I have had such problem with two six-years old APSsl. I received informal help from Tadiran tech support - they suggested to change capacitor 211 on APSsl board. It is originally 25V 100mF, and have to be replaced with high ripple current, low E.S.R and long life capacitor, which value should be 150°C, 100mF, 35V, 20% (Nippon Chemi-Con - capacitor LXJ-35VB120MH12, Jamicon - capacitor WGR101M1VF, Nichikon - capacitor PL-1V101MPH).
By the way, Tadiran have good specialists - tech support gave me fast, clear and correct answer - I have changed those capacitors and now everything is good
