For testing and troubleshooting analog party-line (PL) intercom circuits we think that there’s nothing better than the MR-PRO from NTI Instruments. (https://www.nti-audio.com/en/products/minirator-mr-pro-mr2). While it was designed as a general-purpose audio generator with some test capabilities it ends up being fantastic for analog PL intercom work. It will directly measure impedance and DC voltage, independently from pin 2 to pin 1 and from pin 3 to pin 1, exactly what you need to understand a PL circuit’s characteristics.
Typical PL intercom circuits have a nominal impedance of 200 ohms and in reality will usually be in the range of 150-180 ohms. This will result in good performance. If you get lower than 130 ohms or so you’ll have issues. If you get something on the order of 80-90 ohms that’s indicative of a “double termination”, something that’s typically easy to correct. (Check the termination switches on the back of PL power supplies.) Maintaining an acceptable impedance is the only way to ensure that the auto-null function provided by many Studio Technologies products will function correctly. If a good null can’t be reached then most likely there’s an issue with the impedance of the PL circuit.
DC voltages associated with PL circuits will typically be in the range of 28-32 volts. (Some systems use 24 volts DC, acceptable but not as good.) If you measure 20 or less volts DC there’s probably an issue that should be resolved. A DC voltage that’s too low can result in incorrect operation of user devices such as belt-packs, speaker stations, etc.