> From: sndgroom@...
>
> I made some tests with the scope yesterday night:
> First thing I noticed is that rubbing with the scope's
> negative crocodile the main motherboard's ground, the digital
> noise tended to get higher in pitch to almost a pitched tone.
> Even if I didn't test all of the ICs, I noticed that the
> line Z18, Z28, Z30 had no pulse at their pins, only noise.
> All changed after touching TP2 with the same probe, the noise
> totally stopped and I had the digital pulse back to those ICs
> pins. he problem now is that the synth is totally mute, even
> with the scratch patch.
> Doing some diagnostics tests this morning, I found this:
> D3= all lights on, but only when I'm holding the switch. As
> soon as I remove the finger the LEDs turn off (it wasn't like that)
> D4=ADC Range, still I cannot turn on the LED while moving
> R45, but until I do not fix the A/D, I believe it is normal.

You should always have a connection between the scope ground and a circuit ground near where you're trying to look. It's best if you use the clip on the probe, but running a separate ground line and clipping it to one of the bare lugs sticking up on the board is useful. I seem to recall there's one near the DAC, or maybe between the memory array and the rest of the board. Or, you could use TP1 as a ground. But if you're grounded far from where you're looking, you'll get a noiser view. If you're only grounded through the AC line cord, you'll get tons of noise on the display.

Touching the ground clip to TP2 will prevent the DAC from working at all, probably driving the DAC line to one rail or the other, depending upon the input offset.

If you have a decent scope ground, what do you see on the DAC line? You ought to see a repeating pattern of voltages, as it refreshes the sample-and-hold circuits. You might be able to lock onto it by adjusting the variable time base of your scope, or just triggering off the most positive or negative voltage in the sequence. I'd be curious to know what you see. To get anything meaningful, though, you'd have to force all the voices to be enabled by using LF, E, and then making sure 1 to 6 are turned on. Playing should then cause various voltages in the DAC sequence to change. I would expect to see voltages that cover most of the -5V to +5V range.

And I'm still curious if you can vary the MAIN VOL line on the output board by adjusting the master volume slider.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...