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About these pesky capacitors....
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t-rexky



Joined: 09 Jan 2011
Posts: 272
Location: Snowy Canada

PostPosted: Sat Mar 04, 2017 11:10 am    Post subject: Reply with quote

bobo68 wrote:
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
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wayne123



Joined: 13 Feb 2017
Posts: 10

PostPosted: Sat Mar 04, 2017 12:36 pm    Post subject: Reply with quote

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.
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bobo68



Joined: 03 Jun 2015
Posts: 194
Location: Germany

PostPosted: Sat Mar 04, 2017 4:34 pm    Post subject: Reply with quote

t-rexky wrote:
bobo68 wrote:
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: Smile

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

Ok, the last part is similar. Very Happy
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Mike



Joined: 03 Feb 2017
Posts: 83
Location: Canada: Oshawa, Ontario

PostPosted: Sun Mar 05, 2017 6:57 pm    Post subject: Reply with quote

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



Mike
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bobo68



Joined: 03 Jun 2015
Posts: 194
Location: Germany

PostPosted: Mon Mar 06, 2017 1:06 am    Post subject: Reply with quote

Nice. That looks like the innards of a multimeter. Very Happy
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Scutboy



Joined: 04 Mar 2010
Posts: 40
Location: Northfield, MN USA

PostPosted: Mon Mar 06, 2017 6:49 am    Post subject: Reply with quote

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

Mike


If there were a "like" button, I'd press it.
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t-rexky



Joined: 09 Jan 2011
Posts: 272
Location: Snowy Canada

PostPosted: Mon Mar 06, 2017 9:23 am    Post subject: Reply with quote

bobo68 wrote:
t-rexky wrote:
bobo68 wrote:
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: Smile

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

Ok, the last part is similar. Very Happy


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".

Razz
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t-rexky



Joined: 09 Jan 2011
Posts: 272
Location: Snowy Canada

PostPosted: Mon Mar 06, 2017 1:26 pm    Post subject: Reply with quote

@Mike, that's me in the blue hat kneeling in front of the SMD electrolytic capacitor...
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wayne123



Joined: 13 Feb 2017
Posts: 10

PostPosted: Thu Mar 09, 2017 12:39 pm    Post subject: Reply with quote

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.
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