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| Agilent 54845A Failing all trigger self tests |
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| ARF:
Let's continue... I was right, it's a digital problem. After it became clear where to look for the Logic Trigger IC, I looked for and checked the digital interface. In this case, I was just lucky and didn’t have to search for long. On the LT substrate there is a TTL - ECL translator U1 100324 (https://www.mouser.com/datasheet/2/308/100324-1190082.pdf). This is the data input for the LT ASIC. On one of the bits, I noticed strange signals in some parts of the data block. Sometimes (not always!) the logical 0 of the input signal was about 1.57V and had a slight ringing. 1.57V is an undefined state for the TTL logic. Because of this, sometimes a logic 0 was converted by the 100324 chip into a false signal sequence. This always happened on a specific input data pattern. If someone is looking for this problem, then I advise you to stop the scope, select “trigger group” or "logic trigger" in the self-test menu and run the self-test every time to catch the digital signals for the LT. Then I looked where logical 0 was lost. This bit is connected via U166 SN74ALS273 (https://www.ti.com/lit/gpn/SN74ALS273) to the common data bus. U166 is used for data bus arbitration. You can see that the data bus itself does not have a very nice signal, but this is not a defect. I'll write about this a little later. U166 must set the output by the edge of the clock and we should have a clean signal on the output. Instead, I see copying artifacts from the data bus, slow edges, and sometimes not full swing on the output. The interesting thing is that due to the fact that U166 is located on the back side of the ACQ board, I soldered a regular 2.54" 0.5m flat cable so that it should be convenient to connect the oscilloscope probes. After soldering the cable, the oscilloscope started working, the errors completely disappeared and I was able to calibrate all channels The ringing disappeared. I connected this with the fact that the cable is an unmatched load on the data line and because of this, the level of logical 0 was shifted slightly. Later I soldered this cable to other ICs for other measurements and when I returned it back, I already received LT other errors. Perhaps this is due to the heating and cooling of the U166 during soldering of the cable. One way or another, my U166 was degraded. I ordered the components, but it won't be fast. I'll post the results when I receive the new chip. |
| MarkL:
I've been following your progress on this thread, but I can't really add much since I don't have this scope. However, I will point out that the Agilent 16533A and 16534A scope cards for their logic analyzer line (which I do have) uses the same Logic Trigger 1FJ9-0002 ASIC or 1821-4786 carrier board, and the same Time Base 1SE6-0056 ASIC. Other scopes from that era also use this chip set. I'm sure HPAK re-used this design as much as they could. I think you're right that the Logic Trigger carrier card replaced the ASIC at some point for unknown reasons. The 16533A/16534A cards also exhibit this change in their lineage as well. I have some of both types and the behavior and specifications are identical. However, I don't think the 1821-4786 carrier card is a complete drop-in for the 1FJ9-0002 ASIC because there are other changes in that area on the 16533A/16534A cards between the two versions. I'm just pointing this out since these cards might be a possible source for ASIC replacements, if you should determine you need one. I don't want to send you on a wild goose chase since you are working on a completely different device, but I can also say the scope cards have a history of bad resistors, either completely open or wrongly high value. I've had a couple of bad termination resistors, one of which was on a clock line which caused multiple errors during self-test. I've also bad resistors on the output setting pins of local voltage regulators that caused the wrong voltage to be output. In one instance, it was on a regulator used to supply ECL termination networks on the card, again causing multiple and seemingly unrelated self-tests to fail. |
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