Re: tech: Stern displays issues
- From: AL@xxxxxxxxxxx
- Date: 29 Apr 2007 12:05:23 -0700
On Apr 29, 11:38 am, "dew" <david.warde...@xxxxxxxxxxxxxxxxxx> wrote:
From Stern F2K - two displays have missing digits.
The easy one (I think) - The six-digit match/credit display is missing the
10'000's digit (which is the tens digit of the credits left half of the
display), so I don't know if I have 2 remaining credits, or 12, or 22, or
32. I checked the display board out - all resistors appear to be okay, but
when checking the transistors, I found one Q that showed a reading of 1.56
on one side. All other Qs register around .67 on both legs (as does the
other side of this one). So I'm guessing this Q is bad and replacing it will
take care of the problem (it's the digit driver transistor Q (Q11), not the
level shifter Q). Two questions - one, is this genuinely likely to be the
problem, and two, the 100K resistor in this circuit registers 97K ohms,
while all other 100K resistors for the other digits hit 100 or up - I would
think that's within tolerance but am not sure - should I replace it while
doing the Q?
The hard one (I know!) - player four seven-digit display does not show the
two left digits, 1M & 100K. Swapping other displays with it shows that the
problem is definitely in the display board, not coming from the MPU. All
100K resistors checked okay (anywhere from 100 to 104K), and all Qs seemed
to check okay. While checking other components, the 300K resistor on one of
those circuits read only 290 - is that within tolerance, or could that be
the problem? (I couldn't really tell if the band was silver or gold). The
300K resistor in the other bad circuit would show 297 for a single beat,
then blank out again to my DMM's default state of all zeroes with a leading
3 blinking, which is what it does when you turn it on until you are
measuring something. That sounds to me like that resistor is bad, although
I'm surprised it registers the correct impedance before blanking out. Could
that be causing this digit to not show? But the odd part here is, the same
resistor on the 10's circuit shows the same lack of value
(297-then-nothing), yet that digit shows up fine on the display.
Any help on this would be appreciated... Clearly I'm still at the beginner's
stage, but after years of just wanting to play my games with only doing as
little routine maintenance as is necessary to keep them running nicely, I
find that as I've finally started to delve deeper into the true repair
realm, I'm starting to enjoy it a little too much! Next thing you know I'll
be looking for a project machine!!
Thanks!
David
David,
In the resistor color code, the fourth band indicates tolerance. Gold
= +/- 5%. Silver = +/- 10%. In any case, your measurements are -1%.
Well within tolerance.
Possible causes of your meter going onto reset when taking a
resistance measurement:
~ Low Battery in DMM
~ Transient charge due to a large capacitance in circuit
~ Circuit is powered up.
Two other observations:
~ Static measurements on semiconductors can be misleading. (the most
accurate measurement would be voltage during opertion)
~ Borderline HV display voltage levels can cause these symptoms as
well.
This area of a pin carries DC Voltages in excess of +230VDC. If you
are experienced in measurements, exercise extreme caution when
testing. If you're not experienced you may want to seek assistance.
AL CARGPB 33(1/3)
www.Team-EM.com
.
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