To test the voltage regulator's functions, you can do the SCR crowbar test without having to trip a large fuse or breaker.
The SCR crowbar section can be powered up and tested separate from the regulating/field-winding portion. Crowbar gets power from either BUS or SENSE (via D8,D9) but monitors BUS voltage via R15.
So a current-limiting light bulb (i.e. 1157) in series with your power supply to BUS, no load on the field, SENSE I would run from the PSU output ahead of the light bulb.
Increasing BUS >16.2V should trip U2/Q4/Q3/SCR. There is also OV_TEST point to inject an over-voltage trip there.
Once the SCR trips, it crowbars BUS down to a volt or so, and SENSE would have to be up to keep the circuit powered, although that doesn't matter once the SCR is latched. It does not seem to activate the blinky UV warning lamp.
There are six zener diodes for over-voltage/reverse-voltage transient protection.
1N4752A 33V 1W
D5 protects power to U1, D12 protects power to U2.
D3, D4 protect U2's inputs (in theory) but 33V is too high to actually protect the IC, and the resistors help there.
D13 protects Q5 gate. For the field circuit 1N6282A 30V TVS D6 protects Q1.
The voltage regulating portion is your usual voltage reference LM336-5.0 and op-amp LM201 U1 as a comparator looking at the SENSE voltage. Realistically, none of this operates in a linear-mode, the whole thing oscillates pulsing the field-winding on and off at a few Hz. Thus many capacitors to keep things stable. You would see this running the regulator in a running engine+battery, not on a workbench.
If this is for aircraft, I would do a test plan that checks all functions of the LR3B. The design here is made to be solid but I see a few weaknesses, from my armchair.
What parts did you end up replacing? Sometimes understanding why it failed can make repairs easier.