Wier WA505 SMPSU output failed

[nath1977]

Hi,
Im about to repair a failed Wier WA505 power supply which hasn't been used for many years. It was working for 30 mins or so, then started to smell bad so switched it off quick.
On closer inspection a 1500uF cap across the 5v output has swollen. Adjacent to this is an overheated (open circuit) resistor.
Ive removed the electrolytic cap and resistor - the cap replacement is straightforward and will replace the adjacent ones at the same time. The resistor looks like it sits across the output. Any ideas what value this is likely to be? Was thinking 100R 1/2watt would be suitable.

I have removed the red 0.1uF too, as it looks to have suffered from the nearby heat.

Any advice very welcome. Thanks!

[CapLeaker]

Re: Wier WA505 SMPSU output failed

If it would be mine, I’d replace all the electrolytic capacitors. Since we don’t know anything much of that burnt resistor nor the value, look at where it is connected to. Is it a load resistor that goes from the output to gnd? Maybe take some pictures of the bottom side of the PCB. Also all the black stuff from the resistor has to be totally removed and the PCB repaired if needed.

[nath1977]

Re: Wier WA505 SMPSU output failed

Thanks - I failed to mention the resistor is also across the 5vdc output.

[CapLeaker]

Re: Wier WA505 SMPSU output failed

Yeah, the load resistor isn’t that critical value wise. 100 Ohm 2W is something I’d start with too. Whatever fits. Doesn’t look like there is much room in there to start with. The load resistor just needs to draw a little current for the PSU’s regulation to work properly. The PSU may need a little more or less load. It really depends on the PSU. You can play around with the values a bit. Some want more current others need less.

[nath1977]

Re: Wier WA505 SMPSU output failed

Thanks. Can I ask what would be the likely cause of this fault. I realise caps dry out with age and fail - so why did the resistor frazzle too? Will the output voltage have risen when the cap failed?

[eccerr0r]

Re: Wier WA505 SMPSU output failed

The load resistor will always generate heat, and yes if the capacitors go bad, voltage could spike and resistor takes more heat...and probably the main reason for failure. Most of the times, though the resistor blackens, survive, you should measure it and see what it comes back as.

Usually the resistor is sized to take a fraction of the total amperage of current that the power supply is rated. A 100Ω resistor on a 5V line (50mA, ¼W), unless that line was not rated for many amps, seems awfully low of a draw.

[CapLeaker]

Re: Wier WA505 SMPSU output failed

Can be various reasons why that resistor burnt up. Like I said before, the load resistor isn’t that critical value wise. Depending on what power supply and age, what regulation circuitry also play a role. There is no one resistor fits all approach here. Just play around with the resistance. Start with a 100 ohm and keep going down in resistance if you need more current in order for the PSU’s regulation circuit to stabilize. It’s not rocket science.

[nath1977]

Re: Wier WA505 SMPSU output failed

Having removed the resistor and measured it again, it’s 18k. Seems very high to have ever been that hot. I’m assuming this is a false reading as a result of it overheating.

[harp]

Re: Wier WA505 SMPSU output failed

If it is burned and have 18k, scrap the burned farbe till resistive layer on very middle on resistor, and with multimeter probe gently scrap around that point and see if you get some low ohm reading, at most only on one side. Multiple with two and use logic to determine standard value.

[nath1977]

Re: Wier WA505 SMPSU output failed

If it is burned and have 18k, scrap the burned farbe till resistive layer on very middle on resistor, and with multimeter probe gently scrap around that point and see if you get some low ohm reading, at most only on one side. Multiple with two and use logic to determine standard value.

What a top tip! After a little scraping and grinding I managed to get some readings. 100R it is! Thank you so much 😊

[nath1977]

Re: Wier WA505 SMPSU output failed

Still looking for some help if I can. Load resistor, and several output smoothing capacitors replaced.

I can now get all of the low current voltages present 24v, -5, 15v etc. However once loaded with say the cooling fans on the 24v, there isn't enough power to drive the fans. Also, the high current 5v is only reaching 3v.

No obvious faulty components elsewhere on the board. Any obvious things I need to go after first?

Thank you!

[CapLeaker]

Re: Wier WA505 SMPSU output failed

Hmm… maybe upload some straight shot high resolution pictures from the whole PSU. For shits and giggles, check with a kill a watt or similar how much power the PSU is drawing.

[nath1977]

Re: Wier WA505 SMPSU output failed

Pics enclosed. Thank you.

[stj]

Re: Wier WA505 SMPSU output failed

check the input smoothing caps maybe
after checking them for voltage - incase it has no discharge resistors!!!!!!!

[stj]

Re: Wier WA505 SMPSU output failed

what caps did you fit?

[CapLeaker]

Re: Wier WA505 SMPSU output failed

As stj already mentioned, if the main filter caps in the primary are bad, you won’t be getting anything much out of the secondary either.

[nath1977]

Re: Wier WA505 SMPSU output failed

what caps did you fit?

Rubycon 35v and 10v electrolytics on the outputs.

[stj]

Re: Wier WA505 SMPSU output failed

so only 2 or 3 then.
what series? you really need YXF or ZL or similar in a psu

[CapLeaker]

Re: Wier WA505 SMPSU output failed

so only 2 or 3 then.
what series? you really need YXF or ZL or similar in a psu

I couldn’t get any yxf on my last DigiKey order anymore. BTW, only because a capacitor looks good from the outside, doesn’t mean they are good.

[nath1977]

Re: Wier WA505 SMPSU output failed

so only 2 or 3 then.
what series? you really need YXF or ZL or similar in a psu

9 in total, Rubycon ZLK and some Samwah WL

I guess these are not top spec - Im not looking to run this thing extensively or put it in to live operation. Its a hobby repair to some old broadcasting kit I want to see running once in a while.

Using a DMM I can get 2000uF measured on those parallel input caps. I realise this isn't a rigorous test so should look at replacing them next.