Dead psu where to start diagnosing?

[Groppo91]

I have a psu with unknown history it looked clean but I went ahead with a recap with a kit from this store. I have since tested for the first time and everything outputs 0v.

I know I probably should have tested prior to recap but was an error on my part. Is there any hope of saving this psu if so what are the common parts to test first and any tips on how to test? I currently only have a dvm to hand.

I have this photo of the top

IMG_20240329_160433.jpg

And this photo of underneath

IMG_20240329_160606.jpg

I can take some zoomed in photos of suspect parts/areas on request if that will help in diagnosing. The ceramic resistor might be faulty as I posted on another post but not sure if I was testing it correctly.

[rubber_jonnie]

First off to test this PSU you'll need load resistors, which I think you have. Just be careful to mount them somewhere where they can dissipate heat as they do get pretty hot.

Next obligatory warning - The heatsinks have high voltages on them, so DO NOT TOUCH when running the PSU for testing. Also the large cap on the high voltage side is pretty nasty when charged.

Safety first is the key here. Do as many checks as you can with power off and unplugged.

I always start with the switch. Does the socket to the switch buzz out for the live and neutral lines, and if you follow all the way to the PCB with the switch on, does it buzz out all the way.

Also is the fuse good? Also, the Bourns fusible resistor (The white ceramic block) can blow and if that's dead then the PSU won't work.

You may also want to remove the transistors one at a time and test them, either with a multimeter in diode mode or with a component tester. Do them one at a time to avoid mixing them up.

Is the mains voltage getting to the bridge rectifier? If the high voltage side is not working, then you need to figure that out first.

[Groppo91]

First off to test this PSU you'll need load resistors, which I think you have. Just be careful to mount them somewhere where they can dissipate heat as they do get pretty hot.

Next obligatory warning - The heatsinks have high voltages on them, so DO NOT TOUCH when running the PSU for testing. Also the large cap on the high voltage side is pretty nasty when charged.

Safety first is the key here. Do as many checks as you can with power off and unplugged.

I always start with the switch. Does the socket to the switch buzz out for the live and neutral lines, and if you follow all the way to the PCB with the switch on, does it buzz out all the way.

Also is the fuse good? Also, the Bourns fusible resistor (The white ceramic block) can blow and if that's dead then the PSU won't work.

You may also want to remove the transistors one at a time and test them, either with a multimeter in diode mode or with a component tester. Do them one at a time to avoid mixing them up.

Is the mains voltage getting to the bridge rectifier? If the high voltage side is not working, then you need to figure that out first.

Thankyou I will try and work through this list but may need some extra help.

1. Yes load resistors obtained from the store but were both 10w22ohmJ I believe I can still test with these I did try both one at a time for 12v and 5v rails but got no readings.

2. I have just had a go at testing the switch I assume I test across both black cables for continuety then switch on to see if a bleep occurs. Same with both white cables. That was the result I got so believe it to be a working switch unless I am missing something.

3. The glass fuse is good but I suspect the ceramic one might be dead as the readings were odd. Any recommendations for the replacement part?

4. I will work on testing the transistors when i get some more time.

5. Any tips on safely testing the bridge rectifier or the high voltage side in general. I havent tested a bridge rectifier since college in 2010 so fairly rusty on my knowledge and prefer to stay away from the high voltage side unless I know where to safely test it from amd how.

Hopefully we can get this one going again to save it from the scrap man.

[Groppo91]

I was looking at the schematic for the ceramic resistor labelled r101 and it is described as 8.2ohm 3W wire wound cement

Will any work as a replacement as the only one I have found look completely different.

Part to replace

IMG_20240329_161925.jpg

Part I can find (description states they come in 8.2ohm 3W)

864967669-985665747.jpg

[exxos]

I don't recall if the datasheet mentions it's a fuse as such, but when the main switching transistor fails, it blows that resistor just like a fuse. If you use a normal resistor and the transistor fails, there will be much smoke and likely a fire risk as you will have 400vdc with some amps from the capacitor into a short-circuit. So I strongly advise not using a normal resistor.

From what I remember mouser had some fusable resistors but I think there wasn't much selection. I was looking to sell replacements in my store but I didn't in the end. I don't remember why. I think they may have only have lower resistance types. But any fusable resistor would be better than a regular resistor.

[stephen_usher]

The only alternative would be to somehow use a normal resistor and a fuse. You would need to work out what the nominal current would be and then source the correct fuse.

[rubber_jonnie]

As the others have said, you can't replace that component with a straight resistor, it need to be a fusible resistor.

I did replace one in mine with a fusible resistor from Bourns and it worked just fine, but it was so long ago I can't remember which one I used.

There is some info here though: Bourns fusible power resistor

If this component has gone, then one of the transistors is likely to have gone too.

[Groppo91]

As the others have said, you can't replace that component with a straight resistor, it need to be a fusible resistor.

I did replace one in mine with a fusible resistor from Bourns and it worked just fine, but it was so long ago I can't remember which one I used.

There is some info here though: Bourns fusible power resistor

If this component has gone, then one of the transistors is likely to have gone too.

Ah it didnt mention being fusible on the schematics I will check out the bourns ones. So I take it the shape doesn't matter too much replacing the block type for the cylinder shape as long as it specs the same and says fusible?

Also the transistor I see it lists two on the schematics but not sure where they are. Is it the big looking block marked tdk in my original top shot image? Are they easy to source?

[rubber_jonnie]

As the others have said, you can't replace that component with a straight resistor, it need to be a fusible resistor.

I did replace one in mine with a fusible resistor from Bourns and it worked just fine, but it was so long ago I can't remember which one I used.

There is some info here though: Bourns fusible power resistor

If this component has gone, then one of the transistors is likely to have gone too.

Ah it didnt mention being fusible on the schematics I will check out the bourns ones. So I take it the shape doesn't matter too much replacing the block type for the cylinder shape as long as it specs the same and says fusible?

Also the transistor I see it lists two on the schematics but not sure where they are. Is it the big looking block marked tdk in my original top shot image? Are they easy to source?

It's certainly possible that the schematics don't show it as fusible, but it definitely is and it's fusible for a reason. I used a Bourns in one of mine.

No the item marked TDK is a transformer, the transistors are the items connected to the heatsinks and are better tested removed from the board, but shouldn't be mixed up. I would say that if you're not sure about what the components are, then you probably shouldn't be working on PSUs, they can bite you badly.

I don't mean this in a mean way by the way, but having been bitten by various PSUs over the years, my advice is usually if you're not sure, don't touch them. It's your choice of course, but I always err on the side of safety first.

[Groppo91]

Ah it didnt mention being fusible on the schematics I will check out the bourns ones. So I take it the shape doesn't matter too much replacing the block type for the cylinder shape as long as it specs the same and says fusible?

Also the transistor I see it lists two on the schematics but not sure where they are. Is it the big looking block marked tdk in my original top shot image? Are they easy to source?

It's certainly possible that the schematics don't show it as fusible, but it definitely is and it's fusible for a reason. I used a Bourns in one of mine.

No the item marked TDK is a transformer, the transistors are the items connected to the heatsinks and are better tested removed from the board, but shouldn't be mixed up. I would say that if you're not sure about what the components are, then you probably shouldn't be working on PSUs, they can bite you badly.

I don't mean this in a mean way by the way, but having been bitten by various PSUs over the years, my advice is usually if you're not sure, don't touch them. It's your choice of course, but I always err on the side of safety first.

Thankyou, I did electronics as part of my qualification but was back in 2010 so have a small understanding and feel happy testing it/ replacing part. One of the transistors has been swapped as part of the recap process so will test the remaining one when I get a moment.