As shown in the attached picture, I removed CN1-4 connected to the board from the display and tested it, but again, +15V is lowered to +3.9V.
So, I cut off the +15V pattern line going into T1 (high voltage transformer) and turned on the power again, but it is still lowered to +3.9.
(As the situation progressed to this point, it also made me think that it is normal for the voltage to be lowered. If the voltage is lowered, IC4 cannot be driven.)
Now all that is left from the +15V supply is IC4, C8 (35V, 100uF, ok), D2 (Zener, ok), RA2 (10K, pin3 to 1, ok), TR1 & 2 (2sk739, N-Channel FET, short Since it did not work, it is judged as normal)
Now, as a final decision, I decided to replace the IC4 and applied for 5 units of $10 to Ali.
If you have any advice on the process I'm going through, I'll listen to anything.
Thanks a lot for your advice.
If +15V is lowered to +3.9V even after replacing IC4, the cause is somewhere else.
(If you look at the picture, you can see the photo coupler, PC1 & 2, and also the black mod chip, but the situation is complicated. The fact that there was also a bad line pattern contact in the electrolytic capacitor 10V, 470uF, which was replaced with a leak before, leads to the failure of many parts. Makes me think.)
Therefore, if it fails to replace IC4, it is concluded that it cannot be touched for a long time.