> internal ~100 ohm resistor between the output and sense lines
The latest service manual shows 10K not 100R. Which could be different from the old units I've got.? These resistors are in series with 243K resistors which drive the "virtual earth" points of the sense amp. Expected error with no links ~4%.
Anyway, out of a sample size of three old units (two of them mine, one was someone else's) all of them showed a significant error without the links, so it would be prudent to make sure they're fitted.?
That PSU had a remarkably long lifetime: 1993 to quite recently (was listed by Farnell until a year or two ago).