Electronics > Repair
Series defect on agilent 167xx boards?
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John_ITIC:
The updates for this weekend:

1) I have managed to get the 52" cables to work. I found that there is too much resistance (or inductance) in the few GND leads in the long flat cables. The trick with using longer cables is to use additional lab cables to create additional GND paths between the 16700 system and the board under test on the bench. I used four additional regular lab cables between the 16700 chassis and the 16754A and managed to pass all self-tests of my 16754A board. Note that these boards are designed to get additional grounding via the rear panel. While working on the boards, I have removed this panel so very little ground return path remains via the few pins in the backplane connector.

2) A 16754A board that previously passed all memory tests suddenly failed them again.  See attached p554. pv told me that the the following SDRAM ICs were bad: U91, U93, U95, U97 and U92, U94, U96, U98 as well as U89. These are all the SDRAMs on one of the two SDRAM controller FPGAs/ASICs. I traced this down to a shorted power rail on that SDRAM controller. Just like another board, one of the 100nF decoupling capacitors had developed an internal short (< 4 ohm), which collapsed that power rail. See attached p560b and p560c. The short was in the capacitor most to the bottom in image p560 although that cap looked good. The ones above looked crusty and corroded so I swapped them out too, although they measured okay.

3) I now get an intermittent cmpTest (and vOffsetTest) error on this board. See p561, p563, p564. I'm not sure why this pops up now as I never have seen that before. I will have to review under microscope again to see if I can spot anything unusual. And, I have to look into where the DACs are located...

So, some issues have been resolved but new issues have arrived. Never a dull moment...

Thanks,
/John.
John_ITIC:
It seems I have a bad comparator. The (attached) pv cmpTest and vOffsetTest outputs told me that there is an unexpected voltage on the 2nd comparator on pod 1. I measured across the input shunt resistors and found one that measured 1.5KOhm across while all the others were 4.38K in-circuit. I removed two of the resistors (both measured 20K out-of-circuit) and I found that the input resistance to ground on the bad channel was 435R rather than 4K like on the neighboring good channel.

I recall that others have reworked the comparators before but I don't think it was posted how to get the heat sink off. Or, what to use to put it back once the board has been reworked.

I may be able to heat it sufficiently with the heat sink attached as I have a rather hefty rework machine, with a bottom pre-heater:
https://www.zeph.com/bgarework_stations_systems_qfn_smd_hot_air_repair.htm

The ZT-7 rework machine has a shroud that completely will contain the heat sink inside. So, the heat will not be able to escape when de-soldering.

I have my burnt out 16754A that I could borrow the comparator from. Or, I could order a new comparator as I believe it was mentioned in this thread that they are still available for purchase. I will go back and re-read to see if I can find that post.

Thanks,
/John.

Hamster:
i thought the comparitors were the smaller agilent chips by the pod inputs without heatsinks?  I posted a some 16700/16751 cards for sale if anything is useful.
MarkL:

--- Quote from: John_ITIC on March 12, 2024, 03:08:53 am ---...
I recall that others have reworked the comparators before but I don't think it was posted how to get the heat sink off. Or, what to use to put it back once the board has been reworked.

I may be able to heat it sufficiently with the heat sink attached as I have a rather hefty rework machine, with a bottom pre-heater:
https://www.zeph.com/bgarework_stations_systems_qfn_smd_hot_air_repair.htm
...

--- End quote ---
I've done rework on the comparators and have never been able to get the heatsink off.  I just moved the whole thing.  I used a pre-heater, as you are suggesting.  Below is a photo of what's underneath if it helps.


--- Quote from: Hamster on March 12, 2024, 04:05:37 am ---i thought the comparitors were the smaller agilent chips by the pod inputs without heatsinks?  I posted a some 16700/16751 cards for sale if anything is useful.

--- End quote ---
That's on other cards (40-pin cards).  The 16753A/54A/55A/56A and 16760A (90-pin cards) have a very different front end with differential input comparators.
John_ITIC:
Thank you all for the help. I would be interested in purchasing a parts 16754A so I can get a replacement comparator as well as a replacement U128 Xilinx XC2V100 FPGA, which is used to implement the bus interface. I have confirmed that mine got damaged when using those 80-lead extension flat cables. U128 gets very hot when the damaged board is powered up. All power regulation otherwise appears fine on that board.

I have done some BGA removal, reballing and re-assembly in the past so there is a good chance I can get the board up and running by swapping out the FPGA. There is no programming involved as the FPGAs configuration image is stored in the PLCC XC18V00 PROM, which is next to the FPGA.

Based on this thread, there should be quite a few piles of 16754A boards where the repair attempts have been given up. Please PM me if anyone is interested in selling me a parts board.

Thanks,
/John.
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