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]> 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]> 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]> 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 > >> > >> 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://photos.app.goo.gl/K1AIj3Pmt15xXfR72 > >> > >> > >> -- > >> bkw > >> > > >
