Hello and welcome back. If you follow the channel, you know that we recently finished
repairing a Xerox Alto owned by Bruce Damer at the Digibarn. Our friend Josh Dersch had repaired
all the boards for us. This Alto was in such poor shape that we expected a lot of the ICs to be bad.
However, they were all mounted on sockets, so Josh reasoned that it might be faster and easier
to just take them all out, and test them all. Which was a good idea, because he ended up with
a whole box of bad chips. And for testing the chips test, he recommended the board he used:
a Tauntek IC tester, a kit designed by Bob Grieb, who also happens to follow this channel.
I am now facing a similar situation with my fried HP 9825,
with a mounting pile of bad ICs. We had much fun testing them with a breadboard and fashioned LEDs,
but I think it is time I build one of the Tauntek testers for myself.
So this is a Tauntech Logic IC Tester, and that's what Josh used for testing
TTLs to repair the Digibarn Alto, and I wanted to give it a try. So it's a kit,
really a minimum kit: you just get the board and the two programmed PICs. And then you have to buy
all the other components. And do it à la Heathkit, it's a good old-fashioned way to do stuff,
but it's fairly well documented. So it shouldn't take very long, should be fairly relaxing.
This kit is a lot of resistors and diodes!
But the reason there are so many resistors and diodes is a very good one:
the board is able to measure the voltage of each pin under different load conditions,
allowing it to ferret out marginal parts that would otherwise pass a simpler tester.
Okay, there she is, that was definitely old school! I gave her my best sockets here. The
PICs are really millipedes, they're 28 pins! So I put two 14 [pin sockets]. But those are
Swiss-turned sockets, nothing but the best. Gold plated. None of that nickel plated pincer rubbish!
All right, time to test. And here I have the Tauntek and here I have my TL 866, which I
didn't know could test the logic IC circuits. It's a great unit, I use it to program EEPROMs.
But yes, it can also test some TTLs. Thanks for the commenters who pointed me to that. And
here we're going to test a bunch of failed chips that were from the 9825 restoration,
here. And see how it compares. First, this one, it doesn't tell you very much when you
plug it in. There is not even an LED. But it becomes pretty chatty once you connect it to
a terminal. So it's kind of an old-fashioned way to do things. But it's efficient enough.
So for added fun we will compare these two testers side by side, the TL866 EEPROM programmer
and the Tauntek I just built. Both are pretty good you’ll see, but let’s see if we can pick a winner.
Okay, let's start with one circuit that actually works, or I think it does. That's one of the first
ones that we pulled with Carl, and actually we knew it was one of either two ICs. All right,
so we suspect either of those ICs. We don't know which one of the two it is, so we might as
well replace the two. And it turns out this one was good, and the other one was bad of course.
And this one is a flip-flop. It's a [74LS]112. I have it somewhere here. And here's the 112. That
is a dual flip-flop. And by the way, this is the Texas Instruments TTL catalog. A very early one,
first edition, 1971. So it doesn't even have the LS
series in it. And this one exists only in the S series at this point.
Carl had tested that circuit before the good old-fashioned way using our fancy
breadboard. But although it worked logically, we weren't quite sure if the weak signals we
observed where due because of the circuit, or because of the circuit that followed it.
What does it say, Carl?
[Carl] Well, it seems to work but we don't know about the quality of the output signal.
[Marc] So that’s a perfect candidate, because the Tauntek should enable
us to check the logic level output voltages. But let’s start with the TL866.
Test! And it tells you: flip-flop 1 set and reset test, flip-flop two set and reset test. Normal,
normal, normal. It passed all the tests. So let's see how it does on the Tauntek.
So the Tauntek is an old-fashioned terminal interface with RS-232. And you just say: LS112.
And it says: found LS112 in our database. And then you just say "t" for test,
and it gives you a whole bunch of information. It measures the current in the chip:
4.4 milliamps. It figures out if there's any shorts on the input,
and then it does a logic test with threshold voltages that depends on the series you test:
the S, the LS, etc... And then it gives you the vector test and the results. And it actually
reads the voltages, so it gives you a lot more information than the TL866, which is pretty nice.
So the Tauntek confirms that the output voltages were fine,
the other IC was causing the problem. And indeed its inputs were short circuits.
However we changed both for good measure. But let’s try another circuit
that tests good logically but where the Tauntek allows us to see a hidden problem.
This is an LS04. Actually this is not from the 9825. This is from my collection, when I was
looking for replacement of a failed LS04. LS04, this one. So this one just does LS and HC, but
I found it works for almost any 74 series, independent on what series they are.
By the way, series of ICs: there is the regular one, there is the S, which is Schottky, which is
fast. Then came the L which was low power, then came the LS which was low power Schottky, which
was about as fast as the original, but in lower power. Then came the H and the ALS and the AS,
advanced version thereof. But most of what you'll will see is the regular, the S and the LS.
Look at that: normal, normal, normal, no problem. It tests fine. However, when I test it on the
Tauntek - by the way, Tauntek, the chip is at the bottom and TL866, the chip is at the top.
That got me going a few times. So, what is that, LS04. It knows what it is. Test.
And look at that! Test pass, chip looks okay. However, if you look at the power consumption,
it's 33.7 milliamps. So that's not a good LS chip, that's totally wrong!
I'll get you a good LS04, here's one.
And if I test this one, it gives me also a good table. But it's 4.9 milliamps.
So the other one is a factor 10 off, and I think it's just about to die. So this board
allowed me to ferret a circuit that this one wouldn't have found. So that's pretty nice!
We found next defective chip in episode 9 of our HP 9825 repair.
It came from the extended memory Skoal board on the top.
[Ken] It's supposed to be a hex inverter, but after testing it's a penta inverter.
So this inverter, the light lights up. The pin 8 - pin 9 inverter: it's dead.
[Marc] So here is the Tauntek. This is an S04, so different series.
And this is the S04, hex inverter in the Schottky version. So speedy, speedy: three nanoseconds.
At the expense of higher power consumption, so it's 19 milliwatts. Found it! Test...
And so this one is 32.2 [mA]. So it's an S-series, so that is the high power consumption that you
expect. And it found everything was fine, except pin nine, it had a problem with it.
And then it puts in the voltage it
expects for the S series. So it has to be higher than 3V for a one, lower than 0.35V for a zero.
And it tells you that they all work, except [pin] eight. And it highlights it.
So now if we do the same over here. Oops, no. See, I made a mistake,
put it at the bottom. Here you go, at the top. And here, this guy doesn't know the difference between
an S04 and a LS04. So I'll just reuse the same test. And it gives you: normal normal, normal,
error, normal, normal. So I think the two are perfectly fine here to ferret a bad circuit.
This is a S64. So that's one from the 9825 that failed. And it's a weird combo of AND and
OR gates. So, they are called AND-OR-INVERT gates. And it turns out the reason they make
those is that it's easy to make with very few transistors, so it maps really well on the TTL.
So what does it say Master Ken? [Ken] So we're not getting any output out of the AND-OR-INVERT.
We've grounded all the and gates. So the and gate should give
zeros into the NOR gate. We should be getting a one out, but the LED is not lighting up.
[Marc] S64, test. And this one, it just doesn't even try to test it, because one of the pins
seems shorted to ground. So it just abandons right there. And if I try it over here, search,
64... It doesn't have it! 164, 564, blah blah blah, 64-something. So, this one has some chips
that this one doesn't have. So that's good to know. So, one more point for Mr. Tauntek.
This is a 7425. It's a weird one. It's a four input NOR gate, but it has an extra
gate input. And I couldn't find any at my usual surplus stores here. And that's one
from an episode not yet released. It's one of the chips that killed our our keyboard in the HP 9825.
So fairly rare chip for some reason. It's actually not known in either [of the testers]. If
I do 25 here, it doesn't know it at all. And if I do 25 here: 7425...
"I couldn't find that part number". Not to worry though, because you can
ask Bob. And he said that he would add a chip if you ask him to. So, when I replace this one,
I'll buy two chips, and I'll send one chip to Bob, and he will add it to his database!
And you might wonder which one I recommend after this test.
And the answer is: both of them! If you need only one, the TL866 is such a fantastic device.
Professionally made, very cheap, you get a ton of adapters. It tests TTL, but it's it its side
job. It's a great programmer and reader of EEPROMs. For the price it can't be beat.
And then if you want more details on your TTL and also want to catch the marginal ones,
the Tauntek as you've seen is very very good. So I'm going to keep that one and use it a lot.
And of course in case neither of those work, never underestimate the power of the breadboard,
the simple one or the complicated one. You can always test TTL chips on the breadboard!
Alright, see you in the next episode!