Oh well, 2020 was really a rotten year to the end. I was making my last video of the year
with the 9825, and all of a sudden
it made a weird noise, and then some bad smell came out, and the display went on the fritz.
Hello and welcome back. If you follow the channel, you know that I recently played a
lot with my HP 9825, trying to imitate Mr. Fancy Pants from the HP catalog.
It gave us impeccable support while repairing the HP 98035 clocks,
and endured a thousand power up cycles. But when I was filming my video celebrating the 100,000
subscribers milestone, it decided it had had enough, and suddenly gave up the ghost.
And the machine stopped working. Of course I turned it off, and what happened is that
the power supply - one of the transistors just let go, became a short. And the 5 Volts
became 13 Volts all of a sudden. So one of the circuits, the TTL circuits,
exploded, actually in my clock interface. And that's what's left of the circuit that
exploded in my clock interface. So I don't know if the circuit went short and blew the supply,
or the supply went short and blew the circuit. And I suspect it's actually the latter, and that the
power went high in the whole thing and that this was the first circuit that took the brunt of it.
And it might have zapped that plenty of circuits on this
poor 9825. So I don't know how catastrophic it is.
So Carl is helping me taking care of the HP 9825 that has been
dead since the beginning of the year actually. What happens is that [in] the power supply
a transistor let go, and if the transistor goes and shorts it shorts basically they unregulated to
the 5 volts so I got 13 volts on my 5 volt [rail]. It's one of these. This is a 30 amp transistor
actually. So it's not an usual one. They were pushing it. But I think once we repair that,
we should have a working supply. The problem is, it got the 13 volts all over the place over here.
So how much damage we have done, I don't know. Depends on how quickly it went, right? Yeah.
So here you have the schematics, and it's regulated. And I believe the transistor that
went down is this one. And it's an early switcher, actually has some IC in there.
And then you can see the Shottky diode - that's this big boy over here.
And then there should be a coil. So that's probably this one, stores the energy.
Actually there are two, so go figure, one of those things. And the problem with that design is that,
if the transistor goes bad, then your high voltage is going straight through
to the five volts, and it fries all your TTL. Which is exactly what just happened.
So this power supply is just a buck converter, which I explained during a previous epic repair
of a more modern HP switching power supply. That poor supply kept blowing itself up, and I wondered
at the time why they made it so complicated and had first a buck converter, then followed by
another DC-DC converter. And now it suddenly makes sense: it’s to avoid this very failure mode, where
a dead buck switching transistor would kill the whole equipment the power supply is connected to.
In this more complicated modern design, my dead transistor in the buck converter just
killed the DC-DC converter following it. It was still very annoying,
but it stopped right there, with damage only in the power supply.
And that explains why my other supply, my other HP supply, I was wondering why do
they do a DC to DC converter followed by a switcher, that's stupid right? But no,
it's not stupid! It protects you against that fault, right? So, in the later supplies,
they first bring down the voltage, and then they bring it back up with with a switcher
and a transformer, which means if either stage has failed you never have a continuous feed from the
unregulated to the regulated [voltage rail]. But in this one, they didn't do it. Yeah. This is a
4398. Must be this one, right next to the the main switching transistor. This one says 4393
right? No, it's an eight I think. Is that right? Yeah, I think so. Okay, we'll see.
One Ohm - zero Ohm! So it's a complete short, but that might - the guy
is as much as short as you can get. That makes it a great test result. The emitter is just connected
to the collector! It would be interesting to open it up and see how much damage there is in there.
I think the transistor may have literally blown. Okay, so that's a transistor that has
definitely lost its mojo. And it overheated and sled from its pad - soldering pad maybe?
I think I see the former footprint to the left. I think it's not centered anymore.
And it both fused and broke in pieces.
That's no fun
Okay, not a happy transistor anymore.
So as expected, pretty straightforward failure, the main switching transistor was a solid short.
After a quick check with the HP community, we discovered that we were not the first one with
that very problem. We did the customary checks for the caps and other ancillary stuff, but all was
well. So hopefully when we replace the transistor, we’ll have a good supply again. But God only knows
how much damage the overvoltage has done to the complicated circuitry of our beloved 9825.
All right, so, we've put the power supply back in, we've taken all the power connectors out. So it's
not hopefully connected to anything. And that's a hail Mary because we just replaced the transistor,
checked the caps quickly. Should I go on, see if we have 5 volts?
We do have 5 volts. 5.1 volts. Okay, so it was the transistor.
Let's test all the other voltages, and then we'll just plug and see what damage we have.
So Carl had the good idea to check the resistance from five volts to ground as we plug things in.
And this board should not consume too much, that's the memory and stuff. So 69 ohms, fine.
That's the main computer board, so that should suck a lot of current.
It's 3.9 ohms. So there we don't know, could be normal. Yeah, 1.2
amps. It's not zero. It's just Watts. And then we have to plug in the keyboard. So we connected
the keyboard, so now it's everything, and it's 2.2 ohms. The 12 [V] is 3 amps,
I bet you 5 is way more than that. Yes. It's a pretty power hungry processor in there.
Yes they had a big beefy transistor Yeah. Okay we'll try a hail Mary which we don't know but...
You got 5 Volts!
Okay, it's unhappy with something. But your 5 volts was right there.
Well, but we fried something else.
We had forgotten the data connector back to the keyboard display. So maybe that was it.
Aha! No beeps, but no T, not starting up.
OK, our power supply was a simple fault and a quick repair.
But unfortunately, and not too surprisingly, our HP 9825 did not boot back up. The overvoltage has
done much damage to the computer circuitry. We are still in the middle of trying to revive it,
and so far it has been anything but simple. Actually I am still not quite sure that we
will be able to bring it back to life. But this is a noble and groundbreaking 16-bit machine,
and you can bet that we’ll try our best to save it. So, hopefully, see you in the next episode.