About these pesky capacitors....

Mike here again:

As you may have gathered, I am relatively new to NexT hardware, having received a really nice clean complete NeXTStation system with a failing hard drive that has been replaced. The computer is (now) working perfectly.

As far as I can tell, it has seen very little use and has been handled gently and stored well.

Reading back through the archives on this forum I have been spooked by the frequent references to the capacitors in this and some other NeXT models.

How will I know if and when my capacitors fail ?  

What are the symptoms ?

Will it simply stop working ?

I have examined the capacitors in this unit very carefully (and looked on the internet at lots of pictures of swollen capacitors) and mine show no indications of distress.

I have also looked at the brilliant NextStation Mono Logic Board images at:   http://www.asterontech.com/Asterontech/nextstation_mono_retore.htm  

Neat-o !!

Mike

No experience with NeXT motherboards, I've replaced caps before they started giving troubles. On MO drives, faulty caps result into malfunctioning drives.
Worst thing that can happen though, is that leaked fluid damages the board and makes capacitor replacement quite tricky. Better replace them sooner than later.

You definitely want to replace the caps if they start to spill cap goo. The SMD caps tend to do that. That stuff will eat up other components and traces.

OK....

Thanks for the information and suggestions.

I have the pictures that show where all of the "caps" are on each type of NeXTStation mono board.  I will identify my board type and diagram the locations.

I will make a point of popping the cover off the slab every couple of months and checking the suspect components for visible swelling, discolouration, and the board for signs of possible leakage.

If these "caps" have been in the computer for about 25 years without failing.... maybe they are immortal.

M

Nope. It depends on the quality but they will die at some time. Have a look at 68k Mac forums e.g. and you will see that this is a common problem of 80's Macs. I replaced leaking caps in my SE/30 and that machine is from 1989.

Big, radial ones are easy to check visually, they tend to bulge and crack on the upper face, which is normally marked with a cross for giving an easy way out to the pressure building up inside the case.

My experience, though, is that they are usually lasting longer than surface mounted ones, unless they are operating at higher end of the temperature range (like those ones in the MO drives).

SMD are the nasty ones, I have replaced many, many of them on all my stuff (mainly late eighties / early nineties Macs and NeXT machines) and most of them were in poor conditions with fluid leaking out.

As long as you have some barely decent equipment and a steady hand, you just need some practice on a dismissed board before being able to successfully replace the caps on your NeXT machine.

Is it necessary to replace the small ceramic caps? I thought they last virtually forever.

No, it is not. Actually, you should try to replace the SMD caps with ceramic or tantalum ones: since they are solid state, they last longer.

Quote from: MikeMike here again:

Reading back through the archives on this forum I have been spooked by the frequent references to the capacitors in this and some other NeXT models.

How will I know if and when my capacitors fail ? 

What are the symptoms ?

Will it simply stop working ?

I have examined the capacitors in this unit very carefully (and looked on the internet at lots of pictures of swollen capacitors) and mine show no indications of distress.

I have also looked at the brilliant NextStation Mono Logic Board images at:  http://www.asterontech.com/Asterontech/nextstation_mono_retore.htm 

Neat-o !!

Mike

You can read my original rant on the TurboColor capacitors here:

http://www.nextcomputers.org/forums/index.php?topic=2803 (http://www.nextcomputers.org/forums/index.php?topic=2803)

Since your machine has likely no surface mounted (SMD) electrolytic capacitors your risk is much lower.  <- Note per the posts below that this is not a correct statement.  It is mostly the SMD capacitors of that era that are most susceptible to failure.  Electrolytic capacitors are filled with a roll of sandwiched aluminum plates with a "wet" electrolyte solution.  The electrolyte is either acidic or alkaline and is therefore corrosive.  NeXT appears to have used at least two supply sources for electrolytic SMD capacitors.  One of them appear to be "bad" and the other one "good"  I have seen a number of TurboColor main boards and many audio box boards with both types.  None of the "good" types have leaked on the units I have seen.  All of the "bad" types have leaked, even on new old-stock audio box boards that have never been used.

The through-hole capacitors that your board is equipped with can fail in a similar way, but are much more durable and significantly less likely to experience such failures.  Now, all electrolytic capacitors have finite life expectancy.  But I have some test equipment manufactured by HP in early 70s that still has fully functional electrolytic capacitors.  The more recent bad rap that electrolytics had was associated with industrial espionage where electrolyte formula with a missing key ingredient was stolen from a "high-end" Japanese capacitor manufacturer and the knock-offs manufactured elsewhere in Asia started swelling, venting and exploding after short time of use.  That's the "capacitor plague" of a few years ago and is not related to normal electrolytic capacitor wear-out and aging.

As the electrolytics age in a "normal" way, they may lose their capacitance either through drying out or leakage.  Loss of capacitance on the computer main board with result in increase in electrical noise.  This noise increase may eventually reach a threshold where it will start affecting integrated circuits and may lead to unexplained behaviour, freeze-ups, crashes, etc.  If the electrolytics in the power supply go, they may result in a somewhat more spectacular failures including a catastrophic power supply failure that could take out other computer components.

You can see a 20 year old SparcStation power supply that was very well designed and built, with high quality components, showing signs of capacitor failure here:

https://www.flickr.com/photos/t-rexky/albums/72157640695991374 (https://www.flickr.com/photos/t-rexky/albums/72157640695991374)

In that album you can also see my rework of that power supply with all the electrolytic capacitors replaced by extremely high quality modern equivalents.

There was a question about ceramics.  These do not need to be replaced as their life is effectively "infinite" compared to the electrolytics.  Tantalum capacitors also have virtually "infinite" life in comparison and are often used as replacements for old SMD electrolytics.  Polymer capacitors also last for extremely long time.  All of this assumes, of course, that the equipment is well designed and the capacitors are appropriately de-rated for their application.

Quote from: "t-rexky"Since your machine has likely no surface mounted (SMD) electrolytic capacitors your risk is much lower.

Maybe I missed something but why should his NeXTstation have no SMD electrolytic caps?

Looks like out of all board revisions only one has a single SMD electrolytic, while others have only through-hole electrolytics.

Sorry, my images might be misleading. I had already replaced the caps before taking the images. The green circles are where the SMD's would be. I've updated the site to mention this.

Well, that makes more sense now.  I was wondering why NeXT would have used ceramics and why you'd be replacing them.  I edited my response above to avoid confusion.  Thank you for the clarification.

What about the NeXT power supply caps. Is there any info on recapping them.

There is info but the NeXTstation's psus are a pita to open.

Quote from: "bobo68"There is info but the NeXTstation's psus are a pita to open.

I have done a few.  They are not that bad to open, but very difficult to work with.

The main board is a silk-screened conductive artwork on top of aluminum plate that is the bottom of the power supply.  This aluminum plate transfers heat to the case of the NeXTstation.  It is virtually impossible to work with it without a gas torch in a home environment.  The high voltage mains filter capacitor and one tiny SMD capacitor are mounted on this plate.  The output filter capacitors are mounted on a daughter board that is electrically connected to the main "plate" board with conductive posts.  The daughter card is reasonably easy to desolder and rework.  But the posts are prone to break due to solder cracking at the "plate" board.  I believe this cracking is caused by differential thermal expansion during on/off cycles.

Overall it is a nice and compact design that is extremely difficult to repair and rework.  I have some photos of my rework but I am unable to locate them.  On one PSU I threw-in the towel and accepted i'd probably cook it, then used a plumbing torch to reflow the solder on the posts.  Somehow it worked and even the high power switching transistors mounted on the screened plate survived the ordeal.  But I do not trust that power supply in the long term as the silicon may have been overstressed.

Here is an excellent repair page that has good internal photos:

http://heller.no-ip.org/~heller/NeXT/power_supply/power_supply.html

Nice repair link, looks like a major P.I.T.A. to work on and nice to read they are a weak point that will fail.

I just got my NeXTStation recently and it has a NOS power supply but sooner or later I will need to fix it.

My computer already has had a rough life in shipping thanks to the Post Office, they damaged the BNC connector on the back and broke a small chunk out of the case.

I need to find some old computer motherboards to practice on, luckily there are millions of old PCs that no one will care about if I destroy one.

Quote from: "t-rexky"

Quote from: "bobo68"There is info but the NeXTstation's psus are a pita to open.

Overall it is a nice and compact design that is extremely difficult to repair and rework.

Helmut had a bit of a different view on that: :-)

"In my view the power supply is built very shitty and its construction makes repairs very hard"

Ok, the last part is similar. :D

Do you have to be very small.... and have the right tools ??



Mike

Nice. That looks like the innards of a multimeter.  :D

Quote from: "Mike"Do you have to be very small.... and have the right tools ??

Mike

If there were a "like" button, I'd press it.

Quote from: "bobo68"

Quote from: "t-rexky"

Quote from: "bobo68"There is info but the NeXTstation's psus are a pita to open.

Overall it is a nice and compact design that is extremely difficult to repair and rework.

Helmut had a bit of a different view on that: :-)

"In my view the power supply is built very shitty and its construction makes repairs very hard"

Ok, the last part is similar. :D

I still stand by my original statement as written.  It is an awesome design for it's design constraints, which in this case was the super compact size and thermal dissipation path through the bottom plate into the pizza box chassis.  I think Helmut was intending to say: "In my view the power supply is built very shitty because its construction makes repairs very hard".

:P

@Mike, that's me in the blue hat kneeling in front of the SMD electrolytic capacitor...

Those pesky capacitors, where did I put them?

I just got my new vacuum desoldering tool and thought I would start by replacing the caps in an old Amiga I have, it is all through hole and should
have been a fairly quick job. I had a complete set of caps I ordered that I was sure I had put inside the case so I wouldn't lose them, oops, I guess I forgot that part. Getting old is so much fun, at least I am not to the point of forgetting my pants when I go out.