I did the full set of electrolytics, a total of 17 – C49, C50, C52, C54, C55,
C75, C76, C77, C78, C82, C83, C84, C85, C86, C90, C92, C103. (I know the
majority of these have nothing to do with the power supply, it’s mostly meant
to prevent future issues due to capacitor failure.)
jim
From: M100 [mailto:[email protected]] On Behalf Of Stephen
Adolph
Sent: Thursday, October 15, 2020 04:18
To: [email protected]
Subject: Re: [M100] low VEE voltage - help me?
CAUTION External Sender: Do not click links or open attachments unless you
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Which capacitors in particular did you replace?
On Thu, Oct 15, 2020 at 2:21 AM Jim Anderson
<[email protected]<mailto:[email protected]>> wrote:
Hi,
My first M100 died on me about two years ago with a problem where the VEE
supply voltage is low (between -1.4 and -1.7 volts, instead of the expected
-5v). At the time I posted here and the best guesses centered around one or
more bad capacitors, particularly C85. I did get around to removing C85 at the
time and it did have a tiny amount of fuzzy crud on the bottom around the
leads. I put the whole thing aside and swapped in a motherboard from another
parts machine.
Fast forward to this year, after continuously putting off ordering the
capacitors from digikey, I finally ordered several capacitor replacement kits
and nimh memory batteries from arcadeshopper and today sat down to re-cap this
motherboard. Well, it seems maybe I should have started with a different
machine because all I've done is blow a few hours and frustrate myself. I
checked after replacing C85 and VEE was still only -1.7 volts, so I soldiered
on in the vain hope that maybe another bad cap was at fault - replaced all the
caps and sure enough it was still only -1.7 volts when I was done. :(
VDD is correct (+4.97 volts) and the memory nimh is charging when power is
connected (voltage at the battery rises). I'm testing the motherboard without
the lcd or keyboard connected as I'm pretty sure it's supposed to be able to
make -5v on VEE without those components connected...
I'm not really familiar with expected diode test values, but to give you guys
some additional info to go on (in case any of these are bad) I lifted one end
on each of D13, D14, and D15 to test them, measured 0.22, 0.73, and 0.54 volts
forward drop respectively and open circuit in reverse. I don't know if those
are appropriate values. I also checked R97 and it is 180 ohms as expected.
Help!!! I feel like I've been banging my head against the wall all evening.
jim
