[8] Radio Technology (500:10/12.1) RADIO_TEK
Msg : #3305 [157] Uns Loc
From : Bill Cheek 1:202/731.1 Sat 06 Feb 93 00:24
To : All
Subj : PRO-43 S-Meter!

.MSGID: 1:202/731.1 2b73580c
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

--- GoldED 2.41.B0108+
* Origin: Hertzian Intercept, San Diego (6pm-1pm) 619-578-9247 (1:202/731.1)