Friday Vector Trivia #001

  Hi All...

I have about 10 Asteroids PCB's that are dead. I will post a new one
every friday with a synopsis of the issue and an attempt at a rationale
of the issue up front. Im sure some of them might be complex enough that
we might need to do a few rounds but this will be something fun for the
archives either way. In 'CarTalk' fashion, I will draw a name from all
that made correct guesses and send them some free beer schwag from New
Belgium Brewing here in Fort Collins, CO (my humble employer). If Im
correct on my guess at the solution, then my name goes in the hat too
(the house has to have an angle doesn't it?)

'Ditto' guesses don't count, but you can come up with a different
rationale that names the same defective part as someone else and I will
count it.

Monitor in my cabinet is known good (except for blooming until I get my
HV diodes from Mark C. :-)

Asteroids PCB #1 (Rev C)

I haven't pulled any chips on this guy yet, but the cool thing about the
vector engines is that you can tell a lot by just surmising info from
the test display...

http://www.ipsnd.net/images/asteroids_badimage_001.jpg

As you can see, there is a step in the diagonal, the horizontal line at
top shows some retrace from defect and if you look ever so closely on
the vertical lines, there is a slight skip to the left.

The issue is not...

1. The monitor
2. The circuitry after the first buffer op-amp (eliminates the 4016's
and the final Op-Amp on the PCB)

I don't think it relates to 'Big Blue' as I figured the defect would
move if it were an analog noise issue (however, this *is* an assumption).

Here is my rationale... but it might have a flaw in it...

1. the defect seems to show mostly in the X axis and not the Y (Im going
to dismiss the slight deviation on the vertical line as I just can't
seem to explain it). If you look close at the image, when moving
diagonally, the X vector counter seems to count backwards one(maybe two)
step(s) every certain count.
2. There are 15 defects moving along the X count axis. Since the X axis
counter is 1024 steps, that would mean that it is happening every
1024/16 (16 spaces resulting from the 15 defects)... or 64 counts, that
puts us on the defect occuring when bit 7 (one based) is going from low
to high.
3. Looking at the X counters (IC position on Asteroids PCB's depend on
the revision and there are G(7)? revisions) this falls into the two
lower end counters LS191's. There isn't anything complicated here, but
it seems that there isn't any way that the UNMDACX5-UNMDACX8 counter
could cause the UNMDACX1-UNMDACX4 lines to change since the lower
counter feeds the upper counter via the RIP (ripple) output. So, I
didn't think it would be a counter issue.
4. the next step in the circuit is the counter limit multiplexers
(LS157s) which limit the count range to 0-1023 to prevent wraparound
(overcounting). Again in this circuit, I don't see a way that bit 7
could cause a change in bits 0 or 1 since they are in physically
different IC's.
5. The next stop is the DAC, since there is lots going on in the DAC
there certainly could be a small effect when bit 7 flips potentially.
Both the X and Y DAC use the +5VDAC supply, there is a chance I suppose
that if the XDAC is messed up, that it might be putting noise onto the
VDAC line causing that little glitch in the X axis on vertical lines?

So, my guess is the X DAC.

Thoughts? Guesses? Please give a nice rationale for those newbies out
there!!! Only original rationales get counted, first come first serve
based on vectorlist timestamp.

have a great weekend everyone!!!

jess

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Received on Fri Aug 27 20:08:21 2010