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| Replacement for Fluke 700013 IC (quad SPST analog switch) |
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| JoergR:
So the voltages appear ok at TP302 - in ratio 1:1 for ranges 0.2, 2, 20 and 1:100 for ranges 200 and 2000. The divider Z301 measures correctly as 18k, 2k. The switches for 4WOhm, IDC and AC path U301D, U302D und U302C are all open - as should be I suppose. Oddly, the switches U301A, U302A and U301B as well as U302B are also all open - so no signal appears at the input of Track and Hold Pin 14 of U301C. I would have expected that shorting any of these switches before the input to Track and hold would give sensible readings, but no such luck. |
| Kleinstein:
The switches at the input to the S&H stage should turn on 1 at a time. In the non AZ mode, this may be just the 1 selected input. In the AZ mode there should be some switching between the zero reading U301C and the selected input. For 4 wire ohms this should be with U301D (for ohms sense low) instead.For some inputs there is also a precharge phase with U301A. The sequence should be visible at the input to the S&H amplifier and also at the control signals to U301-U303. Besides the CMOS switches also the JFETs Q311 7 Q315 have to switch - a possible defect would be U305 (= LM339 and thus cheap). |
| JoergR:
Hm, the LM339 is probably ok, the FET also send the signal through. I checked the logic inputs to the U301, U302, U303 and they are totally off. I hadn't done that before as with self tests and display working, I didn't suspect an error on the digital side. There is not a single high in AD0-AD7 that is supposed to come over the bus that appears on the inputs.... Other digital signals do appear to come in ok. I checked the uC, U202, with the procedure described in the manual and the signals come out as described (the 1kHz and the interrupts etc). Since the display is working and the self tests are performed and even the right voltages appear on the test point 302 during the self tests, I guess the uC is good. That leave U201 as the next suspect or the Adress Latch U219. I'll see what I can figure out there when checking the timing of signals between chips. |
| JoergR:
So maybe there is progress. I checked the uC in my faulty device - a 8840A/AF and compared with my functional 8840A I found that the clock signal on pin#2 is very different. See the scope picture. Also, the clock pulses on the faulty device appear to have significant jitter on them. I tried exchanging the uCs between the devices. The uC from the faulty AF device would not work at all in the functional A. The uC from the functional A shows error 50 "guard crossing error" when placed in the faulty AF. I'm a bit at a loss here as I don't know whether I should expect compatebility anyhow. Chips are marked different. The AF has the chip seen in the picture. The functional A has a chip marked Zilog Z8. |
| Kleinstein:
Probing the clock signal is tricky, as the cystal part can already react to the extra proble capacitance. AFAIR there are also differences in the clock pins - causeing much of the trouble with the replacement earlier in the thread. So changing the µCs may not work, even if both chips are OK. I the CPU is passing the self test, the AD0-7 lines should work OK, as reading the memory works. The switch specific part is mainly the CS2 signal for the latches. The control signal for the switches has 2 paths: one via AD0-7 directly from the CPU and one directly from the ADC circuit. The settings from the CPU side may not update regularly, more like only with a function change. The other meter to compare should show the expected signal, if not in the service manual. |
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