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| Replacement for Fluke 700013 IC (quad SPST analog switch) |
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| coromonadalix:
what probes do you have on your scope ? Do you have an x10 multiplier on them, it helps to get an higher impedance for measurements, you have x100 probes too ... |
| JoergR:
probes are Tektronix P6133, 1:10, 15pF - they come with the 100MHz scope, but of course it all depends on the capacitance in the oscillator, doesn't? Probe capacitance must be small with respect to that, or? |
| Kleinstein:
The capacitance at the crystal is in the 20 pF range - so even a rather low capacitance probe can disturb the signal and cause asymmetry in the loading. The clock seems to be good enough, as the µC seems to be running. |
| JoergR:
So back to the drawing board. I went back to measure the digital signals on the switching chips, this time not with the multimeter as I had done the first time, but with the scope ... :palm: they are ok and match what I measure in my functional device. I also figured out that in the 20V range, I can get a signal through to the track and hold. Then I get a decent signal on TP 103 - see the scope image and a reading that is stable albeit off by a factor of two. Note how the Track and Hold signal is not as symmetrical as in the functional device. I then checked the Lm339 again, too. In contrast to the manual, it shows -26.5V on pin 12. Its supposed to have -5.5V at zero input. However, here it seems the defective 8840A/AF and the functional 8840A differ in construction. The zener CR303 is connected to -15V in the functional 8840A and to -30V in the defective 8840A/AF. So this could be by design and I just don't have the right manual - or the zener is broken. The CR303 in the defective 8840A/AF has the same marking 74 6 610 as CR309/11 which are both 3.9V Zeners that clamp the output of the track and hold. So I guess its by design. That would be odd, though, as the LM339 is rated for symmetric supplies with +/- 18V max. Now +5/-26.5 doesn't exceed that differential, but still. The PCB of the defective AF states 8840A/AF-3001 Rev C, the functional A is Rev E - both are dated 1983, though. |
| Kleinstein:
The -26 V at pin 12 of the LM399 should not be a big issue. The supply for the LM399 sould still be in the safe range. The difference would be mainly with the ability to also turn off a rather negative input signal, e.g. in the 4 Wire ohms mode or maybe (if supported) some ratio mode that uses the 4 Wire sense input as a 2nd input. On the othe side a more negative voltage needs the switching JFET to withstand a higher gate voltage with a large positive input votlage. Worst case there many be up to some +24 V before the clamps engage. So with a -26 V supply the JFET would have to withstand -50 V gate to source. With a negative signal from the active signal this case may still happen though. So I don't think the difference between the 2 vesions is that relevant, both should be OK, at least for the test / normal operation. If the 20 V range kind of works, but the 2 V range not, this indicates a problem allread before the S&H stage, like with U301B Getting at least some sensible signal out shows the U303 seems to be at least partially working. It may be interesting to see the input signal to the S&H stage constant in this test. It should change between 0 and the signal. A factor of 2 error would also be visible on the scope - so is the factor 2 error allready at the input to the S&H stage or only at the output or maybe even a problem with the ADC ? AFAIK the signal may be slightly asymmetric from an amplifiers offset, but not that much. |
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