Create gm300.md

wiki/gm300.md

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+# Hardware modifications
+
+There are 2 things to do:
+
+    Solder a 100k THT resistor in parallel with R552
+    Move jumper JU551 to position A
+
+Optionally, you can cut open pin 4 of the power amplifier IC. This allows an access to the internal speaker via 
+pins 1 and 15 on the back of the radio. JU551 jumper is visible on the left side, between an electrolytic 
+capacitor and 2 (MKP?) blue ones.
+
+![Image of the area on a GM300 mainboard on where to make the modification.](gm300_mod.png)