Extron RGB Matrix power supply repair (switch mode)

[Konsolkongen]

Hi. I bought this Extron VGA Matrix of ebay, and found that while it did work the audio was bad. I then discovered that some of the caps on the power supply was bulging and I could hear a fizzling noise coming from it. I replaced the the capacitors, checked orientation and that sound is now gone, but is replaced by a constant clicking sound. This is just plugging the PSU in to the wall outlet, I haven't tried it connected to the actual matrix, as I suspect something else is wrong it with.

The PSU is +/-15V.

Any ideas?

[Psi]

If it has no load connected the clicking might be normal as the controller shuts down, due to no load using power from output caps, and then restarts later once the output capacitor voltage has fallen enough to need a top-up.

Try connecting a load to the 15V output and see if the clicking goes away.

[Konsolkongen]

Thank you I don't have an electronic load I can test it with. Can I use something else to test it?

I do however have another Extron Matrix which uses a PSU that is 99% identical (same layout, but one of the caps are a different value). I can try that without load too tomorrow. If it clicks also then I guess it's a good sign that this one is working correctly, right?

[james_s]

Use lightbulbs. 12V automotive bulbs will be fine on 15V for testing purposes. It's very common for SMPSs to require a load in order to function properly, especially those that are designed to be tightly integrated into a product.

[Psi]

yep, automotive 12V lamp will be fine on 15V to test with.
Don't use a car headlamp but a normal interior light will be fine. Maybe a indicator/park/brake lamp if your psu can handle that current.??
But interior lamps are easy to get, pull the cover off and remove the lamp. Should take a few seconds.

[Konsolkongen]

The other power supply does not click when I test it without load. I disconnected it from the rest of the matrix switch (not shown in picture). This power supply is practically identical.

As someone who knows nothing about cars, are you talking about the bulb for the roof lighting that shines when you open the door on a car?

My landlord owns a car repair shop down the road so I could probably get them there. Do I need two, to test both plus and minus 15V at the same time, or would just one on +15V work?

[Psi]

If the other "good" one does not tick without a load then yes, the bad one is fault and the tick is a sign of that.

Probably a short circuit on one of the converters and the switchmode controller is tripping out from overcurrent/heat, and after a while trying to start again.

[Konsolkongen]

Is it likely that it only shorted out once I put new good capacitors in? It was working before that, although with that aforementioned fizzling sound.

By converters are you referring to the two IC's in TO-220 packages attached to the heatsink? Can I test these?

[Psi]

Is it likely that it only shorted out once I put new good capacitors in? It was working before that, although with that aforementioned fizzling sound.

By converters are you referring to the two IC's in TO-220 packages attached to the heatsink? Can I test these?

The TO-220 things attached to the heatsink are mosfets, the driver IC is likely that DIP8 package.
There is also a DIP6 optoisolator that i can see in the image

Replacing the capacitors shouldn't cause a problem, assuming they are all the right way around and approximately the same value

Could be a dead mosfet, or the chip that drives the mosfets has a blown output pin.
Could also be a shorted rectifier diode. Or maybe even the optoisolater has failed.

[Konsolkongen]

Is it likely that it only shorted out once I put new good capacitors in? It was working before that, although with that aforementioned fizzling sound.

By converters are you referring to the two IC's in TO-220 packages attached to the heatsink? Can I test these?

The TO-220 attached to the heatsink are mosfets, the driver IC is the little DIP8 packages,

Replacing the capacitors shouldn't cause a problem, assuming they are all the right way around and approximately the same value

Could be a dead mosfet, or the chip that drives the mosfets has a blown output pin.
Could also be a shorted rectifier diode. Or maybe even the optoisolater has failed.

They are oriented correctly and the values are the same as the originals

There is an IRF840 mosfet - and two CQ821G's on the output attached to the large heatsink, that I can't find a datasheet on. None of them have shorted pins if that tells you anything

I will check diodes

[CJay]

The TO-220 things attached to the heatsink are mosfets, the driver IC is likely that DIP8 package.
There is also a DIP6 optoisolator that i can see in the image

Replacing the capacitors shouldn't cause a problem, assuming they are all the right way around and approximately the same value

Could be a dead mosfet, or the chip that drives the mosfets has a blown output pin.
Could also be a shorted rectifier diode. Or maybe even the optoisolater has failed.

The MOSFET is connected to the black heatsink on the primary.

The parts connected to the silver heatsink are almost definitely rectifier diodes and a linear regulator.

If the PSU is clicking/chirping then the MOSFET on the heatsink is OK, the SMPSU "driver" chip is also working, it is extremely unlikely to click/chirp if they're not, the opto might be faulty but we are getting *WAY* ahead of ourselves here.

We need to go right back to the basics.

As the PSU was working before the caps were replaced then it *HIGHLY* likely that the cap replacement caused the problem so the single most important thing to do is re-check the work done before starting to speculate on more complex things and causing real damage that might end up unrepairable.

So:

Visual inspection first, look for poor soldering and shorts caused by that or stray component leads.

Check the orientation of the capacitors, compare them to the other working PSU if it's the same make/model or the board markings if not.

If the OP posts a decent picture of the underside of the board it will be obvious where the GND and output rails are so we can check cap orientation from that.

Take a multimeter and with the power removed form the board check the output pins for shorts to ground.

Report back results.

[Konsolkongen]

Here is a photo of the underside.

I see no shorts. Some of the pads lifted when I replaced the capacitors so I just bent the legs over to strengthen things.

No shorts on the output pins.