The X-Sustain board and the Y-Sustain board are common problems on these Vizio P50HDM and P50HDTV sets. The Y-Sustain board (P/N: 6870QYC004A 6870QYC004B 6870QYC004C 6871QYH039A 6871QYH039B 6871QYH039C, 6870QYC104A, 6870QYC104B, 6870QYC104C, 6870QYC104D, EBR30597501) is the most common failure on these TV sets. You will normally hear a pop noise or a loud zapping noise when either of these Sustain boards (or high voltage boards) go bad.
The power from the power supply flows through the Y-SUSTAIN first then to the X-SUSTAIN (P/N: 6870QZC004A, 6870QZC004B, 6870QZC004C, 6871QZH044A, 6871QZH044B, 6871QZH044C, 6870QZC104A, 6870QZC104B, 6870QZC104C). If you unplug the X-SUS, you should still get power to flow through the Y-SUS...
Power Supply sends ~196V DC to the Y-SUS,
Then the Y-SUS sends ~196V & 60V out to the X-SUS.
Have the technician check with that information to see if the Y-SUS is sending the proper voltage to the X-SUS. There you should be able to tell whether one board or both boards are bad.
In very rare cases, when the X-SUS goes bad, so does the Y-SUS (and vice versa)... And at times the lower buffer boards on the TV short out to cause the X-SUS to go bad.
In short: 3 - Common problems with these TVs:
1 - Most common = Y-SUS and can be found here for $154.99:http://www.discount-merchant.com/Y-SUSTAIN-PN-6870QYC004A-6870QYC004B-6870QYC004C-p/vizio-sku9501.htm
2 - 2nd Most Common = X-SUS and can be found here for $154.99:http://www.discount-merchant.com/We-provide-factory-original-parts-worldwide-p/vizio-sku9502.htm
3 - 3rd Most Common (but rare) = Mid lower buffer board
I haven't found anywhere else to sell these parts close to this price with the same quality and peace of mind as Discount-Merchant.com... I highly recommend them.
Let us know how it goes.