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 17th 21, 02:38 AM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

Hi all, I know there are some very experienced / SMPSU folk on here so
I'll run this by you if I may.

The PSU on the MendelMax 3D printer I built with / for a friend over 5
years ago (then bought off him) died the other day. ;-(

I noticed it had stopped towards the end of a 90 minute print and the
printer was off (very unusual).

I power cycled it and it appeared to come up again, but when I told it
to preheat (extruder and bed) it nearly immediately powered down.

Disconnecting the heated bed, the printer would both power up and pre
heat the extruder but the software then halted the printer because it
noticed the bed wasn't heating. ;-)

So I dug out an old linear 13.5 x 12A PSU I had from my Packet radio
station and used that to power the heated bed and the printer then ran
ok and I was able to re-print the job.

When I powered it down it didn't seem to go off as normal and after
taking it out and removing the top, I noticed one bulging cap (one of
4 x 1000uF, 16V) on the output / smoothing stage, so replaced all 4.
Alas (and sorta as expected), it didn't really help so I'm not sure
what to do (and if it's worth doing) now (knowing how little I know
about SMPSUs).

However, I am fairly familiar with working on stuff that has mains and
have been a component level field support tech for some of my career
(Tele/datacomms/Microfilm and built all my own disco lighting) so
don't mind giving stuff a go, if anyone is willing to point me in the
right direction and thinks it would be worthwhile.

This is the unit out of it's frame.

Front
https://ibb.co/5McshGF

Rear
https://ibb.co/w6Wq6hj

I think it had a small fan, stuck to the side of the big transformer
but it got noisy so we took it out and the PSU only used to run warm.

That said, with everything on I think it was pulling current close to
it's max rating so we can't complain that it's lasted that this long
(heated bed ~10A (measured 11.5 on 13.5V)), extruder 3.5A, 5 x stepper
motors at 1A each and a few fans and the Arduino Mega and LCD display
etc).

After changing the caps, I ran it up without a load and it was a
'flaky' 12V (onboard LED on but the voltage was waving about a bit)
and when I fitted a 5A load, it wouldn't start up at all.
(crowbarred?).

I've measured the bridge (diode check on my DMM) and that seems ok. No
strange noises, I did sense that 'hot electronics' smell when going
back to the printer and it looks like the windings of the output choke
thing are darkened but nothing obviously burned or blown.

I assume the 3 legged device on the LV side is a switching transistor
/ FET of some sort and the 5 legged thing on the HV side, some sort of
SMPSU controller device / IC? I can't tell what they are without
desoldering them (but can do so etc).

R7 looks like it's been warm but measured at around 4K7 I think.

As an aside, a quick Google for LJYD-0540 suggests (and it's name g)
it, or the PCB was designed to be a 5V 40A PSU and so has been built
into a 12V 20A model somewhere along the line?

Cheers, T i m

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Old May 17th 21, 05:00 PM posted to sci.electronics.repair
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Posts: 436
Default Faulty SMPSU.

On Mon, 17 May 2021 01:38:56 +0100, T i m wrote:

Hi all, I know there are some very experienced / SMPSU folk on here so
I'll run this by you if I may.

The PSU on the MendelMax 3D printer I built with / for a friend over 5
years ago (then bought off him) died the other day. ;-(

I noticed it had stopped towards the end of a 90 minute print and the
printer was off (very unusual).

I power cycled it and it appeared to come up again, but when I told it
to preheat (extruder and bed) it nearly immediately powered down.

Disconnecting the heated bed, the printer would both power up and pre
heat the extruder but the software then halted the printer because it
noticed the bed wasn't heating. ;-)

So I dug out an old linear 13.5 x 12A PSU I had from my Packet radio
station and used that to power the heated bed and the printer then ran
ok and I was able to re-print the job.

When I powered it down it didn't seem to go off as normal and after
taking it out and removing the top, I noticed one bulging cap (one of
4 x 1000uF, 16V) on the output / smoothing stage, so replaced all 4.
Alas (and sorta as expected), it didn't really help so I'm not sure
what to do (and if it's worth doing) now (knowing how little I know
about SMPSUs).

However, I am fairly familiar with working on stuff that has mains and
have been a component level field support tech for some of my career
(Tele/datacomms/Microfilm and built all my own disco lighting) so
don't mind giving stuff a go, if anyone is willing to point me in the
right direction and thinks it would be worthwhile.

This is the unit out of it's frame.

Front
https://ibb.co/5McshGF

Rear
https://ibb.co/w6Wq6hj

I think it had a small fan, stuck to the side of the big transformer
but it got noisy so we took it out and the PSU only used to run warm.

That said, with everything on I think it was pulling current close to
it's max rating so we can't complain that it's lasted that this long
(heated bed ~10A (measured 11.5 on 13.5V)), extruder 3.5A, 5 x stepper
motors at 1A each and a few fans and the Arduino Mega and LCD display
etc).

After changing the caps, I ran it up without a load and it was a
'flaky' 12V (onboard LED on but the voltage was waving about a bit)
and when I fitted a 5A load, it wouldn't start up at all.
(crowbarred?).

I've measured the bridge (diode check on my DMM) and that seems ok. No
strange noises, I did sense that 'hot electronics' smell when going
back to the printer and it looks like the windings of the output choke
thing are darkened but nothing obviously burned or blown.

I assume the 3 legged device on the LV side is a switching transistor
/ FET of some sort and the 5 legged thing on the HV side, some sort of
SMPSU controller device / IC? I can't tell what they are without
desoldering them (but can do so etc).

R7 looks like it's been warm but measured at around 4K7 I think.

As an aside, a quick Google for LJYD-0540 suggests (and it's name g)
it, or the PCB was designed to be a 5V 40A PSU and so has been built
into a 12V 20A model somewhere along the line?

Cheers, T i m


You're running at 220V? OK

Make sure the choke ends aren't touching as they leave the toroid
center. Wires not shorting to core. No sign of cracked cores.

Check big primary bulk electros - voltages should be roughly
equal with small low-frequency ripple.

Replace small electros on primary side, then optocoupler.

A basic forward converter. Integrated primary switch should
be replaceable, but check transformer Lp first.

RL
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Old May 17th 21, 10:11 PM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

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

Wires not shorting to core.


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

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?

Check big primary bulk electros - voltages should be roughly
equal


I was inspecting those earlier and noted the caps were spanning an
'island' and each cap only 200V (so in series where the working
voltages add)?

with small low-frequency ripple.


Would AC volts on my DMM be ok for that RL (I do have a USB scope but
I wouldn't want to use it on mains and I haven't modded it to be AC
coupled yet).

Replace small electros on primary side,


The ones near C18? (just for reference, C16 is 100uF, 25V and C17 is
4.7uF, 50V). I have one of those automatic component testers. Would an
'ok' from one of those be ok?

then optocoupler.


An EL817. [1] (there is a TL431A (voltage reg?) beside the opto FWIW).


A basic forward converter.


I agree it doesn't look 'overpopulated' as such (thank goodness!). ;-)

Integrated primary switch should
be replaceable,


Is that 5 legged device on the underside?

but check transformer Lp first.


Sorry, you have lost me on that one. ;-(

Cheers, T i m

[1] When I was a datacomms guy we used to make and sell a Current loop
'Line driver' that used an opto for the receiver. They would often be
sent back marked as faulty, we would bench test them and loop the
output back to the input and they would often pass (BERT). Heat, cool,
tapping, still ok. Then I designed and built a small box where we
could vary the input current to better represent true line (loss)
conditions and lo-and-behold, some of those that were NFF now failed
because their sensitivity had dropped, putting the sense threshold
right near the TX current level. We found a new opto would trigger at
around 10mA, the max current from the TX was 20mA and some of these
returned 'faulty' ones were nearer 18. I had to build testers for all
the techs. ;-)
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Old May 17th 21, 10:22 PM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

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

snip

Check big primary bulk electros - voltages should be roughly
equal with small low-frequency ripple.


166V across each, no real measurable ripple (off load of course).

Cheers, T i m
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Old May 17th 21, 10:47 PM posted to sci.electronics.repair
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First recorded activity by DIYBanter: Jun 2020
Posts: 15
Default Faulty SMPSU.

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

Wires not shorting to core.


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

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?

Check big primary bulk electros - voltages should be roughly
equal


I was inspecting those earlier and noted the caps were spanning an
'island' and each cap only 200V (so in series where the working
voltages add)?

with small low-frequency ripple.


Would AC volts on my DMM be ok for that RL (I do have a USB scope but
I wouldn't want to use it on mains and I haven't modded it to be AC
coupled yet).

Replace small electros on primary side,


The ones near C18? (just for reference, C16 is 100uF, 25V and C17 is
4.7uF, 50V). I have one of those automatic component testers. Would an
'ok' from one of those be ok?

then optocoupler.


An EL817. [1] (there is a TL431A (voltage reg?) beside the opto FWIW).


A basic forward converter.


I agree it doesn't look 'overpopulated' as such (thank goodness!). ;-)

Integrated primary switch should
be replaceable,


Is that 5 legged device on the underside?

but check transformer Lp first.


Sorry, you have lost me on that one. ;-(

Cheers, T i m

[1] When I was a datacomms guy we used to make and sell a Current loop
'Line driver' that used an opto for the receiver. They would often be
sent back marked as faulty, we would bench test them and loop the
output back to the input and they would often pass (BERT). Heat, cool,
tapping, still ok. Then I designed and built a small box where we
could vary the input current to better represent true line (loss)
conditions and lo-and-behold, some of those that were NFF now failed
because their sensitivity had dropped, putting the sense threshold
right near the TX current level. We found a new opto would trigger at
around 10mA, the max current from the TX was 20mA and some of these
returned 'faulty' ones were nearer 18. I had to build testers for all
the techs. ;-)

To test C16 and 17 you would need an ESR tester. It is a pretty good
bet that both are bad. Make sure that you use 105C caps.


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Old May 18th 21, 12:43 AM posted to sci.electronics.repair
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Posts: 2,017
Default Faulty SMPSU.

On Sunday, May 16, 2021 at 5:38:56 PM UTC-7, T i m wrote:
Hi all, I know there are some very experienced / SMPSU folk on here so
I'll run this by you if I may.

The PSU on the MendelMax 3D printer I built with / for a friend over 5
years ago (then bought off him) died the other day. ;-( ...


When I powered it down it didn't seem to go off as normal and after
taking it out and removing the top, I noticed one bulging cap (one of
4 x 1000uF, 16V) on the output / smoothing stage, so replaced all 4.
Alas (and sorta as expected), it didn't really help


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.
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Old May 18th 21, 11:15 AM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

On Mon, 17 May 2021 15:47:30 -0500, Chuck
wrote:

snip

Replace small electros on primary side,


The ones near C18? (just for reference, C16 is 100uF, 25V and C17 is
4.7uF, 50V). I have one of those automatic component testers. Would an
'ok' from one of those be ok?


snip

To test C16 and 17 you would need an ESR tester.


Yes, I have one. Of the 4 x 1000uF smoothing caps across the output
(where one had blown) 3 had an ESR around 0.1 and the blown one a
touch higher? Is that right / normal (FWIW they also showed a Vloss of
~2% and were all around 1100uF)? [1]

It is a pretty good
bet that both are bad.


Ok, I'll pull test / replace those next.

Make sure that you use 105C caps.


Check (I did so with the 4 smoothing caps, but didn't check the lead
spacing on my order (doh)).

Cheers, T i m

[1] Could it be bulging because it just happened to be the closest to
the output choke and that had been running fairly hot, rather than it
had failed by drying out or whatever? (they were nichicon 105 1000uF
16V YV(M) H1305)
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Old May 18th 21, 11:27 AM 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:

On Sunday, May 16, 2021 at 5:38:56 PM UTC-7, T i m wrote:
Hi all, I know there are some very experienced / SMPSU folk on here so
I'll run this by you if I may.

The PSU on the MendelMax 3D printer I built with / for a friend over 5
years ago (then bought off him) died the other day. ;-( ...


When I powered it down it didn't seem to go off as normal and after
taking it out and removing the top, I noticed one bulging cap (one of
4 x 1000uF, 16V) on the output / smoothing stage, so replaced all 4.
Alas (and sorta as expected), it didn't really help


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);


The two TO(xxx?) packaged devices under the board were bolted to the
square U section case with insulators. eg, the whole ally plate
chassis and perforated ally 'lid' would be a heatsink.

OOI, as it seems this PCB was originally designed to be 5V / 40A, so
is it likely they would have used the same component there for 12V /
20A (I know it has to dissipate the same number of watts but wondered
if it might be able to handle 20A easier if it was originally specced
to take 40A)?

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.


Ah. ;-(

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test.


Understood.

Sometimes there's two diodes, center
pin common, in this form factor.


Yes, I think I've seen such on the various SMPSU diagrams I've been
checking. ;-)

As advised elsewhere, I think I'll pull and test (and possibly replace
*anyway*) the two small electrolytics first and then move to the
rectifier.

FWIW, I do read around 10V DC on the output but it's fluctuating. I
could put my PC digiscope on it (after checking the output for AC on
my DMM)?

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Old May 18th 21, 03:33 PM posted to sci.electronics.repair
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Posts: 436
Default Faulty SMPSU.

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.
You should be concerned that wires exiting core in different
directions are not visibly or physically separated. Force
these apart with a flat screw driver and insert biz card
thick paper stock.

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.
Discoloration is sign of point-contact heating.

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.

Check big primary bulk electros - voltages should be roughly
equal


I was inspecting those earlier and noted the caps were spanning an
'island' and each cap only 200V (so in series where the working
voltages add)?

with small low-frequency ripple.


Would AC volts on my DMM be ok for that RL (I do have a USB scope but
I wouldn't want to use it on mains and I haven't modded it to be AC
coupled yet).


Your later description sound like these are OK. Large voltage
imbalance or large ripple would indicate failure of one or both.

Replace small electros on primary side,


The ones near C18? (just for reference, C16 is 100uF, 25V and C17 is
4.7uF, 50V). I have one of those automatic component testers. Would an
'ok' from one of those be ok?

then optocoupler.


An EL817. [1] (there is a TL431A (voltage reg?) beside the opto FWIW).


A basic forward converter.


I agree it doesn't look 'overpopulated' as such (thank goodness!). ;-)

Integrated primary switch should
be replaceable,


Is that 5 legged device on the underside?

but check transformer Lp first.


Sorry, you have lost me on that one. ;-(

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

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

RL
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Old May 18th 21, 08:16 PM posted to sci.electronics.repair
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Posts: 13,431
Default Faulty SMPSU.

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?

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?

snip

Replace small electros on primary side,


The ones near C18? (just for reference, C16 is 100uF, 25V and C17 is
4.7uF, 50V). I have one of those automatic component testers. Would an
'ok' from one of those be ok?


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.

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

but check transformer Lp first.


Sorry, you have lost me on that one. ;-(

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.

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.

Does any of that help?

Cheers, T i m


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