Some years ago I watched a Techmoan video on Linear Tracking Turntables, and instantly felt the need to own one being somewhat attracted to devices that work a little differently to the usual. Within a relatively short amount of time I'd picked up a Marantz TT-243 from eBay, and after learning about the need for a phono-preamp I had it up and running. I gave it a quick service of sorts, cleaning it out, lubricating some parts on which the grease had dried up long ago, and replacing the main belt with some fresh rubber. It's worked well, but ditching the 5.1 receiver I was using during a house move I needed to find a new amplifier to use with it. Again I took to eBay, and found an SM-143 and tuner going for all of £10, listed as for parts: the description said the units work but the output levels were very low. The visual design matched the TT 243 so knowing nothing else I made the purchase.
As far as I can tell, the XX-x143 and XX-243 series are components sold as a system, with each separate connecting to the others through a plug/socket mechanism built into the tops/bottoms of the units - the turntable(s) are different and connect using standard leads. When I received the amplifier and tuner, I hooked up some speakers and powered them on. Sure enough, once the tuner was tuned in correctly I could hear a broadcast, but it was very faint even with the volume turned right up. Digging around online for information resulted in a very handy resource: the schematic for the amplifier. Armed with the pinout for the inter-unit connectors, I hooked some jumper leads to an audio cable and poked around to see if I could get more sound out of the amp.
The top of the amp has a connector for the tuner, and the bottom a connector for the tape deck, this shows two lots of Left/Right/Ground as well as a power line and a connection labelled EASY. I (incorrectly) assumed at this point that the two sets of L/R/G were for two differnet inputs and the amp would somehow select between them based on the state of the EASY line. Touching my jumper leads to the appropriate pins resulted in no output for one set of L/R/G, but I did get full-volme, good sound out of the other set. Taking to Reddit the obivous was pointed out to me - on the schematic only one set of those went to the amplification circuit, and the L/R/G lines from the tuner actually got passed through to the other set going down to the casette deck.
Some more searhcing online resulted in finding some images of said Cassette Deck (SD-143) and it's plain to see that it's this unit that does the channel switching, it has inputs for phono (presumably with a preamp inside), inputs for Auxillary, and selector buttons for all of the input sources, namely Phono, Auxillary, Cassette and Tuner. It also has a 4 pin connector on the back labelled EASY, clearly indicating this is some kind of proprietary Marantz bus for easy connection or control. I've as yet been unable to find any useful information on this bus, nor a schematic for the deck, so I do not know how the Casette player implements channel seelection.
In order to make use of the amplifier in the short term I've made rather ugly, but reversible modification: I removed the connector to the tuner, de-soldered the connector that goes down to the cassette deck, and have rigged up to RCA sockets poking up out of the top of the unit, wired straight to the L/G/R input triple. This gives me something I can use, with my turntable and other sources, but of course it's only one input, so I may attach some kind of swtich box until I can find out more about the Easy bus and cassette deck - if I can source one that would be great, but otherwise perhaps simulating one is the way forward. The balance control also appears to be faulty on the unit, but given it's split 50/50 I can live without that.