BCD325P2/BCD996P2: How Come BCD996P2 Doesn't Always Show The PL/DCS During Custom Search?

[jonwienke]

Maybe this will help:

In the diagram below, R1 limits the current that can be drawn from the discriminator tap, C1 blocks the flow of DC current, and R2 is a shunt that prevents any DC bias on the output to the sound card. R1 should be in the 10KΩ-100KΩ range, and R2 should be about 10x the value of R1. The interaction between C1 and R1 and R2 determines the low frequency cutoff on the output side. The larger the value, the more low-frequency audio will pass. A 10μF capacitor should be fine.

 

[CanesFan95]

Awesome, yes that helps. Thanks for drawing that! Is this low-frequency audio you mention referring to the sub-audible CTCSS and DCS? So not having R2 is what could be somehow messing up the scanner trying to decode CTCSS & DCS?

 

[jonwienke]

Not exactly. If the value of R1 is too low, then the overall audio level on the input side will be decreased, and the scanner won't be able to decode CTCSS/DCS from the remaining signal. If you still have trouble after increasing R1 to 100KΩ, then you'll need to use an op amp to buffer the discriminator input from the output.

 

[Ubbe]

Don't use polarized capacitors like the tantalium and you also don't need that high value. It's enough with a 220nF or 470nF non polarized capacitor. If you connect it to the mic in on a computer it will output a positive voltage to drive an electret mic and could polarize a tantal the wrong way and start to DC conduct and upset the discriminator. A 100k resistor might be to high to get a signal level suitable for the computer but you can always experiment with that. Try to use line in or at least turn off any DC supply in mic settings.

/Ubbe

 

[CanesFan95]

This is on a Windows 7 64-bit machine and am not sure how to turn off DC voltage like you say. Is there a setting somewhere in Windows for that?

And would 50k be a better middle ground / compromise?

 

[milcom_chaser]

This is on a Windows 7 64-bit machine and am not sure how to turn off DC voltage like you say. Is there a setting somewhere in Windows for that?

And would 50k be a better middle ground / compromise?

On Windows, no. On a special sound card that supplies Phantom power perhaps.

Here's a thread: Discriminator Tap. Real simple too.
https://forums.radioreference.com/uniden-thread-archives/82413-bct15-discriminator-tap-how-too.html

 

[CanesFan95]

On Windows, no. On a special sound card that supplies Phantom power perhaps.

Here's a thread: Discriminator Tap. Real simple too.
https://forums.radioreference.com/uniden-thread-archives/82413-bct15-discriminator-tap-how-too.html

Those are the same components in that thread that I used for the BCT15X. A 10kΩ resistor and a 10μF tantalum capacitor. I've already ordered some 100kΩ and 1MΩ resistors off of eBay. But now I'm confused. How do we tell exactly what the right components are to use? 10kΩ? 50kΩ? 100kΩ??

If polarized capacitors should be avoided, then should I get ceramic capacitors or something else? What capacity and voltage rating should I get?

 

[jonwienke]

If the capacitor value is too large it won't hurt anything--the worst possible outcome is that it passes subsonic frequencies. Polarized is OK if you have the shunt resistor to ensure that the DC voltage across the capacitor matches its polarity.

 

[CanesFan95]

So If I use a ceramic capacitor instead of the polarized one, then I get to nix having to do the shunt resistor?

 

[jonwienke]

True.

 

[CanesFan95]

Well shucks, I wish I knew all this 2-3 years ago. I have like 3-4 scanners I wanna re-mod now. LOL.

 

[CanesFan95]

Went ahead and re-did the mod with a 470nF ceramic capacitor and opted to stay with the 10k resistor and no shunt. I also opted to nix the ground wire and let the 1/8" jack just use the chassis ground and am happy to report the PLs are behaving the way they're supposed to now. Not sure what I did, but it's all fixed now and the scanner is back to normal. Maybe the first mod I screwed something up or had a bad component.

The new discriminator mod is working good and I literally have every jack on this scanner hooked up and in use (DB9 for ProScan, recording jack, external speaker, discriminator tap, power, antenna, etc.). Now, back to normal!