thanks for that Jim. I checked and I have the "silicon diode + resistor" version of this mod on my M100 now.
In my M100 (powered off): Wall wart voltage = 7.5V Voltage @ 4xAA = 5.5V (while charging) Voltage across resistor/diode = 2V I'll monitor this as it charges up. Ideally I think one would want to see the AA NiMH batts at 1.45V each, or 5.8V max during charging, and then maybe 1.4V each for 5.6V when initially loaded. Thinking in this case that the silicon diode is probably ok. On Mon, Nov 23, 2020 at 4:47 PM Jim Anderson <[email protected]> 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 > >
