The "older" units are probably Motorola SpectraTac comparators. These were state of the art for many years, but are being replaced in droves (largely by JPS voters). The SpectraTac is an analog device. It is also a device largely modified by folks (using a razor blade and soldering iron), so you run the risk of picking up one in the second hand market that won't perform the way the book says. Also, SpectraTacs used components that have passed their design useful life. Replacement parts can be hard to find, and Motorola no longer repairs them.
The basic SpectraTac comparator consists of a card cage and a power supply. You will also need one receiver card (a/k/a SQM, for "Signal Quality Module") for each receiver (including the one at the main site). You will need a Tone Keying Module to connect the comparator to the transmitter at the main site, and your transmitter must either be one that responds to EIA tone signalling or one that is connected a "tone remote adapter" (converts EIA tones to relay outputs). Likewise, your receivers must either be capable of sending the 2175 Hz "idle tone" over the wireline when squelched or must be connected up to a module that will do that for you. (JPS sells those modules.)
If your comparator is to be connected also to a console by wireline, you will also need a Tone Priority Module. The Tone Priority Module keys the transmitter in response to console commands, while the Tone Keying Module keys the transmitter in response to receiver unsquelching (via the "voted" bus line).
Two things to bear in mind:
One: as previously adverted to, the key to having a voted system work as it should is level setting. This is a two-man job and it requires some knowledge of the backhaul system you are using and the radio equipment at either end. And some equipment. The bottom line is that each receiver wants to output to the wireline at a level such that at the comparator end (after all line losses) all receivers have the same level. Back in the day when phone lines were the norm, one standard rule of thumb was -13 dBm for average voice and -3 dBm for peak. (Phone lines typically are intolerant of signals that stray above 0 dBm.)
Two: a box like the MTR2000 actually contains three elements: a receiver, a transmitter, and an internal controller. When used as a stand-alone repeater (a/k/a an "in-cabinet repeater"), the MTR's internal controller handles transmitter keying and audio routing. However, when you go to a voted system, these functions are handled by the comparator, and you have to disable the MTR's internal controller. You do this under system configuration (or the like; I don't have my radio computer with me where I am now) and set the unit for "Station" or "Base," vs. "Repeater."
Not to preach, but setting up and then maintaining voted systems (particularly those that use Ma Bell lines for backhaul) is a bit of an art. You will save yourself some heartache if you can latch onto a professional to help you out.
Finally, a note for those engaging with the JPS Voter for the first time: The JPS Voter is a superb device. It will do all that a SpectraTac will do, and a lot more. However and for that reason, the JPS Voter is a complex instrument to set up. It is accompanied by one of the finest manuals I have ever seen, which is in one sense unfortunate because too many techs don't take the time to read it! Ignore the manual and you'll triple the time it takes you to get your JPS up and running.
The "repeater disable" function: there are two ways to do this with a JPS Voter. You can disable the repeater via a console EIA command, or you can use a pin (just like on the back of the SpectraTac). Because so many JPS Voters are installed as replacements for aged-out SpectraTacs, using the pin is more common. However: the polarity of the JPS "repeater disable" pin is the reverse of the SpectraTac. If your existing system uses a Form C relay for the repeat/disable function, just move the line to the pin from the NO terminal to the NC terminal. If you're wiring directly to a console Aux Output that pulls to ground, you'll have to insert a polarity inverting relay. And, finally, note that the repeat/disable pin on the back of the JPS Voter has to be enabled by DIP switches.
Likewise re: main/standby switching. On the SpectraTac, you could only install one Tone Priority Module, so the norm was the handle main/standby transmitter steering via an Aux Output and a 2 Form C relay. The JPS Voter allows you to hook up one transmitter to each receiver card, with Card #1 being the Main and Card #2 being the standby, and then switching between them via an EIA command. Most of us, however, have found it simpler to leave our old main/standby relays in place, and simply wire the common terminals of the relay to the transmitter terminals of Card #1.
End of sermon. Good luck.
