Dead PROOPS MULTITESTER Model TS-55A

[NottheDan]

The movement still works and I guess the problem lies in the bolded part in the schematic, which I'm not quite sure what it actually is.

A selenium bridge rectifier?

If it is that, can I actually straight replace it with one made from diodes? Or does it need more?

[SeanB]

It is a few diodes in series. There to provide meter protection from 2 diodes in reverse parallel across the movement, and as well contains another diode to allow use on AC voltage.  The 2 diodes in reverse parallel can be 1N4007 diodes, there to protect the 500R meter movement (meter plus trim resistor) from minor overloads, and then a 1N4007 for the AC rectifier.

If you are willing to lose the AC voltage ranges just connect the 2 reverse parallel diodes across the meter movement terminals, and then connect the plus of the meter movement to the pot terminal that used to connect to the diode block, and leave the other 2 wires not connected. That will at least allow you to see if there is drift in the voltage divider resistors, and if the resistance range still works somewhat.

[NottheDan]

I assumed selenium rectifier because I've never seen diodes in this form factor or any similar one while it has at least a passing resemblance to the other one.