The DC balance should be stable right off the get-go.
Unplug the jumpers. You can leave them out for now.
Check the Balance once again. Report your results.
Even with running the set on the DBT you should get the relay to work.
Now that you know what the replay is doing. When operating the set normally (Without the jumpers), does the relay engage (Albeit ever so lightly)?
Still the NTE components must be replaced.
The relays can be a small turd to replace. If you have a spare relay - wire it into the circuit with only the coil connected and see if it works normally.
I'm worried that the wrong relay is installed, maybe a 48 volt one(?), as the previous owner had made questionable choices. Can you tell if the protection relay has been tampered with?
Try to figure a way to do this and not have the original relay connected. Maybe: Lift a pin (9) from the protection board.
Or just rebuild the protect board and replace the relay.
If you need a relay, below is the bible on the replacement. Lots of images on the web about this replacement procedure.
From my MTF archive:
--------
start of: these models use ONE chassis mounted protection relay:------------------
models:
SX-727 SX-737 xxxxxxx
SX-828 SX-838 SX-850
!!!!!!!!!! SX-939 SX-950
!!!!!!!!!! SX-1010
4pdt
chassis mount using upper ears 24v 653-LY4F-DC24 MOUSER.COM stock level 14 @ $11.61
or a substitute:
dpdt
chassis mount using upper ears 24v 653-LY2F-DC24 MOUSER.COM stock level 51 @ $ 6.55
being chassis mount, conversion from double connected 4pdt to dpdt contacts is easy
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
to mount this relay use standoffs (mouser.com parts numbers) 15mm M-F aluminum,
need two per side for 30mm: 728-FM2110-3005-A due to the ears being on TOP instead of on the bottom!!
stainless preferred standoff from mouser 15mm (need two per side): 728-FM2110-3005-SS
728-FM2110-3005-A is 4.5mm od 3mm x 0.5 thread 15mm male female
aluminum 0.63 /100: 0.43
728-FM2110-3005-SS is 4.5mm od 3mm x 0.5 thread 15mm male female
stainless steel 1.07 /100: 0.74
728-FM1262-3005-A is 4.5mm od 3mm x 0.5 thread 15mm female female
aluminum 0.59 /100: 0.40
728-FM1262-3005-SS is 4.5mm od 3mm x 0.5 thread 15mm female female
stainless steel 0.97 /100: 0.67