Not that I recall - I've been pretty careful handling this thing. EDIT mode works fine, but that's not much help. :-(
***EDIT*** - See the following post, it may be as simple as a stuck EDIT button on the front panel. If not, read on.
OK, so as far as I can tell, the _DUMP line is a signal coming out of the processor, not going in. It does two things: disable the right reel motor, and also sends M-STOP to the capstan servo - presumably telling it to keep the motor running even though the right-hand tension arm has gone slack.
Assuming the machine really has gone into Dump mode, it either means that something is accidentally pulling the _DUMP line low, e.g. a piece of something has shorted it to ground, or else the CPU is deliberately pulling it low for reasons we don't understand.
Are all the controls on the command panel working, e.g. RTZ, LOC1 etc? If so, that would probably rule out a phantom keypress from the control panel. I'm assuming you've turned the machine off and on again just in case it's a transient fault?
If it were me, I'd first verify that it's not something mechanical like the brake drum seizing up - if the capstan motor is still turning while the tape is being dumped, it's probably a good indicator that it's somehow got into Dump mode because the right-hand tension arm going slack usually shuts off the capstan. Running the machine with the front panel off may help too, so you can see what the brake mechanism is doing.
If that doesn't yield further clues, I would take the back off the machine, flip down the control board (remove the 3 screws at the top) and have a close look around the big chip, U9 just in case something has shorted out pin 30. U17 and U29 are also implicated.
It might be worth reassembling the machine afterwards and testing it again, since if it really was a piece of metal swarf the act of inspecting the control board will likely have moved it elsewhere.
After that, I would get worried because we'd be going into "Find out what the logic levels is on pin 30 of the CPU" territory and that would be bad.