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Icom IC-37A modification info
Collected from various places on the web
by Mike Morris WA6ILQ
The Icom IC-37A is a 25W/5W 222MHz FM mobile rig from the mid 1980s. To improve the receiver sensitivity, power output, SWR at the radio, and reliability, remove the SO-239 RG-58 coaxial pigtail extending from the rear of the radio. Install a BNC chassis mount, screw-in connector (UG-1094/U) in the hole left in the back of the radio. The new connector will screw in as if the modification was done at the factory. One note on the above - the BNC connector mentioned for the IC-27 / IC-37 / IC-47 series radios is sold by Radio Shack as their part number 278-105 at around $1.39 each (2005 price). This is a cheap nickel plated connector and is a piece of junk. If you can get one a better choice is a UG-1094/U Amphenol or Kings brand of the same connector (they are silver plated). To restore high-frequency receiver audio response, replace C-103 on the main unit with a .0022 uf mylar capacitor (thats 2200 pf). To restore LOW-frequency receiver audio response, replace C-106 with a .22 uf mylar capacitor. To eliminate the distortion in the transmit audio you will need to lower the audio level from the microphone. Open the microphone case and locate the the white lead coming from microphone element. Install a 4.7k ohm resistor in series with the white lead. Once the resistor is installed, reset the DTMF level pot to the middle of its range (this pot is the only one inside the microphone case). To complete the mod, set R88 inside the radio to the desired "mike" level. The PL tone is displayed as a code rather than the actual frequency, so unless you've memorized the codes you need the manual to set the tone. There are only 9 memories, two of which are used to set the programmed scan limits. These rigs are known for a voltage regulator problem that causes the displays to eventually burn out one segment at a time. It is possible to repair the display by removing the spare LEDs on the panel and using them to replace burned out LEDs. The LEDs are driven hard for maximum brightness. Replacement LEDs are not available from Icom. The LED supply drive regulator is a hack at best. R67 is often burned up and damages the circuit board. The voltage to the LED is 6 volts plus the voltage drop across D42. The simplest way to reduce the LED voltage is to jumper across (short out) D42. It is on the underside of the Logic Board. This is not a proper fix... you really need to replace the regulator circuit with a better one. You can reduce chassis heating in the receive mode by completely removing the shoddy regulator. Remove R66, R67, R68, Q32, D42 and D44 from the main unit. Install a 7808 (3-pin 8v DC voltage regulator) on the transmitter power amp shield behind the space reserved for the optional voice synthesizer. Thermal compound is recommended between the tab and the metal. Hook the input of the 7808 to the point where R67 and R68 used to connect. Connect the output of the 7808 to the anode end of a 1N4001 diode. Connect the cathode of the diode to J14-1. Or use a 7805 (3-pin 5v DC voltage regulator) and leave the diode out. The LEDs will be dimmer but usable as a base station.
The author can be contacted at: his-callsign // at // repeater-builder // dot // com.
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Text, artistic layout and hand-coded HTML © Copyright 2008 and date of last
update by Mike Morris WA6ILQ.
This page originally posted on 27-Dec-2008.
This web site, and the information presented in and on its pages is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors.