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I last repaired/re-calibrated one of these nearly 30 years ago but in my notes I did not come up against a fault in the section you are interested in. I did however come across a 2215 and a 2235 with similar problems in their power supplies that may be of help/interest if your scope should have similar problems.

Tests I made on faulty power supply of a 2235.

Both pre-regulator FET Q9070 and its driver Q908 were blown to pieces.
After removal of these devices, a power supply was connected across C940, Positive connected to the lug which has print traveling to R945 and C943.
While carefully watching the current that is being drawn as the voltage was slowly increased from 0V, it was possible to power up the scope to a final power supply setting of +40V at 0.8A.
With the external power supply current limit set to just before limiting, the supply voltage was increased to approximately 52V when the crowbar operated (Q935), sending the remote power supply into fold back current limiting. Thus it was possible to prove basic oscilloscope operation and the over voltage protection circuit.
The external DC supply was now applied to the cathode of diode CR920 in order to prove Q930, Q928 and the pulse width modulator chip U930.
However, the supply tried to power up the main inverter so it was necessary to remove the control chip U930. With a Multimeter connected to pin 12 of the vacant I.C. position, the DC supply was increased from zero. The Multimeter climbed slowly then suddenly jumped to +14V. This action could only be repeated by first removing the supply and constituted a reasonable test of the circuitry around Q930, Q928 and the Zener diode VR925.
After checking the remaining diodes CR920, CR908 (FOUND SHORT CIRCUIT), it now remained to replace the chip U930 and blown transistors prior to re-applying the mains supply.

Model 2215

This fault appeared to be similar to the 2235 above. Internal circuit board appeared identical.
The FET appeared OK with a resistance check. Investigation showed two diodes in the bridge rectifier had gone short circuit, these being CR904 and CR902 on the negative side of the bridge.
U930 and the FET were removed so that the 0 to 40V test could be done using a remote power supply connected across C940. After adjusting the front panel controls it was found that the oscilloscope was working OK. So as before for the 2235, positive of the remote supply was connected to CR920 cathode. The slow start circuit can now be checked (an AVO is best used for observation) where a slowly rising voltage should be observed until 14V is reached, when there is a quick upward swing. This oscilloscope however showed no swing and Q930 was found very ill. This TEK 151-0164 is a 2N2907A PNP Transistor and after replacement the slow start operates.
Replaced U930 and monitored AC current whilst oscilloscope was powered up on a Variac.
The +40V rail measured as +42.5V and the oscilloscope was working.
NOTE:
Use only sufficient voltage to check the Slow Start circuit, it is not a regulator and a maximum of 15V is recommended. The diodes were 1A rated and were replaced with 1N4007 as they have a higher PIV rating.