Icom UX-49 UHF Band Unit Modifications
For quicker PLL lock-up
By Kevin K. Custer  W3KKC

The Icom IC-900 band modules are used generally with the ACC FC-900 remote base interface.  The UHF band unit, UX-49, has a problem locking up on TX quickly. This is especially notable when transmitting a CTCSS (PL Tone) and the transmit frequency is different from the receive frequency (having a shift).  When you key up, you have to wait at least a second to start talking due to the slow locking action.  This article explains what to do to help improve the lock time.  The modification is not for the beginner, as it involves some component changes in the band unit.  Advanced soldering ability is a must.  After modification has been completed, lock time is about the same as other band units.  I have performed this modification and I know that it works.

The following needs changed to these values listed (original values forgotten.)
Resistors:
Replace R 60 with a 100K (this is a 1/8 watt resistor)
Replace R 49 with a 2K2 (2.2K) (this is a 1/8 watt resistor)

Capacitors:
Replace C 87 with a .1 uF (point one uF) @ 35 volt tantalum type capacitor.
Replace C 97 with a 10 uF @ 35 volt tantalum type capacitor.

Also a 2.2 uF 16 volt tantalum capacitor should added from the collector of Q 15 to ground.
The positive side of the capacitor goes to the transistors collector lead solder pad.
Install this capacitor on the solder side of the circuit board.

The board is silk-screened with the component numbers for location.  I have used surface mount resistors for the modification and they work great.
 
 
 

Html Copyright 12-01-2002  Kevin Custer  W3KKC
All Rights Reserved.