TDS 380 calibration error

[siggi]

Nice pic, great idea to use the iPad as a signal generator.

So you have precisely one wonky decade, and it behaves identically across channels. Not knowing what's inside the attenuators, I'm not sure whether that's good or bad news.
Hopefully it's a configuration problem rather than identical damage across channels.

In your shoes I'd try turning on the cal menu, and try and find that initialization option. The reasoning is that either you have a config issue due to scuttled NVRAM, or else you need new attenuators. Either way you'll need to re-cal, so you've nothing to lose by trying this.

If you prefer to keep hunting, then I'd do this:
1. Measure the input impedance of both channels at all vertical settings (again). Measure inside the bypass cap for CH2.
2. Measure how the gain control works. When you play with the vertical vernier control, it probably runs through a range of voltages between each 5/2/1 attenuator step.
3. Check the attenuator offset (pin 15/16) - this is the last input to the attenuators I could imagine being messed up by configuration.

Good luck!


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[siggi]

To understand how the attenuators might work, download www.tek.com/oscilloscope/tds520b-manual/tds520b-mod-cm-component-service-manual and read from 1-5.
It wouldn't surprise me if the preamps in your hybrids were the same as the 520b. Six gain steps fits, as you see the gain adjust voltage change for the 10V/div setting, IIRC.



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[Dw11in]

I will look into what you have for me for review and testing, in the mean time I found the input gain voltage going into both attenuators coming from the daculator. Interestingly the voltage did not vary continuously throughout the entire range of the vertical scale. The illustration below describes what is going on.

Vertical scale/division
|10        5             2             1            .5            .2            .1          .05          .02          .01         .005   .002|
|------------------------------------------------------------------------------------------------------------|
|.9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8 | .9   1.8|
attenuator input voltage variation

As the vertical scale is adjusted, attenuator voltage varies from about -.9 volts to -1.8 volts. This variation occurs repeatedly, when the variation starts at .9 and ends at 1.8, it starts over again at .9. My theory on why this is occur is it might be a result of multiple soft attenuation circuits being used and cycled through. As each soft att is selected the voltage varies on it and then passing off to the next one. Well maybe at least