DIGICOM
From: WB3JDI @ NM3G
Subject: C.A.R.S.DIGICOM BOARD MOD.
MODIFICATIONS TO THE CARS DIGICOM BOARD
FOR ALL THAT HAVE CARS DIGICOM BOARDS. THIS MODIFICATION WILL IMPROVE
THE RECEIVE PERFORMANCE OF THE BOARD.
REQUIRED PARTS:
1. 18K 1/8 WATT RESISTOR OR 75K
1. 100K RESISTORS
1. .0022 MYLAR DISC CAP.
1. .047 DISC CAP.
1. 2N3904 NPN TRANSISTOR
3. .001 CAPS
RCV TOP VIEW
ADJ
:--------H--H--H--------------:
: H H H FSK HH :
: H H H 0 HH :
: H H H :
: R1 ======== X 1
: 2206 X 2
: ======= C1======== Z1 X 3
: 2211 X 4
: ======= X 5
: X 6
: 0 0 :
: RCV C2 PTT :
: :
:-----------------------------:
R1 56K LIFT ONE END AND ADD 18K OR REPLACE WITH A 75K
C1 .0018 OR .001 REPLACE WITH A .0022
C2 .1 REPLACE WITH A .047
Z1 5V ZENER DIODE REMOVE
========================================================
TRACE SIDE VIEW
: :
: :
: TO 2211 :
: 6/7 :
: GND ~ :
: + + :
: + + :
: 100K+ + :
: B>-####-->+ :
: + + +:
:+ ++ >+++++++ / CUT TRACE:
:+ + E / +:
:+ GROUND + C>++++++ GND +:
:+++++++++++ + + ++++++++:
.......... + + + + + + .........:
: * * * * * * :
: * * * * * * :
:=*==*==*==*==*==*=:
1 2 3 4 5 6
1.CUT THE TRAC
E AS SHOWN BETWEEN 6/7 2211 TO 4/6 EDGE CONNECTOR
2.ADD THE 2N3904 THIS WAY: COLLECTOR: TO 4/6 EDGE CONNECTOR
OK TO MOUNt ON TRACE EMITTER: TO GROUND PIN 1 EDGE CONNECTOR
SIDE. BASE: IN SERIES WITH A 100K RESISTOR
TO PIN 6/7 OF 2211 CHIP
3. ADD .001 CAPS TO EDGE CONNECTOR PIN 3 TO GND
PIN 5 TO GND
PIN 4/6 TO GND
THE REASON FOR THE MODIFICATIONS: SOME RADIOS ARE PRODUCING A STRONG
SECOND HARMONIC AUDIO SIGNAL AT 2400 HZ. WHEN THEY RECEIVE A STRONG
STATION. WE NARROW THE BAND PASS TO 2200 HZ AND DOWN SO THE BOARD WILL
NOT HEAR 2400 HZ. R1 AND C1 DOES THIS.
WE ALSO INCREASE THE LOC RESPONSE TO A FASTER VALUE SO THE 2211 LOCKS IN
ON A SIGNAL FASTER. THIS IS THE C2 FUNCTION.
ADDING THE TRANSISTOR LOWERS THE LOAD ON THE 2211 AND HELPS WITH ITS
CROSS OVER FREQ RESPONSE.
THE 3 .001 CAPS REDUSES THE RF PRODUCED BY THE 64 AND BYPASSES IT TO GROUND.
A SMALL AMOUNT OF RETUNING OF THE RECEIVE ADJ IS NECESSARY.
THIS WILL HELP MOST BOARDS WITH STRONG PACKET SIGNAL AND RECEIVE PROBLEMS.
FOR COMPLETE DRAWINGS SEND TO C.A.R.S. PO BOX 653 MEADVILLE PA. 16335
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-=*=- EDITED BY WB3JDI MEADVILLE PA. -=*=-
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RELAYED BY...N0KGX...GENE