Mac 128k Keyboard Recapping - 1uf 63V

[JDW]

Yes, even the Macintosh 128k/512k KEYBOARD needs to be recapped:



But does it really need to be 63V?  

Here's the keyboard schematic, with the 1uF cap showing in the upper right area nearest U2:



And U2 is the 74LS123 shown here:



Sure, I can find a Mouser $3.23 replacement, but why is the stock cap rated at 63V?  Is it because people might be stupid and connect the keyboard to a phone jack, or is there some other reason?  

The reason I even wonder about this is because if lower voltage 1uF caps can safely be used in this keyboard, then more replacement options present themselves that are better and cheaper too.

I look forward to your insightful replies!

 

[JDW]

28 views of this thread as of the time of this writing but no replies, and here I am back to talking to myself again. :-(

According to the Operating Rules on page 3 of this 74LS123 data sheet, the 1uF capacitor in combination with the 200k-ohm resistor forms a timing circuit that determines pulse width.  Since the 1uF capacitor sits behind a 200k resistor tied to 5V, it's not clear why Apple's 1uF capacitor is rated at 63V.  Certainly, the capacitor would have been cheaper had they used a lower voltage rating.  And even though the positive side of that 1uF capacitor is also tied to the Rext/Cext pin of the 74LS123 chip, it's not possible that pin will see anywhere close to 63 volts.  Why then did Apple choose a 1uF capacitor rated at 63V?  Well, we know that the higher the voltage rating of an electrolytic capacitor the lower the ESR, and since 1uF electrolytic capacitors have a large ESR even when brand new, choosing 63V (probably the highest voltage available back in 1983) would have yielded the lowest ESR.  But that is just my speculation.  And why Apple would have wanted lower ESR is not clear because the 74LS123 data sheet makes no mention of capacitor ESR, and I have checked several 74LS123 data sheets even as far back as 1983.  So I don't really know what to think if that voltage rating choice is NOT tied to ESR.

So the question remains, what capacitor should we use as a replacement?

Since that aforementioned datasheet says Tantalum capacitors can be used, it is a matter of choosing the voltage rating.  The Vishay 173D105X9050VWE3 is a perfect length and diameter axial tantalum capacitor rated for 50V but can handle a surge voltage of 65V.  It currently costs $1.17 at Mouser, which is fairly high but much lower than the $3.23 100V 1uF electrolytic I mentioned in my opening post.  And because this $1.17 cap is Tantalum and rated at a fairly high voltage, the ESR would be less than the 63V electrolytic Apple uses, but yet not super-low like a ceramic capacitor would be.  But again, I don't think the rated voltage matters in terms of voltage surges, because I doubt we'd even get double Vcc (10V) across that capacitor.  So I would assume that an even lower voltage rated tantalum axial capacitor would suffice.  However, going down to a 35V tantalum axial capacitor only drops the price to $1.06.  And going to a 25V part only drops the price to $0.97.  So we might as well choose the 50V rated Vishay tantalum I linked to.

Why choose a tantalum rather than an electrolytic?  Because we don't want to swap out an electrolytic replacement again 20 years hence.  Even the best electrolytic caps will eventually dry out and/or leak.  A tantalum capacitor will last the life of the keyboard.

So I think I will go with the 50V Vishay cap, unless some of you present good reasons to avoid it.  Feel free to share you thoughts.  No, actually, please share your thoughts.  I feel lonely talking to myself in this forum.

Thanks.

 

[PB145B]

Is the keyboard malfunctioning? If not, the cap may not need replacing at all.

But, I’d say it probably doesn’t have to be 63v. The 50v would probably do fine. I can’t imagine there being anywhere near that much voltage anywhere in that keyboard.

 

[JDW]

It is not malfunctioning.  But I've had SE/30 motherboards that had never been recapped and seemed to work OK, and all the while the electrolyte was leaking out and slowing eating traces.  The point here is that just because something is working doesn't mean it shouldn't be recapped, especially when the electrolytic capacitor in question is 35 years old. 

 

[cruff]

That cap doesn't need a voltage rating of 63V at all.  They probably had a big stock of them and just selected them for the capacitance value.  I suspect that the ESR rating isn't much of a factor either, since the keyboard is not a power supply that needs smoothing, nor is it operating at high frequency.  Just choose a convenient cap with a voltage rating of at least 6.3V.

 

[PB145B]

But, there’s a big difference in the quality of these axial caps, and the cheap SMDs used on the SE/30.

The axial cap in that keyboard is the exact style of one used on the SE main board (except for the fact it’s orange, but I can tell it’s the same brand)[SIZE=1.4rem], and I’ve never heard of those leaking or giving any signs of malfunction before.[/SIZE]

 

[JDW]

Arguably, this is but one capacitor inside a keyboard.  And if it leaked, it would leak on the top side of the PCB where there are no traces.  It would not harm anything, most likely.  ESR would slowly rise until the capacitor is no longer usable due to leaked or evaporated electrolyte.  And while no reports have been made to date about these keyboards, not everyone knows why these keyboards go bad and may just replace them in silence without reporting anything.  Also, I would suspect that with it now being 35 years or so since manufacture that we would eventually see problems pertaining to this capacitor because no fluid-filled electrolytic capacitor lasts forever.  That's the key here.  No fluid filled capacitor lasts forever, and after 35 years, you're lucky indeed if you still have half of the original electrolyte in there.

So let's forget all the "if it ain't broke, don't fix it" thinking for a moment and consider replacements in light of what @cruff and I have already said.  6.3V is probably a reasonable rating for an electrolytic replacement since this is a 5V IC it is connected to.  However, it's silly to replace it with another fluid filled cap for reasons already stated.  It therefore makes the most sense to use either ceramic or tantalum, since both are solid electrolytes that will indeed last as long as the circuit board itself.  Axial ceramics will fit but they cost a lot more than tantalum. I checked lower voltages for the 1uF tantalum cap on Mouser.  Surprisingly, those actually cost more.  And since over-voltage on tantalum caps results in fire and burning, you always get a rated voltage much higher than an electrolytic.  At least twice.  So 10V would be pushing it.  I'd feel safer with 15V or higher.  But again, the axial tantalum caps on Mouser are all about $1 each, so one might as well get the 50V Vishay 173D105X9050VWE3.  If the price is roughly the same and if it fits, get the best part.

After all this discussion, there will still be some who won't replace the cap until it dies.  I think that's probably ok in this case for reasons already stated.  But for those who are doing recapping jobs like I am right now, we put together a bunch of caps to recap various things, which is a good idea as a sufficiently large enough order could qualify you for free shipping.  And when you live outside the USA like I do, that matters.

Thank you for the input!  And if anyone reading this has experienced a bad cap in a Mac 128k or 512k or Plus keyboard, please report that here.  Surely there must be at least one of you out there.

 

[superjer2000]

However, it's silly to replace it with another fluid filled cap for reasons already stated.  It therefore makes the most sense to use either ceramic or tantalum, since both are solid electrolytes that will indeed last as long as the circuit board itself.  

I'm not sure this is true.  The current cap has lasted for 25 years - If you can get another 25 years out of a replacement electrolytic that's pretty good.

 

[JDW]

I'm not sure this is true.  The current cap has lasted for 25 years - If you can get another 25 years out of a replacement electrolytic that's pretty good.

There’s no guarantee that the chosen replacement fluid filled capacitor will last as long. And 25 years hence, I will be too old to want to replace it again, probably. And that’s precisely why tantalum is a good choice because 25 years from now you won’t need to replace it again. 

By the way, I first started replacing capacitors in keyboards when I experienced a problem with the IIgs keyboard attached to my SE:30. Sometimes I would be using the computer and typing on it and nothing would happen. I opened it up one day only to find fluid filled capacitors inside. I replaced them with tantalums and the problem was solved. 

 

[superjer2000]

That's good to note (that you had fixed a keyboard by replacing a cap - I'll keep that in mind if I see a similar issue)

On the longevity of the caps though - if you're putting in caps that last 30, 40 years you're taking away the excitement of recapping from the next generation!

 

[JDW]

Here's a photo of my recapped IIgs keyboard, and note the hot glue around the ADB connectors that I added is important to prevent broken solder joints:



And here's a photo of the Apple Mouse G5431 with stock capacitors (3pcs).  Width and Height of replacement caps is a very important consideration because there's not a lot of space.  Suffice it to say, if you've been having tracking issues even though your rollers are clean, it could be the caps!  Normally you want to spec Tantalum caps higher than electrolytics to avoid a fiery explosion in the event of over-voltage, but ADB supplies only 5V.  So you could safely use 16V radial dipped tantalums without problem, and at that voltage rating they should fit fine.

 

 

[LaPorta]

Indeed, I have made notes on when either myself or whoever later takes on my Macs (hopefully one of my kids) will need to replace the ones I have replaced.

 

[JDW]

@LaPorta  What I do is use a thick black Sharpie pen to handwrite the replacement date clearly on one of the big, easily noticeable caps, and that serves as a note to me (or those two whom I sell something) that ALL the caps were replaced on that date.  Paper notes get lost but a pen-mark on the cap won't go away.

 

[LaPorta]

You are correct. On the machines I have re-capped for other people, I have always left a label that states that I did the job, and what date it was done.

 

[JDW]

I just recapped the same style of mouse but with a very different PCB, as shown below.



The ball is much heavier than the ball in my other mouse, and the clicker switch feels a tad different too.

BEFORE Recap (C1=1uF 50V, C3=4.7uF 35V -- both D=4.2mm, H=7.6mm):



AFTER Recap with Tantalums (C1=1uF 25V, C3=4.7uF 35V -- size about the same as stock):



I used old stock tantalums I had on hand which are about 20 years old.  Since they are solid electrolyte it doesn't matter.  I measured ESR of the stock caps and my replacements and they were about the same.  Since this is a 5V mouse (ADB), even a 25V rating is fine for the tantalums.

It is interesting Apple used a PIC MCU:



And the weak point of these mice in terms of durability over time appears to be the rollers which rust!

 

[ktkm]

On the longevity of the caps though - if you're putting in caps that last 30, 40 years you're taking away the excitement of recapping from the next generation!

:lol:

 

[JDW]

As you can see from the state of those mouse rollers though, the next generation will still have the excitement of trying to figure out how to fix that rust, which I must say is more difficult than a recap job!

 

[trag]

Electroplating?     https://www.caswellplating.com/electroplating-anodizing.html#

 

[JDW]

Electroplating?     https://www.caswellplating.com/electroplating-anodizing.html#

 I look forward to seeing your video on that, Jeff!