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
SOURCE: The QRZ Windows Ham Radio CDROM |
