Servicing Pioneer TX-520 Tuner
This unit was brought to me with the complaint that the audio output stops after playing for a few minutes. I opened the set and subjected it to a thorough cleaning and brought it to my work desk and observed that the 2SD880 Power Regulator Transistor leg had dry solder and over all there were several dry solders.
I downloaded a free service manual from http://www.hifi-manuals.com/Pioneer/TX-520/downloads and checked the input voltages at the power regulator Transistor and found it to be ok. Then disconnected the jumper wire from its output and connected my multimeter to check what was the current drawn. It was negligible, i.e., in the order of around 140mA. But the Transistor was getting hot within a few minutes. Checked the ESR of filter capacitors and a few others on random and found these to be either abnormal or at the brim. So, replaced all the electrolytic capacitors on the board and retouched all the solder joints thoroughly.
But inspite of all these, the Transistor was getting hot! Removed and rechecked the transistor using Peak Atlas, as well as analogue multimeter (x1K and x10K tests suggested by Jestine Yong in his book). There was nothing wrong with any of the components. The current drawn got reduced to around 100mA after replacing the electrolytic capacitors. (There is no mention of any current readings on the service manual) So, I decided to provide heat zinc to the Transistor. Got one from my junk collections and fixed it.
The set worked normally for hours together. Now, why the transistor was getting hot? I have no clue. (If any of you know the probable reasons, kindly do let me know in your comments) I even tried replacing the transistor with an equivalent one, replacing the Zener and Rectifier Diodes etc. Same result! So, this was the only solution! In fact, this is the second time I am coming across a similar problem in the power supply regulators. I have one BPL component system. That too had the same problem. It could be small leaks somewhere down the positive rail, probably, and it is very difficult to locate such faults, unless a failure occurs! Here are pictures of the set working as well as the capacitors replaced:
A job completed not so satisfactorily to me!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and 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.
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