HI, I did a similar modification to this when I was much younger.
It worked beautifully. Later I installed a LM317 voltage regulator on a heatsink inside the uinti so I could power it with 12V, like the rest of my Amateur Radio gear. The unit would stay plugged into permanent power for days. Then one day I realised that the Nicads had heated to the point where the battery springs had melted into the plastic retainers. It took ages to repair the case melting issues around the battery compartment. Lesson learned: Do not leave NiCads on charge for too long. I still use this machine for lots of interesting things, including talking to a Nicolet Z80 pod when debugging other machines. Kindest regards, Doug Jackson em: d...@doughq.com ph: 0414 986878 Check out my awesome clocks at www.dougswordclocks.com Follow my amateur radio adventures at vk1zdj.net ----------------------------------------------------------- Just like an old fashioned letter, this email and any files transmitted with it should probably be treated as confidential and intended solely for your own use. Please note that any interesting spelling is usually my own and may have been caused by fat thumbs on a tiny tiny keyboard. Should any part of this message prove to be useful in the event of the imminent Zombie Apocalypse then the sender bears no personal, legal, or moral responsibility for any outcome resulting from its usage unless the result of said usage is the unlikely defeat of the Zombie Hordes in which case the sender takes full credit without any theoretical or actual legal liability. :-) Be nice to your parents. Go outside and do something awesome - Draw, paint, walk, setup a radio station, go fishing or sailing - just do something that makes you happy. ^G ^G ^G ^G ^G ^G ^G ^G- In more laid back days this line would literally sing ^G ^G ^G ^G ^G ^G ^G ^G On Tue, 24 Nov 2020 at 08:47, Jim Anderson <jim.ander...@kpu.ca> wrote: > > -----Original Message----- > > Option 1: 47 ohm > > - works, but does not prevent reverse conduction from batteries to an > > unplugged wallwart > > > > option 2: 10 ohms + silicon diode: > > - protects from reverse current, but drops the voltage for charging > > > > option 3: 10 ohms + schottky diode > > - also protects , maybe a little less voltage drop? > > I had a look at the T200 service manual to see what the official solution > was for internal ni-cd charging, and essentially the jumper on J302 just > places a 27 ohm 1/2 watt resistor between the barrel jack sleeve input and > the AA-holder positive terminal. So, same modification, but now we have > three different resistance value suggestions. :) > > I don't know enough about differences between charging ni-cd and ni-mh to > comment on which resistor value would be most appropriate for each type. > > Also, your mention of a diode reminded me of the service bulletin 200:3 > which called for using a 1N914 diode in place of the jumper at J302 to > prevent problems with lockup of the power-on circuit of the T200 in the > event that the wall wart is unplugged and the power button is pressed while > the barrel plug is still inserted... The service bulletin did not suggest > changing the resistor value from 27 ohms to compensate for the voltage drop > across the diode. > > The other half of the T200 mod is not essential, but nice - J301 which > puts a 94.4k resistor (R93) in parallel with 22.6k resistor (R94) to shift > down the trigger voltages for the low battery LED and auto power-off. This > could be done in exactly the same way by piggybacking a 94.4k 1/4W resistor > parallel onto R108 in the M100. Calculating it out, the resistance value > would only be 1.1% higher by using a 100k resistor instead of 94.4k so that > might make this part of the mod more appealing for those who have common > resistors on hand. :) (The 94.4k resistor shifts the combined value of > the two resistors 19.3% lower whereas a 100k resistor shifts the combined > value 18.4% lower - not a huge deal imho.) > > All this just made me realize that this is why negative tip/positive > barrel was so common on older gear like this - the insertion switch in the > jack is connected to the barrel contact, so you have to use the barrel for > whichever polarity you want to switch away from the batteries - to use tip > positive/barrel negative you'd have to switch the negative side of the > battery pack... > > > > > > > > jim > >
