Question Concerning Discriminator Taps

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Dewey

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Does anyone know if a discriminator tap has any bad long term effects on the scanner? I tapped my Pro-2004 back in the 80's, and got about 6 or 7 more years out of it before death. While I know the Pro-2004's were notorious for quirks like cold solder joints, I sometimes wonder if constantly powering a data slicer shortened the life of the radio.

I'm now wondering because I recently tapped 2 more of my scanners... a Pro-92, which doesn't concern me a great deal, and a Pro-96, which I would like to get long life from. Since my novice mind still can't tell which would be better with a tap, a resistor, or a capacitor, I installed straight line to the jacks. Now I'm wondering if a resistor or capacitor would help ensure the longevity of the radio (I used the capacitor recommended by Bill Cheek on the 2004) if regularly loaded with a data slicer, or fed to the computer's sound card.

Thanks to all,
Dewey
 

WayneH

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I've had a BC-8500 and 895 tapped for more than six years, the 8500 even longer. There don't seem to be any effects I've noticed.

I would say you're safe provided you don't accidentally send anything IN via the tap.

-Wayne
 

windigofer

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As I understood it, the purpose of the resistor was to attempt to prevent anything going in the "wrong way" (I'd think a diode would work as well).

The purpose of caps, as I understood it, was for discriminator taps with lower voltage that might not be able to power an external interface otherwise.

As for myself, I have an old Pro-29 as well as a Pro-2067 that are tapped (the latter is actually my main Trunker machine), and I also have a Pro-96 that I may get around to tapping.

I've not noticed any worsening of reception on the Pro-2067 post-tap (I used a resistor rather than a cap) and, like several others, get sufficient signal to actually decode successfully even in Windows using Trunker (!).

I have also read (via several mailinglists for the Pro-96 and on radioreference.com) that the Pro-96 is actually one of the easiest handhelds TO tap (most GRE-manufactured Radio Shack scanners can actually be tapped via a test pin).

Let us know how it goes :3
 
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N_Jay

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windigofer said:
As I understood it, the purpose of the resistor was to attempt to prevent anything going in the "wrong way" (I'd think a diode would work as well).

A resistor reduces the loading on the point and decouples the scanner circuit from what is connected to the tap.

A diode would NOT do the same thing.

windigofer said:
The purpose of caps, as I understood it, was for discriminator taps with lower voltage that might not be able to power an external interface otherwise.

A capacitor decouples any (or most) DC from the scanner circuit, and will also drop the AC loading to some extent.


The long term effects would depend on the scanner, the way it is tapped, what is hooked to the tap and a good amount of dumb luck.

You are adding some loading to a point that may not be designed to drive much more than the scanner circuits.

You may be affecting the DC bias on that point.

You may be introducing static (ESD) to an internal point in the circuit that is not as well protected as a designed connection point.

And all this might or might not be significant.

The best way (and maybe over-kill) would be to tap the circuit with a very small capacitor driving the input of an Op-Amp (preferable a MOSFET Op-Amp) which is internal to the scanner and run of the scanner power. The output of this Op-Amp should be provided to the outside connection through a capacitor for further isolation. It would be good if this connection was bypassed to ground through a small capacitor also.

(Like I said, probably over-kill) :wink:
 

DaveH

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windigofer said:
The purpose of caps, as I understood it, was for discriminator taps with lower voltage that might not be able to power an external interface otherwise.

The radio does not "power" the slicer. Power for the slicer comes from the RS-232 interface signals. If you are talking about signal "loading" that is different. The life of the radio might be reduced by static or other foreign stuff getting back into the radio and doing damage. It's possible to install some protection, but I don't have specifics at the moment.

Dave
 

windigofer

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DaveH said:
windigofer said:
The purpose of caps, as I understood it, was for discriminator taps with lower voltage that might not be able to power an external interface otherwise.

The radio does not "power" the slicer. Power for the slicer comes from the RS-232 interface signals. If you are talking about signal "loading" that is different. The life of the radio might be reduced by static or other foreign stuff getting back into the radio and doing damage. It's possible to install some protection, but I don't have specifics at the moment.

Dave

Yes, I was referring to signal loading--thanks for the clarification.

(I think the post above actually has gone into great detail on that, incidentially, though further comment is appreciated)
 

windigofer

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N_Jay said:
A resistor reduces the loading on the point and decouples the scanner circuit from what is connected to the tap.

A diode would NOT do the same thing.

Ah, thanks for clarification. (As I understood it, the purpose was basically to prevent static or overload damage from current flowing into the tap)

N_Jay said:
A capacitor decouples any (or most) DC from the scanner circuit, and will also drop the AC loading to some extent.

I think that your explanation may be the first I've seen as to why some tap instructions specify caps rather than resistors. (At least one site questioned capacitor usage, recommending resistors)

N_Jay said:
The best way (and maybe over-kill) would be to tap the circuit with a very small capacitor driving the input of an Op-Amp (preferable a MOSFET Op-Amp) which is internal to the scanner and run of the scanner power. The output of this Op-Amp should be provided to the outside connection through a capacitor for further isolation. It would be good if this connection was bypassed to ground through a small capacitor also.

(Like I said, probably over-kill) :wink:

LOL...if you know of a way to fit it into a handheld... :3

Seriously, though, the issue of caps vs resistors in a tap *is* one that does cause confusion and probably should be discussed more.

As for the GRE-manufactured scanners (I can't really speak on Uniden kit, ICOM kit et al) the usual method of tapping involves soldering to a test point. (One would actually think GRE knows someone is going to discriminator tap and wants to make it as easy as possible...especially on the Pro-2067 and Pro-96/2096).

I've so far hesitated with the '96 I own because I have no real desire to risk a $500 radio till I hear more reports of success and what specific caps/resistors may be needed :3 (And as there IS sufficient room in these handhelds to put in equipment easily, it probably isn't much to put an op-amp, lol)
 
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N_Jay

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windigofer said:
(I think the post above actually has gone into great detail on that, incidentially, though further comment is appreciated)

Just ask.
 

Dewey

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I thank everyone for all of the quality replies! While I don't see any definitive answers, it look's like the consensus appears to be use a resistor with the GRE equipment???

Dewey
 
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N_Jay

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pro92b said:

A resistor in Series with a capacitor, with a small shunt capacitor tp ground for static protection wopuld be even better.

The less signal you tap off the less loading you are causing. So a sensitive "input" on the slicer/sound card would help.

Of course low signal levels are also most susceptable to noise.
(AHhh, the fun of design trade-offs).

Of course like most efforts "Good enough" is GOOD ENOUGH, and "better" is not any BETTER! :twisted: :wink:
 

pro92b

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A series capacitor serves no purpose as the referenced URL's explain.

The output impedance of the Discriminator pin is around 500 Ohms. A 10K series resistor provides all the isolation needed. It should be mounted right at the IC pin for best results. The device connected to the tap should be 100K Ohms or higher to avoid attenuation of the signal. Every slicer and soundcard input I know of meets this requirement.
 
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