For the sake of helping anybody else who may run across this particular piece of equipment and have this particular problem with it I will record the solution to the "error 171 - channel X overloaded" on my HP 3326A function generator.
Being relatively new to the repair game it took me some time to work through everything and get a sense of what was going on, and what was causing the failure. As I mentioned earlier, I narrowed it down to the high voltage board (opt 002) for channel B.
The symptoms showing on the board were:
A: Zener diode CR117 had failed
B: Pico fuse F502 was open
C: Capacitors C113 and C114 were shorted to ground
It took me a while to get see that the capacitors had failed as I was playing around with the fuses and zener diode (which I found first), thinking that they were somehow at fault (rather than a symptom). Pico fuse F502 is not shown in the attached diagram, but is in series with the -28V circuit coming from the power supply.
In hind sight it is fairly easy to guess what was may have happened. Somehow someone overloaded the output and this shorted the caps on the output (C113 and C114). This internal short caused the perpetual error on the generator.
Of course as everybody has already mentioned, the answer was a short on the output. Just took me a while to find it
. Attached is the relevant part of the schematic. Hope it helps someone.