Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old May 18th 21, 09:03 PM posted to sci.electronics.repair
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Posts: 436
Default Faulty SMPSU.

On Tue, 18 May 2021 19:16:44 +0100, T i m wrote:

On Tue, 18 May 2021 09:33:03 -0400, legg wrote:

On Mon, 17 May 2021 21:11:45 +0100, T i m wrote:

On Mon, 17 May 2021 11:00:05 -0400, legg wrote:

snip

Front
https://ibb.co/5McshGF

Rear
https://ibb.co/w6Wq6hj


You're running at 220V? OK

Check.

Make sure the choke ends aren't touching as they leave the toroid
center.

Do you mean the opposite ends RL, in as it passes past out? If so, no
they seem to be a good 2mm apart.

https://ibb.co/YQg7Lp4


The wires without magnet wire enamel look pretty close to me.


I think it may be my bad picture, IF we are talking about either of
the pairs of wires as they pass in and out of the ring choke?

You should be concerned that wires exiting core in different
directions are not visibly or physically separated.


Ah, right, then yes, there is a good 2mm between them.

Force
these apart with a flat screw driver and insert biz card
thick paper stock.


A stack of 5 biz cars would fall out. ;-)

Can I just confirm something please? The output choke seems to be
bi-wound, two wires wound side-by-side so in parallel, both going to
the same pad at each end so carrying the output current between them?

So, it wouldn't matter if the 'pairs' of wires touched as such, as
long as the input and output wires didn't as they will be at a
different potential / phase?


They're shorted on the printed wiring, so just simple 2wires in
parallel.

On closer examination of choke, I see it's been impregnated, so
there's little likelihood of fault due to enamel failure or
vibration.

Wires not shorting to core.

Measurable just with my DMM? (if so, I can't measure anything).


You're looking for discoloration, crumbling of core where magnet
wires are forced against core edges due to winding force.


Check.

Discoloration is sign of point-contact heating.


Well I think the output choke windings have been running hot as the
enamel looks darkened compared to those bits that were likely cooler
as they were in the air and had heatsinks in the form of the soldered
connections to the board but I can see no obvious signs of failure of
the enamel.

No sign of cracked cores.

Do I only have one on the board (the big one in the final stage (is
it?)) and if so, not that I can easily see?


If you don't hear grinding noise or see shifting, when manipulated,
then there's probably no issue.


There doesn't seem to be?


Anything serious would have showed up measuring Lp.

snip
Ok, I just pulled these had found:

The 100uF, 25V:
VLoss= 1.8%
Capacity= 90uF
ESR= initially 2.1 ohm then dropped to .69 when repeating the test.


These are often part of feedback network for simplest integrated
switcher ICs. Probably not the culprit, but low side of tolerance
on an electrolytic is indication of end-of-life. Changing esr can
indicated corroded/intermitent inner contact.


The 4.7uF, 50V:
VLoss= 3.0%
Capacity 4755nF
ESR= 5.5 Ohm and stayed around that after several repeats of the test.

So, is that our 'bad' cap?

snip
Small 4u7 can have esr in the mid-ohms range. Probably not an issue
on housekeeping supply, but parts with 1R esr will allow start up
always in cold weather. Check rectifier feeding it for typical
forward voltage. If open or shorted, the unit would only run
intermittently (short would affect Lp measurement, but only if
in-circuit)

Main transformer primary inductance of a forward converter
running off mains will be more than 400uH - typically 2mH.


Ok, I'll try that now ('strike whilst the iron is hot'). ;-)

https://ibb.co/N33cKSf

Lower values suggest shorting of windings or terminal
connections/traces.


Understood.

What I think is the primary (marked in red) was:
.4 ohms, 17.4mH.


That's ok as long as measuring units are correct (common mistake).
Fairly high value for this power level and topology, but only
low side limit is important.

Another winding on the primary side (marked yellow and going to the
small caps etc) showed:
.3 ohms, .08mH

FWIW, the output seems to be made of 3 windings in parallel (marked in
green and tested wired that way) measured:
.02 ohm, .25mH.


No surprises.

Integrated switchers can be finicky when it comes to self-generated
noise. This makes snubbers and clamps pretty important. As a last
resort, check primary parts wired across main primary winding and
secondary side RC before replacing the big integrated switch.

RL

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Old May 18th 21, 11:17 PM posted to sci.electronics.repair
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Default Faulty SMPSU.

On Tue, 18 May 2021 16:03:31 -0400, legg wrote:

snip

They're shorted on the printed wiring, so just simple 2wires in
parallel.


Ok, so just to increase the current carrying capacity or something
more subtle? I'm more of a DC / LV / serial datastream sorta guy and
know 'frequencies' can do weird stuff sometimes (skin effect etc). ;-)

On closer examination of choke, I see it's been impregnated, so
there's little likelihood of fault due to enamel failure or
vibration.


Good news, thanks.

snip

Ok, I just pulled these had found:

The 100uF, 25V:
VLoss= 1.8%
Capacity= 90uF
ESR= initially 2.1 ohm then dropped to .69 when repeating the test.


These are often part of feedback network for simplest integrated
switcher ICs. Probably not the culprit, but low side of tolerance
on an electrolytic is indication of end-of-life.


Noted.

Changing esr can
indicated corroded/intermitent inner contact.


Ok, and cheap / easy enough to replace so ...


The 4.7uF, 50V:
VLoss= 3.0%
Capacity 4755nF
ESR= 5.5 Ohm and stayed around that after several repeats of the test.

So, is that our 'bad' cap?

snip
Small 4u7 can have esr in the mid-ohms range. Probably not an issue
on housekeeping supply, but parts with 1R esr will allow start up
always in cold weather.


Ok.

Check rectifier feeding it for typical
forward voltage. If open or shorted, the unit would only run
intermittently (short would affect Lp measurement, but only if
in-circuit)


D7 checks out as a std rectifier on my DMM (ohms Diode test).

Main transformer primary inductance of a forward converter
running off mains will be more than 400uH - typically 2mH.


Ok, I'll try that now ('strike whilst the iron is hot'). ;-)

https://ibb.co/N33cKSf

Lower values suggest shorting of windings or terminal
connections/traces.


Understood.

What I think is the primary (marked in red) was:
.4 ohms, 17.4mH.


That's ok as long as measuring units are correct (common mistake).


My tester actually displays the values as quoted so I'm happy there.
;-)

Fairly high value for this power level and topology, but only
low side limit is important.


Ok.

Another winding on the primary side (marked yellow and going to the
small caps etc) showed:
.3 ohms, .08mH

FWIW, the output seems to be made of 3 windings in parallel (marked in
green and tested wired that way) measured:
.02 ohm, .25mH.


No surprises.


Good news. The txfmr also 'looks' ok FWIW, no burn marks etc.

Integrated switchers can be finicky when it comes to self-generated
noise.


So that's the 5 pin TOxxx case device?

This makes snubbers and clamps pretty important.


Do you mean the small electrolytic's, diode and other caps (is that
what they are / doing)?

As a last
resort, check primary parts wired across main primary winding and
secondary side RC before replacing the big integrated switch.


So are we still on for replacing the opto device RL? I don't think my
tester can do that, other than the opto side as a diode possibly and
if it wasn't so cheap and already fairly old, I'd set up a test for it
myself on some breadboard. ;-)

Just to give me a better visualisation on all this, could you point me
to a schematic of something similar perchance please?

Cheers, T i m
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Old May 19th 21, 02:58 AM posted to sci.electronics.repair
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Default Faulty SMPSU.

On Tue, 18 May 2021 23:17:32 +0100, T i m wrote:

On Tue, 18 May 2021 16:03:31 -0400, legg wrote:

snip

snip

So are we still on for replacing the opto device RL? I don't think my
tester can do that, other than the opto side as a diode possibly and
if it wasn't so cheap and already fairly old, I'd set up a test for it
myself on some breadboard. ;-)


Optos get leaky, light seals break. - in most feedback loops that
means it's turning the circuit off without assistance from the TL431.


Just to give me a better visualisation on all this, could you point me
to a schematic of something similar perchance please?

Cheers, T i m


https://www.power.com/design-support...gn-methodology

You'd probably get better app notes chasing down the actual
part number in the unit.

RL
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Old May 19th 21, 09:13 PM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:
snip

I assume the 3 legged device on the LV side is a switching transistor


TNo, that has to be the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test. Sometimes there's two diodes, center
pin common, in this form factor.


Update: Whilst waiting for the smaller components to turn up I've de
soldered the two devices on the rear of the board to check the part
numbers and (as you say), the three legged one is a rectifier, a
'30H150C' and I've tested it with my DMM (diode test) and it checks
out ok on that (centre pin to both outsides, both ways round etc). I
hope it's good under load as they don't look as easy to find as the
switcher, a KA1M0800?

Cheers, T i m


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Old May 20th 21, 01:32 PM posted to sci.electronics.repair
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Posts: 436
Default Faulty SMPSU.

On Wed, 19 May 2021 21:13:02 +0100, T i m wrote:

On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:
snip

I assume the 3 legged device on the LV side is a switching transistor


TNo, that has to be the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test. Sometimes there's two diodes, center
pin common, in this form factor.


Update: Whilst waiting for the smaller components to turn up I've de
soldered the two devices on the rear of the board to check the part
numbers and (as you say), the three legged one is a rectifier, a
'30H150C' and I've tested it with my DMM (diode test) and it checks
out ok on that (centre pin to both outsides, both ways round etc). I
hope it's good under load as they don't look as easy to find as the
switcher, a KA1M0800?

Cheers, T i m


Any change in performance with ANY of these part subs?

RL


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Old May 21st 21, 10:48 AM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

On Thu, 20 May 2021 08:32:09 -0400, legg wrote:

On Wed, 19 May 2021 21:13:02 +0100, T i m wrote:

On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:
snip

I assume the 3 legged device on the LV side is a switching transistor

TNo, that has to be the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test. Sometimes there's two diodes, center
pin common, in this form factor.


Update: Whilst waiting for the smaller components to turn up I've de
soldered the two devices on the rear of the board to check the part
numbers and (as you say), the three legged one is a rectifier, a
'30H150C' and I've tested it with my DMM (diode test) and it checks
out ok on that (centre pin to both outsides, both ways round etc). I
hope it's good under load as they don't look as easy to find as the
switcher, a KA1M0800?


Any change in performance with ANY of these part subs?

I'm still waiting for the small caps (might be here today, whilst I
have some stock of caps, I never seem to have the spec I want), and I
intend, assuming this wouldn't risk any other stuff, to replace the
two caps, then opto, voltage reg then switcher in that order (as I
believe that was the order you mentioned the likely cause)?

The idea being that it might be interesting to see what it actually
was?

As an aside, I replaced the 4 x 1000uF, 105, LESR output smoothing
caps with new caps but with the wrong wire spacing (I was jumping
about on eBay and clicked the wrong one) but have some good s/h 1500uF
(16V 105, LESR) caps with the right pin spacing that would fit better.
Would using higher capacity be 'better', given it's soft start etc or
would it negatively influence the L-C of the output and so end up
being 'worse'?

Cheers, T i m

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Old May 21st 21, 04:06 PM posted to sci.electronics.repair
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Posts: 436
Default Faulty SMPSU.

On Fri, 21 May 2021 10:48:32 +0100, T i m wrote:

On Thu, 20 May 2021 08:32:09 -0400, legg wrote:

On Wed, 19 May 2021 21:13:02 +0100, T i m wrote:

On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:
snip

I assume the 3 legged device on the LV side is a switching transistor

TNo, that has to be the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test. Sometimes there's two diodes, center
pin common, in this form factor.

Update: Whilst waiting for the smaller components to turn up I've de
soldered the two devices on the rear of the board to check the part
numbers and (as you say), the three legged one is a rectifier, a
'30H150C' and I've tested it with my DMM (diode test) and it checks
out ok on that (centre pin to both outsides, both ways round etc). I
hope it's good under load as they don't look as easy to find as the
switcher, a KA1M0800?


Any change in performance with ANY of these part subs?

I'm still waiting for the small caps (might be here today, whilst I
have some stock of caps, I never seem to have the spec I want), and I
intend, assuming this wouldn't risk any other stuff, to replace the
two caps, then opto, voltage reg then switcher in that order (as I
believe that was the order you mentioned the likely cause)?

The idea being that it might be interesting to see what it actually
was?

As an aside, I replaced the 4 x 1000uF, 105, LESR output smoothing
caps with new caps but with the wrong wire spacing (I was jumping
about on eBay and clicked the wrong one) but have some good s/h 1500uF
(16V 105, LESR) caps with the right pin spacing that would fit better.
Would using higher capacity be 'better', given it's soft start etc or
would it negatively influence the L-C of the output and so end up
being 'worse'?

Cheers, T i m


You are, occasionally, reapplying power to check function?

RL
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Old May 21st 21, 04:08 PM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

On Fri, 21 May 2021 10:48:32 +0100, T i m wrote:

snip

Any change in performance with ANY of these part subs?

I'm still waiting for the small caps (might be here today, whilst I
have some stock of caps, I never seem to have the spec I want), and I
intend, assuming this wouldn't risk any other stuff, to replace the
two caps, then opto, voltage reg then switcher in that order (as I
believe that was the order you mentioned the likely cause)?

Update: The caps arrived this morning so I re-fitted the txfmr, the
switcher and rectifier, then fitted the new caps and tested it. DMM
shows around 10V on the DC range (as before off load) but it's jumping
about a lot and about .5V on AC.

I then replaced the opto and then the voltage reg with the output
looking the same each time (allowing for the voltage on the main caps
to bleed down each time etc).

So are we now looking at the switcher itself and / or is there
anything else I can check in the meantime please?

Cheers, T i m


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Old May 21st 21, 04:13 PM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

On Fri, 21 May 2021 11:06:41 -0400, legg wrote:

snip


You are, occasionally, reapplying power to check function?


I think we just keyed up at the same time? ;-)

After re-assembling and between replacing the components, yes.

Cheers, T i m
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Old May 22nd 21, 10:30 AM posted to sci.electronics.repair
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First recorded activity by DIYBanter: Jul 2006
Posts: 13,431
Default Faulty SMPSU.

On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:

snip

I assume the 3 legged device on the LV side is a switching transistor


TNo, that has to be the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test. Sometimes there's two diodes, center
pin common, in this form factor.


Coming back to this before buying / replacing the switcher, I tested
it out of circuit on my DMM (diode test, all ways etc) and it seemed
to check out ok?

Whilst rummaging though my spares last night I came across a similar
looking device (with the two diodes facing the centre pin marked on
the case) so assuming the spec checks out similar (at least 12V etc),
should I try swapping that in and see what happens, in case the other
is breaking down in use?

FWIW I measure about 10V DC on the output (no load) and only about .5V
on the AC range (ripple / noise)?

Cheers, T i m


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