DAC failure mode

From the looks of it, I'd say the bit with weight 4000 is stuck on. If that isn't visible at the DAC input bus, then we must conclude that something inside the chip is shot.

Regards, Jeroen Belleman

It looks like the dac's 2nd to the MSB bit is stuck high all the time, perhaps a problem on the PCB? At any rate, that's an issue on the input side of the dac, not on the output side, so "touching" the card couldn't be a cause.

Yeah, looks like it. Now I've got to find an ISA raiser card to look into that problem, and get my head ripped off again if caught doing it.

robert

Inspect the soldering. Most problems like this turn out to be visible.

John

In fact I did, and it looked promising: The DAC712 (not the AD669, contrary to the board's datasheet), had obviously been reworked in the past. It was hand-soldered and came from a different batch than the other two. Its D14 pin (being in a corner of the package) was easily inspectable under a microscope and looked suspicious, and indeed, I could get a scalpel into where there should have been solid solder. Rather than taking the chance of a pad that might have been damaged during the rework, I directly soldered a wire to the D14 pin of the neighboring DAC712 (both on the same data bus).

Seldom had I been more confident of having found the right fix, so I was quite disappointed when the problem remained (intermittent, by the way, as I've found out in the meantime. Sometimes the darn thing works).

Lacking proper logic analysis equipment, I clipped a scope to my wire and played with the DAC's inputs (unfortunately I can't set them directly but have to mouse-move a slider on the application that talks to the board). The D14 line sits frozen at 0V, although it should of course cycle high and low at times when it should be set high for this particular DAC.

Then I started the part of the application that ramps the other DACs (all of which work flawlessly). Once this is running, the D14 line starts hopping merrily between 2.5 and 5V (!!!). When I stop the scanning process, the D14 line sits back at ground. Capacitive coupling through a faulty trace? Why the three-level thingy, then? Could it be a measurement thing -- because I don't have an ISA riser board, I couldn't fit a proper scope probe onto the board but had to use clips and banana-plug cables.

More headache. I'm actually not hell-bent on fixing this; having it repaired externally is no problem. The problem is the time factor involved in shipping this thing back and forth overseas, and my piece of wire would have been just so beautiful.

robert

Not capacitive. Just a dead short between 2 outputs, and the drivers' RDSon divide according to their status.

Go hunting for another data/signal line showing the same pattern and you're almost done.

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Thanks,
Fred.

Or (but more unlikely) something going into tri-state.

Or trace it back to the respective bus driver chip and see if that looks healthy. Quite possibly a solder joint has come off at that chip.

--
Regards, Joerg

http://www.analogconsultants.com

Tssttt... With a sig line hoping between 2.5V and 5V? Not impossible. But very unlikely.

--
Thanks,
Fred.

It depends on what else is hanging on that line. For example, you can have that very scenario with a thevenin termination. High -> 5V or whatever VCC is, tri-state -> 2.5V or whatever the thevenin ratio has been set to.

--
Regards, Joerg

http://www.analogconsultants.com

OK, you almost win. Almost, because it's an ISA card and I don't see Thevenin terms used there :-)

--
Thanks,
Fred.

Well, sans Thevenin you theory about two outputs fighting each other looks more likely. That should be quite easily detectable via touching the chips. When it goes "Ouch!", then that's one of 'em ;-)

--
Regards, Joerg

http://www.analogconsultants.com

[I'm assuming that your DAC is a high-density package, with a zillion pins on 4 sides.]

The first thing I'd look for is a short (likely a solder whisker, from your description) between the D14 pin & the next pin over. If so, you should be able to detect it (powered down, for preference) with a multimeter. It's a very typical kind of fault on boards with high density chip packages (eg; QIPs, etc), especially ones that've ben banged around by service staff, because they pins are so close that a tiny bump can bend them just enough to touch. I fix those by sliding the point of a scalpel between the pins to separate them.

Another common one is an invisible dry joint *under* the 'L' part of the pin, which typicially shows up as an intermittant fault that gets progressively worse over time, or with heat or vibration. The scalpel trick will often reveal that problem as well. The easy repair is to lift the pin with a blade or fine needle, solder-wick & tin the pad, do the same for the pin, then re-tin the pin, bend it down, & dab it onto the pad with a *freshly cleaned* fine iron. Test for shorts between the pin & the ones on either side, & you should be all fixed.

--
   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      
  \\|/  \\|/              Perna condita delenda est
---^----^---------------------------------------------------------------

The 2.5V will be because it's shorted to another line, & you're getting 2.5V when one is trying to go high, but the other is trying to go low. Betcha the chip[s] driving the short are both running hot. ;)

--
   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      
  \\|/  \\|/              Perna condita delenda est
---^----^---------------------------------------------------------------

Right. Sounds reasonable. At the moment I don't have access to the card so I need to speculate a bit (an interestingly nested problem here):

1. If this short exists, shouldn't all four DACs (or even more parts; I don't know how many share this particular segment of the data bus) show weird behavior?
2. The 712's logic threshold is somewhere between 0.8 and 2 volts. Yet it only works properly while I see all the 2.5-5V stuff going on on the D14 line. When D14 is solid low, the 712 behaves as if it were stuck on high. Of course a lot could be clarified here if I could just have a quiet sit-down at the bench, triggering on CS and whatnot. Suffice it to say that this is not possible at the moment.
3. The fact that this particular 712 had been replaced before (by whom I don't know -- the card has a somewhat muddy history) suggests that this problem had been there earlier, and that it was erroneously traced to the

712 when in fact it was shorted outputs or some other cause.

1. All this is moot because the card will be sent out for repair for two reasons: a) I'm currently being blamed for having caused the defect (which is ludicrous because I don't have telekinetic capacities). If I were to try my own hands at a repair, one of the following three things will happen: - I manage to fix it but I won't gain anything because in the eyes of the beholder I just righted my own wrong as I damn well should. - If I can't fix it it will still have to be sent out. - If I break the thing altogether I'll take even more blame. In short: I can't win but can only lose. b) I neither have the schematics to this (6-layer) board, nor the proper measurement outfit nor SMD rework equipment.

But thanks for bearing with me. The shorted-output idea nicely fits the D14 behaviour on the scope, but not the proper operation of the other devices on the bus.

robert

This signal isn't on the ISA bus but on the data bus of the TMS320 on the board. Or at least the part of the bus that feeds the DACs; there may be drivers in between (I don't have any schematics).

robert

It's a big 28-pin SOIC (1.27mm lead spacing)

That's exactly the way in which the D14 pin of that chip looked suspicious. I fixed it (with a proper piece of wire, not just reflowing that pin) but to no avail).

Yeah, it seemed to. I mentioned it in an earlier post.

robert

Jumpering it is not that good a way to fix that kind of fault. You'll be more likely to get a reliable repair via the method I described (or any other method that ensures that the pin is soldered back where it should be).

Seriously, ditch the jumper & redo the pin properly. You might be surprised what you find under it when you lift the pin under a magnifier & a really bright light.

--
   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      
  \\|/  \\|/              Perna condita delenda est
---^----^---------------------------------------------------------------

Oh boy, I can really relate to your situation. I've recently parted company with an obnoxious & rabidly clueless client who repeatedly blamed his network (that I built) for faults that he'd caused himself. His most recent self+foot+gun disaster was when he crashed the servers & switches by scheduling builders to perform structural work in his office - in the middle of the working day - without informing me, or taking any precautions, or at least shutting down all the electronics. He then put the cherry on top by letting the builders plug their saws, grinders & **ARC WELDER** into the same power circuit supplying the server, etc. Needless to say, the moron blamed the failure on me.

Sorry Robert, I hadn't seen this part of the thread when I wrote my posts. Obviously, it makes more sense for you to wash your hands of the whole thing. ;)

--
   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      
  \\|/  \\|/              Perna condita delenda est
---^----^---------------------------------------------------------------

Never mind. I learned a great deal in this thread. Thanks, everybody, for contributing.

Now I have a little electronics problem that's more domestic. See another thread.

robert