I would caution builders to make sure that the chassis surface under the soft thermopads beneath the power transistors is very clean and smooth. I had a nagging "loss of insulation" problem that was resolved when I discovered that a fine, hardly visible, aluminum burr on the chassis had actually cut through the pad to short the collector tab.

Also (by the way) I've noticed that the latest Mouser catalog (Cat. 621) refers to an incorrect illustration and pinout for the NEC relay type MR-82-12USR (K-1 on the main board). The part number for ordering is correct (551-MR82-12USR, and the illustrations themselves are actually labeled correctly.