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Birth of a good amplifier from the junkyard

By on May 30, 2015











I had one component system brought to me for repair long back. It had got hit by lightning and almost all its control functions, including Micro Controller became bad. The customer wanted me to try to bring alive only its amplifier, leaving the 3 CD changer, 2 deck Tape and AM/FM Tuner.   Since the power amp IC STK407-070B was short, along with the protective components, I had to import this IC from Utsource, Hongkong.   I ordered for three, anticipating that should there be a problem, I need to have spare of two. Two ICs had to be used, as one failed within a short time. The other one was lying in my stock for quite sometime now.   Then, one day, when I was having some free time and mood, an idea struck as to why not I assemble an amplifier using the STK407-070B lying in my stock. Following is the result of this idea:


The power supply was made from a 3A, 15-12-0-12-15, 230V Transformer, leaving the 12V leads untouched. I got (+/-) 20V from this, as this IC uses dual power supply. The PS circuit is given in the circuit diagram. The components used are: 4x6A rectifier diodes, 4×103/100V (.01) ceramic capacitors, 2x4700mfd/100V electrolytic capacitors, 3A Fuse, AC on/off switch, power on LED directly connected to AC ground and one output of TX, using a current limiting resistor.  The DC was checked for ripple free output.

The circuit diagram is given below:

 amplifier schematic

Connected the IC to a common board, clearly marking the pin numbers on the back side. Provided a very good, heavy heat sink.  Populated the PCB with components as per the circuit diagram, making some changes. The input from two RC sockets were fed to a dual 100K Variable Resistor. The centre points were fed to positive side of the two (left and right) 1mfd/63V capacitors, for line in to IC.


The Resistors R3315/16, Capacitors C3315/16 were omitted. C3313/14 bipolar caps were replaced with the 1mfd 63V normal electrolytic caps. R3323/24 and R3351/52 were omitted.  In place of D3301/02, IN4148 Diodes were used.  1A Fuses were provided to the output that goes to speakers. I provided a headphone socket also, just for easy trouble shooting. Here are some pictures:




amplifier repairing


For the speakers, I got a couple of unused speaker boxes, woofers (Dual, West German, 50W) and Philips Tweeters from a friend of mine. The tweeters were having open coil. So I pried open the front ring, removed screws holding the rear magnet, took out the coil assembly, traced the connecting wires were corroded (these were lying unused since 1976). Replaced with thin strands of took out of shielded cable.  Both had same problem, and the coils were in tact, only the connecting wires were cut. Please see picture below to know, which ring of the tweeter has to be pried open:

tweeter repair

The front panel of the speaker boxes were worn out, as it belonged to 1976! I got it replaced by fresh 1/2″ thick plywood by a local carpenter, who did a good job. The woofers and tweeters were fit in. I used a 3.3Mfd/50V bipolar capacitor for connecting to tweeter from the positive side of the the woofer. Woofer was directly connected to the outputs.


 wood speaker

 amplifier tweeter repair

The result is excellent!

Another friend of mine wanted this amplifier for his home use, after listening to its wonderful performance!  So, now I am in the process of adding a cooling fan outside, by putting appropriate holes. Since there is no space inside, I may have to fit it outside at the rear side of the amp, where the heat sink is seen.I will put a few holes and put the fan there and put a few holes at the bottom, so that the air comes directly on to the IC from the bottom.For its DC supply, I will use one of the unused 12V of the transformer, and use a rectifier diode and capacitor with a 7812 IC for regulation. So, another Amplifier took birth from the junkyard!


Here is the sequence of self-speaking pictures to pry open a tweeter, dismantle the coil, check its continuity, patch up corroded coil ends that reach the two pins: (Since the connector ends, the strands and the patch up strand provided are too tiny, my mobile camera could not capture it).  Please remember to mark +/- on the coil, before removing it from the magnet slot.

tweeter repairtweeter repairstweeter repairing

So another Tweeter is saved from going into junkyard!


Best regards and thanks

An existence expressed as Parasuraman in this birth!

This article was prepared for you by Parasuraman Subramanian from India. He has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company and presently working as Consultant Manager, Purchase & IT, in Irinjalakuda Cooperative Hospital.

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You may check out his previous repair article in the below link:




  1. Gumby

    May 30, 2015 at 10:36 am

    Thanks for sharing this article ... clever enclosure! Good job!!

    • Humberto

      June 1, 2015 at 7:54 pm

      Thanks Gumby

  2. Merlin Marquardt

    May 30, 2015 at 10:41 am


  3. Yogesh Panchal

    May 30, 2015 at 1:41 pm

    Good job thanks for sharing.

  4. Anthony

    May 30, 2015 at 2:07 pm

    Nice work and a great outcome Sir , Thank you for sharing.

  5. Paris Azis

    May 30, 2015 at 3:32 pm

    Hi Parasuraman,

    Thank you for this time-travel to the past through your article!
    I guess also that if this assembly fails for some reason, you will be the unique person on earth able to repair it!

    Best Regards

  6. Robert Calk

    May 30, 2015 at 5:28 pm

    Good job. I love to see devices saved from the trash bin!

  7. mike

    May 30, 2015 at 6:09 pm

    A tip which may of saved the first ic is to put a bulb inbetween the live ac supply and the item your testing..a simple jig like this is handy for finding if there is still a short in one of the componenets.. if the bulb goes bright look for the short if the bulb goes dim the problem has gone.. this can also be used for automotive electrical repair by making a jig that allows you to connect an automotive (12v bulb) in to the fuse box of a car.. whenever a fuse blows plugging the bulb in will give a visual indication that something is wrong with the circuit.

  8. Capt36

    May 30, 2015 at 6:46 pm

    I smiled from the moment I read the title!

    It brought back a good memory!

    I did a similar arrangement, many years ago, only not stereo.

    I also added batteries to my 'box', and used it in the park often!

    Thanks for sharing your creativeness!


  9. Albert

    May 30, 2015 at 6:57 pm

    Hi Parasuraman Subramanian, nice that you could repair a broken tweeter. And thank you for sharing this repair.
    It is always difficult to get hold on thin very flexible electrical wires for special repairs.
    In your repair the wires need not move since they are steady glued to the Speaker itself.

    A friend of mine also put a special kind of protective selfhealing fuses in serie with his expensive Speakers. (I forgot their names). This way if the current should become too high, and would certainly destroy the Speaker, it will be stopped immediately from comming through by raising its resistance. And after the overcurrent is gone everything works normally automatically. As I understand you can buy them appropiate in the right value for any Speaker.

  10. Chris

    May 30, 2015 at 7:09 pm

    You are my man! I love to modify stuff and make new things from waste electronic part.
    I really love your article and sharing, and I can feel your passion until you finished your project.
    Brilliant stuff.

  11. Dagoberto Avalos

    May 30, 2015 at 10:50 pm

    very good job, from mexico

  12. Anthony Reo

    May 31, 2015 at 10:14 am

    Thank you for such a creative article and idea! I look forward to reading more articles like it!

  13. Corriete

    May 31, 2015 at 10:39 am

    wow!wonderfully done.keep up the great electronic work even for saving from the dump.

  14. Andrew

    May 31, 2015 at 5:04 pm

    Good fun and great patience on you behalf, IMHO I would replace the in put capacitors to a pair of Bi-polar even though the input signal is around 800m Volts max an electrolytic still does not like reverse voltage
    Maybe try 2 x 2.2uF in parallel and listen to the difference. I have spent hours trying different combinations of capacitor types and values in different Amps, DAC's and pre-amps all having different results
    Best regards

  15. Luis Estrada

    June 1, 2015 at 1:37 am

    Your handiwork and resourcefulness is admirable. It's refreshing there are people in this world that are not part of the throw-away society. Keep up the good work.

  16. mahmoud_tajpour

    June 1, 2015 at 3:28 am

    hi parasuraman congratulation you are wonderful electronical man. Mahmoud from Iran

  17. Humberto

    June 1, 2015 at 9:48 pm

    Hi Parasuraman Subramanian, you are an expert. Congratulations.

  18. Johan

    June 2, 2015 at 3:02 am

    Hi, I want to ask any one for some advice. a Client hit his Samsung TFT LCD TV screen with his cell phone. It is not cracked but it made a round mark on the screen, look like a splash. There are also lines H & V on the screen. Can it be repaired or must it be replaced.

  19. christopher

    June 10, 2015 at 10:03 pm


  20. christopher

    June 10, 2015 at 10:05 pm

    I really like what comes out from you and the crew.

  21. Taring K Arioka

    June 11, 2015 at 1:45 pm

    Helpful information. Need to know more and once again thanks for sharing

  22. Mendall

    February 13, 2016 at 9:29 pm

    Thanks. I have tried similar idea before but the result was not successful. Now got an intuitive hint from your article, thus I shall try it again. Thank you very much.

  23. Dananjaya

    September 8, 2018 at 11:17 pm

    Thanks verymuch.


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