The 1N5355 helped with the -18voltage, but the positive voltage still was too low. I resolved the positive voltage by changing the dropping resistor down to a value of 20 ohms. I also connected the center tap of the transformer secondary to the ground. With these three changes, the positive and negative 18 volts seem pretty stable.

However, my next problem is that the second channel on my Tektronix’s 465 oscilloscope has become intermittent. Looks like something is wrong on the vertical preamp board. A few hours of debugging hasn’t revealed any obvious component fault. Seems like an intermittent connection somewhere close the input jack is the culprit. I can’t very well test the SCELBI oscope interface until I get both channels working again, so I’ll have to keep working on it.