10 Feb
2009
10 Feb
'09
11:45 a.m.
Tony Duell wrote:
I am pretty sure my Microwriter, an MW4 model, has a zener diode across
the battery pack to protect the machine if the battery goes open.
Incidnetally, one thing to watch for on this machine is that the power
'swtich (actually a pair of VMOS FETs in parallel) is in the -ve rail.
There's a common positive to all the ICs and seaprate awitched and
unswitched grounds.
Alas my MW4 seems to have a ROM problem (in that some chords are
misrecognised, it is not a switch/wirign/diode fault). If anyone has a
klnown-good dump of the 2564 in an MW4, I'd be interested...
-tony
Worse than that, as you mention in passing, in
some designs the battery
acts as a shunt regulator and stabilises the votlage on one of the main
supply lines.
The Microwriter does this, with four NiCad cells acting as a
"5V regulator". Naturally, my Microwriter came with four dud cells,
but I only paid a pound for it at a Bristol car boot sale!