I replaced MPSA05 and MPSA55 transistors that I suspected of being reversed, and bingo, the power supply is working again. I’m still upset with myself that I put them in backward, but in the end no other damage was done. Thank goodness.
Now I have a good, stable 5V supply, and I’ve adjusted the output correctly. That means I can get back to the business of debugging the CPU and seeing why the machine won’t run.