Icom UX-49 Mods for Quicker Lock-up

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. (see below for option)
Replace C 97 with a 10 uF @ 35 volt Tantalum type capacitor. (see below for option)

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 in substitution of 1/8 watt resistors for the modification and they work great too.

Andy WJ9J provides the following update after modifying eleven of these UHF modules in the summer of 2022:

Hi Kevin,

I wanted to follow up and say I have bench modded and tested 11 UX-49A band units. (I still have #12 to do but it's at a site 80 miles away).

There is some variableness on how some of them behaved.
These values additionally help reduce a "ping" that's sometimes heard as the PLL re-locks.
I have found the following changes to improve the mod slightly:

1. The value of C97 - What I have found is that a 15uF Tantalum works every time with no distortion on the low tones so that is what I settled on. By low tones, I mean 67-74 Hz.
2. The value on C87 - Seems to work better as a 1uF. I prefer the response at 1uF.

Thnaks,
Andy