The full-blown IGM system actually would consist of three clocks(we just had one).  They would be called clock I, II, and III.  We modified it by changing one of the unused rows overnight and making it work for every hour.  That allowed us to change the format and not make it uniform for each half of each hour.  Make sense??   The clocks were set to ready up 1 1/2 minutes before the actual time.  This would allow for events to air closer to when it was scheduled, such as the IDs at the top of the hour, spot sets in the mid-quarter hour, etc.

The order of the sources for each "break" was determined by the order in the source control knobs that were physically placed in the rack panel.  For example, if your break consisted of a buffer, commercials, and time announce, then the components had to be in the rack, in that order.  With a little creativity and ingenuity, this simple automation system could do some really nice things.  In fact, when I was made PD of the FM when it was WEEE(FM), I took it AC, and by switching a couple of jumpers on a circuit board, we actually had it crossfading music.  The following source was started by the preceding source's "step tone" which, on the Scully was a 22.5 cycle tone, and the usual secondary tone on the cart.  There was also no dead role on the source, so when the "step tone" was over, the tape stopped and was rescued to the next song.

The time announcement machine was a double cart machine in a drawer of the central unit, which put both carts facing each other and shared a capstan but with two separate rollers.  One cart contained the even minutes, the other the odd.  Each cart cued itself at the: 30-second mark every other minute.  If you didn't use the time announcements, they could play jingles.  These machines were obviously in constant use and always needed cleaning and maintenance.

In order to program the correct commercials to play for each commercial break, the operator would insert diode pins of varying lengths into a "battleship game-like" board with each length representing either the A carousel or the B carousel.  Each carousel would contain 24 cartridges (containing the recorded commercial on an endless loop tape cartridge).  Each cartridge could contain multiple commercials from the same advertiser that would rotate.  Generally, they would be all 30-second announcements or 60-second announcements.  There were 24 holes representing a tray that contained the commercial.   A short diode pin would represent the A carousel, and a longer pin would represent the B carousel.  An even shorter pin in a designated hole would indicate a return to the music format and the commercial set was over.

The third carousel was set up to be sequential, it was simply a single-play cart machine in which the carousel would cue to the next cart after playing the first one, and so on.  I used it to play "buffers" and would only play 4 carts in it.  One would play before each spot set and would consist of a brief weather forecast, or a local PSA, an hourly news brief, a station promo/jingle,  and the "Weather Watch," a sponsored complete weathercast from an outside meteorologist that ran at the: 30 positions. 

By programming it using clock "I", I had it play a station promo at :05,  ran local news brief at :15,  station promo jingle at :25, Weather Watch at :30, a local PSA at :40, and a weather brief :50.    Spot sets ran at  :17(after the news-news sponsor), :30(after the Weather Watch-sponsor spot), :40 and :50.   Time announcements ran after spot sets before a re-entry jingle at :15: 40 and :50.

One of the major problems of this system is that it didn't react that quickly.  It would completely stop if it detected more than one "step tone" going through it simultaneously.   That made it necessary to keep the tones lengths on our "home-made" music reels as short as possible, and song-to-jingle-to-song segments were risky.  Jingles had to be longer than the longest "step tone" on a music reel.  Another major problem was the 22.5-cycle tone needed on the music reels.  Most of the syndicators used 25-cycle tones, which didn't work on this one.  IGM had its own music service, and I guess it was a way to keep subscribers to their own music services.  

I used a tone generator in the mike channel of a Pioneer reel-to-reel recorder when I recorded reels.  By flipping a switch on the generator, it placed the "step tone" on the reel, the music was faded and the recorder stopped.  A record was cued on the single turntable, and the recorder was started about a quarter second before the song started.  In fact, we didn't even have a stereo studio, so I rewired the turntable to feed a preamp and from there used RCA inputs to the Pioneer recorder, bypassing the mono control board.   There were other stations, which I found out much later, that were using Drake-Chenault formats on IGM systems(such as WLRW).  There was a way to modify the detectable tones.  The audible 22.5 tones on the music reels were "filtered" by the automation.

The only other local stations I remember using IGM systems were WDZ in Decatur, WLRW in Champaign, and former WTIM/WTIM-FM sister station WMDH located in New Castle, Indiana.  WMDH was using a massive system with at least 6 reel-to-reel decks.  WDZ was using carousels to play carted music.

Overall, the IGM systems were pretty reliable, simple little automation systems.  A good engineer could modify the systems to do everything you wanted.  WDZ in Decatur probably modified one the most and actually used an IGM to "talk track" and talk up song intros in a "like live" top 40 format in the mid to late '70s!

The question is, are there any more of these out there....and are they being used??