> -----Original Message-----
> While the +5 and -5 are off the same transformer, the +5 is designed for
> a much greater current draw. Regulation on the -5 is simply by
> zener. A back of the envelope analysis of the Zener supply suggests
> that with the 180 ohm R97, and a -7.6v supply, it is only rated at 15mA
> or so, which is not much, so any excessive load will drop the -5v.
I learned something today! After reading this and googling 'zener diode
regulation' I now understand what is going on - I did not previously know about
this property of zener diodes and I was assuming (given its orientation) that
it was just there to help rectification by shunting any positive voltage to
ground (although this made no sense since D15 already ensures there won't be
any positive voltage at that point in the circuit). I had assumed that somehow
the value of R97 had been chosen so that they would end up with -5v assuming a
constant load, which also made little sense since other motherboards I've
checked produce a pretty solid -5v with or without the LCD module plugged in...
> I would be looking at the 4585 chips driver chip, or the opamps used for
> the modem, or those caps etc.
>
> You could remove the -5v supply to M29 and M38, by carefully cutting the
> trace to pin 11 - that would remove the -5v and see if that
> helped. Same for M35, pin 7 is the -5v supply.
Ouch. Well, I guess I can try that by finding points on the board where it
would be easy to bridge the cut with solder later. I keep hoping these
desoldering needles are going to show up...
One thing I've got which may be helpful is an old Fluke digital multimeter I've
had since I was a pre-teen (Dad gave it to me because his employer was throwing
it out!!). It's an 8030A which is an LED-based 3.5-digit unit from the '70s
(runs on 4 NiCd C-cells) but it still works great and it has a nice DC current
function that goes from 2000 milliamps down to a 200 microamp scale (with
decimal point for a 1/10 microamp resolution). I guess I can just as easily
tell whether I've isolated the problem component by whether or not the voltage
can maintain -5v :) but it would be nice to know what exactly it's trying to
draw.
> This will be a complex repair - but when you find it it will be
> extremely satisfying.
Thanks so much for chiming in on this issue - I would be lost without your
guidance! :) My kids are here this week so I may not have time to get back at
it until after next Sunday, I'll post again when I have more info.
jim
