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 >> >
