Radio Shack Pro-405 Discriminator Tap

So I had an old Radio Shack Pro-405 scanner lying around that hadn't seen much action since I upgraded to a 996XT, and after reading recently about discriminator taps I figured it would be the perfect test bed since if I messed up I wasn't losing much.



There is an extensive list of how-to's for DIY taps on this site but the 405 isn't listed. After a bit of careful poking around I found the tap point and have had excellent results, here's my setup...



1/8 mic jack installed into right hand side of scanner towards the back

Ground wire soldered to main ground of the external antenna connector

Signal wire soldered to "TP5" post with 10K ohm resistor inline

Mono audio patch cable running from tap to line in jack (not mic jack)



I'm running DSD+ in P25 only mode (-f1) and decode quality is about 80% of that of my 996. A few notes on the audio input to the computer...



This will not work with all line in jacks... some cause the scanner to lock into a single channel... I believe this is due to some kind of attempt by the computer to detect a connection sending RF back down the cable to the scanner tricking the squealch into opening.



Adjust the mic gain so that when receiving it just about maxes out the input s-meter... this will mean when not receiving the meter will be pegged but I don't think that will hurt anything.



Hope this helps someone... I own two digital capable scanners but this little project was a ton of fun!!!

Radio Shack Pro-405 Discriminator Tap