...two left to go and I will have 10 working beautiful U67's.
(...) Close visual inspection can reveal residue from cap leakage, but missing residue doesn’t mean the cap is fine.
...If you don‘t break one by plugging into a faulty PSU.Use resistive dummy loads for testing, not a valuable mic:33 Ohms / 2 W (this simulates additional 1.5 Ohms cable resistance) and 240 kOhms.
C1-C3 caps can be basically checked in place by looking at their AC ripple voltage UNDER LOAD and comparing the result to a known 100% working unit (preferably with fresh caps).
Close visual inspection can reveal residue from cap leakage, but missing residue doesn’t mean the cap is fine.
Electronic’s symbols vary all over the place, remember the 60 years that passed along since.American symbols are often quite different to european’s, even denoted for resistors in the schematic!
...and electrolyte leakage from the vent holes in the top of electrolytic capacitors does likewise not indicate damage! Lift the plus (in the case of NU67 heater rail minus) connections of the cap and use a capacitance tester to determine health.