FBT Found Failed In Samsung CRT TV + Tips And Tricks For Troubleshooting CRT TVs
MAKE AND MODEL: Samsung CRT TV Model # CS21C370FL
COMPLAINT REPORTED: Dead
TROUBLESHOOTING AND RECTIFICATION DONE: Opened, dismantled the board after discharging the anode and cleaned the inside and board thoroughly. Looked for any visible damage to any of the components. Did not find any. Did the ring test on the SMPS, which showed healthy. Same was on the FBT too. Disconnected the B+ rail to the FBT, connected a 100W Bulb as load, switched on. The B+ was normal and steady. The Horizontal Output Transistor (HOT) was ok on checking. An LED remained LIT when connected to Base and Emitter of HOT, indicating that the Horizontal oscillation was taking place. Looked for any defective diodes on the secondary of the FBT, did not find any. Reconnected the B+ rail to FBT, loaded the board to CRT and switched on. Set did not work and the B+ dropped indicating a short in the FBT. Discharged the anode and tank capacitor for any residual high voltages. Disconnected the PCB and removed the FBT. Connected one end of the primary that goes to the Collector of the HOT, to ground, and applied the B+ to the other pin. Voltage dropped, indicating that there indeed is a short within the FBT. As mentioned in yet another article, this is a sure method to check the condition of the FBT. If the voltage does not drop, we have to give a 40 to 60W Bulb Load to its secondary that goes to RGB supply on the CRT Board. If the voltage drops, then also the FBT is defective for sure. Another method is to check the current drawn by FBT, by measuring it on the point where B+ enters the FBT primary. If the current is beyond 150mA to 250mA, the FBT or its secondary loads (if checked without removing the FBT from the board) is defective. Better to go for a change. Even if there is a leak in the HOT or its surrounding components, more current will be drawn. Another tip is to provide heat zinc to the HOT (that has no heat zinc) whenever we change FBT as the replacements available in the market are not having good quality and might draw a little more current resulting in overheating of HOT. (These are certain tips and tricks that can be useful to new comers).
Obtained a replacement matching its number, fit the FBT and checked that the TV was working. After dismantling the PCB once again, did my routine maintenance of replacing all the electrolytic capacitors on the board and re-touching all the solder joints on main board and CRT PCB. Rechecked all the electrolytic capacitors to ensure that these were fit correctly following the polarity marked, before soldering the caps and cutting the leads. (This is a tip to the new comers. This is very essential so as to avoid a human error and possible damages when power is applied) Then cleaned the board thoroughly using IPA, to clean the residual flux that can cause problems later due to formation of capacitance. Reconnected the board, switched on, and I was thrilled to see the result. But I noticed that there were slight gaps on both sides. When I checked with my techie friends, one of them wanted me to check whether the button magnet on any of the PIN AMP coils was missing. He said that usually there will be an additional magnet and sometimes this could fall off when we do cleaning using blower and/or brush. That was exactly the case in this TV. I picked up one from a discarded board, from the top of the horizontal coil and put it on the PIN AMP coil of this TV. The width became perfect. (This is yet another tip for those who are doing servicing of CRT TV. Before jumping to adjusting the width by entering service mode, we should ensure this is checked first) Mission accomplished and satisfaction added, along with new lessons learnt!
For the sake of new comers and those who are interested in learning further on CRT TV service, kindly buy these books and see the links: (1) https://jestineyong.com/crt-tv-repair-ebook/ (2) https://jestineyong.com/crt-television-troubleshooting-guide/ (3) https://www.repairfaq.org/sam/crtfaq.htm (4) http://www.electronicrepairguide.com/high-voltage-circuit.html
Here Are Some Pictures:
This article was prepared for you by Parasuraman Subramanian from India. He is 70 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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