weird k7500 SMPS issue and fix

[lilshawn]

Going into a superfast overload "tick" (very faint fast bzzzzzzzzzzzzzzzzztttttt that when you cut the power, gradually slows until the voltage in the primary bleeds off.)

Isolated the SMPS load with 60 watt incandescent on output of L100. (lift D118 and "output side" L100) nothing output.

unload all ceramic capacitors to rule out short to ground (C111, C121, C119) no change

found R104 (10k 7watt) out of spec (read about 16k) and R120 (68K 2watt) completely open. replaced both as well as D117 for good measure.

not sure what caused R120 go open no signs of damage or overheating, just... open. 

and replaced the feedback and startup caps for the SMPS to be sure there too.

Get dim incandescent output from SMPS now. Fluke can't lock onto read voltage just glitches out trying... manually set 400 volts range and read frequency and it's like 102 khz. Filter cap no good.

Replace C115 main filter B+ cap 100uF 200V.

Good bright incandescent light and 120v SMPS output now.