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, died from a power supply
failure that caused an over-voltage. The computer died that is, not Mr. Fancy Pants fortunately.
But the 5 Volts became 13 Volts, which destroyed many TTL chips throughout the machine.
It was our version of the Apollo 13 disaster, with everything broken but just the custom
multi-chip processor still working, which was enough to keep the hope going.
Over the previous 13 episodes, we methodically nursed the machine back to life,
and finally, it is back to full functionality.
In the end, we found no less than 29 fried
chips in it. And it wouldn’t surprise me if a few more failed in the coming month or years.
But we had accomplished much more than saving a rare machine:
we had recovered the original schematics, understood how each board works,
met with some of the original designers, and interviewed Mr. Fancy Pants,
who had spent his whole carrier at HP and ended up as Keysight’s principal metrologist emeritus.
But of course, we don’t want this power supply failure to happen again. And for that,
we are going to design and install an over-voltage protection circuit, called a crowbar. Actually,
we don’t have to design it even, because we are going to copy the one designed by HP.
Yes, we are not the first ones with this mishap. HP must have had its share of fried 9825s coming
back to the factory for a very expensive repair, and decided to provide a fix of their own.
Fellow HP restorer Rik Bos discovered the fix in an HP 9835, the successor to the HP 9825.
The 9835 shares the same form factor, and uses an almost identical power supply. He sent a photo of
the board he found. It is a simple fix, a small add-on board next to the edge connector contacts.
But you might ask, what is a crowbar circuit? Could we not simply use a Zener diode to prevent
over voltage? Indeed, many commenters suggested adding a 6V Zener across the output to fix the
problem. The thinking is that if the output goes too high, the Zener kicks in, and clamps the
output to 6 Volts. But some experienced viewers are probably shouting at the screen right now,
because this not only is not going to protect from the over-voltage at all,
but it’s going to end up in a pretty bad ElectroBoom moment.
You see, to use a Zener, you need to feed it with a resistor. The diode works by
regulating the current in the circuit so that the voltage drop across the Zener remains at 6
Volts, and the resistor takes the rest of the voltage drop.
The optimal resistor value for correct regulation is given by the Zener manufacturer,
but is typically chosen so that around 10 mA flow into the Zener, within a fairly narrow
band. This leaves you with a regulated voltage from which you can extract a few
milliamps. This is typically used as a reference voltage, or to power a very small circuit.
If we omitted the current limit resistor and connected the diode directly to our supply,
which is capable of 30A, bad things would happen. The typical equivalent circuit of
a 6 Volt Zener is a 10 Ohm resistor in series with a 6 Volts voltage source. So let’s imagine
we feed it with our faulty supply, now at 13 Volts, but still capable of 30A.
The junction of the Zener drops 6 Volts, which means the internal resistor will be dropping 7
Volts. At 10 Ohms, it means approximately 700 mA will got trough it and the junction. Meanwhile,
power output voltage has not changed a bit, it will still be at 13 Volts, but now robbed of
700 mA, so it can only deliver 29.3A. Which will of course still fry our entire 9825.
In addition, the Zener now dissipates 10W at will go up in smoke. So that won’t work at all.
But we are not far from the real solution. We just need to add another component to help the Zener.
Something that can maintain the output close to zero volts, while sinking the entire 30A. And yes,
you guessed it, this is called a short. We need to create a temporary short at the output,
for example by putting a real crowbar across the terminals. And that’s exactly what our
aptly named crowbar circuit does: it creates an electronic short if the voltage goes too high.
We are still going to use the Zener, but only to help us detect the over-voltage condition.
We are just going to use it to trigger a Silicon Controlled Rectifier to create our short.
The Silicon Controlled Rectifier, or SCR, is a magical component indeed. Its symbol
is like a diode, except it has an extra control electrode. The diode is normally completely off,
but as soon as the gate control voltage is above a threshold, on the order of half a volt or a volt,
it suddenly behaves like a real diode, with about 1V across it. Moreover, once triggered,
it does not let go, as long as there is positive voltage across the diode.
Like a real power diode, it can also tolerate very large currents without destroying itself.
The one on the HP board can take 80 Amps for a short while.
That’s enough to sink all of the power supply current and blow the input fuse. At which point
the SCR resets as if nothing had happened, and our machine has powered down safely. The crowbar
circuit using an SCR is a very old trick . Our IBM 1401 at the Computer History Museum, one of
the first transistorized computers from 1958, has solid state crowbars on all of its power supplies.
So if you look at the circuit over here, that they have added, this is actually a crowbar.
And you can kind of figure it out fairly easily. Here is the
SCR. You can read the number of it, it is a 2N4441, which is an early Silicon
Controlled Rectifier. And here are not one, but two Zeners, because they want to protect both
against an overpower in the 5 volts and in the 12 volts. And here are two additional components.
And basically the grounds come here, and shorts - actually, they shorted not on
the output of the power supply, but on the input. They short the 20 volts at the input.
Small aside here, both the regulated 5V and the 12V are derived from the unregulated 20V
just after the transformer. So they chose to monitor the 5 and the 12 volts outputs,
but put the crowbar further upstream on the unregulated 20V,
to make sure they’d blow the fuse and not the components in between.
And they monitor - one of them is 5 volts and the other one is 12 volts. So basically,
if you look at it, I reverse engineered it, it's most probably that circuit. You have our SCR.
There are some modern ones, but it turns out, thanks to the micro-fiches that were given to
me by a viewer, that I actually had a previously unidentified part in my collection. And this is
exactly what they used. This is a version of the 2N4441. So I have the exact original part!
And what just happens, is here you have the two Zeners, and let's take for example the 5V one.
If this raises above 5.6 Volts, the voltage here will start to grow, and it when it becomes above
the triggering voltage of the SCR - in this case it's actually half a Volt - it will trigger the
SCR. And the SCR will just pull down the 20 Volt to ground. It will try to get all the current it
can out of the transformer, and out of the mains. And it will blow the fuse.
I have made a reproduction of the circuit, and while i was at it i
made it exactly the same. So if you look at that, you can see how I made it. And I even
reproduced the little HP logo, and made it all exactly the same.
So off it goes to PCBWay, and hopefully we will get a very good replica of our original circuit.
So off to PCBWay we go.
2.2 archaic units times 1 archaic units. And we do 5 [mils]. Two layers.
We're going to do no silk screen, and we're going to do like in the good old days, with gold on it.
And then, we just dump our Gerber files in it. And we submit.
And a few minutes later you should have it. And, oh, it's going to look exactly like that,
like I wanted. With the HP copper silk screen. [One week later...]
And PCBWay PCBs have arrived. I'm actually kind of excited to see how they turned
out. We always have our PCBWay pens, they're all over the lab. And? Okay!
Oh yeah, my HP logo turned out cool, all right! Let me make sure I didn't screw it up
in terms of size. Nope, I did not. So that's where this goes.
So the other version should be a little bit more modern, and have a silk screen.
That's the only difference. Actually this one looks even cooler, I think.
Here is the modernized version with silkscreen,
and the more authentic original version where the only marking is
copper traces under the solder mask. Both of them with gold plate. Very nice!
And here's the finished circuit. To make it look as much as I could like the original,
I found the resistor with the same color scheme. The capacitors, they make them yellow now,
they don't make them teal like they used to. And I couldn't find the Zener diodes with the
opaque body, they don't make them anymore. But it looks like the real thing, pretty pretty close.
And here is a more original one without the silk screen, but frankly, it looks almost the same.
I'll put this one the 9825 because it looks more original, but if you want to build one,
take the one with the silk screen, it's way easier to build.
And the question is: does it work? So I've connected it like it's in the HP 9825.
It controls the 20 Volts and senses the 5 or the 12 Volts. Here I have connected it to the 5.
So if the output goes above 5, it should bring down the 20 Volts. So here is my 20 Volt supply,
and here is my 5. So it should go something above 6. Ding, there you go just at six.
Then even if that fault goes out, it's an SCR, so it's still clamped. And to reset it,
I have to turn it off, turn it on again. And it clamps at 6 Volts. Perfect, excellent!
Let's try the 12 Volt. So, same experiment, I connected it on the
12 Volt sense input. And I think I have a 13 Volts Zener, so it should be out at 13.6.
13... there we go! 13 point something, it protects it. Okay, so the circuit works perfectly
And I have taken the 9825 power supply out, and it should fit right
in, if I haven't messed up anything.
Goes here, goes there, and voila. There you go! So all you do, you solder it from the bottom,
and then you should be done. You should have a crowbar protection modification for your machine.
And voila, the modification is done. And I was wondering why there was a window here in the
PCB. And that's for the cap behind it! If not, it doesn't work. So it's a little too thick,
it goes through the PCB. So that explains it now! All right, so it goes [in] perfect.
Well that’s too funny! In the original picture from Rik’s machine, the 9835 supply is different,
and it made no sense why there was this square opening. But now that I have it in the 9825,
it all makes sense! The caps fits exactly in. Good thing that I made a faithful copy.
And I'm not brave enough to cause an overall voltage on purpose,
but we'll check if at least it turns on without tripping the crowbar.
And it does!
Looks fine. And does it print?
Yep, works fine. Yeah, it doesn't like the command,
but it did print with the 20 Volts and all the power supplies on, so it works.
Protected now!
So I have now upgraded my four fully repaired HP 9825’s with the crowbar modification, and I’m
happy to report that they are all working fine. So if you are the lucky owner of one of these,
I highly recommend that you build your own and upgrade your machine before you even try
to turn it on. I have uploaded the project both on my website and on PCBWay’s project website,
so you can order it straight from there with one click if you want. I would probably skip
the gold plating option though, which adds a huge amount to the cost and is just purely cosmetic.
So on this good news, I hope we will see the HP 9825s at work in future episodes
and I feel much better now that they are all protected. See you soon!