It might be better to use the CLK (37) output as a reference if it is needed. The CLK is derived from the X1,X2 XTAL inputs and should not be impacted by the loading of the scope probes.
The M100 reference manual is quite good for troubleshooting some parts, although it does have a few broad instructions like “Check All IC’s”. :-) From: M100 <[email protected]<mailto:[email protected]>> on behalf of Brian White <[email protected]<mailto:[email protected]>> Reply-To: <[email protected]<mailto:[email protected]>> Date: Friday, December 8, 2017 at 9:55 AM To: <[email protected]<mailto:[email protected]>> Subject: Re: [M100] Fwd: Re: diagnose I noticed something a couple days ago. Those noisy gnd and 5v lines were only like that because I happened to have probes on both X1 and X2 at the same time. Later I was looking at a good M100, and happened to only be watching X1 and others but not X2, and everything was much better. But just by touching a probe to X2 while another was on X1, both gnd and 5v went exactly like those pics I posted from the bad M100. I can probe either X1 or X2 and it's fine, but not bith at the same time. I don't actually see any special reason to monitor either of those signals except briefly to verify they exist, or to provide something steady to trigger on while probing others. Just pointing out something I saw that explains those ugly traces. On Dec 7, 2017 11:45 AM, "Josh Malone" <[email protected]<mailto:[email protected]>> wrote: Yeah - there is something very weird with the ground on that Tandy. Maybe some problematic traces? Or a bad IC leaking current to ground? Can you measure the DC power consumption? It should be ~60-70mA IIRC. On Wed, Dec 6, 2017 at 10:14 PM, Brian White <[email protected]<mailto:[email protected]>> wrote: > After watching a youtube video about how to use an oscilloscope by sparkfun, > I switched all probes and channels to 10x and adjusted the trim caps in the > probes against the built in test signal to make the square wave look square. > > With gnd ref on the cpu gnd pin 20 instead of chassis gnd, and a couple > probes on X1 and reset_in, and one probe on chassis ground, now the x1 and > reset_in signals are steady, reset_in looks flatter, and only the trace for > chassis ground floats and bobs up and down now. > > So chassis ground simply floats around in relation to the rest of the board, > or at least in relation to the cpu. Not a bad afe in the scope. *chagrin* > > > -- > bkw > > On Dec 6, 2017 7:38 PM, "Brian White" > <[email protected]<mailto:[email protected]>> wrote: >> >> This is the bobbing I was talking about. It's dumb for me to even be >> talking about it yet since I haven't even done a basic comparison check on >> some other reference yet. >> >> https://youtu.be/b9jAYsfTOzQ<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fyoutu.be%2Fb9jAYsfTOzQ&data=02%7C01%7C%7C33b7c3a9b7cd4909b28f08d53e64ed91%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636483525556528212&sdata=UvTz5UI2LRigb2jv0YVX%2BQqcwLR1LS1JpfYvVsiA3Ts%3D&reserved=0> >> >> And here's another look at a different signal hooked up a different way. >> Here, I have the ground ref hooked to the gnd pin 20 on the 8085, NOT the >> chassis ground. And CH4 is on pin 18, AD6. The offsets, trigger, and >> vertical scales are all visible on-screen. >> >> https://photos.app.goo.gl/mel4CKym2EtcV1352<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fphotos.app.goo.gl%2Fmel4CKym2EtcV1352&data=02%7C01%7C%7C33b7c3a9b7cd4909b28f08d53e64ed91%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636483525556528212&sdata=Z%2FNVGP3BjFaZW64aT0rMQteNAfXinZKhIC0XL6c%2BK%2Bk%3D&reserved=0> >> >> https://photos.app.goo.gl/K1AIj3Pmt15xXfR72<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fphotos.app.goo.gl%2FK1AIj3Pmt15xXfR72&data=02%7C01%7C%7C33b7c3a9b7cd4909b28f08d53e64ed91%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636483525556528212&sdata=vX%2FNje%2BZK0f1cDmSAeohlNWJlD%2BZi1%2FHbUawQmBi0y8%3D&reserved=0> >> >> >> -- >> bkw >> >
