RUNNING AN APPLE-1 FROM ONE CHEAP SWITCHMODE PSU

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RUNNING AN APPLE-1 FROM ONE CHEAP SWITCHMODE PSU

Hi !

 

A few weeks ago, a buyer of one of my famous 100% tested and burned-in IC Kits for Apple-1 builders (he lives in  Sweden, the only Western world country without destructive and tyrannical covid countermeasures, nice !) nudged me into exploring the possibilities of running an Apple-1 from one +5V/-12V/+12V switchmode power supply.

 

Not that use of switchmode power supplies ("switchers") with the Apple-1 is new, many builders did that,  and even I gave some hints in my "Tips & Tricks" pdf on how to use two (or three, if cheap, safe, isolated "wall warts") switchmode power supplies in lieu of transformers, but all these previous recipes were geared to be as foolproof as possible: no modifications whatsoever to the basic Apple-1 motherboard, and all the on-board rectifier diodes, filter capacitors and regulators still in the circuit. This means feeding +9Vdc and +/-15Vdc and this means, alas, no single switchmode power supply to be found being able to provide these non-standard voltages from one unit. So you had to take at least two switchers, one for 9Vdc, and one for +/-15 Vdc, which are readily available as standard off-the-shelf items. Or, for people scared to death from tinkering with potentially lethal line voltages, three "wall wart" type switchmode power supplies, one 9Vdc, and two 15Vdc, again readily available off the shelf.

 

The irony is that all these solutions typically come cheaper than buying two suitable transformers and paying the shipping costs for these heavy items. And switchers, as long as they are properly insulated, are much safer than transformers. Which have numerous issues, such as being a fire hazard if not surrounded by (several) properly sized  fuses.  Still, switchers also do have their pitfalls - don't even think of grabbing any "open frame" switcher using the fingers of one hand to move it while it is in operation ! - it will definitely hurt and if unlucky, may kill you.  (Don't dare to ask how I know ;-)

 

Now, the drawback of these previous switchmode power supply solutions was that it ain't just one switcher doing the job. Which is as ugly and awkward and wasteful as having two transformers instead of one (BTW, I have found a few nice vintage looking 110Vac transformers which can run an Apple-1 (or more) from one transformer - send me a message if interested in one of them).

 

So it was inevitable to explore how to run an Apple-1 from one +5V/-12V/+12V switchmode power supply. Without opening traces and if possible, keeping the original looks of the Apple-1. In the end, the same requirements as with all my other mods: keep the looks and make the mods easily reversible.

 

Here is the result:

 

 

 

This Apple-1 happily runs from one switchmode power supply, and all the on-board rectifier diodes, filter capacitors and regulators still being there, for correct vintage looks. The only mod to the Apple-1 motherboard was adding three wires on the backside (sorry, no photo yet due to liability reasons). A custom cable to connect the switcher to the Apple-1 also must be made. The switcher suggested by my Swedish fan happens to have the same 6-pin header as the J1 on the Apple-1, so the same type of connector can be used to make the cable.  Nice !

 

This mod also is easily reversible buy just removing these three wires. Then you can plug in the transformer(s) again to feed the Apple-1.

 

Here are the risks as far as I can see them: make one little mistake when implementing this mod, and - poof ! - your Apple-1 will go up in smoke or, even if no smoke happens, all these precious and rare ICs may get fried (aka "dead") instantly once the power is turned on. Of course, proper procedures can mitigate this risk, but it's definitely not a foolproof mod at all. And, of course, mess with the switcher in any incompetent way, such as blatantly evident in the above photo, you (or a pet, or a kid...) may touch lethal high voltages which may kill all living things - every switcher has a large capacitor on the "hot" side with enough charge to kill a horse, so always put these beasts into insulated enclosures, but allow enough cooling air flow (typically, for the Apple-1, convection cooling is enough).  The only reason why I took the risk of not putting this switcher in such an insulated enclosure is that I had to take this (and other) photos to show details which would be invisible if the switcher had an enclosure. So don't follow my footsteps in this case ! Always insulate your switcher, no exception !

 

Yet another risk is if you have the Apple-1 modded to be fed from the switcher, and you still have a transformer based power supply (see it lurking in the background of the photo ?), and plug it into the modified Apple-1, and power it up, you may instantly kill all these precious and rare ICs again. This time by AC, not by DC, but I was not able to ask these ICs for their opinion which type of death penalty they prefer. I tried to mitigate this risk by putting a red (!) Dymo placard at J1 spelling out "DC ONLY".  But placards have the property to fall off or getting ignored. Such as "AUTOPILOT  INOP" placards regularly found in small aircraft.  Known to be an irresistible lure for curious, cocky and stupid pilots ! And I expect to find a few fools like that also among Apple-1 owners. Which even might sue me for their own stupidity if their Apple-1 is toast due to their own ineptitude or attitude.

 

This said, and the pros and cons listed, I would like to hear your opinion / comments on this mod. Do you think you could implement it safely ? Do you think it is fit for less experienced builders ? Do you see some other  risks / dangers lurking in the mod which I did not list (a trick question, of course !)

 

Now it's your turn. Comments invited !

 

- Uncle Bernie 

 

 

 

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There are, of course,

There are, of course, alternatives to the deadly open-frame power supplies; such as the industrial closed-frame units from Meanwell, Astrodyne, Acopian, Vicor... of course these are expensive but you can get deals often.

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Hi Uncle Bernie!

The topic of alternative power sources is very interesting to me. Our mains voltage is 220v, I, like many people in Europe, use Hammond 186D10 and 186D28 transformers. If you buy them in Russia through a network of electronic supermarkets "Chip and Dip" is expensive and you have to wait almost 2 months. I found a store in Pennsylvania, they sell them for only $ 13, the minimum order is 2pcs of each type, which in principle is acceptable. However, I am not in favor of any modifications to the board, it is beautiful as it is. Can I see a picture of the wiring on the underside of the board? Thanks in advance!

 

PS. Very cool "pigtail" connecting the board and power supply, I like it.

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Some linear regulators (78xx,

Some linear regulators (78xx, etc.) can be damaged by having the output powered but not the input. This varies by manufacturer, so cannot be determined solely by the part number. Anyhow, having been bitten by this a few times in the early 1980s, I have since always put a diode (e.g. 1N4001) from the linear regulator output back to its input. This has no effect when the regulator is used normally, but protects the regulator when it is powered by the output pin.

 

WARNING: make sure you wire the diode so that it is reverse-biased when the regulator is used normally. If it is forward biased, it will allow input power to bypass the regulator, so the output will exceed the regulator voltage. For a positive regulator, the anode of the diode connects to the output of the regulator, and the diode cathode to the regulator input.

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That looks suspiciously like

That looks suspiciously like the Meanwell PT-65-B I used to build an new PSU for my Atari 520 ST https://www.exxoshost.co.uk/forum/viewtopic.php?f=9&t=1714&hilit=meanwell

My build has a link to a 3d printable case that may be of interest.

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I think running that

I think running that switching supply would make the ACI not work very well if you had -12V.   I would actually use a supply with 9V, -9V, 18V,-18V and run them across the normal regulators on the Apple-1 board, so that the ACI got -18V.  With regulated -12V, the volume would have to be so high on the cassette player (forget using an iPod), that noise from the actual player will interfere with loading software.

 

Cheers,

Corey

 

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WOW - so many responses and comments !

So let me address all the issues and questions raised !

 

In post #3, macintosh_nik wrote:

 

"The topic of alternative power sources is very interesting to me. Our mains voltage is 220v, I, like many people in Europe, use Hammond 186D10 and 186D28 transformers. "

 

 "Very cool "pigtail" connecting the board and power supply, I like it."

 

Uncle Bernie answers:

 

I know ! It has been a huge problem for me. This is why my lab runs from a huge isolation transformer sourcing  110Vac and 220Vac on its secondary side. So I can use equipment from all over the world, and I can touch line voltage powered items on the lab bench without getting an electrical shock - it's only safe however if the old rule is followed to only use one hand/arm at a time, never two. Every professional learns this rule (and others).  For typical hobbyist types, it is not advised to do these things that way, but for professionals it's not always inevitable to work on "live" high voltage powered items.

As for the transformer conundrum, in my "Tips & Tricks" I recommend TRIAD "World Series" transformers which have a split primary winding with four pins. You can put these primary windings in parallel for 110Vac operation and in series for 220Vac operation. This is a very flexible solution because you can configure the line voltage input with four pins and then plug in the different line cords, which then select the series or parallel configuration.

Modern switchmode power supplies however can be fed with a wide range of line voltages, i.e. from 90Vac to 260Vac (or even DC). Which makes them "universal" they work anywhere in the world and they are ideal for places with a third-world standard power grid (such as the People's Republic of California). They have a big filter capacitor in their input circuit which has the benefit not only being able to kill you if you touch it, but it also allows the switchmode power supply to "ride" through brownouts.

So use of switchmode power supplies has merits !

About the braided cable between the PSU and the Apple-1: not only does it look nice, but it is also robust, still flexible, and it stays put. It is also cheaper in terms of materials (no heat shrink tubing which is expensive). So I recommend braiding such wires !

 

 

In post #4, brouhaha wrote:

 

"Some linear regulators (78xx, etc.) can be damaged by having the output powered but not the input. This varies by manufacturer, so cannot be determined solely by the part number".

 

Oh, I know. I worked as an IC designer for that famous semiconductor company which is known as "the switchmode power supply IC company with an attached luxury shop for precision analog ICs at boutique prices".  I know regulator ICs inside out, literally, at the transistor and layout level. Guess what the two weeks of daily work in my lab were for ? I characterized every aspect of every suitable regulator I found in my lab, but with emphasis on the regulators I furnish with my IC kits. They are all from the same manufacturer and the same production lot. They were found fit for this type of application. Some regulators from other manufacturers failed my criteria. Unless you are willing to repeat my tedious lab work,

DO NOT ATTEMPT TO USE ANY REGULATORS NOT FOUND IN MY RECENT KITS FOR THIS DIRECT FEED METHOD !  BE WARNED !

However, these operating conditions do also exist in any normal application when the power is turned off and there is a sufficiently large bypass capacitance on the output of the regulator. This is what happens: the power is turned off, the voltage on the input of the regulator drops, depending on the size of the input filter capacitor and the load current, and when the input/output voltage difference drops below the "dropout" voltage of the regulator, it enters the dropout region of operation where it can't keep the output voltage constant anymore, and despite the error amplifier tries to turn the series transistor up, it won't source enough current anymore to the load. At this point, the output voltage of the regulator starts to collapse. Where it gets interesting is when the output side has a large enough capacitance such that the input voltage of the regulator collapses faster than its output voltage.  This is reverse mode operation which is abnormal. As it can only happen under specific and rare conditions, which almost never is the case unless the input voltage is suddenly shorted to ground,  and there is a huge capacitor on the output side, early three pin linear regulators could get damaged and destroyed by this "reverse mode". It was simply a design oversight.  This problem was mitigated by adding the "reverse" diode you mentioned in the application notes. It protects the regulator from the "reverse mode" condition. Now, if you draw the circuit of the Apple-1 power supply and add a connection from the J1 pins 1 and 2 to the output side of the LM323K, you can see that the rectifier diodes will turn into these protection diode configuration, and all is good.

I agree with your statement that the inner circuit of the regulator IC may vary from manufacturer to manufacturer, even for the same type number. But once an IC is put on the "Released Product List", no manufacturer will ever change the inner circuit without notifying the customer(s) and without mentioning the change in the datasheet. These changes are extremly nasty and have huge repercussions as the ICs typically will need to be re-qualified in the customer's application(s). This process involves a lot of expensive lab work at the customer. To avoid, customers would prefer to just blacklist this IC from this manufacturer and use second source parts instead. So IC manufacturers avoid such running changes of a RPL'd product like the plague. They rather make an "improved" version with a slightly different type number instead, to address issues, but they won't change the original part unless absolutely necessary.

So I'm confident "my" regulators will not cause nasty surprises. I have overstock and would sell them as sets (no other ICs/parts) if need be.

 

 

In post #5, Ronin47 wrote:

 

"That looks suspiciously like the Meanwell PT-65-B"

 

Uncle Bernie answers:

 

It is a "Mean Well" PT-65B which is available from distributors all over the world. Thanks for providing the link to your 3D printable case !

 

 

In post #6, Corey986 wrote:

 

"I think running that switching supply would make the ACI not work very well if you had -12V. "

 

 

Uncle Bernie answers:

 

I tested all my ACIs with this configuration (the LM311 based original circuit and the 741 based circuit copied from the Apple-II) and all worked well, despite running from -12V coming from the switcher. No increase of volume was necessary. The performance / reliability of the ACI was as before, no noticable difference, perfect with AIFF files from a sound card / notebook (both recording, playback, and synthetic AIFF files were tried), and the usual mediocre performance with a real cassette recorder. You mileage may vary, though.

 

Despite I have spent a lot of time in my lab testing and investigating the ACI, I never really got to the bottom of its lousy performance and its sensitivity to the negative supply voltage, which definitely is there and can be observed both in the Apple-1 and on the lab bench. The irony is that neither the LM311 nor the 741 should have any influence of the negative supply voltage per se, as long as it within the bounds where the comparator / opamp can do its job. If you don't believe me, just study the transistor level circuit of at least the LM311. There may be, however, influences of the power supply once the input differential pair is unbalanced. I have some ideas on what could be going on, especially when considering that the unregulated V- supply on the 44-pin bus has a lot of ripple coming from the rectified AC. If you reduce the AC voltage using a variac, the ratio of the DC content and the ripple changes. This may be the reason why we see an influence of the magnitude of the negative supply voltage and why some people believe  the more voltage, the better. But this is speculation. I never had the time to really get to the root cause. And since my ACIs work fine with a sound card, and my cassette recorder has poor quality replacement drive belts made out of Chinesium, so I hate to use it anyways, any further investigation of the ACI topic has no priority for me. I have more important things to do. But if I ever get bored (maybe this Winter ?) I might look into the ACI again.

 

Hope now all the concerns have been properly addressed !

 

Nobody so fas has commented on the electrical hazards less experienced hobbyists might be bitten by if they try to do this. One 3D printed isolation case was contributed by Ronin47. But would them fools - which certainly exist even among the elite group of Apple-1 builders - use such a insulation case all the time or would they fool around and get killed (or fry their Apple-1) ?  This is my biggest concern. Technically, my proposed "direct feed" method is sound and it works. I now run two Apple-1 with it 24/7.

 

Comments invited !

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Thanks for explaining in

Thanks for explaining in detail the potential linear regulator issue I raised! I'll have to take a closer look at the Apple 1 power supply schematic.

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With this trick ... you can use "golden base" TO-3 regulators !

Hi !

 

Of course, once "Uncle Bernie" has learned a new trick, his mind starts spinning about what else could be accomplished with the new trick. Here is an example:

 

Many "purists" know that the original Apple-1 came with LM323K regulators that had a "golden" base. For correct looks, they would be much desired, even in clones. Alas, National Semiconductors soon switched the LM323K to a steel base  and stamped the nasty word "Steel" under the type number. Consequently, "golden" base LM323K are exceedingly rare nowadays, I consider them to be "unobtanium". But after a long search I was able to find a dozen or so National Semiconductor LM309K with the elusive "golden" base. Alas,  a LM309K is specified for 1.5A output current which is too weak for an typical Apple-1 using TTL ICs and an ACI plugged in. I measured the current limit of the LM309K specimen I have to kick in at 1.7A load current, which seems as if it might work in an Apple1, but it didn't (so far I had no time to get to the root of it).

 

Now, I also qualified these LM309K for the "direct feed" method using the switchmode power supply. And so I have an Apple-1 clone with a "golden base" regulator that really looks nice. Here it is:

 

 

Don't get upset about the low profile heatsink, I have full height heatsinks, too, but this particular build is one of "Uncle Bernie's Specials" which are super low profile and deliberatly deviate from the originals. These are meant to go into super slim acrylic cases which are only about 1" high and have a nice looking handsome tropical wood frame. These can be used as wall hangers (as they are low profile) and then taken off the hooks, put on a table, connected to PSU, keyboard and monitor, and actually be used to demonstrate.

 

This was an idea of a customer of mine and it's really cool (literally, if a switcher is used) unless you are one of these "purists".

 

If you are such a purist, you would of course use the correct height heatsink and the full size filter capacitors, and the "golden" base LM309K, but you would carefully remove the "LM309K" type number using a solvent like Acetone and Q-tips and then rubber stamp the correct "LM323K" type number on it using the correct font.

 

I just wanted to show you how a "golden base" regulator could be obtained and used in a fully functional Apple-1 (with some invisible trickery such as the "direct feed" from a switcher involved).

 

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I'm sure I posted a thread on

I'm sure I posted a thread on this years ago, but you can get some LM323K regulators in steel where you use a gold plating system make them look correct.  Then you can remove the printing with some fine steel wool.   The next step involves using a laser printer, wax paper and a clothing iron or heat transfer iron (available in craft stores) to transfer a new logo onto the regulator.   I used this on my personal Mimeo.

 

As for the ACI reliability with the switching supply.  I did extensive testing for Vince Briel when he was planning on creating an ACI for the replica-1 using both his prototype card and a Mimeo ACI.  What I found was that there was a sweet spot of about negative 16.5 to 18v where the ACI is rock solid with a digital audio source from an iPod.   When the voltage is lower, I would need to increase the volume beyond what an iPod could drive (a laptop and real cassette player would work, just less consistently).     I don't recall what my scope showed, but I did a lot of probing around trying to figure out my options to making the ACI work on the replica-1 without issue.   My goal was to make audio files that could be shared between a real ACI on a real Apple-1 and the Replica-1 with an ACI.  

 

Cheers,

Corey

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About the ACI negative supply mystery ...

In post #10, Corey986 wrote:

 

"As for the ACI reliability with the switching supply.  I did extensive testing for Vince Briel when he was planning on creating an ACI for the replica-1 using both his prototype card and a Mimeo ACI.  What I found was that there was a sweet spot of about negative 16.5 to 18v where the ACI is rock solid with a digital audio source from an iPod."

 

Uncle Bernie answers:

 

Alas, I never did search for "sweet spots" in the ACI negative power supply. I am a lazy guy and just do the minimum work necessary to draw the conclusions needed for the task at hand. So, when I did the experiment to find out which transformer secondary voltages would work, before I bought the transformers, I made a quick rig with a variac and noted down which secondary voltages did work. In the range I was interested in I found none which would not work. And I saw no "sweet spots" either.  The ACI was as quirky and unreliable as usual with my cassette recorder which is known to be a POS, and worked well with the soundcard. Alas, I am too cheap to buy an iPod. I can't tolerate any in-ear devices anyways ... the aftermath of all the nasty ear infections from scuba diving (last dive 18 years ago, never again, unless antigravity generators are invented to make the tanks lighter). So having neither an iPod nor a good cassette recorder I don't qualify to discuss "sweet spots" and other magic places as far as the ACI is concerned. I still remember the location of those on women ... 

 

But I can read IC datasheets and the transistor level circuit diagram of the LM311 comparator and so I would suggest the following experiment:

 

Take an ACI that works (it should have Mike Willegals 100nF input cap mod). Remove LM311 from socket. Bend pin #4 up about 45 degrees such that it won't touch the contact the in the socket anymore. Same trick as with activating the diagnostics page in my PROMs. Then solder a small jumper from pin #4 to pin #1 without getting solder on the thin portion of the pins (which would go into the socket contacts, no solder wanted there).  Plug back the LM311 into the socket. Take instrument and make sure pin #1 and #4 now are connected to circuit ground but not to bus connector "Z". Now the comparator is configured for the +5V only operation that is advertised in the LM311 datasheet. Try the modified ACI out. Does it still work ?

 

I did that and the mine works. I also did some actual measurements involving signal generator and an oscilloscope and the performance / sensitivity of my ACI did not change over the whole investigated pin #4 voltage range from 0V (grounded)  to -24Vdc in any significant way.

 

This begs the question where these different reported behaviours of the various ACI's could come from.  My money is on the possibility that the LM311 you guys are using which apparently need negative supply voltages ain't no genuine LM311 but Chinese counterfeits which are re-stamped older, obsolete comparators that needed negative supply voltages. But this is bizarre. I have no other explanation, though. The opposite could also be true - the "LM311" which I use could be counterfeits,  but based on a much more modern die, which allows them to work with no negative supply voltage whatsoever. All the "LM311" I ever had pretend to be TI and they also had the signature tarnish that plagued many TI parts (silver content too high). I'd think the Chinese counterfeiters did not perfect the reproduction of the genuine "TI" tarnish yet.  It is characteristic and involves a lot of "secret sauce".

 

In this crazy world, everything is possible when it comes to surprises from obsolete ICs coming from dubious sources ! (Remember the counterfeit Signetics 2504V which were re-stamped 555 timers ?)

 

And then we speculate why our "genuine" ICs can do things which they should not be able to do. Or vice versa.

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My LM311 are not from China

My LM311 are not from China since I scavanged most of them myself from vintage cards and all are pre-1980, so it could be that there are differences in newer LM311. 

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PROPOSAL TO DEBUNK THE ACI MYTHS ONCE AND FOR ALL

Corey986 wrote:

 

" My LM311 are not from China since I scavanged most of them myself from vintage cards and all are pre-1980, so it could be that there are differences in newer LM311. "

 

Uncle Bernie answers:

 

The mine are TI LM311 and were bought from www.mouser.com or Digikey, I don't remember. But I have heard of enough stories about Chinese counterfeits of the products of my last employer before I retired as an IC designer, that I think anything is possible, not only with semiconductors. They found counterfeit brake rotors in the landing gear of 747 jets, for instance. Nothing is safe from these counterfeiters !

 

Contrary to your suspicion that "newer LM311" may have "differences" to older ones, in the special case of  the industry standard LM311 comparator I'm quite certain they all have the same inner circuitry, 1:1 copycats from the original,  if they are genuine and not counterfeits. There may be some differences in the bipolar process technology being used, so maybe some small layout differences, too, but any manufacturer who wants a slice of that pie - which is huge - will make damn sure their "LM311" are drop-in compatible. IMHO, they will continue to make the LM311 until the end of this technical civilization (which may be in my lifetime = soon).

 

The LM311 is THE universal bipolar comparator which can be found almost anywhere. It's one of these great IC designs which are timeless, like the LM324, the 555 and the 741. They even make them in tiny, modern BGA packages.  They just give you the best "bang for the buck". An example of another "sweet spot" in terms of performance / price tradeoff. Its only distractor is that is does not work with a 3.3V only power supply. But it will work fine beginning at 3.5V !

 

This praise however should not distract from the fact that any comparator, if incompetently designed into an application, may act up in various modes of misbehaviour. Don't have proper power supply bypassing, don't balance the differential input signals, have no good hysteresis, and add a lousy layout, and you got youself a problem, guaranteed. Slow opamps like the 741 are much, much more benign. Maybe Woz could not see the mistakes in the ACI design and so he chose the 741 for the cassette interface in the Apple-II.

 

I have put the same 741 based circuit copied from the Apple-II schematics onto an ACI card and got the impression that it works more reliably than the LM311 based circuit, but it is still temperamental, and due to slower output slopes it introduces yet another quirk which the LM311 based circuit does not have, or at least does not have all too often.  This work can be seen in this thread:

 

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

 

I never had the time to really get to the bottom of the ACI's problems and their causation. No magnetic tape storage system is trivial to characterize and measure properly, and the lab instrumentation needed is demanding, if you want real science and not just cargo cult and wishful thinking. The whole issue is exacerbated by the cassette players we can get today being low quality Chinese made new out-of-the-box junk or decades old high quality ones in various stages of disrepair and decomposition / rot. It's just too many unknown parameters in the parameter space of this riddle.

 

THE PROPOSED EXPERIMENT (+5V only ACI mod):

 

So what I propose is a phenomenological approach, like what pill doctors do: just take your ACI that - hopefully - works, at least somewhat, and then put the +5V only supply voltage mod I proposed in post #11 above in. It is easy to undo without leaving a trace, and there is no risk of damage if done right. Check it out if your ACI still works or not. If it does, you can be sure that the "direct feed" from the switchmode power supply will also work and have no influence on the ACI performance.

 

IF THE EXPERIMENT FAILS (ACI does not work from +5V only supply):

 

If your ACI stops working due to the +5V only mod, we now know it is possessed by a demon which makes it dependent on obscure (unholy ?) features of the negative power supply rail named V- on the Apple-1 44-pin bus. Remember, the Apple-1 was sold at $666.66 which invites demons, because it's a bad, bad number from the holy book !

 

If the demon is in there, you are out of luck for now. You must first find a witch doctor who can work his magic, using holy smoke, dances and chants, and burns sacrifices to exorcise the demon. I suggest you buy and bring a live pig so the demon can find a new host and is more inclined to leave your ACI. The trick with the pigs is also in the holy book !

 

Oh yeah, "Uncle Bernie" can be snarky.

 

I am in this snarky mood because after a careful analysis of the ACI I just can't see any possible reason why its - albeit mediocre - performance should depend on the negative supply voltage at all. Inside the LM311 there is nothing which could cause any significant effect on its gain or offset voltage from its negative supply rail. It has a PNP input stage in which everything references upwards to the positive supply rail, even if it is imbalanced. The following NPN differential pair has a good PSRR from the V- supply pin (according to my SPICE simulations). BTW, most of the "random" timing jitter seen at the output of the LM311 comes via the 47K hysteresis resistor and the noisy +5V supply. Adding a 100nF power supply bypass capacitor helps to reduce the jitter, but it should be put where noise is generated, and these are the PROMs (location 3 and 4). I started to use DIL 16 sockets with built-in bypass capacitors in these two locations and measurements show the jitter is significantly reduced. But, alas, it does not really help much with the mediocre performance of the ACI.

 

WHY IS THE ACI SO BAD ?

 

So what is really wrong with the ACI ? I don't know. Maybe instead of speculating and trying to analyze what is wrong with the ACI circuit design and layout we should just toss it into the trash can and design a new ACI from scratch. I intend to do this if there is enough space on my floppy disk controller card. Not that I'm out of ideas what may be wrong with the ACI circuit, but each of these ideas would take many hours of lab measurements to confirm or debunk. And in a fraction of that time I could design an ACI which really works. It may even use a LM311 again. It's a great comparator that stood the test of time. But there is no way to make a good tape read channel circuit out of it with that minimum number of passive elements Woz did use. Making a good circuit would require more resistors and capacitors. Filtering of certain nodes being the most important task. The ACI in its present form does not have that. And it shows. Such a circuit may work fine on the lab bench when outside of the Apple-1 and powered by a clean lab supply. Plug it into the supply from hell (the Apple-1 with all its noise and ringing) and its performance falls apart. This is how I see it.

 

CONCLUSION

 

We don't know why the ACI is so temperamental.   ("These are not the wizards you are looking for !")

 

But we can do an easy and quick experiment to find out if it would work with the "direct feed" from a switchmode power supply without even having bought that switchmode power supply yet.

 

So, dear readers, do this little experiment with the +5V only operation of the ACI. And report here what your observations are.

Can you see any of the gain loss issues ? Or not ?

 

I'm curious !

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Hi, I just finished my first

Hi, 

I just finished my first (and probably only) Apple 1 build using Uncle Bernie's IC Kits and a Newton NTI PCB. The build process was a lot of fun and went very smooth. For that, I have to thank Uncle Bernie and this awesome community.

I opted for the direct feed method proposed by Uncle Bernie. For me, this seamed the safest and most cost-effective solution. All voltages are spot on. For safety, I still need to find an appropriate case for the Mean Well PT65.

Next on the list is a keyboard solution. 

Oliver

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Posts: 611
oreimer wrote:Hi, I just
oreimer wrote:

Hi, 

I just finished my first (and probably only) Apple 1 build using Uncle Bernie's IC Kits and a Newton NTI PCB. The build process was a lot of fun and went very smooth. For that, I have to thank Uncle Bernie and this awesome community.

[[{"fid":"33198","view_mode":"default","fields":{"format":"default","alignment":"","field_file_image_alt_text[und][0][value]":false

 

 

Make sure if you use a metal case that you use plastic or insulated standoffs for mounting it.  Looks like yours is siting on those now, which is a good idea.  Probably want something that is ventilated and possibly with an AC powered cooling fan.  Maybe you can adapt an old PC power supply case?

 

 

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