PRO-94 Discriminator Tap

[clbarker]

I have 2 PRO-94's and am considering making one of those a data feed scanner. What do I need to do this??? I found a previous post about this after searching the site and it didn't help all that much. Do any of you out there have any pictures of your tap that you would be willing to share? I tried to open the back of mine today and I could not get the back off! I took out all 4 screws, but it still would not come off. (It looked like it was getting stuck at the top...) Any help is much appratiated! One last question...is it worth setting up???

 

[clbarker]

Okay, I'll change my question to be more general. What wires and other equipment do I need to a discriminator tap on ANY scanner? Is it worth doing??

 

[Al42]

You need a 10 ufd capacitor, a 10k resistor, a 2.2 nf capacitor and a connector of some sort (an earphone jack, usually) and, maybe, a piece of small shielded wire. Total cost, if you have to buy a piece of mic wire, should be around $5-$10.

You want to connect the positive (+) end (or the end that's not marked "-") of the 10 ufd capacitor to pin 9 of IC-2, a MC3361CDR2. Then connect 1 end (either one) of the resistor to the negative end of the capacitor. The other end of the resistor goes to the center pin of the jack. The ground of the jack goes to a ground point on the scanner (the shell of the antenna jack is grounded - so is anything connected to it or thru wires or PC lands to it is grounded). The 2.2 nf capacitor goes from the tip to ground of the jack. Use wire to extend the connection if the resistor and capacitor leads aren't long enough (they should be). Cover the leads with "spaghetti", shrink tubing or even nail polish, to keep them from shorting to anything.

Some people use just the resistor and a jack, or just the 10 ufd capacitor and a jack, but the extra $2 or so is worth the expense - the resistor and 2.2 nf capacitor form a "low-pass" filter - they filter out some noise and all the residual IF signal, so the decoding is a lot cleaner.

 

[coolblueneon]

To answer your question about getting the Pro-94 apart.. Removing the 4 screws (2 on back on either side of the belt clip and 2 under the battery compartment).. The back will come off with a little force, it is NOT connected to the top piece, but it will come off. I've done the mod as stated above and it works great.


Good luck!


greg

 

[724Gangsta]

I hate to bump an old thread, but I recently performed the dTap mod on my aging Pro-94 and wanted to clarify something for possible future tapping endeavors...

Then connect 1 end (either one) of the resistor to the negative end of the capacitor. The other end of the resistor goes to the center pin of the jack. The ground of the jack goes to a ground point on the scanner (the shell of the antenna jack is grounded - so is anything connected to it or thru wires or PC lands to it is grounded). The 2.2 nf capacitor goes from the tip to ground of the jack.

I wanted to follow this method, but ended up using just a 10ufd cap soldered to pin 9 of MC3361CDR2 and the "center pin" of the mono RCA jack cable because I didn't understand that last sentence.

If we attach the positive end of 10ufd cap to pin 9 of MC3361CDR2 and the negative end to one end of the resistor, the other end of the resistor to the "center pin" wire of the RCA jack and attach the RCA ground to the ground point, where does one attach the 2N2 cap? Is it between the ground and the ground point?

 

[724Gangsta]

No takers?

So no one can elaborate on this "low band pass filter" approach to dTap modding?

 

[slicerwizard]

That bit of filtering made sense back in the data slicer days, but when feeding a sound card, it's not required. A software decoder will not see any residual RF and any unwanted high frequency junk (like static) can be digitally filtered.

You should be more concerned with how you are going to properly impedance match your signal with a sound card's input. As it stands, your signal, which is probably somewhere in the 0.5 to 1 VPP range or so, is far too hot for a mic input, nor is it ideal for a (600 ohm?) line in either.

Why is the Internet so chock full of bad discriminator tap instructions? "Yeah, just run a wire, maybe add a 10k series resistor; add a series cap if it makes you happy - 0.1uF is more than enough" It's not like a tiny cap will distort the hell out the signal, especially when fed to a line in, is it? Oh wait, 0.1uF and a 600 ohm load - do the math - oops. And nobody ever wants to decode low speed data like LTR, etc., so no worries if those low frequencies can't get through a tiny cap. And we can ignore the fact that high speed data - Moto 3600, EDACS, P25 - also have low frequency content, cuz we're rebels, damn it! And proper impedance matching? Clearly, that's just for sissies...

 

[KC2YQW]

Slicerwizard,

Since you are so familiar with what will damage the radio/computer as to speak out about how many people don't know what they are doing, perhaps you can elaborate on your perfect Disc tap so that those of us who are new to these mods, [like myself] will understand whats being done and preform the tap.

Sorry, but I am a firm believer that if you find a problem, you ask for assistance or you give a solution. Complaining about how many "bad discriminator tap instructions" there are without answering the question asked [which was "What is needed to preform the tap" "how to open the radio" and "Is it worth doing"] doesn't help anyone.

With that out of the way, I am looking to preform this tap. Since Mr. Slicer feels the mod above is no good I would like some other opinions about what will work without causing damage to the computer. I am using an old pro94 for the mod rather than the 300 dollar 106 so if I make a mistake I don't blow up my good radio. I plan to run DSD which calls for the discriminator cap.

 

[slicerwizard]

Since you are so familiar with what will damage the radio/computer

Hm, nowhere did I suggest that any hardware would get damaged.

as to speak out about how many people don't know what they are doing, perhaps you can elaborate on your perfect Disc tap so that those of us who are new to these mods, [like myself] will understand whats being done and preform the tap.

I've posted diagrams here a number of times. Look for them.

Sorry, but I am a firm believer that if you find a problem, you ask for assistance or you give a solution. Complaining about how many "bad discriminator tap instructions" there are without answering the question asked [which was "What is needed to preform the tap" "how to open the radio" and "Is it worth doing"] doesn't help anyone.

I was replying to the individual whose tap advice was suitable only for feeding data slicers, something that very few choose to do these days. Why shouldn't I suggest that people refrain from posting bad advice? The diagrams of taps with appropriate voltage dividers are already out there, so I don't need to post another one.

With that out of the way, I am looking to preform this tap. Since Mr. Slicer feels the mod above is no good I would like some other opinions about what will work without causing damage to the computer. I am using an old pro94 for the mod rather than the 300 dollar 106 so if I make a mistake I don't blow up my good radio. I plan to run DSD which calls for the discriminator cap.

DSD is mainly used to decode four level signals, so you really do want the correct interface with a voltage divider. A simple 10k series resistor tap will make for a bad decoding day.

 

[KC2YQW]

As it stands, your signal, which is probably somewhere in the 0.5 to 1 VPP range or so, is far too hot for a mic input, nor is it ideal for a (600 ohm?) line in either.

Hm, nowhere did I suggest that any hardware would get damaged.

From your explanation I am reading that the above mod would overpower the mic/line in on the sound card, which would damage the card, thus hardware would be damaged correct?

I've posted diagrams here a number of times. Look for them.

Where? I've done a couple of searches, pro-94 mod, pro-94 discriminator tap, discriminator tap, This is the only post that seemed to give me any kind of useful information.

I was replying to the individual whose tap advice was suitable only for feeding data slicers, something that very few choose to do these days. Why shouldn't I suggest that people refrain from posting bad advice? The diagrams of taps with appropriate voltage dividers are already out there, so I don't need to post another one.

There's no problem with talking about bad advice, but you didn't offer a solution to the post, but rather that the answer AI42 gave is no good for what we want. Rather than complain about all of the bad taps out there, why not post a link to the numerous posts you have made with the diagrams? It would have probably taken less time and helped more people out.

DSD is mainly used to decode four level signals, so you really do want the correct interface with a voltage divider. A simple 10k series resistor tap will make for a bad decoding day.

I thank you for telling me I need a correct interface, but I don't know what interface you are talking about. Do you mean a separate interface chip between the radio and the computer? the tap itself? computer? If your talking about the interface between the radios tap and the computer, I have seen several of these ranging from a few bucks to the 100 dollar range. Which specific interface are you referencing?

Im not trying to give you a hard time or anything, but I would like to learn what I am doing so that I don't make to many mistakes [and of the mistakes I make, they will hopefully be minor] All you have said is this wont work for what you want, I have the answer, but rather than help you with it, I rather just tell you find it. I have checked radio-reference forums [how do you think I found this post after several years of inactivity] and Google and the best I get is pro-93 then pro-95. If I had found any diagrams you or any other person had posted I wouldn't be asking for help would I?

 

[slicerwizard]

From your explanation I am reading that the above mod would overpower the mic/line in on the sound card, which would damage the card, thus hardware would be damaged correct?

No, it would be TOO LOUD for the mic input to handle. Audio would be badly distorted. I noticed a post in one of the DSD+ threads where a user mentions something about some audio level having to be at 7% for him to get good decodes - sounds like an overloaded input to me.

why not post a link to the numerous posts you have made with the diagrams?

Google found this one: http://forums.radioreference.com/1306751-post11.html

I thank you for telling me I need a correct interface, but I don't know what interface you are talking about. Do you mean a separate interface chip between the radio and the computer? the tap itself? computer? If your talking about the interface between the radios tap and the computer, I have seen several of these ranging from a few bucks to the 100 dollar range. Which specific interface are you referencing?

You just need what's in the diagram. The resistor values should have a ratio that's somewhere in the 5:1 to 10:1 ballpark, so 10k and 2k should be fine, for example. I wouldn't go much below 10k (too much load on the discriminator circuit) or over 27k (higher impedance can pick up more noise)