Complex Servicing Of SONY CRT TV With Tips & Tricks For Easy Troubleshooting
MAKE AND MODEL NO: Sony KV-G21PD1 CRT TV
COMPLAINT REPORTED: Dead
HISTORY: This CRT TV belongs to my neighbor. He reported that only dark screen was visible and not even the LED was getting on. I opened the set in their house, did a thorough cleaning of the inside, discharged the anode for a possible charge and dismantled the PCB. Noticed that the board was modified to work from a separate SMPS board which was found tied to one side of the cabinet. The original SMPS in the board has been bypassed by this external power supply. SMPS Transformer, STR along with a couple of other components were missing in the power supply section. Brought the board home for thorough cleaning and troubleshooting.
TROUBLESHOOTING, RECTIFICATION, RESTORATION AND MODIFICATIONS DONE:
Section – 1 – Troubleshooting: Retouched a few dry solder joints found on the Board. Then disconnected the B+ line jumper going to FBT primary, the other end of which goes to Collector Horizontal Output Transistor. Loaded a 100W Bulb in its place and switched on the set. Checked the voltage present at the output, it was ok. As the CRT Tube that I have is not compatible with Sony, which uses Trinitron Tube, pins of which are different, I could not connect the board to the Testing Tube. Brought the TV to my home with the help of the neighbor, in his cycle. Connected the Board with the same complaint that was reported. Checked the B+ and noticed that it was dropping very low. I realized that something was shorting in the FBT section. I removed the Horizontal Output Transistor (HOT) and checked it. It was ok. Looked for any shorts in diodes used in the secondary of the FBT. Could not find any. Connected an LED between the Base and emitter points of the HOT and switched on. The LED remained LIT indicating that the oscillation was taking place. Everything was OK upto the FBT. Suspicion therefore was on the FBT, though the Ring Tester showed ok. It can always fail when a load is given, which is quite common. Removed the FBT, grounded the pin that goes to collector of the HOT through a wire and supplied B+ to the other pin, which then dropped. A clear indication of short in FBT. An easy method to check that. If , however it does not drop, as there is no load on the output, we can connect a 45 to 60W bulb to its 140V output that goes to RGB drive. That is another easy way to correctly check the FBT.
Section – 2 – Rectifications (a): Obtained a replacement FBT and new H-out Transistor. The old FBT had only two wires coming out, one for Anode and another for Focus, as the Screen voltage is taken out through a pin underneath. But the new one had three wires; Anode, Screen and Focus. Loaded the Board to CRT after replacing the FBT and HOT (D1878), with modifications in Main PCB and CRT PCB (described later in the article). The Blue Raster came on indicating that the TV was now working. Discharged the anode and tank capacitor and removed the board for further work on it.
Rectifications (b): Replaced all the electrolytic capacitors on the board. Did dry solder patch up without leaving even one solder point on both the boards. Decided to restore the original SMPS of this Board, for which I obtained the circuit diagram from my Techie friends and also the details of missing components; i.e., SMPS Transformer, Diode and a resistor. Got one spare SMPS Tx from another friend.
Section – 3 – Restoration of Power Supply: Looked for any defective components in the SMPS primary. Found one Diode showing resistance in both directions. Replaced it. Fixed the SMPS STR6707N, Diode D607 and resistor R606. Checked with circuit diagram and ensured that everything was in its place and position. Powered the SMPS giving the load at the B+ rail (of-course, after removing the jumper to the FBT) and was happy to note that the B+ was showing 115V! What a relief it was to see that the original power supply was restored and the hanging external SMPS board was relieved of its services. Imagined the happiness of a person who got relieved of his externally hanging bypass systems after restoration of an important organ!
Section – 4 – Modifications done: Removed one end of the High Voltage diode from the FBT at the Main Board so as to disable the supply wire that goes to the CRT PCB. Disconnected one end of two resistors on the CRT Board and ensured that the Screen Voltage Wire of the new FBT is linked to Grid 2 of the CRT, bypassing the potentiometer on it. Replaced its filter capacitor as a preventive maintenance.
TESTING DONE: After checking that everything was ok, restored the B+ supply to FBT and loaded the board to CRT. Connected a video input. Adjusted the Screen and Focus voltages to get a peak performance. The picture was so clear like a new TV. Thus a complex servicing of a CRT TV came to an end and satisfaction intensified got added to its collection!
USEFUL AND CONNECTED LINKS: All about CRT tubes : https://www.repairfaq.org/sam/crtfaq.htm
How To Completely Check The High Voltage Circuit In CRT Moitor/Television: http://www.electronicrepairguide.com/high-voltage-circuit.html
Recommended books : https://jestineyong.com/crt-tv-repair-ebook/
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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