I acquired one of these back circa 23 years ago. According to the knowledgeable seller it worked but had a bad time-base.? Ha!
As some of you know, this engineering marvel has >100 SSI/MSI TTL ICs.? It turned out that FOUR of the IC's were defective.? All four were manufactured by National Semiconductor. Three of the 4 were SN7402N (HP 1820--0328) and the 4th was a National Semi 74160 (HP 1820-0899).?
See the trend ??
If I recall, National Semi had a case of the "Purple Plague" in the 1970s, and these failures were likely due to this.
A failure analysis was not done on the parts, but it seems to be more than a coincidence that all of them were made by National.
Dick
Dick Benson