After many years of sitting idle, the batteries became corroded and inoperable. Removed the old pack by popping the top on the HX-20 and removing battery cover plate. Unsoldered and put aside the old wiring harness for later use.
They were in pretty sad shape. Found suitable replacements at Fry's. Tenergy 2200mAh NiCd (with tabs) - they came in a pack of 6.
Connected the tabs in series, and shrink wrapped them together. Re-soldered the wiring harness. and duct taped them into a package of 4 just like original pack configuration.
Attached adapter and she fired right up. Reset the clock and all is good to go for another 20 years.