Oregon Scientific TP-326PMR External Power & Charging Mod
All mention of use with rechargeable batteries seems to have been erased from the instruction manual to the TP-326 version 2.2PMR. This model has a serial number beginning 303. It is apparent, however, that the USA version from which it was derived had this feature and used the larger jack socket to do so. It's a stereo 3.5mm socket and the barrel is ground as expected. The tip and ring don't connect to anything. There is a redundant charge circuit weaving its way through the CPU PCB and this ends up on the +ve battery terminal. Along the way, there are unoccupied positions for a resistor/fuse and a diode. These positions are clearly silk screen printed on the CPU PCB. You will find them below the display module near to the LAMP and CTCSS buttons.
To re-instate this power circuit, repopulate the two
positions with a rectifier diode such as the IN4001 and a 600mA fuse. This is a
nice easy mod which should only take 10mins and cost 50p. The socket will then
accept a 6.8v source, current limited to 60mA which will give a 12hour slow
charge with the radio switched off. It's important to note that there is no
charge regulation within the radio itself, this has to be done externally in the
power supply. If the supply is not regulated, the batteries will be damaged, the
radio may melt and you may injure someone. Even with the correct regulation, it
is not good practice to leave the batteries on charge permanently as they will
You can also use this socket to run the radio from external power. Again you need a well regulated power supply but this time capable of 500mA. You must remove the batteries or slip a piece of thin cardboard under one of the battery terminals. The radio draws 40mA with the receiver muted or scanning and 380mA under transmit. It is a very good idea to fit a ferrite bead to the plug end of the power lead in order to mop up any stray RF and stop it from being radiated from the power leads and even freaking out the power supply under transmit. There is also a vacant position for a decoupling capacitor near the power jack which will help in this respect.
Here's my theory as to why the diode was left out on these models. Oregon Scientific wanted a product with which to compete with the cheap end of the Europe wide PMR446 market and required a single product which would serve all target countries without having to supply a variety of specific power adaptors to suit each locale. In omitting these, they could shave £8 off the production cost plus another £6 off if the rechargeable batteries were left out. In addition, they would reduce their shipping weight to less than a third. They were concerned that an end user, possibly a child might plug in any old adaptor without the correct external charge regulation and cause personal injury, so the diode was omitted.
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