Servicing ONIDA Slim 300 CRT TV
This CRT TV was brought to me in a dead condition. On enquiry with the customer, I learnt that it was having some lines appearing/shaking problems for quite some time, after which it became dead. As usual, I opened the set and subjected to thorough cleaning in and out. Discharged the anode wire that goes on top of the CRT, slowly disconnected the CRT earth wire, CRT PCB, Mains connector, Degaussing coil connector, yoke connector, front panel connector etc. one by one.
On inspection of the PCB, I noticed that the Fuse was blown, board had burnt marks under the Degaussing PTC and a lot of dry solder joints. I removed the PTC to study the extent of damage:
As you can see, the burnt marks were worse! So, I did a thorough cleaning of the spot with IPA and scraped the area to the extent possible. The picture below is that I took when I was half way through, but forgot to click it after I finished:
Then combed the board for any bulgy and/or ESR out capacitors and found many. One Blue Fixed capacitor reading 152, 2KV in the SMPS section had blown (According to my technician friends, this is a common fault in this model)! So did my favourite work of replacing all these capacitors with new ones, followed by thorough retouching of ALL the points on the CRT PCB, Main PCB, Panel PCB etc. which is my routine, whenever I find more than 50% dry solders. Replaced the PTC with a new one. Did a thorough cleaning of the PCB using IPA. Connected a 40W bulb in place of the Fuse, disconnected the secondary B+, which was 125V in this set, that goes to the LOT/FBT and connected a 100W Bulb as load for the secondary, and switched on the set. The 40W/230V bulb light lit up momentarily and went off indicating that all was well with the power supply. So, disconnected and discharged the high voltage capacitor, which was 330uF/450V and replaced the fuse. On switch on, the 100W/230V bulb remained lit. The standby LED was also lit. Checked the secondary voltage and found it to be 125V and the marking on the secondary voltage controller used in the circuit also confirmed it. Disconnected, and discharged the high voltage capacitor. The secondary residual charge if any would have got drained because of the 100W load that I had given.
Connected the board back to the CRT and put back all connectors and ensured that the earth wire to CRT PCB is properly inserted and has a firm connection. Before connecting the anode wire, once again discharged the CRT anode with the earth, as CRTs develop mild charge on its own, probably because the CRT with its anode and earth coating outside serves as a big capacitor and retains some charge in spite of discharging it. I heard a ‘putt’ sound, and felt happy about the precaution I took! Such residual charges can give a shock that might find its way through up to the arm pit or even the chest area and we might experience numbness for some time! Just imagine what will happen if we touch the area when the CRT is on! Switched on the set after re-checking all connections and immediately smelt some plastic part was burning and smoke came from the SMPS primary section. Switched off the set and keeping my body parts safely away from the PCB and CRT anode area, did a ‘dog’ smelling to locate the spot, which most probably was from the Degaussing Coil side. Touched the PTC and got my finger almost burnt as it was very, very hot! So, either the Degaussing coil is short (very, very rare) or the new PTC is a defective piece or of poor quality. Disconnected the degaussing coil and checked it with Ring Tester and found it to be ok.
It was showing a few ohms and not a dead short. The suspect therefore was the new PTC itself. Once again went through the process of removing the Main PCB (I always prefer to work on the PCB isolated and kept on my table rather than tilting it up straining almost all connectors, which can cause lot more problems either immediately or later! Moreover I have to strain my neck too!) Replaced the PTC with one collected from my technician friend, who removed it from a discarded Panasonic CRT Board. Reconnected the PCB following all precautions and switched on. The raster came on, but had some noise lines on it. (That much for the quality of new PTC available in the market now-a-days!)
Tried changing the input mode to AV, using the Remote. Nothing happened! Tried using the panel control, nothing happened! So, I knew there was some problem in that area. So, dismantled the PCB for the third time and probed that area. Replaced the suspected sensor. Found data track from sensor/panel to uController showing resistance of a few ohms. So, bypassed it by a thin wire. Checked SMD components around that area and checked the resistance from the sensor to the uController pin to match the value of resistor connected in the path. Found this to be ok. After this detailed and thorough check up, once again mounted the PCB. Switched on. The TV started working properly, and responded to panel but was intermittent to the RC. So, fetched a replacement of the RC from the market and checked. The set worked perfectly well!
Left the set on for almost 18 hours and could not find any problem. These are the components replaced. (You can notice a cracked PTC and Fixed Capacitor)
Another marathon service work came to an end, providing a lot of satisfaction and knowledge full experiences!
I wish to add my sincere thanks to my technician friends who provide sensible advice whenever I encounter strange problems in my service! In fact, a good credit should go to them for what I am capable of doing today! (The precautions for handling CRT TV contained in this article are intended for novices and for those who handle these occasionally like 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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You may check on his previous repair article below: