Just destroyed my SE/30

GerrySch · Apr 13, 2025

Good morning all. This is heartbreaking to report. I've owned a Mac SE/30 for the last 8 years or so. When I bought it, the motherboard (MB) had been recapped, the CRT was bright and sharp and still is. For the past year, I've been sprucing it up, like replacing the Sony power supply with modern ones that have crowbar circuits (Thank goodness for them! They protected the MB from even more catastrophic damage) and a higher current rating. I replaced the fan with a quieter one and replaced the hard drive with a BlueSCSI desktop that also got the SE/30 on-line. I also added an 68030 accelerator card and replaced the Rominator II with a GW-4402B ROM Card. It was really shaping up and the last thing I wanted to do was re-cap it.

So, I bought replacement capacitor kits for both the Analog board and MB and tonight I sat down and installed the new capacitors. Now the MB had it's capacitors replaced when I bought it but whomever did it, did a really ugly job. So I was going to clean it up with Tantalum and electrolytic caps. I replaced all the caps being careful to observe polarity and properly installed with new solder.

I reassembled and powered up and it bonged! Great! But then the Mac shutdown. Weird. I looked around and checked things but couldn't find the problem. Powered it back up while monitoring the 5v rail. The 5v rail was oscillating between 0 and 5v at about a 2 second period and I saw some magic smoke escaping so I powered off immediately and pulled out the MB. C2 had burned up and was looking terrible. I went through the schematics for the SE/30 and found C2 was a decoupling cap for the 5v rail. After cleaning the board up, I rechecked the 5v rail and it's still shorted to ground.

I am so bummed out. The only way to find where the short is to put the 5v rail on a variable power supply and slowly bring the current up and use a thermal camera to identify where it's getting hot. I don't have those resources so there's little I can do. Also, this is a 5 or 6 layer board and if the short is inside the board between the layers, then it's toast. It could be a great candidate to repopulate a new MB but that's beyond my resources as well.

If anyone has any ideas I'm all ears. My wife is already upset with me that I've spent money to rebuild a 35 year old Mac that's only good to play games with. I just downloaded A/UX because I wanted to explore how that worked. Anyway, if you have a possible solution, I look forward to hearing from you.

A very sad Classic Mac owner,
Gerry

Elemenoh · Apr 13, 2025

Occam's razor would indicate you probably introduced a short somewhere else when you replaced the caps. I'd carefully start undoing your work until the short is cleared. I'm guessing you have a trace running underneath one of the caps that was exposed and then shorted to ground during your repair. If so, you'll need to mask that trace before reinstalling the cap.

phipli · Apr 13, 2025

@GerrySch I doubt you've killed your SE/30 completely.

The most likely thing is that you've put the tantalum capacitors in backwards. Irritatingly, they're marked backwards compared to regular electrolytics. The symptoms you describe are exactly what happens when you put tantalums in backwards.

Its going to be fine

worst case is you should replace all the tants because they've been exposed to a reverse polarity.

jmacz · Apr 14, 2025

And also ensure whatever kit you bought included 25V rated tantalums for the capacitors that will get 12V. There are a few of them. Most kits should have been updated by now but I only saw kits updating to 25V in the past year or two so you never know. It shouldn’t have caused your issue but good to ensure you have 2x rated tantalums on the 12V rails regardless.

GerrySch · Apr 14, 2025

Great point Elemenoh! I found a set of schematics for the SE/30 MB and have identified 5 caps that are used to decouple 5 volts around the board that I replaced. C1, C7(blew up), C11, C12 and C13. I know where they are on the board so I can check them.

Phipli, I asked the exact same question the night I changed these caps. I've not worked with SMD caps, so I googled what the stripe meant. It came back and identified it as negative. According to your pictures, the stripe means positive. Crap, if you're right, I installed them all backwards. I bought the kit from Console5. I'll email them.

Jmacz, I did check the voltage rating on the replacement capacitors and they were 25 volts. So I'm good there. I also prefer 105 degree rated caps over 85 degree rated caps depending upon how warm the interior gets. These caps were all 85 degrees but I had to use a magnifying glass to read them.

Thank you for your responses and I'll get to the bottom of this.

Here's my final question, If C7 was backwards and shorted, causing it to blow up (it's totally gone) why is the 5v rail still shorted? That bothers me.

Gerry

jmacz · Apr 14, 2025

Console5 usually includes a little business card with their capacitor kits where they explicitly warn about getting the polarity reversed with tantalums. The card has diagrams of electrolytic caps vs tantalums.

jmacz · Apr 14, 2025

Oh and as Phipli mentioned, if you installed them backwards, you probably should get new ones and not reuse the ones that were reversed.

GerrySch · Apr 14, 2025

Thank you, jmacz. I've emailed Console5 for replacement capacitors and suggested the metal can types.

Elemenoh · Apr 14, 2025

If C7 was backwards and shorted, causing it to blow up (it's totally gone) why is the 5v rail still shorted? That bothers me.

Tantalum caps fail short. If you installed them all backwards and some failed, they'd likely fail short and those caps you installed on the 5V rail would short it to ground. Or the other possibility I mentioned where you might have shorted a cap to an exposed trace somewhere.

Good luck patching it up!

JDW · Apr 15, 2025

GerrySch said:
Thank you, jmacz. I've emailed Console5 for replacement capacitors and suggested the metal can types.

If the metal can types are fluid-filled electrolytic types and not aluminum polymer types, I say you should pass on those because those will need to be recapped again 20+ years hence. These days, I use OS CON caps because they have very low ESR, yet look stock. They'll be destroyed if you reverse them and power on, but they normally don't fail shorted like solid Tantalums do. Polymer tantalums don't fail shorted but they cost more than OS CON caps, plus their ESR, while lower than Solid Tantalum, is still higher than OS CON (polymer aluminum) caps.

If you absolutely MUST use Solid Tantalum caps, you also MUST properly derate them, regardless of what everyone else out there seems to be doing and getting away with. That means you NEVER use a 16V rated Solid Tantalum cap on a 12V-rail. No! You must use a 25V rated Solid Tantalum cap, and if you don't know or don't want to check schematics or be bothered, just get 25V caps, ensure the body will fit the motherboard pads, and then you are good to go, so long as you solder them in correctly.

But soldering something in correctly is yet another reason to go with OS CON. It's hard to put those in the wrong way. With Tantalum, you have to really be careful to avoid putting them in the wrong way.

GerrySch · Saturday at 11:42 PM

Sorry, I've been very busy so I haven't had a chance to respond. I'm awaiting delivery of replacement capacitors and I hoped they would arrive before today and get my SE/30 up and running. But, the parts haven't arrived so I'm stuck. Oh, for the days of local brick-and-mortar electronics stores to get parts quickly.

Anyway, I wanted to clarify the need to replace capacitors in vintage computers. JDW, in his previous post, discusses using polymer over fluid-filled capacitors and if I stick with fluid-filled, then I will need to replace them again in 20 years. But, as I understand it, these bad capacitors need replacement because of faulty capacitors manufactured in the late 80's and 90's that caused corrosion, damaging circuit boards and exploding. I've certainly run it that but modern capacitors have corrected these issues so I won't need to replace them in another 20 years.

If I'm mixing up different issues please clarify this. As far as I know, replacing leaking, smelly capacitors after 20 years is in the past.

Thank you,
Gerry

JDW · Sunday at 1:07 AM

Gerry, that’s incorrect.

No fluid filled capacitors have eternal life. The gaskets where the legs come out breakdown overtime, and the fluid either leaks out or evaporates, slowly turning them into a resistor. That truth has nothing to do with capacitor plague in the early 2000s.

There’s absolutely nothing wrong with choosing fluid filled capacitors as replacements, because the fact is some of them have no other alternative, such as on Analog boards. But on motherboards, you do have alternatives and so it’s usually prudent to pursue those other (better) types rather than pursue a capacitor type that will have to be changed every 20 to 25 years.

GerrySch · Sunday at 3:27 AM

Okay, perhaps you are right. I own an HP-85A computer that was made back in 1980 and has a 6" B&W CRT internal display, a thermal printer and a tape drive for internal mass storage. I also own external 3.5 and 5.25 floppy drives that are connected by HPIB to the 85A and they all still work fine. The floppy drives need mechanical cleaning and lubricating but they still spin and record and playback files. The HP-85A has Mallery capacitors in it's power supply and it still functions as it did back in 1980. The tape drive's pinch roller turned to mush and has been rebuilt.

Perhaps today's capacitors are not manufactured to the same level of quality as capacitors made 45 years ago, but there are examples where liquid-filled electrolytic capacitors still function and have never leaked.

S. Pupp · Sunday at 4:02 AM

I also have electronics from as far back as the 1950’s that still work. I’ve also had plenty of TV’s, receivers, VCR’s, printers, and computers from the 1980’s to 2000’s that have failed because of leaking capacitors. While most of the leakages have been from surface mount devices, every radial electrolytic capacitor on my wife’s Mac Classic analog board was leaking electrolytic fluid.

My own choice is to replace leaking capacitors with ones that don’t leak. The cost differential is worth it both for the preservation of my electronics, as well as for my peace of mind.

JDW · Sunday at 5:00 AM

Actually, devices will keep working… until they don’t.

Or until you start getting unexplainable quirks.

As stated previously, aging caps slowly age and their transformation into a resistor of sorts is not readily apparent, especially when you have multiple bypass capacitors in a circuit that help balance out one or more caps which are no longer up to par.

I also get a few HAM radio guys who comment under my capacitor video claiming nothing ever needs to be recapped because they have oil filled caps from the 1930s which still work, or so they say. But aside from the construction of those ancient monsters being quite a bit different from modern caps, they also are physically huge, and yes, size does matter. I’ve found the physically smaller caps, especially SMD types, to leak or go bad faster than large caps that are something like 3” tall and 1” in diameter.

When in doubt, replace with the appropriate replacement and you normally won’t go wrong, unless you’re recapping a power supply which has an output stage that requires a minimum ESR or when recapping timer circuits that may have a specialized capacitor in use. But motherboard bypass caps which basically are there to keep the 5V or 12v rails from sagging during a transient are okay to recap with low

Mk.558 · Sunday at 5:02 PM

I use electrolytic replacements for my motherboards.

I also only chose Japanese capacitors, Nichicon is preferred. Or Chemi-Con I think they're know by now.

Everything ages over time. Of course we can point to 1950s hardware that's still working but we can't forget about survivalship bias: "it's really good! look at it today! Still good as new!" And we can turn our hand to how many of those still exist, and how many are in landfills.

Having low ESR is great. The lower it is, the more efficient the capacitor is at doing capaciting, and doesn't turn electricity into heat, but also not so much resistance to current flow. JDW is right that for larger capacitors, you're usually stuck with electrolytic, and while they have a lot of disadvantages, they also work fine for a lot of scenarios.

I'm not putting tantalums in because electrolytics made from a reputable brand only need to be changed out every 10-20 years ish, they're cheaper, they do their job just fine, and it's not a conflict material. Of course you could also argue back that Apple chose 47uF 16v caps e-caps because of cost, and that's fair. But they also work fine as is, and I have yet to see a scenario where a higher quality capacitor actually means something on our old machines.

If you are pushing the limits of your 5V rail however, yeah you probably should use better capacitors. But I think e-caps get more flak than they really deserve, and if we want to be honest the world's electronics basically relies on them in some form or another.

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