Well, that did not go well. Magic smoke. I don't have a schematic for my unit, and it apparently has a different design than the Gottlieb. Anyway, I tried the recharge process using an old 9v 600ma adapter. Cut the ends off, added alligator clips. My mistake was not watching closely, instead I was fumbling with my phone to take a picture. I first saw the smoke in the screen of my phone as I was taking a picture. I got a good picture of the glowing red filaments and the smoke:
I quickly unhooked the alligator clip and investigated. There is a resistor tied into the VFD filaments. It looks like it is coming from the supply side. Maybe it is to limit the current flowing to the filaments. The resistor smoked pretty bad, and smelled bad, to the point it was very difficult to read the rating of the resistor.
I tested the resistor and it read about 3.9 or 4 ohms, and luckily the side of the resistor facing the board was still barely readable, and the 4 band markings tend to indicate 3.9 ohms (orange gray gold gold).
It looks like 3.9 ohms is a common size and luckily the resistor still appears to work (although I should order a new one and replace it). Some of the coating wore off but it looks like all metal underneath which would be appropriate for a 3.9 ohm resistor (as a much higher resistor would have a coiled wire look underneath the cover):
I cant figure out Ohm's Law in this case. If the resister is 3.9 ohms, and the likely voltage feeding the filaments is 4 or 5 volts (typical for a VFD), then that resistor in normal operation is passing about 1 amp of current. It looks like a 1/4 watt rated resistor, which does not make sense because if it is passing 1 amp of current, it is passing about 5 watts. Anyway, since the resistor still appeared to be working, and since I did not have a spare on hand, I reassembled the unit and it appears to be working in the oven with no change in appearance in the display. If the resistor shorted, the filaments would be glowing red and eventually burn out but they are not. I think I will leave good enough alone at this point and live with a dim display.
Thanks for any thoughts. Some pictures attached.