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Revisit Of Jinxed AMP With Torroid Transformer Failure

By on April 17, 2020
Revisit Of Jinxed AMP With Torroid Transformer Failure








Those of you who have missed the story behind this Audiosource Amp One, please read it using this link:

Well, as this Transformer failed again at the customer’s place, he brought it back with a sad face to my dismay! (I visualized one ‘satisfaction grain’ jumping out of the collection bag!)

Opened the set and found the primary was open! Gave it back to my friend, who had done the rewinding last time. You won’t believe me if I tell you that more or less same calamities, one after the other continued at his end and it was stuck with him for almost 8 months this time! Finally I got it back and this time as he told me to connect a 100W bulb as a load and just allow it to run for one day fully, which I did. He also asked me to add a protective delay circuit, schematic of which is given below. I used a 12V relay and tapped power from Amps’ secondary. The circuit was assembled on a Vero Board. I housed the board in the gap that was available between the TX and main board.

amplifier diagram

how to fix amplifier with bad transformer

torroidal transformer amplifier repair

The idea was to provide a few seconds pause, before the full voltage of the secondary is pumped into the circuit, allowing the transformer to get charged, before large startup current is drawn. This protection was added, as almost all the time, the TX has failed on switch on!

I used the set continuously for several hours and days, with switch off and on. It worked fine and so, returned it to the customer, who reported that he was afraid to use the set for fear of another failure, but on my reassurance, he started using the set. Hope everything will be fine and let us keep our fingers crossed!

So, I put the ‘satisfaction grain’ on hold from jumping back into the bag!

This article was prepared for you by Parasuraman Subramanian from India. He is 70 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.

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You may check on his previous repair article below:




  1. Waleed Rishmawi

    April 17, 2020 at 5:02 pm

    Man you went through a lot of hardship in this repair but you came on top. Good job and hopefully this time it will workout. Have a blessed day.

    • Parasuraman S

      April 17, 2020 at 10:27 pm

      Many thanks. Yes, he confirmed that it was working very well. At-last!

  2. Albert van Bemmelen

    April 17, 2020 at 5:43 pm

    Interesting circuit and repair story! I needed a similar in-rush current limiter to prevent my 25A main power line fuse in my home from burning through which could have cost me a large repair Bill to get it fixed.
    The in-rush power exceeded easily the max 3600 Watt (probably upto 4800 Watt!!) on one group if I used my 709A Spot Welder to weld Li-ion 18650 battery packs. Why I bought an external limiter to prevent this from ever happening. Which I place inbetween my Spot welder and the power line. Sadly it also limits the Spot welder from making good welding results because it needs this extra main line power to work to my satisfactory.

    • Parasuraman S

      April 18, 2020 at 1:05 pm

      Many thanks for your input!

  3. Paris Azis

    April 17, 2020 at 6:16 pm

    Hello Parasuraman. Some times this kind of trouble happens indeed. It is nice that you put an inrush protection at the primary side of the toroidal transformer. This always helps. These transformers are notorious for their high inrush currents. Nevertheless,I don't understand clearly the circuit you installed there. In particular, this type of protection depends upon a time delay before full power is applied at the primary. This time is expressed as the product of resistance and capacitance, that is an RC circuit with a specific time constant. I don't see these elements in your circuit and I also don't know how it works (as a delaying circuit).
    Apart from that, the application of a PTC thermistor, normally used for isolating the degaussing coil of a CRT screen,works in the opposite way than the needed here. Only the power resistor should be there.
    The needed time constant here, is a relatively complicated parameter for calculation, but nevertheless it must be somehow calculated.
    What I want to say is that you should use a small signal transistor for driving the relay coil instead of its direct drive through the 24V stabilizer. And the time delay elements should be connected at its base.
    Another very important parameter for calculating the time delay needed is the value of the smoothing capacitors...
    That's why I said that this calculation is relatively complicated.
    It can be simplified only if one has a digital storage oscilloscope and after storing the curve of the inrush current and analyzing it, finds the needed time constant. The trial and error method is the last resort to that...

    • Parasuraman S

      April 17, 2020 at 10:31 pm

      Many thanks for sharing a lot of valuable inputs on this service. The circuit is working well and the customer informed me that the AMP is working perfectly. So, that solves the problems!

      • Paris Azis

        April 18, 2020 at 4:24 am

        I hope that time will prove the success of this repair dear Parasuraman.
        The last time I had a similar problem, it had driven me crazy... It was a public address amplifier, a high wattage one, used in a multi-floor building. The problem was almost immediate blow of its 20A(!!) fuse in the secondary side, before I could even see that the power was on...And all of that after my repair on one of its push-pull output branches...
        To make the long story short, the culprit was merely a 470nF capacitor at the primary side of the output transformer circuit, found completely degraded and due to that very high frequencies were developed all around the power line and then distributed within all the stages of the amplifier destroying repeatedly one of the power output branches, that is, three 2N3035 transistors in parallel (per branch).
        I was speechless when I saw these oscillations riding on the power line, because there were no other symptoms like squeals in its output. Everything was seemingly perfect until I saw what was happening using the oscilloscope.
        But I was aware that “something was very wrong” with that amplifier, as I was the fourth technician in a raw attempting to repair it. It’s owner had already lost the hope of seeing it alive again...
        Anyway, I hope that your case is not a similar one and the countermeasures you took will effectively eliminate the cause of that problem.
        I am only worry about it because I know that feeling of encountering the same problem again...

        Best Regards!

        • Parasuraman S

          April 18, 2020 at 1:09 pm

          Yes. I do understand how difficult and disturbing it is. There is no capacitor in the primary of this Transformer. You are right, the AC Capacitors provided as a noise filter of electromagnetic waves, can cause problems when they develop partial leak. It is better to replace these during our trouble shooting work.

  4. Henrique J. G. Ulbrich

    April 17, 2020 at 11:00 pm

    Dear Parasuraman, I agree with Paris Azis. It shall be a NTC, and pursuant to the initial idea, there must be a kind of delay, what the circuit does not show. Another doubt is either if this NTC is original of the input circuit or if it was added by you in this implementation. Anyhow, I realize a merit of this implementation: as the relay operates and so remains, the short-circuit of its contacts remove any voltage on the NTC, keeping it cold to efficiently operate next time.

    • Parasuraman S

      April 18, 2020 at 1:10 pm

      Many thanks for your input. The Amp is reportedly working fine. I shall take care of these inputs for my future troubleshootings.

  5. Lynn Blakely

    April 27, 2020 at 7:16 am

    Gentlemen: This is a very needed type of instruction, and it shows that experiences like these are not forgotten.We need to be reminded of some of the things that others have learned. Thanks to each one who commented to this inrush current problem. It may also show that the original failure may have been caused by the lack of input surge limiting.

    • Parasuraman S

      April 28, 2020 at 8:04 pm

      Yes, very true. Many thanks.

  6. Ulises Aguilar pazzan i

    August 24, 2020 at 11:15 am

    MR Parasuraman Grate Sir ,grate repair Very ingenious, I like that ,very Good Sir


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