The Elusive STRF6456
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Author: Ismail Msheikh
Have you searched the Internet for the elusive STRF6456 ? read this
I was recently called to repair a brand new Zec TV ZT296ST in Nairobi, Kenya, which was perpetually stuck on standby. My first suspicion fell on the power supply. I had no service diagram but voltage checks on the outputs of the chopper transformer showed the HT was at 135v which looked within range for a 29" chassis. The others auxiliary output measured fine. Things remained unchanged with a dummy load. The frequency of the line oscillator was present but the screen remained blanked. I advanced the screen pot and was greeted by retrace lines with a severe pincursion distortion. The OSD was present. There was no remote control function and the preset control function were not working.
Suspicion now fell on the the processor which was being fed a good 5v by a fixed regulator. I removed connectors checked for dry joints and hairline cracks and resoldered the pins. The set came to life albeit with an increased height. I switched off the set and on again but the set could not come out of standby. I then went on a blanket replacement exercise, distrusting all ESR meters, I replaced and upgraded to 105 degree all capacitors standing near the processor and the EEPROM. The set remained on
standby. I now did what cured the fault - resoldered the chroma chip. I was greeted by a beautiful raster and clear picture but the pictures on the screen were minus their heads, there was an EW fault too.
No-one could tell what the preacher on the TV was holding! was it the Bible? my headache had just began!
I tried reducing the B+ but the power supply had no pot. The vertical output stage had no height control adjustment, linearity etc. There was no horizontal hold or centering pots. I tried things with the vertical section, adding resistive loads to get a clue of the source of the fault and checked all passive components, but the fault was not in the vertical section and no remedial action could set back things. I examined three bulky ceramic capacitors all connected in parallel each of 2.2nf 2kV. They read fine on
the ESR meter. I decided to remove one and the picture size was reduced considerably. I was about to say repaired! and the power supply fuse was shattered, STRF6456 shorted, all caused by the reduce capacitance in the line tuning section which killed the line transistor. It was one of the many
The following morning I discovered to my chagrin, that no distributor on the globe had the SMPS chip. I visited the chip manufacturers web site, Sanken.the company described it as quasi-resonant chip for secondary isolated SMPS, no further details. I visited the web site of Allegromicro which also deals
with Sanken products. The chip was not listed anywhere, or anywhere else on the Internet, at least in English. Allegromicro (www.allegromicro.com) were however helpful.They have a pdf file of the superabundantly available STRF66XX series with good information on its configuration and specs. I
chose STRF6656 and compared it with the layout of my power supply, they looked the same. To allow the chip to operate on AC 85-265 volts I reduced the startup resitor connected to terminal 4 of the chip to 68K 2watts. The power supply worked well and it still works.
The cause of the height and EW error was TA8859CP chip described by Toshiba as a "deflection distortion correction chip" for the "flat" screen. Long live information sharing!
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