XTS5000/XTL5000 Discriminator Tap?

[FlashG]

Hi Guys,
I was wondering if it is possible to fit an XTS5000XTL5000 with a discriminator tap? I need a discriminator tap for a project I am working on. The signal is an analog one. I have tried and used various forms of scanners but the sensitivity is really lacking. With the XTS/XTL radios being very sensitive I want to try my hand at modifying one with this tap. If anyone has any information and possibly photos of theirs it would be greatly appreciated!

[FlashG]

I have managed to get my hands on an XTL5000 Service Manual and have found two interesting outputs that may suit my needs in regards to finding a discriminator output on the XTL5000. The outputs in question are RX AUDIO and DET_AUDIO (Pin 9 on accessory plug). I am assuming that the DET in stands for detected audio and is possibly another name for a discriminator output? Maybe someone from this great forum can shed some light on this? I am not sure that the RX AUDIO is going to be usable as from memory it is 1V peak to peak, possibly a bit high for a discriminator output. It is also DC biased. Anyway if anyone could give me some advice it would be greatly appreciated !!

[Spiffy50]

Yes, DET_AUDIO is exactly what you're looking for. I've used the same tap point in a Jedi radio to extract the audio as well. Make sure you feed it through a cap (10uF should do the trick, I use tantalym due to their small size) to block any DC voltages to/from the radio. Not blocking said voltages can easily make your radio take up smoking. You don't want that.

[FlashG]

Excellent news. You say to use a tantalum capacitor around 10uF. What way would I connect tantalum polarity-wise as I wouldn’t like to get this wrong?

I have been doing some more detailed investigation into the XTS5000 to see if I can establish a discriminator tap point for these radios. I think I may have found a way. Because the radio uses a lot of digital conversions on the receive path it is a difficult process but I have found on the VOCODER boards part numbers NNTN4819 and NNTN4717 the GCAP II chip has an output called EXTOUT which is analog audio with minor internal amplification. Post this output further audio pre-amplification and power amplification is done prior to any speaker outputs. This EXTOUT is about as close as I will get with this radio. I passed this info to an Engineer friend of mine for some expert advice. I am hoping that the news will be good. Sadly VOCODER boards with part number NNTN5567 cannot be used as they have a single chip that does the digital decoding and all the audio power amplification prior to output from the chip.

I would love to compare the receive audio path of the XTL5000 radio to see where the DET_AUDIO is tapped from but I don’t have a copy of the Detailed Service Manual, so if anyone would like to lend me one it would be appreciated. My reason being is if the XTLs use the same chipset it may point me to a good tap point.

Oh one other point, I made a mistake in my last post the DET_AUDIO comes from pin 9 on J5 and J6 on the XTL5000 not pin 9 on the accessory port (J2).

[FlashG]

One more question in relation to the tantalum capacitor for the discriminator tap. I have previously modified many scanners with discriminator taps and they a have always used resistors in-line (usually 10k) to reduce the current draw on the tap. Would this suffice for the Motorola radios using the DET_AUDIO or is there another reason for using the tantalum cap? Thanks in advance.

[YouHF]

MaxTracs are great for this, and they can be had for as little as $10-$35 depending on the band. Heck there are lots of options in between a MaxTrac and an XTL5000 that are way more sensitive than a scanner. You're really going to start doing things that risk "your radio take up smoking" with an XTL5000 and relegate it to discriminator duty?

[MotoFAN]

One more question in relation to the tantalum capacitor for the discriminator tap. I have previously modified many scanners with discriminator taps and they a have always used resistors in-line (usually 10k) to reduce the current draw on the tap. Would this suffice for the Motorola radios using the DET_AUDIO or is there another reason for using the tantalum cap? Thanks in advance.

Capacitor is not mandatory, but strongly recommended to prevent smoking radio by DC components. I recommend you 0.33-0.68 uF range to prevent low freqs cut.

You no need to add resistor for XTL, because this output designed to connect to high impendace inputs (greater than about 5 kiloohm). But as I think, it's not designed to connect to low impendace inputs (like is 50-3000 ohm).

[FlashG]

Well over the weekend I managed to make a 25 male connector for J6 on my XTL5000 so that I could tap of the DET_AUDIO output. I assumed that the DET_AUDIO output on J6 would be similar to a discriminator but sadly I was unable to get it work with my pager decoder hardware. I also tried the RX_AUDIO and RX_FILT outputs on the J6 but with the same results. Either this output must be enabled in CPS or the newer Motorola radios that discriminate digitally are no good for pure discriminator output for pager decoding. I guess modifying the XTS5000s would not be a good option now? Oh well I guess they stay intact for now.

On another note, I did manage to get three old Jedi radios and followed the procedures on this site for a discriminator output. After pulling them apart I found that one has the old RF shield and two had the new RF shield. After installing the tap, the ones with the new RF shield worked fine but the one with old shield worked intermittently. Not sure if this was faulty radio or just bad luck. The radio seemed to work fine in all other respects just not with the tap.