Electronics > Repair

[Solved] ChangHong LCD/LED TV boot loop repair.

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shakalnokturn:
This 28" LCD TV had been picked from the dumpster about a year back.
When first powered it worked well, after a few hours test on a restart it went into a boot loop. Sometimes leaving it loop long enough it would finally work.

The board used in this TV is TP.SIS231.P83.
A quick search on internet returned that corrupt / defective SPI flash was common, one of many reports here:
https://www.eevblog.com/forum/repair/broken-tv-(teac-lev3992fhd)/msg1084131/
As the symptoms, matched mine I went looking for a firmware dump and new EEPROM.

The ChangHong website has the latest firmwares updates available if you have the barcode and serial number on the back of the TV, mine didn't. Looking through pictures on internet I found a couple of serial numbers of similar 28" models and downloaded what seemed to be the right firmware. Updating the original EEPROM through USB made no difference so I wrote a new EEPROM with the universal programmer. TV would come on with access to menus but DVB-T reception was unusable my conclusion was that the FW wasn't the right version... TV got pushed aside for a year or so...

Clearing-up a little the other day pushed me to have a last attempt before letting it go for good. This time I started with the basics (as one should rather than looking for the answer on internet before thinking...) and checked the power rails.

A couple of LDO regulators had slowly varying 3.3V on their input and not quite stable 2.5V outputs (tested with DMM).
I couldn't trace where the 3.3V supply was until I downloaded a similar TV's schematic (TP.SIS231.PT85) which showed that my unstable 3.3V should have been 5V.
5V switching regulator was giving 5V, the problem was P-MOS QM32 (WPM2341A) used for 5V standby switching near LVDS connector.

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