Thread: Faulty SMPSU.
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legg legg is offline
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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