Midland 75-820 400 Channel Modification
Disclaimer: The literature below is for informational purposes only and are deemed illegal in many countries.
All 10 meter and export CB radio mods, adjustments, tune-ups, channel/frequency expansions, modulation, power
adjustments are related modifications are also done at the users risk and we assume no liability.
Midland 75-820 400 channel modification
120 channel mode is nice, 400 channels mode rules!!
WARNING, read carefully: This is NOT a simple and straightforward soldering
modification and it will require good soldering skills from you. You will also
need good and sharp soldering iron, a thin aluminum cutter and a very thin chip
picker for picking and placing small chips from a surface mount board.
If you read these instructions, and you don't understand anything about them,
don't even think about trying what is shown here. I am not responsable for any
damage, malfunction or failure done to the units with this modification. I
assume that you have a good knowledge about electronics. And that your soldering
skills are above the normal.
The author has tried this modification on his own radio before making this
information public. He has found it to work fine, and without any problems.
Extra material needed:
Type A3 three pin chip.
1) Opening the Midland 75-820 radio. Remove all the screws that
hold the upper and the lower parts. Take only the silver screw that
holds the upper part. MAKE SURE YOU DON'T NEITHER LOOSE THE TINY SPRING
OF THE BATTERY LOCK NOR BREAK ANY OF THE SPEAKER CABLES!!
2) Separating the CPU Motherboard. Once the radio is opened, you need
to get access to the CPU boot registers. Once separated, make sure you don't
break the thick brown cable that links the CPU motherboard to the rest of
3) Removing the plate. The first thing that appears is a metal plate
that protects the CPU. Use a very sharp cutter for metals to take out this
plate and expose the MAXON CPU. Thats right it's a MAXON design.
4) The register modification. This is when things get more complicated.
Locate the three pin solderings that have no chip on them.
Once located you need to solder one A3 chip type onto the empty socket.
(I got this chip from some spare parts of another radio) This modification
will get the radio to behave like the first models of 75-820, that had
400 channel operation mode instead of the H/L 120 channel mode that
is currently provided.
5) Setting up the 400 channel mode. Once you reassemble the
motherboard with the rest of the radio you need to reboot the MAXON CPU.
However, a new jumper configuration is needed to set the 400 channel mode.
Locate the holes it the front face of the upper part, as it is shown in
picture below, put some thin copper wire that shorts those holes and
reboot the CPU. Turn on the radio ON and if everything went right, then you'll
see on the LCD the number 9 followed by the letter d, which means ... 400