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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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On Saturday, May 22, 2021 at 2:31:00 AM UTC-7, T i m wrote:
On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:


... 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.


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?


They almost always fail with a dead short, couldn't miss that with
a diode check ( 0.2 to 0.8V is a typical good-rectifier reading forward,
and no conduction in reverse).

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


Since the capacitors on output were replaced, are you sure they're low-ESR types?
The ripple current ratings have to add up to about the output (20A) of the unit.
Correct capacitance but low ripple current ratings (aka high ESR) on the replacements
could explain bad output with good rectifier.
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On Sat, 22 May 2021 18:07:17 -0700 (PDT), whit3rd
wrote:

On Saturday, May 22, 2021 at 2:31:00 AM UTC-7, T i m wrote:
On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:


... 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.


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?


They almost always fail with a dead short, couldn't miss that with
a diode check ( 0.2 to 0.8V is a typical good-rectifier reading forward,
and no conduction in reverse).


Agreed. I think I read .2xxV on my DMM diode test in the forward (x2)
and as you say nothing in the reverse (x2 + 2 (between the two
outers)).

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


Since the capacitors on output were replaced, are you sure they're low-ESR types?


I believe they we

"Panasonic 1000uf 16v Electrolytic Capacitor 105 LOW ESR FR Series
EEUFR1C102L" ?

https://industrial.panasonic.com/ww/...ls/EEUFR1C102L

The ripple current ratings have to add up to about the output (20A) of the unit.
Correct capacitance but low ripple current ratings (aka high ESR) on the replacements
could explain bad output with good rectifier.


(Interesting, thanks) From that spec the only value I can see related
to ripple is:

"Rated ripple current-1 (mA) 1560.0"

So the 4 would give 6.24A but would that still impact the off load
voltage?

I might see if I can rig up my USB scope and actually 'see' what the
output is doing.

Cheers, T i m

p.s. Because I need the 3D printer running I've since bought 2 x Mean
Well (SP-200-12) 12V x 16.5A PSUs and have tacked them in to be able
to get going (and need to design and print some new mounting brackets
g). However, whilst it looks like I could end up spending ~10 GBP
(more if you count to total cost when it's more sensible to buy 10 off
something when I only need one) to get this old PSU running (assuming
I ever do etc) and could buy another cheap one new for just a few
pounds more, I am interested to see if I can get it going (with the
guidance of some good folk here etc g), firstly, because I would
find it satisfying, second because I then have a spare PSU and third I
wouldn't them have to throw the rest away. ;-)
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