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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Albert van Bemmelen
November 1, 2016 at 4:33 pm
I wonder how this Audio Set without safety Transformer is sold with only a brief warning that it is dangerous to use it while charging? It is really bad publicity for the Manufacturer. When its USB or other parts like FM antenna are touched it can do more than only give you an electric shock when it costs you your life! No doubt a good repair Parasuraman Subramanian. Changing the charger circuit to a safer design would be almost impossible even for a meticulous and patient person like you are.
I recently finally also bought two fast new PEAK testers, being the SCR-100 for Triacs/Thyristors with Gate currents upto 100mA (because it is also very dangerous to check all those 600V SCRs in primary HV circuits while they are powered on), and the DCA75 Pro that tests Transistors and Diodes with the extra integrated curve tracer with PC software on a USB Dongle. And because the DCA75 and other Transistor Testers only are able to test low Gate Triacs/Thyristors (SCRs with gate currents less than 10mA) we also need the SCR-100. And I love the fact that these good PEAK ATLAS Testers work on only a single cheap AAA Alkaline Battery!
November 1, 2016 at 5:04 pm
Nice modification Parasuraman, as regards the mosfet replacement. I like it.
November 1, 2016 at 5:40 pm
The tricky effect with this type of power supply is that it is not so dangerous when it works fine. This is due the fact that, despite the direct capacitor's connection to the mains, the output voltage is pulled down by the load and kept there in very low levels,usually under 12V. The problem exists when the output is open-circuit or the capacitor gets shorted. In this first case the full line voltage is there,at the output, with the capacitor's role being to just limit the output current to a low level, while in the second case there is no current limit at all and all depends upon the line fuse...
Therefore, although the normal output current level of these supplies is usually of the order of no more than 100mA, it can be fatal. Home RCDS are tripped at 30mA...
This finally means that, overall, your comment about installing such a power supply in a consumer appliance, at least without a warning for the user, is not right at all. I certainly agree with that.
November 1, 2016 at 4:52 pm
Great Artical Mr Subramanian
I liked the part where smd part are replaced and modified.
very detailed explanation and nicely presented.
Thank you for this post.
November 1, 2016 at 5:59 pm
Good Fault finding and SMD Replacement.
very good Job !
November 1, 2016 at 10:30 pm
Good Job! Sir.
November 1, 2016 at 11:07 pm
Hi Parasuraman, good repair and nice explanations.
November 1, 2016 at 11:54 pm
Nice repair thank yoo
November 2, 2016 at 5:52 am
As usual your patience & persistence has paid off.
November 2, 2016 at 8:33 am
Thanks for the article, Parasuraman. It will be easier to check the SOT-23 components with this little adapter from Peak Atlas: http://www.peakelec.co.uk/acatalog/pca23-sot23-test-adapter.html
November 2, 2016 at 12:49 pm
Hi Parasuraman, thanks for sharing this heroic repair. Most people don't read the warnings on back panels. I can imagine someone with the earphones on, in a wet environment.. frightening.