Apple's backwards design mistake and the reversed capacitor
- Reference: 1733160309
- News link: https://www.theregister.co.uk/2024/12/02/apples_design_reversed_capacitor/
- Source link:
Doug Brown [1]outlined his findings in a lengthy post in which he stripped one of the machines of its original capacitors. Leaky capacitors (and leaky batteries) have killed many ancient computers over the years. In the case of the former, electrolytic fluid can seep out and wreak havoc on circuit boards and nearby components.
[2]
Turning a positive into a negative – Pics copyright: Doug Brown , used with permission
This is why replacement is usually a good idea, and the PCB of a Mac LC III with more than 30 years under its belt is a good candidate. It was during the replacement process that Brown was finally able to confirm talk that had been rife in the community for years. Apple had indeed managed to reverse a capacitor.
The problem looks to be with whoever came up with the design. The mounting of the capacitor – C22 – matches the markings on the PCB. However, rather than a positive voltage, the positive marked terminal is connected to -5V from the power connector, meaning that a negative voltage is across the capacitor.
That's bad, but not necessarily catastrophic. The capacitor is rated for 16V, and while pushing -5V across it will damage the component, it's unlikely to cause the dreaded puff of magical white smoke. In addition, according to Brown, "it is only involved with the -5V rail, which is only needed for the RS-422 serial ports. The capacitor might not have been doing its job properly if it was installed backward, but it didn't seem to really be hurting anything."
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However, although the computer might have functioned even with the capacitor fitted the wrong way around, enthusiasts replacing it with a [4]tantalum capacitor might find themselves in trouble. Brown noted posts where other repairers had been left with an incorrect voltage on the -5V rail. He highlighted one repairer who, after installing a tantalum capacitor backward, found it was drawing 1.3 amps - far too much - and the voltage was only -2.3V.
[5]
[6]
"At best, this is going to result in serial port problems. At worst, you're asking for the capacitor to explode or catch on fire," he said.
Apple is hardly the only computer maker to fumble component placement. There have been plenty of errors from other manufacturers, such as [7]Commodore . One poster [8]remarked , "Typical Amiga fanboyism and Apple envy, if a Mac does something they have to prove the Amiga outdid it. Only one model with a reverse polarity capacitor? With Commodore it was a systematic issue!"
[9]
The Register asked Brown about the incident and what the future held for the now-repaired LC III. He told us: "My plan for the LC III is to tinker with it."
Brown added that he was also teaching himself how to make expansion cards for the LC series of Mac and paid tribute to the enthusiastic members of the vintage Mac community who still come up with ways of accelerating, expanding, and keeping the old hardware running via new hardware and logic board recreations.
Capacitor leakage is usually relatively fixable, according to Brown. "You might lose a few pads here and there, need to remove chips to clean under them, or find some traces or vias that the electrolyte completely damaged, but it's usually fixable with bodge wires."
[10]
Damage from a leaking capacitor – Pic: Doug Brown
Battery leakage can be a whole different story. "The worst I've ever seen was a Macintosh Color Classic where the 3.6V half-AA lithium battery on the logic board had spewed disgusting corrosive brown goo everywhere inside the machine.
"It's a very common occurrence with these old Macs. I still see a lot of machines for sale on eBay where they haven't removed the battery and it always worries me. I know that some people do try to repair battery-bombed machines, but I couldn't bring myself to do it."
[11]That time a JPL engineer almost killed a Mars Rover before it left Earth
[12]You would expect a qualified electrician to wire a building to spec, right? Trust... but verify
[13]Shock horror – and there goes the network neighborhood
[14]ARM creators Sophie Wilson and Steve Furber
The Register also spoke to Glen of the [15]Casual Retro Gamer (CRG) channel on YouTube, who noted similar problems with some Commodore Amiga CD32 models. He said: "The problem was caused by the silk screen for the caps being wrong.
"Although, interestingly, at the same position there are points for smd caps, and to the best of my knowledge all these are correct."
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He noted that the components had been installed correctly on his CD32, which he'd owned since 1994. "The easiest thing to remember is that the negative side of the caps points towards the power switch. Or if you want to test it, put your meter on continuity and check the grounds."
The moral of the story is that just because the PCB says something is so does not mean that a poke with a multimeter isn't a good idea. Unless an unexpected puff of magical white smoke is your retro thing. ®
Get our [17]Tech Resources
[1] https://www.downtowndougbrown.com/2024/11/the-capacitor-that-apple-soldered-incorrectly-at-the-factory/
[2] https://regmedia.co.uk/2024/12/02/cap_backward.jpg
[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/personaltech&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2Z048ENFJjItPH3TcefBnZQAAANI&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0
[4] https://llis.nasa.gov/lesson/981
[5] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/personaltech&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44Z048ENFJjItPH3TcefBnZQAAANI&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
[6] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/personaltech&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33Z048ENFJjItPH3TcefBnZQAAANI&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0
[7] https://amiga.resource.cx/exp/a3640
[8] https://news.ycombinator.com/item?id=42253620
[9] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/personaltech&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44Z048ENFJjItPH3TcefBnZQAAANI&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
[10] https://regmedia.co.uk/2024/12/02/leak_damage.jpg
[11] https://www.theregister.com/2023/11/29/jpl_engineer_spirit_mars_rover/
[12] https://www.theregister.com/2021/01/25/who_me/
[13] https://www.theregister.com/2023/11/06/who_me/
[14] https://www.theregister.com/2012/05/03/unsung_heroes_of_tech_arm_creators_sophie_wilson_and_steve_furber/
[15] https://www.youtube.com/@CRG
[16] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_onprem/personaltech&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33Z048ENFJjItPH3TcefBnZQAAANI&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0
[17] https://whitepapers.theregister.com/
Lucky mistake
The good thing is that this didn't affect people. First, a brand new capacitor is likely to form at least OK, if not well, from a reversed polarity, as opposed to a properly used capacitor that's then introduced to the opposite polarity. Second, this just acts to clean the voltage from the power supply, so it's strictly not needed. Third, Macs of that age don't need negative voltage for anything important, so long as the expansion card doesn't need it. Sound works, as does RS-422 and LocalTalk, although it's possible there are machines / devices to which serial can be connected that wouldn't be happy with the lack of proper negative voltage.
I have quite a few 68030 and 68040 Macs that I've built in to various cases, and none need negative voltage, although it's really not hard to add in most instances.
Reverse the polarity of the Coulomb flow...
... as the Third Doctor might have said.
Re: Reverse the polarity of the Coulomb flow...
I am coming to believe that when a scifi protagonist of some kind makes a "reverse the polarity" comment, it really should be regarded as just a short way of saying "I have an idea far too complicated to explain to idiots like you, but can I just get on with it anyway?"
Re: Reverse the polarity of the Coulomb flow...
It means "turn the USB plug over again".
Just goes to show
. . that knowing what is actually happening electricity-wise is still something that needs to be taught.
To think that we're going toward a future where electrical components will be designed by computers . . . if ever there is a glitch, there will be one hell a big puff of smoke.
The words "measure twice, cut once" come to mind.
That's not so bad.
This is nothing compared to the deliberate fail of putting the magic mouse charging port on the wrong side so you can't charge it while using it.
Re: That's not so bad.
I remember that a bunch of graphics artists where I did a short term contract all had the 'magic mouse', the sign of a charging cable in the mouse meant that the occupier of the desk was either in the toilet doing No 2's or in the break room, having a coffee (all drinks were banned from anywhere near the computers). That was all it needed to get enough charge for another few hours of work.
I do remember on manage who would complain loudly (so that everyone shared in his pain) when he came into work in the morning and found it hadn't been charged overnight. No one had the nerve to tell him that it was always unplugged by the last artist to leave the office. He was not liked and this was one way to get one over on him..
Those were the days.
I wonder what Apple's response back then would have been...
"You're charging it wrong" ?