Works in Command but not in Conventional

If you connect a capacitor to the "U" terminal and earth ground, you will create an antenna with the loop, injecting the TMCC signal into the house wiring.  This antenna could still possibly transmit to the locomotives, although not through the normally accepted mode.

Connecting the capacitor to Pin 5 would form a minimum loop, depending upon the connection wiring.

The ouput capacitor inside the Base is .1 uF, which means that a .1 uF externally will form a voltage divider.  That would put a load of about 7 ohms across the output circuit.  That would certainly load down the output signal, but I don't know if the driver transistor will be sending you a Thank You note!

How about taking the path of least resistance and using the TPC to run your conventional engines?  Go with the flow, so to speak.  I remember the TPC as being advertised as having (100?) speed steps, running your conventional engines with greater control than ever.  You can change from command to conventional mode with a couple of presses on the buttons of your Cab1.  Just a thought.  

I believe the TPC is being used for the conventional engines.  The issue is that he wants to run a TMCC locomotive as a conventional, right?

The capacitor can be switched across the track line to ground, and the line from the base can just be left open.  This will shunt any stray signal to ground. and not load the base output circuit.

There's more than one way to skin this cat.

If I had a schematic for the Command unit, I'll bet there is a simple way to disable the RF oscillator.  You would think that Lionel would have built the command unit to allow conventional mode and command mode with a simple switch like they did with the Trainmaster unit. 

Wasn't someone working on a schematic of the command base?  Dale?

http://www.trainfacts.com/trainfacts/?p=298

I don't know if this is okay to do but I found this link on the web that completely describes the command base with schematics.  Lionel should have put a simple slide switch in between the output of Q4 the TIP41C transistor and  the >1 ufd coupling cap.  That would solve the problem of the 455Khz signal always being present, making TMCC devices think they were in command mode.

Good point, I knew I'd seen that schematic somewhere.   I captured it for my internal use, and I also combined them into one PDF file.