Dual output supply dead

[Don]

Hi,

I use a lot of external disk/tape/etc. enclosures or
my computer peripherals. These typically have ~60W,
dual output (5/12) supplies in them.

They are a "popcorn part" so, when one fails, it is usually
easier to grab a spare out of my junk pile and replace the
entire power supply than waste time trying to repair it.

However, I have one such supply that is dead AND has an
atypical form factor (long and skinny). So, finding a
suitable spare in my junk pile has been a problem :<
(the supply in question is a Autec UPS65-2002) It is
functionally equivalent to the dozens of other supplys
that I have so I have temporarily replaced it with one
of those.

I figured it might be worth a few minutes to poke around it
and see if I can steal some components off another supply
to breathe new life into this one. (If not, I'll just pull
the disks/tape out of the enclosure and throw the whole
thing out in favor of another enclosure).

A quick look at the types of components on the PCB leads me
to make some guesses as to what the circuit topology is.

It looks like mains power passes through an input filter
(choke, snubber, etc.) and then rectified. Switching
transformer primary is chopped and controlled by a sense
winding on the xformer. "Center" tapped secondary (no doubt
assymetrically tapped) two legs are rectified and then LC
filtered to provide the final output. I assume some sort
of OVP watches one or both outputs as I see optoisolator
feedback across the transformer no doubt to shutdown the
chopper.

Can someone confirm my guesswork to be correct?

Nothing is visibly toasted. And, assuming this topology,
I'd guess the chopper is "opened". Of course, it is possible
that there is a failure elsewhere on the board/load but I
suspect it may have just opened from fatigue, etc.

I.e. is it worth hunting down a replacement part (out of the
scrap bin) for a "quick fix"? Or, are these a bit more
complicated to repair (in which case, tossing it all in the
hazardous waste recycling bin is more expedient).

Recall I am *not* particularly "attached" to this enclosure... :>

Thanks!
--don

 

[[email protected]]

Hi,

I use a lot of external disk/tape/etc. enclosures or
my computer peripherals. These typically have ~60W,
dual output (5/12) supplies in them.

They are a "popcorn part" so, when one fails, it is usually
easier to grab a spare out of my junk pile and replace the
entire power supply than waste time trying to repair it.

However, I have one such supply that is dead AND has an
atypical form factor (long and skinny). So, finding a
suitable spare in my junk pile has been a problem :<
(the supply in question is a Autec UPS65-2002) It is
functionally equivalent to the dozens of other supplys
that I have so I have temporarily replaced it with one
of those.

I figured it might be worth a few minutes to poke around it
and see if I can steal some components off another supply
to breathe new life into this one. (If not, I'll just pull
the disks/tape out of the enclosure and throw the whole
thing out in favor of another enclosure).

A quick look at the types of components on the PCB leads me
to make some guesses as to what the circuit topology is.

It looks like mains power passes through an input filter
(choke, snubber, etc.) and then rectified. Switching
transformer primary is chopped and controlled by a sense
winding on the xformer. "Center" tapped secondary (no doubt
assymetrically tapped) two legs are rectified and then LC
filtered to provide the final output. I assume some sort
of OVP watches one or both outputs as I see optoisolator
feedback across the transformer no doubt to shutdown the
chopper.

Can someone confirm my guesswork to be correct?

Nothing is visibly toasted. And, assuming this topology,
I'd guess the chopper is "opened". Of course, it is possible
that there is a failure elsewhere on the board/load but I
suspect it may have just opened from fatigue, etc.

I.e. is it worth hunting down a replacement part (out of the
scrap bin) for a "quick fix"? Or, are these a bit more
complicated to repair (in which case, tossing it all in the
hazardous waste recycling bin is more expedient).

Recall I am *not* particularly "attached" to this enclosure... :>

Thanks!
--don

Circuit sounds about right, though there will likely be overcurrent
protection as well. As far as repair goes, just bin it, theyre often
sods to do.

NT

 

[Ancient_Hacker]

Check the fuse?

 

[Sam Goldwasser]

Check the fuse?

Might also be as simple as an open startup resistor.

But as someone else suggested, may not be worth the trouble. For now,
if you need to get your system back up, move the drives to another
enclosure. Then, later if you want a challenge, try to repair it.

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Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Don]

Might also be as simple as an open startup resistor.

Nope on both counts. When I removed it from the enclosure,
I made the mistake of picking it up a short time later and
sitting down on the couch to have a look at it.

Quite annoying to get "bit" from a small device with no
wires attached! (of course, the input filter was still
charged :< )

But as someone else suggested, may not be worth the trouble. For now,
if you need to get your system back up, move the drives to another
enclosure. Then, later if you want a challenge, try to repair it.

Unfortunately, this enclosure handles *3* half height drives
(instead of the one or two typically supported) so I have to
look for a more creative packaging option.

Or, leave the replacement supply sitting atop this enclosure
until I find something else, suitable.

 

[Arfa Daily]

Nope on both counts. When I removed it from the enclosure,
I made the mistake of picking it up a short time later and
sitting down on the couch to have a look at it.

Quite annoying to get "bit" from a small device with no
wires attached! (of course, the input filter was still
charged :< )
until I find something else, suitable.

You can't say " nope " for the contention of open startup resistor, as the
main primary filter cap remaining charged ( often for hours ) is a totally
typical symptom of this failure. Couple of quick checks would be to follow
the print from the " + " terminal of the primary filter cap, and see if a
high-ish value resistor is hooked to it on its way to one of the chopper
tranny pins. Another possibility which is very common on all switchers, and
will also leave the main filter cap charged due to the chopper oscillator
not running, is defective primary-side electrolytics. Look for any small
electros, particularly if they are positioned close to the heatsink or
anything else that gets hot, and ESR them. If you haven't got an ESR meter,
just replace them. It's worth going that far, but beyond that, commit it to
landfill ...

Arfa

 

[Don]

You can't say " nope " for the contention of open startup resistor, as the
main primary filter cap remaining charged ( often for hours ) is a totally
typical symptom of this failure. Couple of quick checks would be to follow
the print from the " + " terminal of the primary filter cap, and see if a
high-ish value resistor is hooked to it on its way to one of the chopper

I repeat, "Nope". :> The fuse is known good (since the input
filter would never charge through an open fuse).

And, this topology does not employ the sort of startup resistor you
are expecting. I.e. there is no DC path to the SMPS controller's
"power" pin from the input filter. (I assume you are expecting a
~1W dropping resistor between the input filter and the SMPS
controller)

To be precise, there are exactly 5 connections to the input filter:
- the input filter cap's + terminal
- the bridge's '+' output terminal
- one side of the transformer primary
- one side of an RC snubber across the xformer primary
(which is *two* components/connections) in series with catch diode

I'll drag out a DVM and verify power is available at the SMPS
controller, though...

tranny pins. Another possibility which is very common on all switchers, and
will also leave the main filter cap charged due to the chopper oscillator
not running, is defective primary-side electrolytics. Look for any small
electros, particularly if they are positioned close to the heatsink or
anything else that gets hot, and ESR them. If you haven't got an ESR meter,

Aside from the input filter (180uF 400VDC) and secondary output
filters, there are exactly two electrolytics in the design. The
first serves as a filter on the sense/feedback coil. I haven't yet
tracked down the role of the second...

just replace them. It's worth going that far, but beyond that, commit it to
landfill ...

This looks tiny enough that I can probably trace the circuit
topology in half an hour. I'll photocopy the foils and see
how quick i can reconstruct the basic design...

 

[Bob Urz]

I repeat, "Nope". :> The fuse is known good (since the input
filter would never charge through an open fuse).

And, this topology does not employ the sort of startup resistor you
are expecting. I.e. there is no DC path to the SMPS controller's
"power" pin from the input filter. (I assume you are expecting a
~1W dropping resistor between the input filter and the SMPS
controller)

To be precise, there are exactly 5 connections to the input filter:
- the input filter cap's + terminal
- the bridge's '+' output terminal
- one side of the transformer primary
- one side of an RC snubber across the xformer primary
(which is *two* components/connections) in series with catch diode

I'll drag out a DVM and verify power is available at the SMPS
controller, though...



Aside from the input filter (180uF 400VDC) and secondary output
filters, there are exactly two electrolytics in the design. The
first serves as a filter on the sense/feedback coil. I haven't yet
tracked down the role of the second...



This looks tiny enough that I can probably trace the circuit
topology in half an hour. I'll photocopy the foils and see
how quick i can reconstruct the basic design...

I assume you checked ALL solder connections for bad joints?
And put your meter on all higher current film resistors and compared the
measured value to the color code?

Bob

 

[Mike]

ok question. we may have to step up a notch now.

What kind of SMPS controller does it have? is the controller and chopper
transistor seperate, or is it an all-in-one such as a VIPIR or STR
regulator. those are notorious for dying out.

 

[Don]

ok question. we may have to step up a notch now.

What kind of SMPS controller does it have? is the controller and chopper
transistor seperate, or is it an all-in-one such as a VIPIR or STR
regulator. those are notorious for dying out.

3842 driving a discrete FET (can't see the P/N on the FET since it
is hiding behind the input filter).

And, to answer "Bob's" questions, below (his post hasn't made it
to my news server):
- first thing I did was reflow all the solder joints (since it is
so easy to do and I've seen problems with fatigue in the past)
- there are only two "higher current" (wattage) resistors on
the board: one in the snubber and the other in the current
sense leg. The current sense R was verified as "approximately
the right value"; the snubber was ignored.

I made a photocopy of the foil patterns last night and will trace
it out this evening if I have time. In the late evening quiet, I
also was able to notice a "ticking" sound -- as if the switcher
was starting up and then shutting down again. I'll pay closer
attention to the current sense circuit when I am tracing the
foil patterns tonight -- along with the OVP shutdown (as those
are the two things that come to mind that could cause this
cyclic start-shutdown behaviour).

But, tomorrow brings another surplus equipment auction so I
may stumble on a suitable replacement and just pitch this... :-/

 

[Arfa Daily]

3842 driving a discrete FET (can't see the P/N on the FET since it
is hiding behind the input filter).

If it uses a 3842 / FET circuit, then it just abiut *must* have a startup
resistor and supply pin decoupling cap somewhere, otherwise, how does it get
a supply to the IC to start it up ? If it's ticking, my money is still on a
cap ...

Arfa

 

[Don]

If it uses a 3842 / FET circuit, then it just abiut *must* have a startup
resistor and supply pin decoupling cap somewhere, otherwise, how does it get
a supply to the IC to start it up ?

Trust me, I can eyeball a 1.5" length of foil with 5 holes in it! :>

As I said in a previous post, only two R's larger than 1/10W:
one as the current sense R; the other as the snubber (it's REALLY
easy to see an RC-D across the primary ... you don't even have to
look hard! :> )

If it's ticking, my money is still on a cap ...

My first guess would be the current sense resistor being open
and immediately shutting the controller down. But, I haven't
yet traced the foils to see what else is going on.

 

[Arfa Daily]

Trust me, I can eyeball a 1.5" length of foil with 5 holes in it! :>

As I said in a previous post, only two R's larger than 1/10W:
one as the current sense R; the other as the snubber (it's REALLY
easy to see an RC-D across the primary ... you don't even have to
look hard! :> )


My first guess would be the current sense resistor being open
and immediately shutting the controller down. But, I haven't
yet traced the foils to see what else is going on.

Did you get to the bottom of it yet, Don ? I'm always interested in SMPS
faults as I do a lot of work with them, and any info that saves a few
minutes, is extra money in the bank ...

Arfa