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 computer died that is,
not Mr. Fancy Pants whom we found very alive and still very kicking as Keysight’s Principal
Metrologist Emeritus. Anyhow, the 5V became 13V, which is a big ouchie if you are a TTL chip.
After many adventures and even help of the original designers,
we finally got the 9825 to boot again, but only when we used the
keyboard display printer assembly, or KDP as it is known, from another 9825.
We attempted to repair the KDP in the previous episode. The board contains the proprietary KDP
chip for which there is no substitute known, so we really hoped this chip had been spared.
The board had several dead chips that drew so much current that it brought everything else down. But
once we replaced all the chips that had shorted, we were pretty confident it would work again.
And instead, we heard that. [Loud beeping]. [Ken] Is that good or bad? [Marc] And that wasn’t good
news, because the buzzer is directly controlled by the KDP chip. Is our precious chip dead?
So before we incriminate our KDP chip further, I wanted to check if it had clock. And it turns out,
it derives its clock from the main clock - and it's hard to see but it's the bottom
here - and it's 6 Mhz and then it's divided by four. So we get 1.5 [MHz] and then it
gets two phases out of it. So, I'm right on this flip flop, the divide by four.
And here is my 6b Mhz in from here. And it should come out on the on the pink trace over here.
And if I turn it on, not only nothing comes out, but this thing, the level changes. So I bet you
that flip-flop is bad. Okay, so we're getting closer to our good average of bad chips, right,
it's four or five. Right now we've found just two maybe three. So let's change that one.
Okay here it is. LS? Actually, not LS, [just] 74107 dual flip flop.
Found in our database. Test it. No good! Which is not unexpected. Okay, so I have to find a 107.
Bad chip! And I have a whole bunch of 107s, but the Tauntek doesn't like it, they all fail
for the same reason: two pins bad. It thinks they have leakage. Let me try it on the TL866.
And the [TL866] doesn't know the 107. So now you have one case where none of the testers know that
flip-flop chip. Dang! But they must be good, I'm just going to replace them.
Okay, try again with the new chip. And a one, and a two, and... Yeah, all right!
And actually it works. And so I have the divide by four. I still don't have my phase one and phase
two, and that got me going for a while. The phase one [and] the phase two being the ones that drive
the circuit. But it turns out, it comes through an amplifier IC that's an analog thingy. So it needs
+12V to it, which is this guy over here. So when I give it +12V, magically my clock appears. So
I think I can drive the board entirely
with -5, +5 and +12V. The -20 and +20V are just for the printer. And, I think, with that, I should
have keyboard scanning. I think the display scanning requires intervention from the processor,
but the keyboard scanning should go all on its own all the time. So, let me see. This is plus five,
minus five, plus twelve. And let's poke-poke. Oh yeah, I see it. So I am scanning.
There you go. So we definitely have some scanning happening at different
frequencies. So that's pretty good. So maybe the KDP chip is not dead.
It's getting warm a little bit, so it's definitely doing something. All right. So let's try it in the
machine, where the processor can talk to it, and see if it boots or something. So back on the 9825
we have now changed five ICs, so we're coming up on our quota here. Is that going to be enough?
And no. Nope nope nope no no no.
So at this point I think the KDP chip must really be bad.
And the one way to know, is to put the KDP chip in the good keyboard,
and see if it works or not. Okay, this is our good keyboard, and I just put the suspect KDP
chip from the other dead keyboard [in it], and see if that does it.
Yes, it works! Oh, that's good news! So the KDP chip is good! I just haven't restored enough TTL
in the other keyboard. Oh, excellent! Okay, so let's work more on the board and see what could
go wrong with the display. Okay, well, that's a giant piece of good news, this appears to work.
I'm suspecting all this is right. This could be bad but at least on the way there it's
good. We know the oscillator is good now, the bell is good, we changed that gate,
and here is the display stuff.
So that gate went bad. So maybe
it could have driven some of these bad. So we have some run-of-the-mill gates,
an inverter and a decoder. And how is the data coming out? Data is coming out here: this gate.
And there's only three circuits. I'll just pick them all up and test them, see what happens.
So here are the external chips involved in the display, and most tested good. So those three are
good, but this one is bad. It's the 7408, and that was the one that was driving the pixel data on the
LED. So definitely that would kill the display. I have a new one of the dead one, and I also have a
new one of the one tests good. Not of that one. So while they are out I'll put some new ones,
those have been stressed definitely. But I don't have a new one of this one,
so it goes back in. And, by the way, this is why I don't like to test circuits at random,
I broke a capacitor. So it causes a lot of damage when you start pulling good circuits out for no
good reason. But in this case, I had no good way to test, so that was the easiest way out.
Okay, it's getting late, I hope this works this time. So we changed the 7408,
which is the only bad circuit in the display that was driving data, so let's see if that works.
And it does not.
Huh.
There's not much else... Actually, it's not completely dead. If I do "beep",
it beeps. So I can input a little program to test: beep
wait 2000
beep
wait 2000
beep.
I should have three beeps.
All right, it works. I must [have missed] a zero in the last one, but... [machine beeps 3 times]
So the machine is running, it's just not displaying. Whoo-hoo!
And there's a couple of ancillary chips that could still - there's
a gate or two that I haven't tested. That could be blocking the display.
Or the LEDs might be bad - I haven't thought about that one.
So I think we are very close, it's booting up, it's wanting to display something but nothing
comes out on the display. So I'm trying to figure out if it's actually sending
data to the display. And the two pins that are of importance are:
Dot Clock and Dot Data. It's what's basically sending the stuff out to the display.
So I'm all probed on the KDP chip down there. I have [holding] my stuff with the
stick. And this is Dot Clock and this is Dot Data. And I have data, it's trying to display something.
And I'm going to press a few keys on the keyboard.
And you see the data has changed, it's displaying a "b" now. And then, the one more thing that we
need to check: each digit has five columns of dots, and it does refresh one column at a time,
from S0 to S4. So this should just ripple through. If that's coming out and that's coming out,
then it's just that the display is bad and not responding. I moved my probe from Data to S0,
to see if a column is selected - that's my red [trace]. And sure enough, there it is! So,
it is perfectly selected. What's going on? So the display should light up at this point, I think.
So maybe it's my display chips that don't work. There's a board here that has little display,
and those little displays are intelligent: you load them dot by dot, so there's a shift
register in there. Maybe that's blown. So I've convinced myself that the display should light up,
but it doesn't. So I took the display portion from the good display, it's easy to remove fortunately.
And all it has is a few driver transistors, and and the display themselves are the smart ones.
You load them with dots in serial fashion, and it arranges them around. So it has a shift register
in them. So I tested the transistors in the board, they look okay. So I think it's the chips.
And for that I have to take one chip from the good board, make sure Ii make a little test set up to
test it, see if it works. And then I pick a chip from the bad board and confirm that it's dead.
And then I have to find new chips, which is not going to be easy.
So I have taken one good chip out of the good display board, and made a little tester. Ignore
this, that's from the previous episode where we were testing the Intel memory controller chip.
It has a clock. Then you load the bits, 28 at a time. And then you light the row that
you have just loaded. And then you do the same for the the other rows.
So right now, I am giving it a constant clock of - let's do one Hertz. And I'm giving it all ones,
so if I do a column, it should be all lit. It is! Then you have the other column, the third column.
But we should see the shift register working. If I inject some zeros in there, you see,
we're loading it. So usually you would load it at 6 MHz, not 1 Hz,
and this would be hidden: you wouldn't show anything while you're loading it, right?
And then, once you have loaded your pattern, you just stop the thing,
and you show it. But what I'm going to do is get the input wired up to the output. Because there's
also an output of the shift register, so the bits that come out here they'll come back there. And
now you see the the bit pattern. If I go faster it's more fun. It's quite psychedelic actually.
I'm just going to take one from the bad board and see if I can light it up in the same way.
Okay. All right, chip from the bad board.
Are you a good chip, or are you a bad chip?
Nothing, nothing, nothing. Dead! So we have probably eight
dead display chips. Actually we don't know, because one display chip might block the
whole content for everything. If one is dead, it will not pass the data to the other one,
and this is number one. But I bet you they're all dead, I'll test them all.
Surprise, surprise! They are all bad. There is not one to save the other. All dead.
I couldn't resist but take a looksy under the microscope. It's such a gorgeous chip.
Here. Come on, focus! Right there.
As you can see here, each chip has four matrices of 5x7 LEDs for four characters. There is a driver
IC chip taking care of two arrays each. And this is it with the glass removed. This is a hybrid
on ceramic that must have cost an absolute fortune to make. It was the first such chip,
and was developed specifically for the long alphanumeric display line of the 9825, which
was very novel at the time. It was later made commercially available, and became quite popular
under the HDSP 2000 designation. These display chips are really worth an episode on their own.
Yeah, I finally received my replacement display chips. The originals had this
tinted window. They were glass-ceramic. This is the commercial part made by Siemens. It's a
HDSP 2000. And they went the cheaper route of course, they put it on a plastic package.
And with - I don't know if it's glass or epoxy in the front. And it's not tinted,
so you can see the whole rows of LEDs and the two chips. And I just popped one in my tester.
And sure enough, if I can get the exposure correct,
they do work fine, I just launched a few bits in the shift register. I'll solder them in the HP
9825 display board, and hopefully our display will be repaired. Here is the new display
with the Siemens chips. I soldered them as straight as I could, I think I did an okay job.
I used a straight edge to do it. Okay, remounted the display, so we'll see if it comes back up.
And it does!
And
it works. Okay, so we repaired that.
And I'll check the printer while I'm at it.
Okay, it works, except a column there. But that didn't work before, so
it's as it was before I got it. This column here, it never works. I don't know why,
because there's nothing specific to it. The head moves, so it must be something mechanical.
Back, back. Delete. Execute. All right it's repaired, the KDP is repaired. Okay, so
we are 90% there. There's still something that doesn't work: it's the tape.
And the tape board is actually the biggest board of the whole thing, so I didn't expect it to work.
And I tried it and of course it doesn't. So the KDP chip lives, and the next time we work on it
,we will try our best at reviving the tape drive. And then we should be done, finally! See you then!