Repairing SSBright SG-SM129D FM/USB/Line In SET
This set was brought to me with the complaint that the Red light comes on, when connected to mains, but the set does not function. I opened the set and noticed that it is an SMD populated, FM/USB/Line-in Player, with earphone socket and two built in speakers. It was connected to a Battery that looked like a large 9V Battery, without any marking on it except polarities.
On checking the voltage of the battery, it indicated 4.5V. When connected to the mains, the charge was coming to the battery, as the voltage increased to 5.4V. Even after charging the battery for more than 6 hours, it did not take the charge. ESR check showed it was faulty. So, replaced the battery with an equivalent looking one which was rated for 4V, 1A. Charged the battery for some hours and switched on. The set worked:
The 230V AC dropping to low level for converting it to DC to the desired value, was achieved by a 155/400V (1.5mfd) Fixed Capacitor, which you can see in the above picture. (Very dangerous and cheap way, generally used by China in almost all such low cost sets). One side of the AC line was directly fed to full wave rectifier diodes, and another to this capacitor, which dropped it and fed to the other side of the full wave rectifier diodes. The DC is smoothed by a 1000mfd/10V Capacitor. On/off function is achieved by a PNP transistor, which triggers a mosfet to feed positive voltage to the circuit. I noticed that the set gets on when connected to battery and/or mains and the off button does not function. I checked the input of the mosfet and found that it was getting the pull down for switching off. So, removed the SMD mosfet which had the marking A1sHB. Connected three terminals to the SMD mosfet and checked it on my Peak Atlas:
It showed ‘Faulty component’! So, downloaded the datasheet and checked that it was a P Channel mosfet with Drain-Source Voltage of (-) 20V. Since getting a replacement of this SMD mosfet was almost impossible in this part of the country, I looked through my stock and found that IRF9Z34 is the nearest. Checked the on/off button switch. It was defective. So, replaced it. Connected the mosfet temporarily and switched it on, and the set worked, with on/off functioning properly:
I removed an SMD capacitor that was connected between the Gate and Source and checked it after connecting a terminal on one side:
It was showing 134 nF. I guessed it could be 104 (100nF), so replaced it with a normal cap. I also replaced the 1000mfd/10V cap with a 1000mfd/25V, as the ESR was at its brim. Removed the mosfet and fixed it on the other side of the board, bending the leads to the PCB bottom and connected wires from the PCB:
Set worked with on/off functioning well. Now I turned my attention to dry solder, as I noticed that some of the SMD components were popping off, because of the dry solder! Retouched all the points with fresh lead and flux. Removed the rectified diodes, cleaned the surfaces well and re-fixed it:
Connected to the mains and noticed that charge was in tact and the LED was brightly lit. So, assembled the set back, after connecting the antenna wire and speaker wires. The set worked well, on USB FM and Line in!
Here is the picture of components replaced: (Battery, 1000mfd/10V Cap, Mosfet, push to on switch and the tiny cap (you cannot see it, it is so small) – Total cost of the components came to around Rs.80, which is a little more than one US$)
Another job done to my satisfaction!
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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