this market, finding interesting used electronic devices to tinker with, repair and use. Thank you for the detailed repair article and the valuable expertise you share with us!

    • Paris Azis

      May 9, 2015 at 11:50 pm

      Hi Fotis,

      Thank you for your good words. At least I am willing to write more articles like this one. First of all I wish that we all have a good health, and all the rest matters will then be easy to manage them!
      Yes, the place is the one you refer to (but there are more as I am informed)!


      • Fotis

        May 10, 2015 at 10:06 pm

        Good Health is indeed the prerequesity for everything else, Paris!! So I sincerely wish you and everyone else to have a good health. I know of a Saturday bazaar-flea market near Nikea General Hospital, on Thivon str, but the Sunday flea market is bigger and better! I am looking forward to your next articles.

  4. Robert Calk

    May 9, 2015 at 2:47 am

    Nice job Paris. But I can't help but wonder why you didn't check the fuse first, and wonder why you didn't open up and inspect your friends unit before applying voltage to it?

    • Paris Azis

      May 10, 2015 at 12:06 am

      Hi Robert,

      I did't check the fuse first because the device was working. So, there was no practical reason for it, at least as a first step. As you have seen, it was olny a matter of a step by step approach until I just checked to see if the fuse was as it should be.
      In cases like this, the chanches are 50% or perhaps even more in favor of a defect to be present than a normal operation of the device. In other words, the device is very likely to be already damaged for good and as such it makes no difference to check its fuse first or open it first before powering it.
      Following the "plug and play" method, if it doesn't play, yes, I open it to see the magnitude of the disaster and fix it!
      By the way, this transformer was not burned out in my hands during the test. This is a 2KVA unit and it needs hours of overloading it before it dies from overheating...


      • Robert Calk

        May 14, 2015 at 5:21 am

        Yes, but you never know what someone else has done to the device. I now always open up and inspect anything I look at first before applying power to it, unless it is mine and I know that nobody has messed with it. And I check the fuse first to make sure nobody has messed with it. Here in the USA, people do all kinds of crazy stuff to bypass fuses.

        • Paris Azis

          May 15, 2015 at 9:35 pm

          Hi Robert,

          Taking into account your last phrase, you are right doing so, but in cases like this one I faced it's really not important to act in this way.
          What has been done is already there, hidden within the case of the device expecting you to see it.
          Talking about a most likely damaged device, all checks will be done anyway, with the "sooner or later" parameter being of secondary importance.
          On the other hand, we are focused on we are doing during the repair process and if these crazy things you refer to happen to appear we can unplug the device in proper time. If again not, the utility's automatic breaker of the house installation will drop instantly to save the case. (And yes, there is no possibility at all to find wire bridging in any fuse holder of my house's installation)!!

  5. Imraz

    May 9, 2015 at 7:23 am

    Nice article and thanks for sharing. Good to have another pioneer in electronics.

    • Paris Azis

      May 10, 2015 at 12:07 am

      Thank you Imraz.


  6. Gary

    May 9, 2015 at 10:10 am

    Very comprehensive and interesting article there Paris it must be hard in Greece to make money at the present time. I too like to find broken equipment and fix it for sale on ebay to make some money.,

    • Paris Azis

      May 10, 2015 at 12:19 am

      Hi Gary,

      Thank you too for your good words. The present time in Greece is really beyond any description but this is not the right place for political analyses. So let me please overcome this, without any further reference to it.
      The point is that you just gave me an idea which didn't occure to me up to now. I have many devices fixed (mostly switching PSUs for computers and voltage regulators, which constitute my beloved field of repairs) standing in my home, which I normally give as presents to friends of mine...


  7. Humberto

    May 11, 2015 at 8:07 pm

    Hi Paris Azis, I liked this article a lot, I read it carefully' good idea with the use of LEDs.

    • Paris Azis

      May 12, 2015 at 12:40 am

      Hi Humberto,

      I also like your articles. As for the use of those LEDs, this is almost a habit for me when it comes to circuits like this one.
      "Let's stay safe, in order to repair as many defective devices as possible"!


  8. Yogesh Panchal

    May 14, 2015 at 2:50 pm

    Good Repair attempt and very informative article
    thanks for sharing.

    • Paris Azis

      May 15, 2015 at 9:38 pm

      Hi Yogesh,

      Thanks for your comment.


  9. Vicken Mardiros

    May 29, 2015 at 3:12 am

    thank you ALL for your comment and a special thanks to you Paris May God Bless you and your work

    • Paris Azis

      September 1, 2015 at 2:54 am

      Hello Vicken,

      Thank you too for your warm wishes! Excuse my delayed answer as well.

      Best Regards

  10. Paul Cohen

    June 2, 2015 at 5:48 pm

    Please send a schematic diagram of the transformer and relays and explanation how they regulate the voltage.

    • Paris Azis

      September 2, 2015 at 12:47 am

      Hello Paul

      Visit please (for example) this site:, in order for you to understand the basic idea. In general, these devices are usually based on autotranformers and operate like the hand-adjusted variacs with the only difference being that the so called tap changing is done automatically,selecting voltage steps for correction by a logic circuit. This in turn can be very simply implemented by using only transistors. In any case they don't have the accuracy of the variac, staying very near to the targeted regulating voltage. The best of course would be to find such a device in a defective status and repair it. Then you will have the best chance to see how it works.
      Anyway, I don't have any drawing to send you. I usually work "in blind". Therefore you have better to use the internet in order for you to seek the proper information answering your question (in total).
      I hope this basic information of mine works for you.

      Best Regards

  11. Mendall

    February 13, 2016 at 8:29 pm

    Thank you very much for the regular helpful tips. Please keep on the good work. I always find some supportive ideas in each and every subscription.
    Thanks a lot.

    • Paris Azis

      March 25, 2016 at 1:57 am

      Hello Mendall

      Thank you too for your kind words and support.

      Best Regards

  12. steve

    July 19, 2017 at 12:08 am

    Thanks Paris but can you help me with my stabilizer. it has always been on "delay" for days now. the unusual light is also on.

    • Paris Azis

      March 13, 2018 at 12:11 am

      Hello Steve

      I have just revisited this article after so long time and saw your request. Just check for cold solder joints first or even some connector with bad contacts. If after that everything seems to be O.K, then most likely your stabilizer uses a microprocessor which is stuck in the delay mode. In this case there is not much to do but replacing the M/P chip perhaps from another junk pcb..I wish this helps, no matter that long delay in answering you...


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