Crackling Noise In Realistic Receiver Amplifier Model STA78 Solved
This lovely receiver amplifier was brought to me with the complaint that loud unbearable crackling noise was heard on switch on. I opened the cover and cleaned the inside thoroughly and downloaded the service manual from website. As the Amp was having this problem even at minimum volume, the problem definitely was at the amplifier stage.
So I took print out of only the relevant board and studied the circuit. The noise was there only in the right channel so only that portion had to be checked. The Amp was using all transistors in complementary symmetry circuit without any coupling capacitors in each stage.
My experience on such problems in this type of circuits has taught me that semiconductors could cause it. I used my oscilloscope to check the stage where the problems were noticed. In the input stage there were three transistors one PNP (2SA659) and two NPN transistors (2SC1570). The problem was noticed at that stage.
Checking of voltages and removing the transistors and checking in the transistor tester did not show any defects. So I replaced these transistors one by one. Then the noise disappeared, indicating that all these three transistors were having defects when on load.
Strange it might look, but from my experience such problems have been noticed several times in Amps. The voltage comparison between the two channels were perfect. Then why these three failed? Could only be due to some inherent defect developed while in use. I then applied audio signal and checked the outputs, which were seen good enough:
The three transistors that were replaced are marked in the above picture.
Mission accomplished with satisfaction getting added to the collection without any crackling noise! (LOL)
This article was prepared for you by Parasuraman Subramanian from India. He is 74 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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Yogesh Panchal
June 7, 2025 at 3:46 pm
Good Job!Sir,
For audio cracking noise these transistors on Driver board are always found faulty.even sometime on testing transistors showing OK condition but it fails on actual Load.
Parasuraman S
June 7, 2025 at 6:05 pm
Yes, very true! But locating the fault on this type of circuit is very difficult! Many thanks, dear Yogesh Ji!
Mark J
June 8, 2025 at 7:34 am
Parasuraman another great radio Receiver repair. Thank you for sharing.
Parasuraman S
June 8, 2025 at 11:24 am
Many thanks!
Albert van Bemmelen
June 14, 2025 at 5:02 pm
Nicely done repair! It is almost as replacing those questionable transistors or mosfets in induction cookers you often fixed!
Parasuraman S
June 14, 2025 at 8:37 pm
Yes, dear Albert! In this type of circuit, that is the only way! Thanks!