This TV was flashing a 2 blink code which is a power supply error.
A service and training manual can be found here.
Power supply board part number is APS-299 (1-474-303-11 147430311). Searching on ebay found repair services and repair kits for this board.
Posts on badcaps indicated potential shorted transistors Q6101, Q6102 and open resistor R6540. When i tested these parts, all three had failed.
Q6101 and Q6102 are shown in the schematic as TK5A50D. These are 500V, 5A N-Channel MOSFETS. I had FDPF7N50U (Digikey #FDPF7N50U-G-ND) spares from a previous repair. R6540 is listed as a 0.1 ohm 1/2W resistor and I temporarily substituted the part with a 0.47 ohm 1/2W fusible resistor I also had. Digikey substitute would be 0.1DMCT-ND (0.1 ohm, 2W fusible).
With these 3 components replaced, the TV powered up. This failure is similar to a previous repair on a Sony KDL-46EX400 here.
This video describes repair of a 2 blink code on a KDL-46EX523. The boards are very similar but the designators and transistors are different.
Update: I had another TV with a 2 blink error but the owner tried to repair it himself and ended up blowing R6001 (15 ohm 5W flameproof cement resistor). He had tried two other boards but they were both dead.
The fault was eventually traced to shorted FETs Q6602 and Q6603 (STP9NK50ZFP, N-CH, 500V, 7.2A, TO-220FP, Digikey 497-12057-5-ND) on the G8 board which is a separate power supply board for the LED backlight. It is powered by the PFC output on the main board. The input to the G8 board was shorted, R6540 was open and Q6101 and Q6102 tested good. Both R6540 and R6001 protect the PFC 400V output so either can blow with a shorted G8 board. It appears if R6540 blows, you will get 2 blinks but if R6001 blows, the TV will be dead.
The other boards the owner purchased were a slightly different part number 147430411 (APS-299W) vs the original 147430311 (APS-299). The difference was two additional connectors by the AC plug that are populated. I believe they are for an external hardwired switch and jumpers JS6000 and JS6002 need to be installed to work in the 55EX620 or else there will be no AC on the rectifier.