Date: 02-06-93 00:24
   From: Bill Cheek
     To: All
Subject: PRO-43 S-Meter!

Greetings Scanner Fans!

It had to happen!  The fundamentals for an S-Meter exist alive and well in the
PRO-43!  Only a simple circuit and your choice of meter, analog or LED, is
required for a jim-dandy S-Meter function!

The key point in the circuit is the cathode of D-302 located on the bottom
(solder side) of the AF Board (middle board).  You will need the Service
Manual for the PRO-43 to find it as it's not marked and it would take me
10,000 words to guide you to exactly where it is located.  Call Tandy National
Parts at (800) 442-2425 to order your manual: approx $10.  Now here is the
deal:

Locate D-302 and either build the following circuit right there on the back of
the AF board in the vicinity of D-302, or solder a wire to the cathode of
D-302 and route it to somewhere else more convenient to feed the below
circuit.  This wire ought not to be very long, though, say not more than a
couple of inches. Refer to the ASCII schematic diagram as follows:

            PRO-43
PRO-43      D-302         existing
 T-302 x---o--|<-------x  PRO-43 ckt
         / |
 Connect   |
  here     |     0.01-uF          1N34A
           |------||------o-------->|----o---o-----------> (+)
                          |              |   |
                          |              \   |              To the
                          |         47-k /   |              S-Meter of
                          |         ohms \   = 0.01-uF      Your Choice
                          |              /   |              (This should
                          |   1N34A      |   |              be a jack
                          |-----|<-------o---o----------> (-) mounted on
                                     ^ PRO-43 Ground^       the case of
Legend:                                                     the scanner.
                    |
1.  ---||---  and   =  are 0.01-uF capacitors; disk, mylar, monolythic, chip
                    |

2.  --->|---  and  ---|<----  are germanium diodes, 1N34A
                              | is cathode
                              < is anode

3.     o       means a connection, preferably soldered

4.  The 47-k resistor is self-explanatory, I hope.

5.  --------  and  |   are wires/conductors.
                   |
                   |

6.     x    is existing PRO-43 circuitry


If you use an analog meter, preferably a real S-Meter like salvaged from an
old junked CB rig, then rig it as follows to connect to the circuit above:

                             5-k Trim Pot
                 (+) --------/\/\/\/\---o--------|
                                |       |        |
        This should             |-------|        |
        be a plug of                             +
        some sort to                          The S-Meter
        connect to the                         Terminals
        mating jack on                           -
        the scanner                              |
                                                 |
                 (-)-----------------------------|

You can also build the 10-segment LED S-Meter as shown in Volume 2 of
my SCANNER MODIFICATION HANDBOOK, available from leading mail order
radio dealers everywhere.  ISBN 0-939780-14-3  published by CRB Research
Books, Inc. Commack, NY.

After connected and working, locate the PRO-43 very near a known strong
transmitter, perhaps a ham transmitter, a police cruiser, your favorite
security guard's handheld or right next to a cordless telephone, or anything
else known to transmit a strong signal.  Tune the scanner to the frequency,
and adjust the 5-k trim pot above so the meter reads exactly full scale.  All
other signals will read proportionally lower to yield "relative signal
strength" measurements!  A hell of a circuit here, boys & girls!  Have fun!

Developed & distributed by:    Bill Cheek
                               COMMtronics Engineering
                               PO Box 262478
                               San Diego, CA 92196-2478

Ok to distribute, but credit appreciated.  Thanks!

Bill Cheek

--- GEcho 1.00/beta+
 * Origin: Hertzian Intercept, San Diego (6pm-1pm) 619-578-9247 (1:202/731.1)