
Title: Mod for HTX-100


      Hi there,
               Remove bottom cover,with the radio upside down and
 display facing you remove the synthesizer board with the four screws,
 this is near the front of the radio.
               This gives access to the small board underneath with
 the surface mount components on it. Locate the processor chip IC401
 (UC-1208).  Locate pins 28 and 29 which are on the right handside of
 the chip and are 4th and 5th from the top.  Note they are bridged with
 solder.  CAREFULLY cut the copper track next to the pins.  DO NOT attempt
 to unsolder the two pins from the board.  Use a grounded soldering iron.
 Locate the two rows of connector pins above the chip.  On the bottom row
 locate the last pin on the right.  Varify with a VOM that this pin has
 + 5 volts.  Solder a 10K (1/4 watt) resistor between this pin and pins
 28 and 29 on IC401. The synthesizor will now tune from 26.000 mhz to
 30.000 mhz.
               *RF wattage to 35 watts*
               With the radio in the same position as before locate VR5
 in the top left hand corner.  This is the RF high powered preset.  Lift
 and bend the wiper so that it is permanetly disconnected from the track.
 Solder a 47K resistor (1/4 watt) between the wiper and the left hand side
 of the track.  This should increase the RF power output to around 35 watts
 but you will now not be able to trim the power output.  The low power trim
 will function as before.
               *Enhancement of microphone frequency response*
               1.  To increase average depth of modulation.
 Remove  top  cover  of  the  radio and with the display facing you and the
 radio upright locate IC3 (MJM45585) near the bottom right hand corner of
 the main large board.  Just to the right of this IC locate C75 (56pf) and
 remove   it.   This  will  increase  the  gain  of  the  microphone preamp
 considerably but without over modulation.  If you find the compression
 excessive (I.E.  background noise ) open the microphone and insert a 470
 ohm resistor between the hot side of the microphone insert and the input
 (white lead).  This will reduce the gain somewhat but will still give you
 much more increased talk power.
              2.   To increase audio response in the high mids.
 This simple mod will increase the audio response in TX in the 2 to 2.7KHZ
 region  giving  higher  articulation  and  more intelligibility under weak
 signal conditions.  Locate IC3 as before on the large main board and the
 radio positioned as before.  Locate a 5.6K and a 4.7k resistor just below
 IC3.  Solder a 0.015 mf (16v) capacitor between the left side of the 4.7k
 to the right side of the 5.6K resistor.  These two mods work together to
 give improved readability on SSB under DX conditions.

              Ok  that's all I have on the HTX-100 for now.


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