Hello and welcome back. If you follow the channel, you already know that my HP 9825, famously used
by Mr. Fancy Pants in the HP catalog, recently died from a power supply failure that caused an
over-voltage. The 5V temporarily became 13V, which is a big ouchy if you are a TTL chip.
In the preceding episode, we finally managed to get some good logic analyzer traces.
But they told us that we were in a disaster of Apollo 13 proportions.
[Gene Kranz in Apollo 13 movie:] Let's look at this thing from a - from a standpoint of status.
Uh - what have we got on the spacecraft that's good?
Well, I’m afraid there is not much good left in our ship.
Aside from our processor, much seems broken. Our RAM appears to have a bad case of amnesia.
It's not the number we had put there. We had put 10101 and now it's 10077.
Our very complicated keyboard assembly is bad, and sends the wrong status byte.
And then one few things we revived, the ROM,
decided to go bad again. It appears that we have lost a bit.
If I put my cursor right here, you'll see nothing but zeros. We've lost a bit in the ROM.
So now, not only our RAM is bad, but the ROM that we repaired has lost a bit.
So that's the problem: we have to repair it faster than it breaks. We're not fast enough!
So, we'd like to fix the ROM so we can execute code and then check the RAM.
But the problem we have, when you have one stuck bit, it's a bit on the bus, so it's shared between
many chips. And you don't know what pulls it high, because it's connected to - of course - the ROM,
where we have the problem right here. But also on this ROM, this ROM, this ROM, this ROM.
And it's also connected in the RAM [on] the chip that does bit five, this one. And also on the buffer.
So the chip might be perfectly good, and pulled high by any of the other six chips. We probably
have a short somewhere, but it's pretty hard when it's all interconnected to figure out where it is.
All right. So we are trying to figure out why we have a stock bit in our ROM. And,
Carl, you have brought the magic instrument to do that. [Carl] So, I brought some HP brand
instruments, because I know how much you like having HP brand instruments involved here.
[Marc] I do? How do you know that? [Carl] This is the logic pulser and a current tracer.
So the pulser is going to inject a pulsating current into the trace at one spot, and then,
this is sensitive to the the current. Basically [it] detects the field. What you basically do, is
inject it into one of the pins in the trace, and then move around, see where the strongest current is.
That's something that is probably pulling it down. [Marc] That's where it sinks.
Here is the RAM data 5 [pin], here is the buffer data five, and then data fives are all
over here on those five chips.
So the principle is very attractive. You inject pulses anywhere on the net, and the current
probe is going to have the strongest signal on the pin that brings the net up or down.
However, in practice, you have to adjust the current light at a threshold value,
and it gives very ambiguous results, a little more here, a little less there.
So we probed around, and got a bigger signal on one ROM and also on the buffer.
But when we did the same test on the good board, it had a very similar signature.
Now if I try to redo it on the good board, I think I see the same. I see the ROM is hot too.
And... Well, I see the buffer, I see the same signature on the good board!
I don't know Carl. [Carl] Including the buffer chip? [Marc] Including [the] buffer chip.
So unfortunately this fancy method did not provide any reliable results,
and we thought that maybe our short might have been partial. You’ll see later that it
was much more interesting than that. Anyhow, we eventually reverted back to less fancy methods.
So we were just trying to see what it does on the powered board
with the fancy instruments. Then I use a very different fancy instrument. I used da finger.
Put your finger on this one Carl, and tell me what you feel.
Toasty yet? [Carl] Getting warm. [Marc] I had it burning before. [Carl] Ow!
[Laughter] Ok!
[Marc] All right, so, it must be this one. It's interesting that we didn't
see it with the fancy HP thing. So a thermal camera, which I don't have, would be better.
Anyhow, we proceeded to remove the toasty ROM, when this happened.
Now, how worse can it get? I was in the middle of desoldering the ROM from the printed circuit
board, when my desoldering iron decided to quit. The wiring to the heating element
just completely delaminated. Things just keep breaking.
Oh well, I continued to desolder the ROM with copper braid,
but man, what a nightmare of failures, it just keeps on happening.
So we got the successful tooth extraction.
I want to see if we have recovered our bits. And... I do this. Got it!
And we haven't!
And dang! We were pretty sure this would be it,
but no! Our bit is still not there! There is yet another fault besides that toasty ROM!
164001 - there are a whole bunch of failures in this thing. Oh man, this is disheartening [sigh].
So desperate times call for desperate measures, and I have done what I should
have done a long time ago, which is put my magic fancy pants on. And
not wanting to wait for my desoldering iron to be repaired, and also trying to avoid desoldering
too much stuff, I'm going to try to snip it to isolate the pins. So, if one chip pulls it down,
if I snip them all, eventually it will come back up. But first I want to try the original ROM,
and see if the bit is really gone or not. Snip! So, with a little bit of luck,
there's enough of a gap that it's not connected anymore, but it's small enough that I can repair
it with a little bit of solder. So let's try it and see if our bit has reappeared.
So, of course it won't boot, because the processor won't see the bit,
but maybe the bit will have reappeared at the ROM itself.
And no! At 040, it got 001, so the bit is not in the ROM, assuming I'm making contact with it.
Oh, but would I re-contact...? Ah, okay. I have to check whether,
after putting my hood, I have not recontacted it by accident.
So I'm wondering if I reconnected it. Yeah, I did! Okay. When I put the probe,
it shorted the two parts of this. So, let me try to put some kapton tape or something.
Okay, I put a little bit of kapton tape over it, and see if... Okay, so now it's isolated. Okay.
Okay, try number two with a bit of kapton tape.
Oh, and the bit has reappeared! Okay. So the bit is in the ROM, aha! But it's short-circuited
by something else. It says 164001 here, because that's on the processor side,
but on the chip side, it sees 041. So, the ROM is good, but it's pulled up by something else.
Okay, so, we repair this, and then we clip other things.
So we're going to try to see if we can heal it. Okay, I think that's healed.
So now that we know the bit exists in the ROM,
we just clip the other chips one by one, until it comes back.
All right. Okay, now the run with the RAM chip disconnected.
Nope! It's still reading 001. I don't know what my next victim is going to be. The
buffer chip, all the way at the back?
And I kept clipping stuff, waiting for my bit to reappear. The buffer chip, and then each of
the ROM chips. And my bit was still stuck! Eventually, I had only one chip remaining.
All right, only one more pin left. All right, so, with the last ROM disconnected, if that doesn't work,
then I am not sure what the next experiment is. A one and a two. And... what the heck?
No! It didn't come back!
Dang it! It looks like even the power the magic pants doesn't help! I am still reading
001 and not 041, and I have cut all the pins. So it's held high somewhere else. Where can that be?
Okay, I think I found it. So, obviously, if it's not any of those pins,
then it must be something I've done to the board. And I've only changed three ICs.
And sure enough, on this one, you can see it: there is a bridge, on this line,
right here. I'll put it under the microscope - ah, you can see it here: there is a bridge.
So, self-inflicted damage, because I replaced the circuit while I didn't have the pants on.
But the sad thing, is that I know exactly where the problem is, right? It's on pin 4 over here.
But if i try to...
If zi try to check it here,
it's not sensitive enough. You hardly see it. Oh, okay.
Okay. Now it works, so it's over here.
So it could have detected it.
So in the end, the review of the clever HP probe is mixed. If you know where the fault is,
you might be able to confirm it. But it was too ambiguous to use otherwise.
In this picture, you can see what happened during the rework. It’s not a solder bridge
as I thought. The trace actually slid and made full contact with the pad nearby.
I don't know if you can see it, but a trace did bend. I have to physically push it away.
Okay, I cleaned it and repaired it under the binoculars. And now it looks a lot better.
And so, the lesson for today is: never solder without your magic pants, and also
check it under the binoculars afterwards. Okay my board is back, I repaired all the pins
and hopefully we will have recovered our bit.
And we did! Okay. Phew! All right.
So yeah, the bit is back! 041, 164041 jump to 10000. Okay, everything is good again.
This was the first piece of good news in a long while!
And then my heater element arrived for my desoldering gun.
Oh yes, chooching good! I made myself a beautiful stand for it.
You have several tips, several diameters. You want to be as close as possible from your pin diameter.
You want to make sure this is nice and unclogged.
Then at the bottom there's actually temperature setting.
And it sucks, which in this case is a good thing. And then it's a piece of cake!
And there she goes.
Hakko FR-300 folks, but don't take the 300, take the 301 that has the heater that doesn't break.
Oh, good to have the tool back.
OK, looks like thanks to the magic pants, we had a better day and got partially out of our
very deep hole. And we have a machine that’s trying to boot again, though not succeeding.
We should now be able to attack our RAM problem. See you in the next episode!
It seems like this is a bad case of amnesia. And now, why is the memory not working
while it checks good in the first part of the boot?
All right, every bit is checked. Uh, so that's a little bit puzzling...