From: moy@xp.psych.nyu.edu
Newsgroups: rec.radio.scanner
Subject: PRO-51 Mod: Shorten squelch tail
Date: 20 Sep 1994 20:34:06 -0500
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Hello fellow scanbots,

Radio Shack PRO-51: Modification to shorton squelch tail

This change was performed successfully on a unit coded "11A3."  The
CPU in this unit was marked "Uniden UC1581."  Use the following
information at your own risk!  Neither I nor my employer are
responsible for any damages you may incur from this information!

Orientations are with the set flat on a work surface, LCD display 
facing down, with antenna BNC and knobs facing away from yourself.

As with any electronic instrumentation, remove all power and take 
all necessary precautions to prevent damage from electrostatic 
discharge (ESD).  This modification requires fine soldering and
familiarity with surface mount devices.

CAUTION:

Be aware that parts of the circuitry will *still* be powered!
There is a capacitor on the digital board that keeps the channel 
memory alive.  This capacitor may contain enough energy to damage 
the electronics should a short occur.

Parts required: One 1/10th watt carbon resistor, 100K to 220K.  Value is
not critical.  A lower value shortens the time the squelch remains open
after incoming signal stops.  The resistor should physically as small
as possible.

Opening Case

Fortunately, this set is very easy to open.  Open battery 
compartment and remove batteries.  Remove four screws: two 
straddle the sticker on the back of the cabinet, two others at the 
upper part of the battery compartment.  There are two small 
plastic detents at the floor of the battery compartment.  They are 
directly downwards from the "-" and "+" legends, which are molded 
on an extension of the removable back.  These detents will need to 
be unhitched to allow the back to separate from the front.
Pull back off gently.

Remove RF board

The RF board is attached to the top panel, which contains the 
volume and squelch controls and the antenna connector.  At this 
point, the board is essentially free and will separate from the 
rest of the set when the header connector at the lower right is 
pulled apart.  This header provides electrical connections between 
the two boards.  Lift the RF board straight out, and set the rest of the
radio aside.  Flip RF board over, keeping BNC pointing away from
yourself.

Add resistor

Refer to the ASCII diagram below.  Bend one of the resistor leads close
to the body of the resistor so that the resistor forms a hairpin shape.
Trim leads and solder the new resistor in parallel with the chip
resistor marked "105."  The latter is positioned between the two
surface-mount IC's towards the bottom of the RF board.  Press the new
resistor parallel to the circuit board, making sure there are no
shorts.

                   +------------+
         [101]     |            |
                   |            |
         [684]     |  NJM2090V  |
                   |            |
         [224]     |            |
                   +------------+
         [   ]   [   ]            [  || ]

         [225]   [105]              [123]
                 |   |
                 |  +-+
                 |  |R|   <---add resistor: 100K - 220K
                 |  +-+
                  \_/       +------------+
                            |            |
                            |            |
                            |  TK11900M  |
                            |            |
                            |            |
                            +------------+


Thanks to Bob Parnass for clarifying the nature of the "squelch tail"
problem and for inspiring me to figure out this modification.

Moy Wong (moy@xp.psych.nyu.edu) Dept. of Psychology, New York University