Icom IC-37 family modification info

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Back to Home 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.
The author had an IC-37 for a few years.  He collected the info below 
as he resolved the lousy audio quality and LED failures.

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. Use a mini-drop of blue Locktite to prevent the 
new connector from unscrewing.

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).
The RS part is a cheap nickel plated junk connector. 
Please get a better one - look for an Amphenol, RFS or Kings 
brand UG-1094/U (a high quality silver plated unit).

To restore high-frequency receiver audio response, replace C-103 
on the main circuit board with a .0022 uf mylar capacitor (that's 
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. You could also put 
the reisitor in the radio, but inside the microphone is easier.  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 microphone
audio 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. Or xerox the manal page, cut the chart out and laminate it, and 
place the card in your wallet.

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.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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