Help identifying pwm controller "67A"
For the TH20594:
I can find references (in russian), here  that states the TH20594MC1.4 is a PWM controller, which makes sense in this context.

I can't read russian (I used google translate) but it seems to indicate that there are similar replacement parts with different numbers, perhaps clones by different manufacturers, e.g. TI UC3845.  Tracing out some of the components and comparing with a typical PWM control IC might give evidence to this possibility.

You can find sources for the TH20594MC1.4 in lots of places in China, but I could not find any datasheet (and it would be probably in Chinese anyway, perhaps why a doc search fails)

For the '67A':
The closest I can find that matches is a 67(space)A, where A is the date code.  This is an ON semi quad TVS diode, NUP4302MR6. Which given the product I doubt is correct, but perhaps I bit of probing with a DVM might yield more info to verify (negatively or positively) this.

I would better guess that given product and the sorts of components around it, it's an opamp or comparator. 

Sorry for the incomplete answer, it's not possible to put a so much text in a comment.
Can give more info, e.g. Trace out the circuit and measure resistance between pins (in both directions).