Trouble shooting a switched power supply

[flussmedel]

Hi,
Have a Toko LW30-11 that gave up on me. When its under load i can hear i sound from one area, sounds like a relay that clicks quite high frequency. Unloaded board measures fine and no sound. I narrowed it down to one area around one inductor. I have soldered off the inductor to identify the black component in between the large caps (thermistor). See attached pic, could it be rectifier, large cap, the inductor or the thermistor? Film caps???
Any one could point out a suspect?

 

[radardoug]

Picture doesn't show.

 

[flussmedel]

Picture doesn't show.

Yeah I saw that now, don't know how to get it to work?

 

[flussmedel]

Fixed it

 

[radardoug]

The most common fault on switchmodes is the output caps drying up, and also the cap across the switching ic power pins. Next would be the dropper resistor to the swiching ic. If you are hearing little ticks it indicates the supply is trying to start.
I would pull all the electro caps and check them first.

 

[Whoops]

Unfortunately besides changing Lytic Caps I was never successful in repairing any SMPS.
Normally just spend a lot of time and not get anywhere.

Sometime it's easier and more cost-efficient to buy  a replacement supply than to troubleshoot a SMPS.

I wish you better luck than what I had

 

[Khron]

If unloaded it starts, but connected to the load it "ticks over", that's an overload-hiccup sign.

Check to see if the load isn't shorted out or something. Then i'd indeed replace the little capacitor(s) near the switching controller on the primary (to eliminate one variable, and to avoid further headaches down the road). Then the capacitors on the rail the feedback is taken from (if there's more than one output) could be out of spec, which will mess up the feedback / regulation loop.

 

[flussmedel]

Thanks, got some good advices here! Seems like the description of failing to start up is what happens.
So i would like to go through this in steps.
Chance for a shortage on a load (assume connecting boards are the load), if i disconnected each boards and it starts, could that indicates which board could have an issue or just that the load get lower and it has enough power to start?

Or should i assume that the issue is on the supply board and focus on that one?

There is NOS Toko NWE30-11 that i can get for aprox 100USD, the have the same spec's and seems to be a newer iteration (hopefully could be a replacement). At the same time it would be great to get this one up and running also.

Maybe just start with the caps except films. Start from load side? Its a bunch of them, especially on the load side (5V 15+/-)
Hate to change a lot and without knowing what fixed the problem

 

[Khron]

Disconnecting the two(?) boards could help - if the power supply still hiccups with one but not the other, the former would then be a suspect (as well). If it works with both individually, then that would indicate that both together are too much of a load for the supply, as it is.

These small flyback supplies usually take the feedback signal from only one of the rails (as opposed to, say, ATX computer supplies, where feedback is taken from all the rails).

If you do wish to start re-capping the load side, i'd recommend seeing if you can determine which rail is the feedback source, and re-cap only that (for starters, anyway).

 

[JohnRoberts]

Round up the usual suspects.....

I have seen capacitors fail short circuit (in the early days of low ESR caps), but caps can also fail open hi Z). Other suspects include diodes (typically fail shorted), discrete switches (can fail open or shorted look for weird gate behavior).  Then there is the controller chip, but look for an easy fault first.

JR

 

[flussmedel]

....
If you do wish to start re-capping the load side, i'd recommend seeing if you can determine which rail is the feedback source, and re-cap only that (for starters, anyway).

Not to familiar with switched supply's, I only have functional diagram where there is a couple of photocoupler picking up from +5V and go to the overload and control circuit. Is that the feedback source then (+5V)?
Thanks for the support!

 

[flussmedel]

Round up the usual suspects.....

I have seen capacitors fail short circuit (in the early days of low ESR caps), but caps can also fail open hi Z). Other suspects include diodes (typically fail shorted), discrete switches (can fail open or shorted look for weird gate behavior).  Then there is the controller chip, but look for an easy fault first.

JR

I will check, nothing obvious right now, need to replace and verify, could not measure any deviations on component level so far.

 

[Khron]

https://sound-au.com/articles/smps-primer.htm

And a few specifics on the flyback in particular: https://sound-au.com/articles/smps-primer.html#s5

 

[flussmedel]

https://sound-au.com/articles/smps-primer.htm

And a few specifics on the flyback in particular: https://sound-au.com/articles/smps-primer.html#s5

Thanks, and that answered my question as well, +5V or V1 in this case acts as feedback

 

[Khron]

I'm more of a "teach a man to fish" type of person But glad to hear that helped

Rod Elliott's site is a literal goldmine of tutorials, articles and projects Has been, even since before i started dabbling in this field, well over 15 years ago

 

[flussmedel]

Looking for caps now, low esr is what I need to look for I suspect?

 

[Khron]

Yes, although if possible, it's usually a good idea to stay within the ESR range of the original ones (assuming datasheets & specs can be found).

I myself prefer to stick with the Japanese brands (Panasonic, Rubycon, Elna, Nichicon, United ChemiCon), and don't really trust much else for longevity.

 

[JohnRoberts]

Yes, although if possible, it's usually a good idea to stay within the ESR range of the original ones (assuming datasheets & specs can be found).

I myself prefer to stick with the Japanese brands (Panasonic, Rubycon, Elna, Nichicon, United ChemiCon), and don't really trust much else for longevity.

+1 a modern low ESR capacitor used in an old design, could result in higher peak current than anticipated. 

Probably OK, but you'll find out if it isn't....

JR

 

[flussmedel]

+1 a modern low ESR capacitor used in an old design, could result in higher peak current than anticipated. 

Probably OK, but you'll find out if it isn't....

JR

Impossible to figure original caps spec's. Maybe i should give up this project, have respect for high voltage and bad nerves

If i source a new replacement unit, up to spec's, any issues i can get? It is used in a EMT 248 reverb, worried about hiccup's or noise etc.

 

[Khron]

Have you already thrown them away? Or how and why is it impossible?

Impossible to figure original caps spec's. Maybe i should give up this project, have respect for high voltage and bad nerves

If i source a new replacement unit, up to spec's, any issues i can get? It is used in a EMT 248 reverb, worried about hiccup's or noise etc.