A floating supply isn't necessary if you have a negative-output supply -- just connect the external PS ground to the PB's ground, and you'll be fine. Or rig up a few batteries (say, a couple of 9V batteries and a couple of AA cells, say).
If adding the substitute supply causes an image to be displayed (you may see a somewhat "ripply" display; don't worry about that -- all we care about at this point is an image of some kind), then the -21V panel bias is your problem for sure. Measure the current being supplied. If it's significantly greater than about 10-20mA, your display is excessively loading the boost inverter, causing it to go into current limiting. If the current is low, your boost inverter is simply dead.
I recall tracing part of the inverter, and there's very little there. There's one impossible-to-find chip, and a handful of discretes. All I can say is, hope that the problem isn't the chip, because you will have a tough time finding a replacement (other than scavenging one from another PB100).
If adding the substitute supply causes an image to be displayed (you may see a somewhat "ripply" display; don't worry about that -- all we care about at this point is an image of some kind), then the -21V panel bias is your problem for sure. Measure the current being supplied. If it's significantly greater than about 10-20mA, your display is excessively loading the boost inverter, causing it to go into current limiting. If the current is low, your boost inverter is simply dead.
I recall tracing part of the inverter, and there's very little there. There's one impossible-to-find chip, and a handful of discretes. All I can say is, hope that the problem isn't the chip, because you will have a tough time finding a replacement (other than scavenging one from another PB100).