Which ACI improvements do exist (and work) ?

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Which ACI improvements do exist (and work) ?

Hi Folks -

 

this week I finally got the PCBs from Newton Mike including an ACI.

I immediately proceeded to build the ACI, but decided not to follow the original circuit with the LM311 comparator.

Instead I did a few little cuts on the PCB and installed the components such that I have the LM741 based circuit from the Apple II.

The reason for this decision was all the bad experiences with the original ACI as documented on Applefritter and elsewhere.

I do not want flakey unreliable junk in my builds, to hell with authenticity here.

Based on the great reputation of the Apple II cassette interface, I expected a great outcome.

No so !

It did not work at all. Despite I had tested it on the lab bench (see attachments).

First I was bold and tried to load BASIC from a 22 year old cheap crappy dictaphone style cassette player with horrible tape wow.

Not one success at any volume setting.

Then I was humbled and tried to load BASIC from a sound file played on a notebook.

No success either.

Then I wrote some diagnostics routines and found the culprit.

A design bug.

After fixing this I could load BASIC even from the crappy dictaphone style cassette player. Every. Single. Time.

At any volume setting except very low and very high.

 

This said, I am getting curious what others may have done to improve the ACI.  Maybe I have just rediscovered a fix that already exists.

 

So, comments from all the Apple I gurus lurking on this lists are much invited.

 

(If anyone wonders why I did not do a search for other solutions before I tried the mine, well, I do this to kill time with some intellectual challenge that prevents brain rot so many retirees are suffering from. Further, did you ever hear the old adage that six months in the laboratory can spare you six hours in the library - - - which would be the internet today).

 

Bernie 
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That's good news UncleBernie

Are you going to share the deatails of your findings?  Schematic of your changes?

I'd be interrested to see your details.  Great work BTW.

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Improved ACI - Schematics

macnoyd wrote:

 

"Are you going to share the details of your findings?  Schematic of your changes?"

 

Uncle Bernie answers:

 

You betcha !

 

Do you know which day is today ?

 

Great celebration, the 70th birthday of the Woz !

 

Maybe he would cherish an improvement to his 45 year old "firstborn" as a birthday gift ?

 

In any case, I love the Apple I for its limitations, and my mission is to make it reliable and eventually add a floppy disk controller based on the Woz Machine.

 

I intend to publish every aspect of this work and make it open source, like I did with my various mid-1980's products for the 8-bit Atari.  Much like the Apple I, they are still in production after all these decades, although only made in small batches by enthusiasts and not anymore by the original business, and none are verbatim clones.

 

But before I do publish my findings on the ACI and the improvements, I need to review which improvements are  already out there. It would be too embarassing to write up my stuff only to find out it already has been tried by others.

 

Hence, again, my humble request to the Apple I "gurus" lurking on this list to point me to what they know about the exisiting / published ACI improvements other than the one by Mike Willegal (larger input capacitor).

 

Any comment invited. If you have a comment you don't want to show to the public, use the Applefritter message system to contact me directly.

 

Bernie

 

P.S.: I also need to go back to the lab and find out whether the original ACI using the LM311 suffers from the same bug I did see in my Apple II ACI knockoff. My current hypothesis is that the 741 based circuit just did exacerbate the effect of the bug over the 311 based circuit just enough to completely render my "improved" ACI useless, which then nudged me to do a thorough  root cause analysis, which then led to finding the bug and a remedy.

The LM311 in the original ACI circuit, may add its own problematic effects. The LM311 is infamous for having nasty habits in certain circuits. For instance, parasitic oscillations that appear under certain output loading conditions even when hysteresis is present: there is a LM311 based LC meter circuit all over the web which exhibits this problem if the LC time constant is large (kHz-ish) and there is a certain output load and some other diffuse features in its surroundings. Otherwise this LC meter circuit is quite brilliant (minimum component count) and works well. So people build it, sell kits, or finished LC meters, and most users seem to be happy with it. I was unhappy with it and over my 40+ years in electronics had enough run-ins with the nasty LM311 that I learned to avoid it like the plague. The LM311 having so many bad habits (under some conditions) did not prevent it from becoming immensly popular and it still is being made in huge numbers by various manufacturers, after half a century. The LT1011 made by my ex-employer is a much better part that does not turn around to bite you without any prior warning signs. Alas, the customers for analog ICs seem to prefer the cheap "jellybean" types and don't cherish much better parts designed by great designers. To cite an old adage: "Eat more sh*t --- millions of flies can't be in error". This is how Microsoft got so immensly "successful" ... but enough of that for now. -B.E.

 

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I presume that you have seen

I presume that you have seen this page on my web site.  The main improvement that Apple made on the Apple II cassette interface was to increase the value of the input coupling capacitor.  With this single change, I found reliability is good enough for my purposes.

 

http://www.willegal.net/appleii/aci.htm

 

Note that I have found that the older and much slower SCELBI audio tape interface is more reliable  and less sensitive to input volume than either of the Apple designs.  The SCELBI design uses FSK encoding instead of zero crossing, which accounts for the speed difference.  The SCELBI recieve circuit has AGC and filter stages, which I think accounts for at least some of it's ability to handle a bigger range of input volume levels.  If you want to experiment with changes that might make make performance better, you might consider adding an AGC  and filter stages to the input.  The downside of all this sophistication in the SCELBI tape interface, is that the hardware is significantly more complex.

 

regards,

Mike Willegal

 

 

 

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I will just say that the

I will just say that the LM311 with the updated input coupling cap is dead on reliable if you are using an iPod.  If you are using a cassette tape, it is very reliable if the cassette deck has a reasonably new drive band.  With an old worn out band the audio to vary a bit in rate and pitch.  

 

if you are doing this with a replica-1  the eeprom can thow extra noise on the bus.  Simply using a regular eprom solves that issue.

 

You may also have problems with an ACI if you are using the edge connector for expansion as that can mess with the unregulated -12V and add noise onto the lines.   

Finally, your choice in transformers can affect reliabiity.   If your out of spec to what is recommended in the manual you can have issues.  I was working on an original Apple-1 which used some radioshack transformers which had lower voltage outputs than the Triad or Stancor which caused all sorts of problems including a very unrelable ACI even with the input coupling capacitor updated.

 

Cheers,

Corey

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Data Check

A further problem with the ACI data is that there is no check sum so you get no warning of a bad load except the visual timing of the CR and the end of the audio if you can hear the audio.  A pretty good substitute is to keep a record the final two bytes of the recording and then see if they are correct.  Since the time length of zeros and ones in the data pattern is different read errors tend to cause a shift in the information sequence and it almost always disturbs the last bits read.  The ACI record format is one of the very few where the record length is data dependent which makes the last bytes read a good check.

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Answer on Mike's comment:

Mike Willegal wrote:

"Note that I have found that the older and much slower SCELBI audio tape interface is more reliable  and less sensitive to input volume than either of the Apple designs.  The SCELBI design uses FSK encoding instead of zero crossing, which accounts for the speed difference.  The SCELBI recieve circuit has AGC and filter stages, which I think accounts for at least some of it's ability to handle a bigger range of input volume levels.  If you want to experiment with changes that might make make performance better, you might consider adding an AGC  and filter stages to the input.  The downside of all this sophistication in the SCELBI tape interface, is that the hardware is significantly more complex."

 

Thanks for the comment, Mike. Of course I knew of your improvement, which I think most ACI builds nowadays do use. I am also quite aware of the SCELBI interface (and others, like Tarbell etc.) because I am ... just that old. At around the time the Woz built the Apple I, I was a teenager who designed and built his own 8080 based machine.  My cassette interface used pulse counts to discern 0's ad 1's such that a single dropped or added pulse would not have made any difference. It was very robust. But much slower than the ACI. No problem though as my machine had only 1KByte of RAM unlike the whopping 8KBytes found in the Apple I ;-)

 

But back to the ACI. As far as my modified ACI goes, I do not seek sophistication and filters and AGC and stuff because I think that the minimalistic approaches both in hardware and software used by Woz are sound. As always, minimum parts count and elegant, small software. I am quite amazed how well the ACI works after my fix, with no mods to the software at all, no AGC, no filtering, etc. If you would see the "wow" of that cheap cassette recorder on the oscilloscope you would never believe it could work as flawlessly as it does. Actually, Woz did a great design here. Except for that one bug.

 

The big open question is whether my current hypothesis is correct: the Apple II ACI circuit based on the 741 exacerbating the impact of the bug. Maybe the 741 based circuit did create the bug (although I believe the bug is real and must also be manifest in the 311 based circuit, but it could strike less frequently, so the 311 circuit does still work OK under certain conditions).

 

As I dislike the LM311 I have none in the house so at this time I can't investigate. But I did order a few LM311 a few days ago and expect them to arrive soon. Then I will change my ACI back to the '311 circuit (without the bug fix) and run the same diagnostics. This experiment will prove or disprove my hypothesis. I have a hunch that it may be that very bug that makes the '311 based ACI circuit so finicky and sensitive to volume settings and power supply, the latter possibly having an effect because of some quirks inherent in the LM311. I learned to avoid that part as it did cost me many pitfalls in the lab.  All of them can be avoided, of course, by doing it right and by paying attention to every detail. But comparators in general are temperamental to work with especially when used in hand-wired lab setups.  The frustrating experience goes like this: while bringing up the lab bench circuit to test your latest and greatest IC design, you need a comparator, so you grab a 311 and slap it in. Big mistake !  You will spend the next few hours beating the 311 into submission instead of doing  productive work. For low speed (i.e. audio) work an opamp has much more benign behaviour. But in case of the ACI, it seems the opamp exacerbated the effect of a latent bug and hence, brought said bug to light. And unlike cockroaches in the kitchen, that bug can't speed away into hiding when that light is turned on. In any case, a lot of fun for me to kill time doing (commercially) useless projects.  This, of course, is intentional ! I have sworn to never create anything anymore that even remotely has any chance to be  looted for profit. Read Atlas Shrugged if you want to find the reason why.

 

Bernie

 

 

 

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Answer to Corey's comment

Corey986 wrote:

"You may also have problems with an ACI if you are using the edge connector for expansion as that can mess with the unregulated -12V and add noise onto the lines.   

Finally, your choice in transformers can affect reliabiity.   If your out of spec to what is recommended in the manual you can have issues.  I was working on an original Apple-1 which used some radioshack transformers which had lower voltage outputs than the Triad or Stancor which caused all sorts of problems including a very unrelable ACI even with the input coupling capacitor updated."

 

Uncle Bernie answers:

 

This are exactly the arguments why I wanted to avoid the original '311 based ACI circuit in the first place. IMHO if any ACI is so sensitive and finicky with the power supply then something must be wrong with it. The problem with any comparator is that they combine high gain with high speed and no internal frequency compensation which is a recipe for inviting disaster in any feedback circuit. You need to pay attention to every detail in the surrounding circuits and in the physical construction to beat these beasts into submission. The venerable LM311 due to its kinks / quirks adds some further possible pitfalls.  On the other hand, I found the '741 based ACI circuit of the Apple II behaves very benign and has none of the temperamental tendencies often seen with comparator based circuits. For instance, in the '741 circuit there are no influences of the negative power supply as long as it exceeds a reasonable value. -5V on the 741's V- supply (pin #4) certainly is good enough. The circuit ceases to work at -2.6V there, so there is plenty of margin left.  There are also opamps which might work in the ACI even when their V- supply is at 0V (GND). I will investigate this option at a later time. This would be great for Apple I replicas lacking a negative power supply.

 

This said, I would not want to go back to the '311 based ACI circuit even if it works OK under certain circumstances. The '741 based ACI circuit is much less temperamental and much more robust. Alas, it just did expose a latent bug. But this bug can be fixed easily. Still, as I did mention above, as soon as I have the '311 in my mailbox I will proceed to try proving whether the bug also is manifest in the '311 based ACI circuit or not. I wouldn't be surprised if a lot of the finicky behaviour of the '311 based circuit would go away after the bug fix is applied. At the moment however all this is just conjecture.

 

If my conjecture happens to be valid, however, this would mean the problems with the ACI are just another case of a "feature" (synonym for any "bug"  when used by the  marketing department) that has a completely different root cause than people think but manifests rarely enough so nobody cares enough to dig down to root cause. The consequence being "cargo cult" like magical behavior patterns of some users seeking best results by turning volume knobs or trying to find the "magic" cassette player that "works", or, in case of the rest of the Apple I main board, find the "magical" DRAM brands, types and date codes that work better than others. All these futile (but sometimes alleviating) efforts to solve a problem by applying magical thinking (like an Amzon tribe's witch doctor) in lieu of proper root cause analysis using a well equipped lab.

 

(Of course I write all this anecdotal drivel such that the casual reader can learn from my mistakes ;-)

 

Bernie

 

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Update on my ACI investigations

I got the LM311 end of this week and proceeded to rip out all the improvements I did to my ACI.

This unfortunately can't be avoided as I do only have one PCB.

This sucks.

With two ACI PCBs a could compare the two circuits without hours of desoldering and resoldering components.

 

Here are the preliminary results on the original ACI circuit:

 

1. The LM311 based original ACI circuit works - - - somewhat.

2. I can load BASIC from a notebook soundfile with no problems.

3. Trying to load BASIC from the same crappy cassette recorder that worked fine with the '741 based circuit always failed, with all possible volume settings and all reasonable negative supply voltages.

 

These results show that my LM311 based circuit was slapped together correctly. Same problems seen as with all other "normal" ACI circuits.

 

My diagnostics were run (briefly) and the bug did not manifest itself, unlike with the 741 based circuit, where it manifested itself.

 

The conclusion so far from my experimental work is that the 741 based ACI circuit from the Apple II indeed is much more robust and can work very well (even with real cassette recorders) but in the Apple 1 ACI subsystem provokes a latent bug to manifest itself, which must be exterminated with some further changes to kill it dead. This vulnerability allowing the bug to rise its ugly head is only present in the Apple 1. It cannot happen in the Apple 2.

 

Further experiments are necessary to see if the bug  ever strikes with the 311  based original Apple 1 ACI circuit.  I'd guess from my experience with this type of bug that it should happen with the LM311, too, but significantly  less often than with the 741. Maybe it never happens, though. Which would surprise me bigly.

 

Even with the 741 it is a rare event so catching it with the 311 is tedious and time-consuming. But it is interesting experimental lab work which is perfect for me to kill time. Much more welcome than cleaning the house ...

 

 

 

 

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HOW TO MAKE THE ACI LED WORK

Some of you may have seen this thread:

 

https://www.applefritter.com/content/which-aci-improvements-do-exist-and-work

 

in which I wrote about my work towards improving the notorious ACI. Alas, there never was enough interest / feedback from the worldwide Apple-1 user community to justify any more investment of my precious RQLT into that project, so it was shelved and I never made a new PCB layout of the improved hardware, which would not have been optimum anyways as the improvement work was not complete. I did have an improved playback circuit I stole from the Apple-II (with some adaptions required for the Apple-1 environment) which IMHO did noticeably improve the tape read operations. I also had written some new firmware (512 bytes, yet another little hardware mod required as the standard ACI circuit only supports 256 bytes of firmware PROM) which provided full Apple-II compatibility (adding the checksum byte) and some more useful features. But at the point I shelved it I still had no solution to make that LED work.

 

The LED on the ACI obviously was meant to give an indication for proper volume setting during tape read operations. But this is just a conjecture because it never worked for me. One cassette recorder would light it when the volume was cranked up to maximum, but then the tape read would fail due to the noise being amplified too much. Another cassette recorder never lit it regardless of volume setting, but lit it when using rewind / fast forward. Any playback of WAV or AIFF audio files from a computer succeeded after the correct volume setting was found by a lot of fiddling, and the loaded software worked, but it never lit the LED. I do not have the original cassette recorder recommended in the ACI manual so I can't tell if the LED indicator works with it, and yields the correct volume setting. Unless somebody proves to me that this is so, I must regard this LED circuit in the ACI to be the biggest POS in the whole sea of Apple-1 quirkyness. You see, there is no properly set DC operating point in it, and taking its input signal from the TAPE IN plug is completely nonsensical ... you don't want to know if the input is moving, you want to know if the output of the comparator is moving. Because just a tad more volume over the point where this happens is the volume setting which gives the best tape read reliability. And you want a LED volume indicator circuit which gives you that capability. But back in August 2020 I did not get around to find an improvement for that circuit. I was quite happy that my 741 opamp based readback circuit did work and I already has spent waaaaay to much time to get there. Lack of a reliable LED volume indicator contributed. But I did not need that as long as I had the oscilloscope near the computer and could look at the output of the comparator to adjust the volume. So I did not really feel the pain from that awfully botched LED circuit and could ignore it.

 

Fast forward in time until last week. I wanted to try out some software on WAV and AIFF files. The oscilloscope was not at the computer and used elsewhere in an ongoing experiment and I was too lazy to disconnect it and move it downstairs. Big mistake ! The volume setting issue nearly drove me nuts ! (This time I used the original LM311 based ACI card). So I finally felt the pain that may be caused by that nonfunctional LED indicator and decided to go to the lab and fix it once and for all. And here is the fix. But be warned, it is not perfect despite it works OK:

 

As I did mention above, the key to make that LED volume indicator work is to make it look at the output of the comparator, and to hook it up in such a way that when the comparator output moves, it will light up the LED. LED shall not light up if the comparator output either is static "L" or "H". This implies we need to add a small capacitor to keep the DC away from the LED amplifier (the two transistors in Darlington configuration). Alas, I found out that any capacitance large enough to give more than a faint glow, if attached to the comparator output, will already impair its signal quality. So another amplifier / buffer stage must be added. Alas, all gates in the ACI are used. Except for the second D-FlipFlop in the 74LS74. So I pressed that into service as a buffer amplifier. The final circuit is like this:

 

 

It works OK but as it is a quick hack that cost me only an hour of my precious RQLT, it is not fully characterized and worked out for all component tolerances. One issue you may run into is transistors with a beta / Hfe too low. Because of the Darlington configuration, the betas multiply which makes this circuit very sensitive to beta variations. If, for instance, beta is 100, total current gain is 100 x 100 = 10000. Which is not really great if you want to make the LED light up from the feeble current pumped into the amplifier input via the capacitor. If you happen to have transistors with a beta of 300 each, total current gain is 300 x 300 = 90000, or nine times of the previous case ! In this case, a 120pF coupling capacitor is enough to light up the LED well. So, if you have a LED too dim, you have to experiment with increasing the coupling capacitor and / or the base pulldown resistor (which is the 4.7 MegaOhm resistor in the above circuit that replaces the 100K resistor of the original ACI). I had some lower beta transistors in the second ACI and had to increase the coupling capacitor to 1 nF to get a nicely lit LED.  The D-FF output would not be happy with that load if it would need to drive anything in addition to this coupling capacitor. But as this is its only job, we are fine.

 

To use this new and improved volume indicator LED, place the tape on the header tone and put the recorder into playback mode (you do not need to start the ACI firmware). Then slowly increase the volume setting until the LED lights up. Just a tad more that that setting is the optimum one.

 

Comments invited ! If I get enough feedback I will add some photos of the mod. Be aware that the one leg of the resistor that must be grounded has traces both on the top and the bottom side of the PCB to the TAPEIN signal, so you need to cut it near the pad of the resistor on both sides. Do not solder near bus finger "A" but follow the trace up until it disappears in a via, and use that via to solder in the wire that connects to U2 / Pin #11.

 

Enjoy ! (And don't forget to tell me if you want the photos)

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Hi Uncle Bernie!

I definitely need pictures, I will definitely do this mod. I'm having a lot of trouble downloading programs through the ACI card. I download everything from my phone. Without problems I download only 2 programs: Basic and Mastermind. I tried downloading the memory test from this thread https://www.applefritter.com/content/apple-1-memory-test-wav-or-mp3 to my phone, it doesn't work, but it does if I run it from Corey's site using the link. I don't know why, tried different players, different phones, volume control and I have 2 of these cards. The LEDs on both do not respond at all. So really need these pics. 

Thank you in advance!

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i also look for a solution

I also had to look for a solution that looks like this for me:

My problem was more with the writing and only a little with the "reading"

In the output circuit behind the 74LS74 I have the voltage divider from 10kOhm to 100Ohm

-> re-soldered to 5kOhm (simply second 10k Ohm in parallel ) to 100-200 Ohm (100Ohm + 100Ohm potentiometer).

At the same time I soldered a 1µF electrolytic capacitor to the supply voltage (Pin7 and Pin14) from behind ...

Since then I have been able to READ and WRITE perfectly ...100% (with a Panasonic Cassette player)And when I play back the sound with a real cassette recorder, the red LED shows the correct answer.(Not necessarily beforehand because there is too much DC voltage on the AC voltage signal).

 

 

 

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Photos for the Volume Indicator LED fix

Hi -

 

here are the photos I have right now. Maybe they are good enough. I never invested in a professional photo studio or a good camera so there is no good lighting and no good background and the autofocus sucks. This is not that I don't know how to do professional grade photos ... 30 years ago I used the equipment of my GF of the time (she had divorced a professional photographer who later became quite famous) and got some really great photos out. A few of them were so excellent I was able to sell then for good money to a tour operator which used them in their catalogs for years (Seychelles, the jewel of the Indian Ocean). But unless you can make lots of money with your photos you never get the investment back.

 

OK, so here is the component side of the modified board:

 

 

 

The red arrow points to the trace that needs to be cut to isolate the one leg of the 100K resistor which will be replaced by the 4.7 MEGohm one. I advise that before doing the cut, remove the 100K resistor and desolder one leg of the 3K resistor to be able to lift that resistor up so it is safely away from the work site when the cutting tools are in action. It is very easy to damage components when you slip with these tools, so better get them out of the danger zone.

 

This is the solder side of the modified ACI board:

 

 

You can see where to make the second cut to isolate the one leg (as before) and how to route the wires around. I use insulated copper wire the kind used to wind transformers and the insulation is conveniently removed by the soldering iron when pre-tinning the ends. This works with most polyurethane based insulations. Buy be aware that the process makes isocyanate fumes, although in minute amounts. I have a solder vapor remover in my lab which sits near the solder work and sucks away the fumes and neutralizes them in some charcoal insert, so I don't need to worry. But if you don't have such equipment, don't inhale the fumes ... unless you are a smoker, then it doesn't matter anymore anyways.

The "A" is at the via where you can solder the wire in to tap the signal from bus finger "A".

 

Tell me how it works for you. Sorry that the photos are such a mess.  I did not clean the PCBs yet from flux because it's still work-in-progress and I had to take them outside which explains the glare. Inside the house the lights were too bad to be able to get any useful photo. It used to be easier. Maybe something on the camera starts to fail. It is 22 years old.

 

I also have a comment to post #12: I can confirm the TAPE OUT signal issue. It has been my experience that with some cassette recorders I tried, the TAPE OUT signal level from both Apple-1 and Apple-II was too weak to get good recordings, and the remedy with modifying the voltage divider is sound and it works. The irony here is that it seems to be the "noise" from the digital logic which is the main culprit, and not lack of sensitivity in the cassette recorder. I had to modify the Apple-II to get a higher TAPE OUT level but I did not need to modify the ACI card for that, as long as I don't try to use the super cheap "dictaphone" style recorder which is my "worst case example". This one would need more signal in any case.

 

As for the TAPE IN amplifier, I have found (as seen in the above thread) that the Apple-II circuit using the 741 opamp works much better for me than the original ACI circuit with the LM311. And this is where I stopped my investigation. I now have two ACI cards, one with the original LM311 circuit and one with the 741 mod. The irony here is that in my LSTTL based Apple-1 the original LM311 card just works fine, once the volume is properly adjusted.

 

This gives us another clue that the problem with the original ACI is  not the LM311 based circuit by itself. In the lab, on the test bench, and powered from laboratory power supplies, even the original ACI card works perfectly fine and I can't pinpoint any measureable problems regardless how I set the signal generator.  I even tried added noise to the signal but could not make the LM311 circuit misbehave (unless the noise exceeded the hysteresis, which, of course, is expected). So most likely, it is a PSRR or CMRR problem which manifests itself when the ACI card is plugged into the "supply from hell" (the ringing and noisy Apple-1 motherboard). But as I need to prioritize what I do with my precious RQLT, I never had the time to get to the bottom of it. And since without knowing the root cause I don't have any proof that my 741 mod to the ACI is not overkill. Which greatly discourages making a PCB layout for that mod. Because the effort for that may be wasted. It could be that a less involved mod to the LM311 circuit exists which brings enough remedy to be a perfectly viable solution. This would mean much less mods to the PCB and staying closer to the original "looks".  Which for me, would be the preferred solution, if such a solution could be found.

 

The observation of AppleMicha in post #12 above gives us a further clue that such a  solution may exist. But I don't have the time nor the motivation to further pursue the improved ACI project, as I now have one hand-wired modified ACI using the 741 which works fine in all my Apple-1, and the original LM311 based ACI which works in my LSTTL based build (it does have Mike Willegals input capacitor mod, though). Had there been much more resonance from the worldwide Apple-1 crowd for the proposed "improved ACI" the situation would be different, of course. But without enough resonance / demand / encouragement, I can't justify to expend more of my precious RQLT on this topic, even more so now, a year later, with my Apple-1 floppy disk controller getting closer to the finish line. So the improved volume LED circuit is all what I've got for you, but I think it's fair enough to at least have published this small but helpful mod which came out of my work on the improved ACI. The improved firmware I wrote for it will never be published, like my APPLE-1 DIAGNOSTIC PROM SET. Not enough takers, sorry.

 

 

 

 

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Hi Uncle Bernie!

Thank you Uncle Bernie! I really appreciate your contribution to our common cause, I will definitely make this modification and write about the results.

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Hello Berni, thank you for

Hello Berni, thank you for showing us your proposed solution.

It doesn't seem that many users are storing data with the ACI at all, it can of course also be the case that less than 10% of the users notice it here, because not everyone discovers or reads "apple fritter" posts? (I hope you understand what I mean, because I also translate my text via google)Therefore, it is certainly often a shame that you do not get a great response, even though these are often really important things.I wouldn't take that "personally".

 

I also don't know exactly how to "optimally" integrate the images here in the forum .. (with me they are displayed quite well on an Applemac "?).here my analog output after the small "conversion".

 

I'll show the picture as it looked before:

 

 

 

 

 

 

 

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Signal to noise (SNR) is the key ...

Hi AppleMicha -

 

thanks for you post #15, it is certainly helpful for the Apple-1 crowd to understand what is going on and why the original ACI does not work all too well.

 

It all boils down to  what electronics professionals call "signal to noise" or SNR. Depending on the circuit you have, the signal must be larger than the noise to allow for proper recovery of the signal. The more primitive the circuit and the signal modulation /encoding is, the better the SNR must be. There are some signal processing techniques where you can actually "fish" out a signal from below the noise floor, so-called "lock-in amplifiers" being one example. But what we have in the ACI is the most primitive encoding and modulation method possible. So we need a really good SNR to get good enough reliability.

 

What most hobbyists (and some professionals) don't understand is that the "noise" part may not only come along with the "signal" out of the "channel" (the transmission medium) but it also may come from parasitic effects coupling unwanted crap into the receiver circuit. And so far all evidence we have at hand points to this being the most likely root cause of the issues with the ACI. Alas, proper diagnosis and a real proof of exactly how this happens and what the parasitic signal path really is requires a lot of lab work and a lot of time. This is why most people (hobbyists and professionals alike) tend to slap some filter capacitors in (or do some other small mods) and if this brings remedy, it's adopted to be "the solution". Which, alas, may be sub-optimum, or not a good solution at all.

 

Time is money (especially when the bonus payments of your local psychopath aka "manager" depend on being in or below budget) and so they call it "done". But this is not how you get stellar products which are ... perfect and very satisfying for both the customer and the maker !

 

There were very few famous companies out there which consistently designed and made stellar products. Names like Hewlett Packard, Tektronix (for instruments) or Burr-Brown and Linear Technology (for ICs) come into mind. But all these once famous and highly regarded companies share a common fate: once the founders were gone and the hired managers took over,  rot and decline set in and crappy products were made and sold which never should have existed at all. In some cases the company ran out of money (or was too small to defend itself against hostile takeover) and then was bought by a much larger corporation, which, by definition, always has internal rot and lousy products, and a management / mindset which spoils and ruins everything. So in the end, all the stellar reputation built over many decades is quickly ruined by the new owners / new management. This is how stellar companies rise and fall. The most sad example properly being Hewlett Packard. Carly Fiorina "managed" to destroy  in a few years what the founders had built in more than 60 years.

 

I have always resisted pressure from management to cut corners and make crappy designs.  So most of my projects were over budget and / or late. But once they went into production, they were always trouble free, had consistently good yields, and never came back to haunt me. So I could fully focus on new projects. Those colleagues who gave in to pressure from moronic managers only concerned with their bonuses had a different fate ... their products more often than not had yield issues or - even worse - field failures requiring mask revisions after the release. Or they were quirky and the customers hated these ICs, so they never became "stellar" products. This is what happens when designers listen to moronic / greedy managers. (Not all managers are bad like that, though. But most are. The "Dilbert" comic strips are popular for a reason.)

 

My policy to never compromise the quality of my work extends even into the hobby realm, so this is another reason why I rather abandon some project that is not yet perfected and would take too much time to get it up to my standards. Some projects are simply not worth doing (or completing). Alas, you can't always tell which project is worthwhile to do at the beginning. At the beginning, all projects look rewarding, otherwise you would not start them in the first place. But once, down the road, you come to a point where any further progress would take unjustifyable amount of effort, time and money, you better stop and put it on the back burner. Or right into the trash can. Depending what it is.

 

If you really want to continue working on the ACI where I stopped, try a resistor and zener diode combination in the negative power supply of the LM311. The zener I used in the 741 circuit was 5.6V but as long as the power supplies are in a range where the circuit would work, any zener voltage would do, but keep in mind that the power dissipation on each component and calculate it before putting them into the ACI. I have a hunch that, together with your supply filter capacitor right at the LM311, this might be a good solution. But I don't have the time nor the motivation to try.

I also did not look into the option to do away with the negative power supply and run the LM311 from a positive power supply to ground only.

 

This is all the hints I have for you. The 741 circuit can be found in the Apple-II manual. But if you can make the LM311 circuit work well enough, then we don't need the 741 solution.

 

Any thoughts ?

 

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