Plus Analog Board C1: Will 4 uF work?

[Bugsi]

My poor Plus just had its second Analog Board die, most likely C1, as I have the vertical line down the middle of the screen. As I inspect the previous Analog Board I had in it, C1 has vented its top. The one on this particular Analog Board is a 3.9 uF, 35 volt electrolytic. I've seen other revs of the motherboard with a 3.9 uF 100 volt cap in it. (Obviously using a cap with a higher voltage rating is not generally a problem.)

I'm looking to get some replacement caps to fix these boards, and I'm finding 4uF caps rated 250 volts. Has anyone fixed their Analog Board using a 4uF cap, and would it be okay? 3.9 and 4 are so close, they're within the 10% tolerance rating of the 3.9 cap, right? I should be able to use a 4 uF cap no problem right? I'm just checking to see if anyone knows if I'm about to make a giant mistake or something.

 

[Osgeld]

considering tolerances you should be ok

 

[JDW]

Why go with 4.0uF when you can get the exact 3.9uF value you need easily from Digikey for only $3 plus shipping? This is mentioned on my analog board parts list spreadsheet here:

http://spreadsheets.google.com/ccc?key=0AnmQLg8d1xThcE96S1Z0S3oxOUFiMkJEYy1nNzNuZEE&hl=en

The 250v-rated Panasonic P/N:ECWF2395JB, Metallized Polypropylene replacement cap (with +/- 5% tolerance) I recommend in my spreadsheet will last about as long as the circuit board will. It is not electrolytic, so there is no worry of fluid leaks 15 years down the line. This is the single best replacement cap you can find for C1.

 

[Bugsi]

Ah, thanks for that, I was looking a Mouser, and the closest I could find there was a 4 uF.

 

[JDW]

Page 2 of this PDF on Mouser lists the 3.9uF cap I mention, with one rated at 250V. I simply cannot pull it up on the Mouser site by part number. But since it is in a catalog listed by Mouser, you have every right to ask them to sell it to you. And if they don't, I still feel this cap is so good it's worth using DigiKey just to purchase it. Indeed, you may make your money back if you buy 10 or so and then present them to users here. Sell the extras (for a reasonable price, of course), and you will make back your shipping and could be a benefit some users here (some of whom live outside the United States). But of course, that would require you to be willing to box and ship them like trag does.

 

[Bugsi]

I ordered four of them from DigiKey, we'll see how they do, I have two analog boards to repair right now, and this will give me two spares. If they work out really well, I'll get some more to supply to people on the forum.

 

[trag]

Sell the extras (for a reasonable price, of course), and you will make back your shipping and could be a benefit some users here (some of whom live outside the United States). But of course, that would require you to be willing to box and ship them like trag does.

Hee, hee. I was looking at that Digi-Key page and thinking that the sweet spot is at 1000+ where the unit price drops to $1.30. Of course, at $3 each, one would have to find 430 people repairing Mac Pluses who need C1 before making one's money back. But it's such a sweet discount over the Qty. 100 price...

Qty 1 => $3.03

Qty 10 => $2.60

Qty 100 => $2.17

Qty 1000 => $1.30

Qty 5000 => $1.22

Qty 10,000 => $1.13

The discounts in the unit price after 1000 are not compelling, but the drop between 100 and 1000 is close to 40%.

 

[Bugsi]

I'm not worried about getting a discount on a $3 part. Mine arrived yesterday, I hope to find some time to do the repair and test the board some time this week. I bought a couple extras, so if there are people needing this part, they can shoot me a PM.

 

[JDW]

If I were set in changing C1 and only C1, it would not matter to me if the price was even $6 total, if that including shipping to me in Japan.

Perhaps it would be more desirable to have a complete cap replacement kit offered for the original Mac 128k/512k series analog boards. And when I say "complete" I don't of course mean every single capacitor, because there are some ceramic disk and other non-fluid filled caps that would never need to be replaced. I am a talking about a kit that would replace all the caps that have the potential to leak or otherwise lose capacity after 20+ years.

Replacing C1 fixes one of the major problems with the earliest analog boards, but replacing only that cap won't solve other problems created by other caps that are either weak or dead or leaking. I must admit that I've not yet seen the same amount of "leaking" from the big caps on the analog boards of early Macs, relative to the massive leaking I see on the silver SMD caps on an SE/30 motherboard. But that doesn't mean all the electrolytic caps on the early analog boards are still working at near maximum capacity. It would be safe to say that if you use your Mac 128k or 512k and notice any kind of screen "glitches" while the floppy drive is in use, it is most likely due to one or more caps on the analog board not doing their job as good as they once did.

Something to consider seriously.

 

[Osgeld]

I must admit that I've not yet seen the same amount of "leaking"

heh from that era I bet they are "sealed" vs newer ones are safety vented, I wonder how many of those shotgun shells have gone off in the assembly / testing stages

 

[trag]

Replacing C1 fixes one of the major problems with the earliest analog boards, but replacing only that cap won't solve other problems created by other caps that are either weak or dead or leaking. I must admit that I've not yet seen the same amount of "leaking" from the big caps on the analog boards of early Macs, relative to the massive leaking I see on the silver SMD caps on an SE/30 motherboard. But that doesn't mean all the electrolytic caps on the early analog boards are still working at near maximum capacity.

One of the old aftermarket houses (perhaps more than one) sold a "Larry Pina" kit. They didn't call it that, but it contained all the parts that Larry recommends replacing on the analog board in his book, except the BU406 transistor. This was back around 1993 or 4. The kits were $50 each or $25 each if you purchased five or more. So I bought 5 way back when. I still have a couple left. They're turning out to be worth it just to have the peace of mind that I have a couple of replacement flybacks on hand. Those seem to be unobtainium now.

Makes me regret the upgrades I did to analog boards where I replaced the flyback with the kit's and probably trashed a serviceable, albeit older flyback.

 

[Bugsi]

I just want to point out in answer to the question in my original post, one should not attempt to use a normal 4uF electrolytic (polarized) cap in place of C1. C1 is a bipolar cap, replace it with the Digikey part mentioned in this thread. I read on another site that replacing C1 with a polarized electrolytic will eventually cause it to explode. I can't confirm that this is true or not, but if you're going to the trouble to replace C1, you might as well just use the nonpolarized metal/poly film cap from Digikey. I replaced mine last night, I'll swap the boards this evening and report back.

 

[Bugsi]

Digikey's 3.9uF cap worked great. Last time C1 blew, I just swapped analog boards. This time I repaired the original analog board by replacing C1 with the Digikey cap, swapped the boards, and it fired right up, working perfectly.

Thanks guys!

I use this Mac Plus to run Altoclock off a bootable floppy. Once it boots and I launch Altoclock, the floppy spins down and there's no moving parts from that point on. It's a completely noise-free digital clock for my kids' room. Yes, it's overkill to use a perfectly working 4MB Mac Plus computer as a digital clock, but it's cool and very retro, and frankly it's the best use I've been able to come up with for it.

Once again, thanks to you guys for the tip on the capacitor!