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Provided Speaker Protection Circuit In Philips Amplifier 22AH393

By on June 14, 2019
amplifier circuit repair












This Amplifier, which was being used by us in the home, suddenly developed problem in the left channel and it became silent. In spite of my year long experience, exposure, knowledge and furthermore, the history of this Amp, when it was restored by me a few years back, it did not strike me to be cautious when testing it!

amplifier repair

I connected that channel to another speaker to get a silent result after a ‘pop’ sound! Result? Coils of two of my good speakers got burnt! I cursed myself for my carefree attitude and over-confidence!

Having done this damage, I opened the set and straightaway looked for shorted output transistors and found B617 and D587 short, delivering around 32V to the speaker output. What a sad thing to happen! Anyhow, I replaced the transistors and looked for any other component failure and found two resistors also open.

open resistor

Got the speakers restored with new coils and having now better sense prevailed, decided to provide speaker protection circuit to this Amp, using UPC1237. I downloaded the pdf file from

Gathered the components from my stock and assembled it on a veroboard (For those who do not know what is veroboard, please view . For providing a separate power supply of 12V, I fixed a separate transformer near to the TX (Transformer) of the Amp, provided the rectifier diodes (IN4007x4 with 0.01 ceramic capacitors across each diode), filter capacitor (4700uF/25V) and 7812 IC for regulation and further added a 220uF cap to its output for better stability, since regulator ICs are a bit noisy. Connected this isolated power supply to the circuit and found it to be working.

Fixed this board inside the AMP, which had ample space in it. Decided to also add two cooling fans for the Amp Output Transistors. So, added one more 7812 and fixed two cooling fans parallel, one each on top of each pair of transistors. The work was completed fully and amplifier was tested for hours together for proper functioning. Since the bottom plate was found rusted, I cleaned it using emery paper and used a canned spray paint and applied a few coats of paint on both sides. I did the same thing for the top cover too. The amplifier looked like brand new one now!

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When one of my regular customer-cum-friends visited my house, he wanted this Amp and I sold it to him, with confidence that I have done what best I can do for its long life. Needless to say that another job got added to my satisfaction bag.


For those of you, who may not know, I am giving a snap of the spray paint I used. This is enamel paint and does not need primer, if the surface is clean enough or that has previous paint on it. The colour selected was 36 Silver to match the cabinet’s original colour. (I forgot to mention that I cleaned the surfaces and scraped rust formations using emery paper, followed by one more round of cleaning, before I applied the paint. I hang the cabinet on a string in my lawn and sprayed the paint standing safely away, after studying the blow of wind, so that it does not come on to my body. I was also wearing a mask in order not to inhale the spray. I repeated the spray after giving time for the previous spray to dry, until the surface was smooth and shining. Whenever, I found thickening or lump formation, I scrubbed it off using thinner and repainted that portion. Let me be honest, this is my first attempt of such a work!)

spray paint

This article was prepared for you by Parasuraman Subramanian from India. He is 69 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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  1. George Greenfield

    June 14, 2019 at 11:07 pm

    Greetings Parasuraman,

    Been there and done that as they say.
    In my early years I fried a few speakers before coming up with 8 Ohm "dummy" loads.
    I like the protection circuit. I may build one myself.

    Well done !


  2. Anwar Shiekh

    June 14, 2019 at 11:13 pm

    I like you self critical attitude; it is very healthy.

  3. Robert Calk Jr.

    June 14, 2019 at 11:41 pm

    Good job, Parasuraman!

  4. Albert van Bemmelen

    June 15, 2019 at 12:59 am

    Good thing that you were able to provide this amplifier with a speaker protection circuit. And it is a good warning to all of us to be careful when connecting the outputs to our speakers!
    I've experienced almost such same problem with my 400 Watt Yamaha amplifier when it suddenly refused to play output to the left channel. The relay in my amplifier protected the amplifier components by being activated because of the reflex 2-way Bass speaker that had a shorted speakercoil. I never expected my speaker to blow or when it had happened. And to prevent this from happening again I now have soldered 2 selfrestoring thermal protection fuses of 2.5A in serie that activate within about 3 seconds if the current through the 50 Watt Bass speakers gets to high. And my 80 Watt ELAC peak 2-way boxes with good Westra speakers are probably safe for a long period of time now. Speakers that I was lucky to buy second hand on eBay because they are no longer manufactured or sold.

  5. S Ramachandran

    June 16, 2019 at 12:21 pm

    Dear, Mr parsuraman ,

    nice idea of including speaker prot ckt .to avoid speaker failures.

    in the early days of my repairs of amplifiers ..we use a capacitor in series with testing speakers to avoid coil getting burnt. also check for dc voltages present across speaker terminals with out load.

    thanks for your nice presentation .

  6. Justice

    June 19, 2019 at 6:39 pm

    Thanks Parasuraman but I could not really understand the protection for the speakers. Is it the 2.5mA self-resetting thermal fuse as Albert's experience or is it something else?

    • Parasuraman Subramanian

      June 19, 2019 at 9:25 pm

      Kindly study the data of the IC used for protection. You can learn how it works and what it does from it.

  7. Chandralal Cooray

    June 4, 2023 at 9:46 pm

    Hi, im chandralal Cooray from Sri Lanka, I have Philips Stereo Aplifier 22ah393 faulty need circuit diagram,if you have please send me great help for recovery of the equipment.

  8. Matthew E

    May 3, 2024 at 8:07 am

    Where did you find the replacement output transistors, or do you know of any equivalent that is available?


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