Looks like someone performed a previous 'repair' and replaced the original Stabilyt cells.
Stabilyt cells have a typical life expectancy of 2 to 3 years and been discontinued for nearly 50 years. These replacements (1,5/300 = 1.5V/300mA) are certain to be dead by now also.
At the time, Neumann made available for a brief transition period a solid-state replacement board SK33d, before redesigned successor supplies were introduced, among them NN48b. With modern voltage regulator devices more effective solutions are possible.
For the rehabilitation of vintage power supplies for use with the precious AC701(k) tube I have developed another 'Stabilyt Replacement' circuit with gently ramping supply which reduces stress on a cold tube filament. While working on these power supplies it does make sense to examine the high voltage plate/bias voltage stabilization and consider replacing the gas discharge tube (150B2) with a suitable 150V/1W Zener diode.
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