| Electronics > Repair |
| Series defect on agilent 167xx boards? |
| << < (46/50) > >> |
| Hamster:
Any of these would work - 16753A, 16754A, 16755A, 16756A |
| John_ITIC:
I successfully transplanted a comparator from my burnt 16754A and I now have one more (almost) good board. The below failures still persist. Per the attached image, I have three 16754As (four with the burnt board) that all fail the final 'comparatorCalTest' and 'dappAddrDataTest' tests. 'comparatorCalTest' is the same test that is made in the GUI 'calibration' tab and it fails 'RC compensation' and 'tap delay' in various combinations for the different boards. The service manual self-test description says: Comparator Calibrations Test. The purpose of this test is to verify that each of the comparator one-time calibrations can successfully be performed. This verifies that all of the calibration circuitry and components are within the tolerance limits required for proper calibration. This test is executed only if all probes are detached. Timing Zoom Memory Addr/Data Test. This test verifies connectivity of components within the analysis chip. It verifies that the address, data, and clock lines of the timing zoom circuitry is correct. I have one good 16754A board that passes these tests too. This is what I wanted to use DogP's extender boards for. I now have a way to do comparative measurements. But, I'm not sure where to start to look. I have a hunch that these remaining errors are not related to trace corrosion. Thanks, /John. |
| MarkL:
I have several cards with the RC failure and have not been able to figure it out. It's been a while since I looked at it, but as I recall, swapping comparators was not the answer. Since the test says that the pods must be disconnected, perhaps start looking on the pod side of the comparator. On the pod side, underneath is an OP184 and a mystery SOT23-5 for each comparator. The SOT23-5 is marked AAAG, and by way of how it's connected, one guess is that it might be a MAX4516 analog SPST switch. They're all ganged together, perhaps turned on when in test or calibration mode. Anyway, some comparator to comparator comparisons on pod side components might turn up something interesting. I don't think I got that far. It can be frustrating trying to fix these cards and I had to put them aside to get other things done. |
| John_ITIC:
I have done some more research on the "RC Comp" and "Tap Delay" failures. The 16754A service manual is a bit fuzzy on the details: "Comparators. The comparators are differential input/differential output devices that interpret incoming data and clock signals as either high or low. A threshold voltage provided by an internal digital-to-analog-converter (DAC) is coupled to the negative side of the differential signal through a precision resistor. Alternatively, this voltage can be provided to the data channels by a user supplied threshold line in the probe cables. There are separate internal DAC driven thresholds for the data and clock in each pod. In order to achieve performance, an extensive calibration is performed on each comparator when the board is manufactured and the results of this calibration are stored as Calibration Constants in non-volatile memory on the logic analyzer board. These constants are loaded into the comparators at power on." The 16760A service manual is phrasing this differently: "Comparators. The comparators are differential input/differential output devices that interpret the incoming data and clock signals as either high or low. Threshold voltage, programmed by the user through the user interface, is set by a digital-to analog converter (DAC) coupled to the negative side of the differential signal through a precision resistor. There are separate DAC-driven threshold voltages for the data and for the clock. In addition, the comparator contains a diode in which the junction temperature is monitored to ensure the module is being properly cooled. Much of the performance optimization for the module is accomplished by the comparators, including channel delay setting (EyeFinder), programming of input resistance, and frequency compensation adjustment. Module operation such as state clock modes and configuration are also done by the comparators. A digitalto- analog convertor (DAC) provides the module threshold voltage for single ended operation. The voltage at the DAC outputs are buffered to prove sufficient line drive. An analog switch is used to channel either the module threshold voltage from the DAC or the threshold voltage input from the system under test to the comparators." So, it looks like comparators themselves handle the adjustments to the "Input R", "OS Null", RC Comp" and "Tap Delay". Could it be that the comparators are "aging" and that it makes the SW unable to bring them back into spec via the available soft adjustments? I played around with the 'vp' debug GUI (started via './vp -debug 255' from the shell) and I was able to "fudge" the various settings until the "RC Comp" and "Tap Delay" calibration passed. See attached p579. I am, however, not certain that such a "fudged" calibration will actually work correctly. Perhaps this just makes the test pass but the H/W may still be out of spec. I know that the 16754A service manual talks about timing zoom performance validation via external pulse generator. I will study this topic some more. I also will make an attempt to replace the FPGA on my burnt board (the one that was connected via 80- lead flat cables). I found what I believe is a direct replacement on Ebay. It is one speed grade faster but that should not matter as it is usually okay to go from a slower device to a faster, but not the other way around. https://www.ebay.com/itm/145127678615 If the FPGA swap works, then I will have to find a replacement comparator. I may take it from my other 16754A that has "bouncy waveforms" after FFFF to 0000 transitions as I'm at this point am not too hopeful of fixing that issue... I will post a couple of IR images in the next post as the web site does not allow more images to be attached... Thanks, /John. |
| John_ITIC:
I also attached a couple of images from my thermal imaging camera, where the FPGA can be seen is very hot after having been shorted out. I could not find any other parts on the board that were unusually hot when comparing with a good board. If the FPGA replacement works then I will have to put back the comparator that I borrowed for another board. Step by step... /John. |
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