Radio Shack Pro-405 Discriminator Tap

[rhombus_000]

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!!!

 

[slicerwizard]

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)

No voltage divider. No DC blocker. Mono plug into a stereo jack...

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.

No DC blocking cap.

 

[rhombus_000]

No voltage divider. No DC blocker. Mono plug into a stereo jack...


No DC blocking cap.

The mono cable going into a stereo jack didn't seem to be a problem... the right channel just gets grounded out and I set DSD+ to only listen on the left channel.

I read elsewhere about the DC blocking cap... I'll give that a try. It should solve the problem of the Mic bias voltage from locking up the scanner. One of my computers has a true line in and that works fine but I figured out the two others I was having trouble with are microphone inputs only.

What is the purpose of the voltage divider? So I need another resistor behind the first one going to ground? What value would you recommend?

Since the original post I have added a 1nf filter capacitor connected from the signal terminal on the mic jack to ground. Didn't seem to make any difference...

I'm getting MOTOTrbo clear as day now even from a fairly distant system, P25 is still not that great even on a system the next town over (I have a pretty decent mast mounted antenna so signal strength isn't an issue, 996XT decodes perfectly).

 

[SCPD]

So I need another resistor behind the first one going to ground? What value would you recommend?

Replace the 10k resistor with a 47k potentiometer.

With the three lugs pointed down and the knob facing you, wire the center lug to the tip of the headphone connector. Wire the left lug to ground and the ring of the headphone connector. Wire the right lug to TP5.

Use an oscilloscope program to view the waveform as you adjust the pot.

 

[rhombus_000]

Replace the 10k resistor with a 47k potentiometer.

With the three lugs pointed down and the knob facing you, wire the center lug to the tip of the headphone connector. Wire the left lug to ground and the ring of the headphone connector. Wire the right lug to TP5.

Use an oscilloscope program to view the waveform as you adjust the pot.

Thanks!!!

 

[rhombus_000]

Replace the 10k resistor with a 47k potentiometer.

With the three lugs pointed down and the knob facing you, wire the center lug to the tip of the headphone connector. Wire the left lug to ground and the ring of the headphone connector. Wire the right lug to TP5.

Use an oscilloscope program to view the waveform as you adjust the pot.

Had a chance to give this a shot tonight... worked great. Tuning in waveform with the pot greatly increased the quality of the decode, almost can't tell the difference between the Pro-405 and my 996XT.

The DC blocking capacitor solved the mic jack issue, but revealed another problem. The two computers that had mic jacks aren't fast enough to decode properly. One is simply old, and the other is pretty fast but it was connected through a 50 foot USB extender with and el-cheapo USB sound card.

Not sure if it's the cheap sound card or the extender that was the problem but the faster computer worked great with the scanner plugged in directly to the built in mic jack without the extender and external card, even with the loss of receive strength using the built in antenna.

Thanks again for your help!

 

[rhombus_000]

Not sure if it's the cheap sound card or the USB extender that was the problem but the faster computer worked great with the scanner plugged in directly to the built in mic jack without the extender and external card, even with the loss of receive strength using the built in antenna.

Update: It was the cheap USB sound card that was causing the problem. I changed it out for a higher quality one, and now I can locate the scanner in the shack where my coax comes down from the antenna, send it 50 feet indoors over the extender to the faster computer, then back over the USB to the line out from the soundcard to an external speaker in the shack.

Decode on P25 and TRBO are crystal clear now... not too shabby for the $10 it cost me to set all this up. Thanks again to all for the help.

 

[wbswetnam]

I know this thread is nearly a year old, but I wanted to write that I did this project, too, with my Radio Shack Pro-405...and it cost me nothing! I used parts from a broken scanner (an dead BC145XLT) from which I salvaged a 100K potentiometer and a 4.7 microfarad capacitor that I unsoldered from the PC board. Now my Pro-405 is my newest tapped scanner. It was a great Saturday project!