I would like to monitor ATCS on the railroad but I need to install/find the discriminator tap on my BC92XLT. I found a site that gives directions on how to do it for a PRO-97 (http://www.naturalgfx.com/pro97_discriminator.htm) but that obviously isn't the same inside as the BC92XLT. On the site, it said to locate TP4, which is the hard part for me. Where is it?? I understand how to do the rest of the steps but I can't find TP4.