FG504/Repairs

Some personal recommendations:

the red var-knobs should be handled with special care. Even isopropanol will damage the lettering.
the big frequency scale seems also to be very sensitive. No isopropanol recomended.
C1630, C1730, C1665, C1765 tend to get a high serial resistance. less than 1 Ω seems to be OK. For replacement I recommend less than 0.1 Ω. The manual allows up to 87,5 µF instead of the built in 50 µF. If there is a problem can be ckecked by doing the adjustment procedure stepp 4: Check Power Supply Ripple and Noise.
the mechanical part 165 can develop whiskers. I recommend cleaning it with a metal brush and lacquer it. Lack will not stop the growing of whiskers but slow it down.