[Coco] Weird CoCo3 problem...

[Robert Gault]

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
>

I've looked at my Coco3 in the past and the observed waveforms closely 
matched those in the service manual.

I think your scope is a dual channel but your TP2&3 pictures seem to 
show just one channel. You won't be able to see the time shift (Eclk 
Qclk) unless you display both TP2&3 at the same time synchronized on one 
or the other channel.
In any case, the waveforms don't look good. They almost look like the 
signals have been filtered by a bad scope probe.
Your TP4&5 again show low pass filtering somewhere. It should not be the 
Tektronix scope but might be the probes.

If there is no doubt about the scope and probes, the Coco will never 
work with those signals. If you have access to a second GIME, you could 
try a swap. I'd look for bad caps or ICs that are placing an excessive 
load on the clock lines.
If you have a 256 or 512K memory board, try removing it and using the 
original 128K memory. See if the waveforms improve.