Curious you should say this: I just had my first (out of 8) build failures, with exactly the same failure mode you are observing.
The LED indicates that the tuner module has received the correct initialisation data from the empeg. It does not, however, imply a handshake with the player, so as you and I discover, it could be a fully functional tuner that is not being recognised by the player due to the player thinking there's no module attached.
Using a scope on the RX and TX lines at the connector to the player, my module receives a stream of data packets during startup of the player, to which it replies with a stream of packets. The player is not seeing this, for some reason. To eliminate the player from the equation, I tried it with a known good factory unit, which then is recognised - so it's not that.
The level convertor chip is an additional Serial Port for the player unit, which is currently unused in the player software. The software for the PIC on the module has support built in to use this port, along with the 12V sense inputs and the two solid-state relays - but the support is not there in the player software to drive them.
The RX and TX data lines between the player and the module are TR4 and 5.
What I need to do is do a capture of the incoming and outgoing data stream on a working, recognised module then compare it with this dead one. Will let you know what I discover.
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One of the few remaining Mk1 owners...
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