Do discriminator taps disable your discriminator entirely?

[saioke]

I wanted to try my hand at installing a discriminator tap in my old Bearcat BC350A. I was hoping I'd be able to use it to scan through a few DMR frequencies. I "think" I've done it correctly, soldering to pin 10 and the other end to ground. Unfortunately I can't seem to find my Aux cable at the moment so I can't attempt to decode anything.

That being said, upon reassembly, my discriminator doesn't work at all. Rotating it left or right doesn't make a difference, so the scanner will stop at basically every frequency now. Is this supposed to happen? If so, wouldn't this method just allow you to decode one stationary frequency instead of scanning through any frequencies added to your private bank?

Edit: I turned it off and unplugged for a bit. Plugged it back in to experiment, and now the discriminator works again. Strange, I need to hunt for my Aux cable to see if it actually works.

 

[Ubbe]

In the application example for the IC they use a 7,5k resistor to pin 10 and then a capacitor to isolate the DC voltage from pin 10, that changes 0,3V per KHz off frequency. If you load pin 10 too much the audio from it lowers and the squelch circuit thinks it's receiving a carrier and stops scan with noise from the audio.

Use at least a 2.2k resistor to pin 10 but a capacitor shouldn't be needed when using a resistor. Any capacitor used will work as a high pass filter so for digital signals to not be too distorted you probably need a 1uF capacitor with its positive leg to pin 10 and then no protection resistor are needed.

/Ubbe

 

[saioke]

Thank you, the guide I've read online didn't mention a resistor or a capacitor being required at all, but I've seen it mentioned on practically other scanner discriminator taps so it makes sense. Changing the AC adapter seems to have corrected the issue though, but I think I have some old electronics I could salvage some resistors from and it's better safe than sorry.

This has been a fun project and if I knew how easy it would be I would've done it sooner. I just wish the scanner went above 512mhz but I can't imagine it would be possible to trunk track anyways. Mostly just wanted to do the mod for 450-460 conventional DMR frequencies.

 

[rgchristy]

mods.dk - Modifications for radioamateur

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