Then the components in the table in the document at Pinwiki are suspect. It could be one of them, or one of the resistors (R1 I believe.)
Can you do board repair? If so, order the components in the table from greatplainselectronics.com and you'll be back in business. If it were me, I'd replace all the Zeners (ZR1-4), and the power resistors at R1 and R4. Be ready to replace some of the other components (Q1-4). We're talking a few dollars worth of parts, not much, so buy everything and replace the common components that fail while you are at it.
If your displays are nice and bright, you can lower the voltage to -91 and 91v by replacing ZR2 and ZR4 with 1N4763A diodes instead of 1N4764A diodes. This will increase the life of your displays. If your displays were dim due to age, put back in 1N4763A diodes to keep the voltage at -100 and 100v.
Also be aware that the SDS201/SDS202 in your power supply at Q1 and Q3 are not available any more and the replacements are MJE15030 and MJE15031. When using the replacements, you need to cross the legs. PM me and I'll give you details.