> -----Original Message-----
>
> The -5V supply is off the same transformer as the +5 v supply.
I know, that's what really baffles me - how the +5 could be fine and -5 be so
low!
> The fault domain could be:
>
> 1) D15 Shorted or Open - Check with multimeter
I did lift a few of the diodes off - D15 shows 0.54 volts forward drop and open
in reverse. I'm not sure if that's a good value for that diode or not, but at
least it isn't shorted or open.
> 2) R97 Open - Check with Multimeter
Checked, and it's 180 ohms.
> 3) Zener D14 Shorted, or incorrect voltage - Check supply voltage Across
> C85
I lifted and checked D14 and got 0.73 volts forward drop and open in reverse
(again, not sure if this is an appropriate value or not). I didn't check the
voltage across C85 before lifting D13, D14 and D15 - would it be any use
testing the voltage between the D15 input (terminal 9 on the transformer) and
ground? Or should I put the diodes back in circuit before powering it on?
> 4) C86 shorted or leaky
I did just finish replacing all the caps and it's pretty much the same as it
was with the old caps. I just double-checked C86 with an ohmmeter and it seems
as expected (starts in the megohms and declines into the hundreds of kilohms
over the space of several seconds).
> 5) Excessive load on -5V supply - Just R97 and see what the available
> supply is between D15-R97 and ground.
I'm not sure what you mean - is 'just' a typo? Did you mean to lift R97 and
then check the voltage (across C85, basically)? I did this (after putting D13
and D15 back first) and I get -7.6 volts which is about what the service manual
says it should be (page 4-32) without R97 and D14 to stabilize it at -5 volts.
(I still have D14 lifted and when I touch R97 back into place the voltage drops
immediately to -1.7 volts, so D14 doesn't seem to be contributing anything to
the fault.)
I've been thinking a bit about this idea of excessive load - I know the LCD
needs VEE but I don't have the LCD module connected to this board, so what else
uses it? I know RS-232 needs negative voltage, and I see it used several
places there for six pulldown resistors and feeding into pin 7 of M35. After
more peering at the schematic (sort of a "where's waldo" for VEE arrows) I
spotted it on M29 and M30 pin 11 (listed as bipolar op amps), at T1 through
R113 (presumably as a pulldown for the buzzer circuit - R113 tests OK at 3.2
kilohms), and of course the LCD connector pin 3. I didn't spot anything else -
could that be all?
Not sure where to go from here - assuming I've identified all the places VEE
goes on the motherboard, I guess I'd have to pull M35, M29, and M30 (one at a
time, of course) to see if one of them is the culprit. I don't have a good
desoldering vacuum (just a springloaded handheld one) and I'm still waiting for
the set of desoldering needles I ordered from China to arrive. I would think
that with the desoldering needles I'd be able to disconnect just one pin from
an IC without pulling the whole thing... but that's just my theory, I haven't
ever tried them.
> Check out my awesome clocks at
I love these! Time to start saving my pennies again... :)
jim
