CTC-4 Director 21

[Electronic M]

Got an analog VOM with a d'arsonval meter? That ought to capture the variation.
Dirty secret to taking under-chassis measurements with the chassis in the cabinet: get some clip leads connect one end of a lead to a relevant test point and run the other out from under the chassis to your meter probe and slide the chassis back in the cabinet....That method has saved my bacon on sweep and boost troubleshooting a few times.

Also table model CTC-4s have enough harness length (except maybe HV, but that can be addressed) that you can set the chassis up on the floor behind the cabinet with all the under-chassis exposed...For a console like yours if you can find a sturdy box or some such as high off the ground as the chassis shelf then you should be able to pull it up to the back of the set place the chassis on it and make everything reach/accessible.