The Harbach soft start module for the SB-220 should work. http://harbachelectronics.com/shop/heathkit-sb220-sb221/ss-221-soft-start-module/
Just curious. Did you replace the 40uF capacitor at C3?
Thanks Bob. The Harbach kit sure is a good solution for soft-start, and I sure could have used it when I had an SB-220. WRT C-3, yes, actually I have two NCX-A's and one has a fresh C-3, the other has the original, and both spike the 280v supply although the steady state ripple is very low. I think the spike is due to an initially unloaded buss on the NCX-5 at power-up until the OA2 kicks in with the regulated B+, and the numerous stages using the unregulated +220v come to life.
I found a couple of gas discharge tubes (GDT's) in my parts bin that are rated at 300v breakdown (nominal). I'm going to try one of them, knowing that the GDT's have a wear-out or limited firing life. The "+280v" supply current is limited to under 2 amps, so maybe I can get some reasonable life from the GDT's. The better (longer-life) solution would be the Harbach soft-start kit, but I'll see how the GDT scheme works out. I'm surprised that the OA2 has survived the pounding that it gets at each power-up.
Thanks again, and I'll post results of the GDT scheme for those who may have an interest in solving similar power supply secondary surges.