[Coco] Weird CoCo3 problem...

[Gene Heskett]

On Monday 04 January 2010, Brian Blake wrote:
>Okay guys, got the o'scope, schematic and CoCo3 service manual, along with
> Frank's 'Tandy's Little Wonder' out for good measure and started reading
> and testing. The results have been posted to my blog at the following
> address so I could post pictures of the actual scope traces.
>
>http://cocom3repack.blogspot.com/2010/01/troubleshooting-perplexing-problem
>.html
>
>I'm not sure if these will be much help or not, as the test results are...
> interesting. I've got the exact same signal from the GIME to places I
> wouldn't think they should be (i.e.: GIME output to TP2, 3, 4 & 5 appears
> to be identical, and NOT what the service manual says they should be). I
> freely admit there are folks here who have forgotten more than I know
> about the CoCo family, so I hope these images help.
>
>
>Thanks guys,
>
>Brian

[huge snip]

First thing, either that scope is slow, or your ground leads from the probes 
are way too long and connected too far away.  Rise and fall times are slower, 
by 5x, than anything I've seen on my Hitachi V1065, a dual trace 100mhz 
scope.  Using 200mhz rated 10x probes, properly compensated.  You are using 
1x probes maybe?

Second, the E and Q clocks will look identical, _unless_ the timebase is 
always triggered from the same clock.  Use the E clock for the triggering 
signal, and if you have dual trace on that tek, which it should, look at both 
clocks at the same time in chopped mode.  The you should see the 90 degree 
phase angle between them.

Third, one of the lower pix/traces seems to indicate a data clash, at the 
left end of the trace is a fairly square pulse that just doesn't make it down 
to a solid logic 0, but the whole pix looks like its a slow scope or 1x 
probes so its not cut and dried.  Is that the one created by running the 
basic program I saw suggested in an earlier post in this thread?  That might 
indicate a stuck input when that chip is enabled, like one of the 74ls245's 
in an mpi that is not properly reversing the data path when told to, and 
while not in any of my coco stuff, I have seen that failure in a 74ls245 
once.

Last, when posting the pix, we need the gain settings of the vertical display 
for both channels, and the sweep speed settings info too.  That adds much to 
the data we can interpret.

-- 
Cheers, Gene
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