> -----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
