I think I’ve mentioned before that your probe graphics overlaid on the schematics is really helpful for those closely following along. Another fantastic episode. It’s obvious that you are doing your homework for these and I’m really enjoying them.

Fake components started to become a problem in the early 2000s, when production was basically outsourced to Asia. A company I used to work for was seriously hit several times by the infamous capacitor plague. Even when sourcing supposed top notch products from very serious distributors, that ended reviewing their own supply chains, and, for what i understand, involved the police as well. It meant millions of pounds to these guys.

Excellent demonstration of the TTL/CMOS differences. That really made a difference in my understanding.

It would be interesting to see the current consumption of the suspect chip. A CMOS one will consume essentially zero (just a few μA) unless it is switching rapidly. A LS TTL one will consume mA or tens of mA in the same state.

Wow, that's shocking that they've have gone so low to start rebadging 74 series...

I just got pointed to this video by a friend and O M G, I'm having a similar issue and now ALL makes sense, I even replicated your test to identify the TTL parts. Thank you SO much for the detailed explanations, great video.

It happens quite a bit. I found it when developing Pi1541. I use a 74LS06 to take a 3.3v signal from the Pi and invert it to drive the open collector IEC bus signals on the c64 low. Some cheap Ebay sellers buy their components in bulk from China and most of the time they don't work as well as the official TI ones from Mouser, Digikey or Element14. Then the end users come to me complaining about my software. After I tell them to replace the 74LS06 with one from a credible source their problems go away. Thanks Noel, now I can refer them to this video.

Great video. Just as a side note, though the 74HC chips may or may not work in those types of TTL circuits, 74HCT chips(the T standing for TTL levels) are specifically made to be backwards compatible with TTL LS and ALS chips, and the input threshold voltages would be the same for both.

Time Error Correction is applied every day to ensure that the frequency of mains supply is correct over every 24 hour period. This ensures that mains-synchronised clocks are extremely accurate.

I really enjoy these videos. It's all good: content, production quality, preparedness, but the outstanding feature is pacing, Noel does it perfectly. Never do the videos seem to drag on. Not a second is wasted.

The analog behaviour of 7400-series logic can vary widely from device to device. I've noticed this myself with multivibrator circuits which were built using 7400 quad AND gates.

I think that Commodore engineer's decision to use the chip in its low threshold mode is a bad design by itself.

Your video quality and graphical explanation are getting better on each video Noel. Good work.

In the 80' days a lot of clocks worked using ac signal as a clock freccuency because it is very accurate in long term. Remember the "radio reloj despertador" all of them worked using the 50/60Hz from mains, in fact most of them have a switch for selecting 60 or 50 Hz.

I repaired a ZX Spectrum programmable joystick interface recently and accidentally fitted a HC ic by accident with the others, instead of a LS. Had me looking for a very intermittent crash for ages, until I realised my mistake. The tipping point for a port crash was just on the edge and was corrupting the data line.

I used to have 3 C64 computers but only one reset when the kitchen fluoro was turned on or off. The fast spike went straight through the psu and any filter Cs. If I remember correctly, tantalum caps filter out higher frequencies than electrolytics so would be better for HF noise. Once reversed, I wouldn't risk using a tant again...I have seen some light up even when installed correctly. Interesting videos , Noel, and nice looking C64 board.

Again great cap from your serial! :P and my gosh! how great video quality and edition!

Great stuff, love your systematic approach. Your explainations are crystal clear. Like to build these repos myself, and this is a great source for learning commodore stuff, the best channel in my opinion!

Another great video!. I really like your way of explaining this stuff. You make it understandable, even for people like me, with almost 0 knowledge of electronics. Thanks!

Something worth mentioning, re: mains frequency synched clocks.. For several decades, clocks have been synched to mains frequency. Before electronic clocks with digital logic, clocks like the one pictured used synchronous motors - a bit like the one that drives my belt-drive turntable. Here in Australia, the lights still flicker around 2am each morning as power utilities trim the rotating machinery driving the grid such that the number of complete cycles in the 24 hour period is (as far as I know) within a certain percentage of 24hrs x 3600 seconds x 50 cycles/sec = 5,184,000 cycles. That is mandated by the fact that traditionally, so much time-keeping relied on it. Nowadays, NTP (the network time protocol servers) means our computers and phones are almost always in sync.

Thanks for showing that breadboard circuit to test the TTL chips. I will check my Chinese so called TTL chips with it. Best wishes.

CSI:Retro Lab ... Where no one has gone before... ;)

Excellent detective work Noel! Very well researched and clearly explained with great diagrams as always. 👍

Thanks for the lesson on CMOS and TTL differences. As always a very well put together video.

Amazing video! The investigation was awesome. Thanks!

Great catch, can't say I would have noticed that re-mark!

with the latest version of xgpro (11.71) you can actually choose the test voltage (5.0, 3.3, 2.5, 1.8). one thing to note is the test menu is no longer listed as an ic option. it has its own menu and dialog window

Great video, many thanks, love the logical fault finding and 'proving' technique...now that's how to find faults, guys. Servicing TVs in the 90's we had a batch of high power 2N3055 TO3 transistors immediately blowing up when switched on ( put on Blast hats and dive under the bench!!) Cutting a 'new' one open, we found that the silicon die was a fraction of the size it should be, yes it would test OK and run in a low power circuit fine... But, give 'em some whoomf and Bang!! .. Yes, they were 'new' from China...... I hope you like the highly technical language and description... 🙂

Excelente job! You are clear and gave the full information. Thank you!

Great episode! Learned something - again!

Again a super informative episode, expertly presented! Thank you!

Great explanation on the zener diode. I had always wondered what an application for one would look like - little did I know I had one next to me!

Love your videos and the effort you make to educate. Never stop

There are two types of 74xx CMOS logic gates, the 74HC range, like the ones you were experimenting with, but there is another range 74HCT specifically designed to have the same transition voltages as standard 74TTL chips. The “T” after the HC denotes standard TTL logic levels. I think if you put a 74HCT08 it would work.

This was great, Noel! Thank you very much!

Interesting and a great vid as usual. Thanks Noel.

CuriousMarc did some videos the first part of this year where he was repairing a bunch of timer modules for an old HP computer. The clock circuit used was similar to the way the squaring up circuit for the CIA timer was done by Commodore, except it was crystal driven. He found that even if he replaced the chip with the same part number from a different manufacturer it would not work properly and he had to change a feedback resistor to a lower value. Evidently this type of operation is 'quasi-analog' (made up but descriptive term) and depends very much on the analog properties of how the exact chip was fabricated. So, in a typical digital signal operation it would work fine but not in this quasi-analog state. Interestingly Ken Shirriff found out from decapping and reverse engineering flip-flip chips that they have a quasi-analog design as well which explains why they tend to fail more often than other logic chips.

Another giveaway that the chip is CMOS is that the OUTPUT voltage goes at or near 5V. A real LS one would barely reach 4V. Not that this affects the circuit any; I'm just bringing it up to add one more way of identifying the line of chip. Excellent video, although I wish that the scope had more prominent lines for each volt.

I love the research you have done on the and gate. Very didactic. I am planning to build a C64 replica, so I will love to see the upcoming videos on the alternative chips. Thanks for the video...

That was interesting. It seem to behave like a CMOS chip with the trigger level. Did you check the current consumption?

To elaborate on what was mentioned near the end of the video, there are actually two kinds of 74 series cmos chips. The ones that are labeled 74C, 74HC, 74AC etc, are the kind you have in the video. They are made with the functions and pinouts of 74 series TTL chips, but with CMOS voltage levels for interfacing to CMOS logic chips like the 4000 series. There are also chips labeled with numbers starting with things like 74CT, 74ACT or 74HCT, etc. These are made with CMOS but they are made to use TTL voltage levels to interface with TTL logic, but with the low power consumption of CMOS. There are also parts labeled with numbers starting with 54 , such as 54LS08, 54ACT08, or 54HC08. These are the same as their 74 series equivalents, but are mil spec components with an increased temperature range.

Hi Noel, me the Chinese mythbuster again! From my China factory experience, they may have shortage on the LS type of chips but not the HC, so they need to fill the shipment, otherwise the business will be ruined. Or someone did placing the wrong order carelessly. Since Chinese character is totally different from English, and Chinese factory is not a place with people with high level education, the foreman may barely can use English and make a batch of HC but customer want LS, so what? Mark them as LS!! Remark chips can cheat TLS866II and work in some situation, just this kind of circuit which require tight voltage level,fail.

Wow! That wasn't unexpected, as I'm following you on Twitter, but it's another example of great troubleshooting!

Wow, that was very informative!!! So good to know, thank you for sharing!! Michael

Great diagnostic video! Yeah, conclusion is shocking. There is no logical sense to fake 74 logic chips, but, apparently, someone gets money from that. One option is, that assembly factory purchased lots of unmarked chips and after job was done, they resell these leftover chips very cheap or thrown away and rebranding was done by some underfloor factory in China who picked these up.. Now I need to verify my LS stock. I think that I have at least one fake, because of strange behavior in one new circuit - replaced with old used IC from different manufacturer and problem disappear.. This reminded me situation with TDA1524, back in days. Purchased originals - both was faulty. For one chip worked only one channel, for other chip worked only other channel. From different local seller I purchased other pair of chips. Seller warned me, that these chips may be fakes. I get home, plugged first chip in socket and it worked as it should.. Both chips, claimed as fakes, worked, but originals didn't.. 😂

I wasn't so about all of being fake but good info as mentioned here about differences in those signal levels. Thanks!

Hello Noel! If you also change the resistor R100, not only the zener diode, to a higher value, you should get a higher voltage level on the AND gate input. So then it should also work with the "original" 74(HC)08. Greetings, Doc64!

Back in the 80's we had a very similar problem when card production swapped the manufacturer of an LS chip in the Power reset circuitry for a banking terminal (Texas Instrument vs Nat SemiConductors I can't remember which way round it was). Needless to say when turning the machine off the replacement chip enabled the processor whilst the voltage was dropping and the processor corrupted the backup memory - it took some time to find that problem. Nothing fake - just manufacturing tolerances.

Another fantastic video! Plus I enjoy seeing you use a simple handheld solder-sucker, as it's all I can afford. 🇨🇦🐧

i always look forward to your new video's and this video certainly didnt disappoint. Keep up the video's they are great

Wow, I found this channel recently, it's so underrated, I hope videos like these receive a lot more views than now, success for you, have a great day :)

Great job Noel, every time better videos!

I taught digital electronics for almost 25 years. Our supplier of lab parts, the lowest bidder, supplied fake chips. It was a nightmare for me because we were analyzing the performance of various families and they were all the same (CMOS) yet labeled differently. Also, several of the kits had chips that were labeled upside-down so had many burnups. Also, some chips were NAND for NOR chips, and NOR for NAND chips. We had to collect all the parts and purchase replacements from a reputable supplier. I have never seen a stand-alone TTL produce an output high greater than 4.4 volts, and a CMOS no less than nearly 5 volts. This is the easiest way to test. Also, the input current unloaded for CMOS is nearly unmeasurable whereas TTL will have a few mA.

Another illuminating video! Thanks!

You can also create a zener diode with an higher voltage by adding a normal diode to it in the reverse direction so its drop adds to the zener voltage. To keep the normal diode conduction mode you have to add another normal diode ok parallel to the zener and the first diode.

For time-of-day keeping, the mains' frequency, at least in the US, is quite accurate over time. Traditionally, the multiple grids in the US have been kept it tight synchronization, partly to ensure clock accuracy.

I mean, you can still get brand new 74LS-series chips from mainstream component sources (Mouser, Digikey, etc). I think this is more of a story about buying from reliable sources.

Great video, Noel. Could you tell me where you got the solder sucker that you use. It really works very well! The ones that I have used don't work that well. Thanks.

You cold try to measure gate input current to see if it is CMOS or TTL. At least if they haven't gone through the trouble of putting in a resistor to the input. Great video btw. 👍

Good video. I work in retro pinball circuit boards and I have dealt with the same thing, usually when trying to source out obsolete parts from China.

Great video. That's really annoying when a new component is out of spec. It might be safer to go with HCT as I can imagine the demand for LS is very small these days.

Noel, this explains why I was told that you could use a TTL chip for a CMOS circuit, but you cannot use a CMOS chip for a TTL... due to the low trigger voltage levels. Blows my mind that a manufacturer would do this!

Very interesting channel! I'm a subscriber now!

Nice repair video and discovery of a mislabeled or (more likely) FAKE TTL chip! One thing you did not mention, Noel, is that CMOS power consumption goes up dramatically as the input voltages are farther from the rail (like 2.5v for high on a 5v rail), and likewise TTL power consumption increases quite a bit as the input levels get closer to the voltage rails (such as 0.1v low input instead of .7v low input). The good thing is that for this application it shouldn't really matter as far as the input from the zener shunt is concerned. However, due to the reliability issues of (seemingly) all Commodore/MOS chips I would personally be concerned about the CMOS 74HC08 driving an input on each of the 6526 CIA chips. It's one more reason not to mix CMOS and TTL logic chips. If you have to do that a series resistor between the CMOS output and the TTL input is a good idea to prevent blowing out either the CMOS output FET's or the TTL input transistor(s).

I was wondering if they accidently mislabled them at the factory. Great vid, and something we will have to watch out for in the future.

Two things: 1. Using the mains frequency as a time base is a very good idea if one does not mind losing the time when power is off. The mains is more stable than most crystal oscillators over time periods of days or weeks. 2. I would be cautious of triggering a digital circuit using marginal logic levels. The trigger voltage can change with power supply voltage, temperature and time. What works today may not work tomorrow if one or more of those parameters change. If LS chips are not available you could try HCT chips as those are meant to have TTL compatible input ranges at the expense of lower output drive.

I fixed the exact same issue fixed it with a higher zener thanks to you. Exact same label as well. I actually also had a similar fake 193, causing wrong clock. Pretty irritating. It is easy to see via the power consumption, around 4ma is too low.

Very nice video as usual, very well researched. The "fake" chips could be regular chips that fail some test (temperature or other) and are therefore rejected on the standard line. The factory could be tempted to sell them anyway where it could be assumed that they "should" work. As far as I remember the voltage specification for high speed CMOS is as follows HC (Standard CMOS) VCC/2 HCT CMOS with TTL 0.8 / 2.0 The reason for the threshold not being absolute values like like 0v and 5v is that TTL uses bipolar transistors. This in turn means that currents flow in and out of circuits which changes the voltage of the logical levels. So there is a safety margin in the TTL levels. CMOS ICs are not susceptible to these level issues as they can drive to "rail to rail" and they use virtually no current on the inputs. CMOS chips will use less power then the equivalent chip in TTL technology but this is only valid in a stable state (not switching). They have a pretty linear consumption to frequency ratio (consumption goes up with frequency). This is much less the case for TTL. TTL or TTL compatible chips (74HCT) are meant to work at 5v +/- 10% (from 4.5v to 5.5v) CMOS chips are not limited to 5v or 3.3v in fact many of the 74HC chips are specified at 6v (from 3v or even 2v to 6v) One last thing: using logic chips in an 'analogish' way like crystal clocks is always risky as some of the characteristics of the circuit will depend on the implementation technology.

i love these little mysteries. gj

Might note that 74HCT08 uses TTL voltage levels. I use those. Usually LS TTL outputs are

Excellent job!

Nicely done.

Taking the timing signal from the AC mains may give a time that is ahead of or behind the real time by up to a minute or so, but the deviation does not get bigger than this. The speed of the generators at the power stations is controlled so that on average over a long period the frequency is exactly the nominal frequency, but at any given moment it could be slightly lower or higher.

Thanks for great content !!!

Nice demonstration! But why didn't you use a 74HCT08? That would have the same switching levels as the LS, so no need to replace the zener diode.

I had a similar problem trying to control a HUB75 display from a Blue Pill board. The display used CMOS chips, and the datasheet said they wanted 70% VCC for a logic 1. That's 3.5 volts, which you aren't going to get from a chip that runs on 3.3 volts. It was close enough to work most of the time, but when actually using it, sometimes a few pixels on the display would glitch because it was right on the noise threshold. I made one failed attempt at a level shifter before I moved on to other things.

Where the un-ANDed signal voltage goes under 0V, I think that can be expected. At he "unused" half wave of the 9VAC the zener diode is working in the forward region with the resistor, leaving 0,6ish V under the zero threshold. I believe that is not a problem for most logic chips, but it could be avoided with a standard diode between the 9VAC and the ballast resistor.

Good way to distinguish TTL logic ICs from CMOS is to measure powier consumtion at idle. TTL will drain few to even 20mA, while CMOS ICs consumpition will be about 0.

Could you swap out the zener diode for a 3.5v diode and have both CMOS and TTL work?

This was super interesting, and your solution was great. I had a question about the test of the chip on the EEPROM programmer. The chip passed the test, but the IC is specified as 74HC(LS)08. Does that mean that the test is really only running HC voltage levels, and not LS levels as well? Would it be possible to set up IC test parameters that specifically target the lower voltage thresholds of an LS chip? I assume that would make this kind of test easier, but that the 2 types are grouped because this problem is such an edge case. I don't have any experience with how that testing works behind the scenes.

For TTL, the output transistors draw current at any input level. For CMOS, the input voltage outside of thresholds, i.e. 2.5 volts, neither P or N transistor conducts so output is indefinite (high resistance).

The problem here is with the original design. The extra feedback resistors are an attempt to create a Schmitt-Trigger to overcome AC line noise, however relying on the linier characteristics of a digital part is very bad practice. My guess is that the designer found he (she) had a spare gate and so decided to save the expense of adding another logic package (with a real Schmitt-Trigger) such as an SN7414. (One extra DIP package took up a fair amount of space) Unfortunately this was done a lot in the early days but the variations in manufacturing process from manufacture to manufacture or even run to run can be large. That's why the data sheets list typical values with a fairly wide range from Min to Max. The moral to the story is don't design a digital logic circuit that depends on the linier characteristics of your logic elements. All of that said, I'm guessing that the zener diode voltage was chosen to keep the transition close to the zero crossing of the AC line where power supply rectifiers (and other AC loads) might generate noise into the wave form.

Uh that's very interesting, i'm excited. I got a KC compact which is a kind of duplicate from the amstrad cpc. It is working fine with his own CPU - a U880 (the gdr type of the z80 processor). Interestingly, this CPU works also fine in a cpc but the Z80 CPU from this cpc doesn't work in the KC compact. This Z80 CPU btw. is a brand new one. I think i take the board out on the weekend and check a few voltage levels with my oszi again. Thanks Noel for this informations. Thumbs Up.

Hi, question: can you still get commadore sound hips?

Tandy had a similar issue in Australia in the early 80's with their model 16 B running Xenix (a derivative of Unix from Microsoft that latter became SCO Unix) in that they had set the system clock run off mains frequency which is 60 Hz in the USA and 50 Hz in Australia. It was fixed with a software patch.

wow very interesting. a really good video :)

Changing the zener diode with one having higier zener voltage is meaningles. With ~ 2 volts on the rail you were not nearly close to that 2.7 volts on the first one so puting one with 3.3 volts zener voltage doesnt make any sense. Placing the second 74LS08(HC actually) may have solved the problem byt that is because of the tolerance of that chip and it seams that it works at the edge of its specification, which is not good because in time as the chip degrades it will probably go out of that range and the clock failure will reaccure.... Better find a genuine 74LS08 as soon as possible. The only thing that I think may help here is changing resistor R5 (560 ohms) with one with lower value (240, 270, 300 or 330 ohms). In that way the voltage drop acros the resistor will be smaller and the rest will go accros the Zener diode wich will reach its zener voltage

I'm wondering, when you swapped the 3.3 v zener, and the first cmos chip didn't trigger, could you lower the value R37 from 2.7k to a 2.4k or 2.2 k? it would slightly increase the current flow back though the zener/R100 and pull the voltage up ever so slightly....

very interesting. I believe I have my Breadbin showing the same error - but the clock is working. I didn't think too much about that to be honest...

If you really want to troubleshoot a power line derived time issue, wait until you have something in a building with solar and battery backup. Both the solar and battery systems use a pll to lock to the grid frequency. If the grid drops, the solar and battery inverters are locked together but may drift a fraction of a second relative to the missing grid power. If you have many power outages, you may find that clock circuits that rely on line sync may be minutes off over time.

It'd be interesting to take this further... 1: Measure the unloaded and loaded high and low OUTPUT voltages to see what the levels and drive currents are 2: Measure the quiescent current consumption of the chip with all I/O pins not connected (CMOS should be near zero) 3: Measure the current consumption with one input grounded (If the input is a BJT as in 74LS08, then the current consumption will increase) etc etc

ummmm, I just ordered 74LS08's from Jameco and 4 of them look exactly like that 'fake' one. Time to test them I guess. Good job catching something so subtle with these btw!

The Atari 8-bit series uses a division of the color clock for the RTC functions. Since this function is subject to interrupts due to bus-mastering by the graphics chipset, the clock on these machines is not very accurate over long periods, something on the order of a superb chronometer (mechanical). The 60hz signal that drove the clocks in my schools as a boy produced much better results over long terms. I was fascinated to find this in use on the otherwise uninspiring C:64. Now I have a good reason to admire it.

20:12 Well, you accidentally found a viable replacement path for at least one LS chip in the C64! Started diagnosing a fake chip, end up slightly improving on the C64 design...

The LS in the part number stands for Low-power Schottky as opposed to the original TTL which consumed quite a bit more power. I am curious why they didn't use a chip with Schmidt triggers, as otherwise you hit that unstable band between high and low input levels, resulting in a lot of random noise, as you found. I'm surprised that using the AND gate input for hysteresis works, clever if unconventional.

I have to ask a question that seems obvious: why didn't the circuit use a five volt zener diode so the input to the AND gate would be the right voltage?

Excellent sleuthing!

I've been experiencing this quite a bit myself. Recently I got a lot of 5 MC6847 VDG chips. 4 out of the 5 worked. The one that didn't work had a notch that was different than the 4 that worked. So I did the acetone wipe and sure enough it was a rebadged chip. But it was rebadged from a 100% comaptible! It just didn't work. Why did they feel like they needed to rebadge that? So I tried the test on the other 4 chips just to see and sure enough they were rebadged too....but they were still all 6847's! I could see the original labeling underneath the paint. They didn't bother sanding them down. The only reason I could think of them rebadging them was to make the date look like they were new old stock. We all know they haven't made these in a long time, so they're not fooling us. It's just a waste of time. I got 5 63C09P's that didn't work, and did the acetone test and I got a nice black q-tip, meaning the badging was fake, but they sanded them down so much, I couldn't tell what they were originally. These printing jobs on the labels even indent into the plastic a little bit, so you can't completely get rid of the badging, but that copperish color sure faded away.

I built the same board here and had exactly the same problem. I have the same mislabeled chip here. Thanks for the solution :) The C64 diag doesn't show any error now, but my VIC still overheats quite fast. I do not know yet what this is? The voltages all seem to be exactly right.

great info.. ohh i have some chips from "ail" i haven't used yet .. nervous now