[morzh]

You cannot check the cap in-circuit, you have to take it out.
Or unsolder 1 pin (any pin) of R5 and then you can check the cap in-circuit.
It is C1 and is the only one electrolytic cap in the whole board it seems like.
measure the capacitance and if possible the leakage.
If you have any new cap of comparable (does not have to be exactly 40uF - could be 30, could be 100) and the voltage rating can be as low as 10V, so it is a cheap junk cap - simply replace it and see if it helps.

Watch the polarity. It will not blow up but the reverse polarity may lead to distortion.