Subject: MAXAR temp drift mod



This file contains information to REDUCE the amount temp drift that the

Motorola Maxar series radios exhibit.  This is a proven modification,

however, Iam not responsible for anything contained in this information.



From day one, the Maxar series of Motorola radios have had a temperature-0

frequency drift problem.  Motorola is aware of it and has issued

Tech Bulletins on the subject.  None of the Tech Bulletins corrected

the problem as well as the following mod does.



The Motorola Maxar 80 radio has a built in temperature-0 compensating

REGULATOR built into the radio.  This regulator works below -10 degrees C.

Therefore, the crystal temperature-0 in Not maintained under most normal

operating conditions.

The heart of the regulator consists of a resistor and an NTC thermistor.

Basically, the increase of temperature-0 at the crystal increases current

flow thru the thermistor which decreases current flow thru the resistor

which lowers the temperature-0 of the crystals.  The values of the standard

components cause the "idle" point of the circuit to be somewhere above

-10 degree C.  When the temperature-0 drops below the "idle" point, current

decreases thru the thermistor which increases current flow thru the

resistor which heats the crystal.

The concept of this mod was to merely increase the "idle" point of this

circuit to insure that the NORMAL average conditions could be delt with

in a reasonable way.  During this mod, it was also found in the original

design, that the regulator is wired to switch battery. this mod also

corrects this.

TOOLS AND SUPPLIES NEEDED:



a.  Soldering iron

b.  Solder

c.  Short length of hook-up wire....6inches??? #20

d.  2k ohm 1/4 watt resistor

e.  Sharp knife or razor

f.  Manual on radio you have



A) Locate the TRANSMIT crystal.  You should find a resistor right next

   to it.  Directly under the resistor, you should find the thermistor.

   it is usually wrapped in heat-shrink along with the resistor.

   Also locate this on the schematic in the manual.  It is located in the

   UNIFIED CHASSIS, exciter section.

B) Locate the trace going to both RT101 and R135. Cut it.

C) Add a jumper wire from the junction of RT101 and R135 to A+.

   Before you cut the trace, the junction was wired to SW A+.  Make sure

   you wire to A+, NOT SW A+.

D) Locate the other side of RT101 and the trace.  Cut it and clean both

   sides of the trace for soldering.

E) Add the 2k resistor across the cut you just made.

    ****YOU ARE DONE****

NOTES:

   As best I can tell, the crystal temp is now maintained at 80 degrees F.

   I have done this mod to a couple of radios and have not had to adjust

   the freq since...bout 2 years!

   This mod does increase the idle current flow of the radio.  Not much but

   you dont get something for nothing.

   73 de Jim, WA6OFT