On Fri, 26 Sep 2014, Robert Jarratt wrote:
On Fri, 26 Sep 2014, Tothwolf wrote:
I had a couple of these PSUs within easy reach.
The machine this one
came out of hasn't been powered in a number of years, but was working
fine last time it was powered up. I have another identical PSU in a
companion expansion chassis which I can also pull if needed.
The board in question can be removed from the main PSU chassis without
desoldering any parts. There are 2 screws in opposite outer corners
plus 5 more accessible through holes in the board (only the 5 with
black plastic bushings, the others don't attach to the main heatsink
but just hold the semiconductors to the two aluminum plates), along
with the 4 shoulder screws holding the self-aligning connector board to
the rear of the PSU's chassis. Once those are removed / loosened, the
board can be lifted out, but be careful not to tear the huge silpad
(use a nylon spudger to carefully separate it from the two aluminum
plates). You don't want to know what a sheet of silpad material that
large would cost to replace...
I have already removed the board in question. Although I did not use a
nylon spludger as I don't know what such a thing is, let alone actually
have one. However, I managed to avoid tearing the pad you mention.
Menda / Desco 622
http://www.all-spec.com/products/35622.html
One of the most versatile tools to have in ones toolkit. Always keep a few
extras around, they wear out quickly. Beware the fakes and counterfeits on
eBay and Amazon though.
Looking at the
connector head-on from the mating side, the two
left-most pins are connected to '+', and the next pin to the right,
along with the next two pins towards the right on the other side of the
connector are connected to '-'. The pin furthest towards the right
seems to be a different supply or sense line of some sort. (Anyone have
a pinout for a H7874?)
You can see the pinout on the backplane itself, at least you can in the
machine that mine came out of. The two leftmost ones are +12V, the next
three are GND, and the last one is +12V.
The rightmost pin looked like it was isolated but I'll have to have
another look at the board (already moved it back off the bench).
Compensating
for lead resistance, with a Fluke 177, I'm measuring 12.1
ohms between the '+' and '-' contacts on my PSU's board.
I have been using the diode testing function, which shows a short, but
when I measure the actual resistance it shows about 12ohms too. So
perhaps those outputs are not bad after all.
I used ohms. When I checked again with the diode function, my PSU showed
as short with my meter as well.
My current plan is to put it all back together and
attach a dummy load
to see what the outputs really are. Trouble is, having broken the
capacitor I still can't get the solder out of the holes (see last answer
below).
Does your board also use a bunch of Nippon
Chemi-Con / United Chemi-Con
LXF capacitors? I've had so much trouble with various series of those
brown Chemi-Con capacitors leaking or popping that I tend to replace
them wholesale on sight.
Don't know, I see three brown ones, marked SXF.
There may me some SXF series parts on mine as well, maybe the 100uF? They
probably used more than one series. I was looking at the taller 330uF
parts.
Which of the
small capacitors was it that you broke on your board?
The one marked 224, behind a brown 100uF electrolytic, and connected
across +12V and GND, right behind the left hand set of outputs.
That one is definitely a ceramic. I couldn't read its value though and it
has markings on both sides. I may have to pull the board back out of my
PSU and desolder the capacitor to measure it with a capacitance meter.
The extra
aluminum plates on the bottom of the board wouldn't make
repair of this supply easy unless you have a vacuum desoldering tool
and a long reach nozzle since you would have to desolder some of the
semiconductors to fully access the solder side of the board.
Indeed, I don't think I have the equipment to be able to remove the
plates without damaging something. Do you know if there is any tool a
hobbyist could afford to do this?
I don't know of an inexpensive tool that would be able to successfully
desolder those parts. I don't think a solder sucker or braid would work
due to the limited clearances. These PSUs were definitely not designed to
be serviced at the component level in the field. If they were serviced at
all in the field, they probably just swapped boards and then sent the
faulty boards back to be repaired.
I have a Hakko 808, which until they recently discontinued it, was about
$200 new (it has since jumped to about $250 when you can still find them).
The "replacement" handheld desoldering tool Hakko came out with is $300+
and I'm not convinced it will hold up as well as the 808. I've heard you
can get a made-in-china benchtop style desoldering tool for ~$100 USD, but
reports are mixed as to how well they hold up long term. I'll see if I can
find the link.