PM3055 Oscilloscope PSU failure (Don't leave your scope turned on)

[(*steve*)]

A great day.

I have measured the resistance of the various resistors (including a couple of 0.01% and 0.005% resistors I have for calibration purposes) to get the baseline resistance of the wirewound resistors so I can try to measure their inductance.

Then, as I prepare to wire up a signal generator and my scope I notice that my scope is turned on... But no lights.

It appears I have left it on. We also had a power outage a while ago and that may not have been to the liking of the scope.

The 1.6A fuse is blown and on replacement I get momentary lights, a glow from within the case, an acrid smell, and the fuse blows...

Oh well. I now have a different job for this weekend...

 

[(*steve*)]

Real time repair thread

I was taking some photos and SWMBO asked what I was doing. The conversation went like this:

SWMBO: What are you doing?
(*steve*): My scope died. Now I'm going to try to fix it.
SWMBO: You haven't had it very long?
(*steve*): It was second hand.
SWMBO: How much is a new one?
(*steve*): [quotes price] -- We can't afford one right now...
SWMBO: (thinks hard) We probably could...

Bless her...

(At present about 100% of our income goes to her medical bills, so no, we can't afford one)

 

[(*steve*)]

OK, so here are the fuses:



In retrospect I shouldn't really have hoped that I was going to get any other result.

However I do have a nice fresh smell to track down.

Here is the scope with the top cover removed:



The power supply runs along the top of the image. The drama happened at the top right corner of the image.

And here is the cover:



It's oriented so the top right of this image is the top right of the other image, however the right edge of the cover as shown goes along the top of the scope as gown previously.

Clearly there's been some action happening.

For anyone playing along at home, the service manual is located here: http://www.qsl.net/vk5bar/AHARS-Resources/Philips PM3055/PM3055 Service Manual.pdf (Thanks QSL.net)

My first thoughts are that since I got some low voltage rail (the backlight of the LCD operated at normal intensity) then the power supply was at least partially working during the failure.

I haven't looked at the schematic yet, but that indicates that either there are multiple power supplies (quite likely) and that not all of them have failed, or that there is a single power supply and it has failed in a way that allows it to maintain a regulated output. That may be valuable input for later...

 

[davenn]

woohooo wonderful gal ya got there just say to her " its an early birthday pressie"

cheers
Dave

 

[(*steve*)]

Page 3-13 gives an overview of the power supply:



From this I am fairly happy that there may be just the one power supply. Not sure how it generates everything from 10V to 14kV, but I'll find out.

The drama happened very close to the incoming mains, so I'm hoping it's as simple as a dead rectifier or filter caps.

If it were a shorted mosfet in the switching power supply I would not expect to see the LCD backlight come on.

 

[(*steve*)]

And here is the power supply schematic... (pp 9-7 to 9-8)



Sure enough, it's a single (I almost wrote simple) power supply.

And I have confirmed (p10-8) that the backlight comes from the -12V rail.

OK, now to take a look at that board...

edit: update that schematic to one that's hopefully readable (take 2)

 

[(*steve*)]

For some amusement, look at the number of warnings and notes compared to actual instructions.

 

[(*steve*)]

"Push both clamping lips"

Hmmm. What does that mean?

The internet comes to the rescue! http://www.eevblog.com/forum/reviews/philips-pm3055-repair-and-teardown/

They're UNDERNEATH! OK, I'll remove the bottom cover too...

 

[(*steve*)]

Removing the bottom cover (which isn't the trivial process described in the manual) reveals some more "yuk"



It's obviously been here for a while, but it's still tacky. What components are full of sticky liquids?



The other side of the cover shows that this liquid has been sprayed around -- great. I am not looking forward to cleaning this up...

But having remove the bottom cover I can now release the clips (and that's not as easy as it could be either). The clips are clearly suffering from the ravages of time and are not in great shape. They're still in enough of one piece to hold the board though, but broken enough to get caught on things. Finally I can lift the power supply board part way out.

And I know where to look. What do you think has failed?

Can we take bets on an electrolytic capacitor?



Wow! What has failed (and we may have other failures too) is a 0.22uF X2 capacitor.

It's great that these fail safely, right?

I've seen these fail similarly before. They shouldn't fail short circuit, but...

 

[(*steve*)]

Aaagh! visitors. Oh well.

Here's some better shots of the carnage.



Should this be in the graveyard section? (No, I think it looks far worse than it is)



I've got a lot to clean up...



Hey! That's not supposed to have a hole in it!



Another shot of the capacitor I'll have to replace (I'll replace both). It was pretty obvious from the mess that one of the capacitors had allowed their insides to get outside. And does that stuff stink...



I don't think I'll be salvaging that cap. And I'm sure glad I have a schematic. I can't quite make out what's written on that other component!



Or that one

 

[(*steve*)]

And after a quick cleanup...



The capacitor obviously got really hot, but seemed only to take off the solder mask.

There's a few more places to clean up, but I'll do that with a toothbrush. The good thing is that the board isn't charred.

The plastic connector has charred a little but I have measured the resistance and there is no conductivity across the plastic, so I think I'm good to go there as well.

Some of the components can go back after they're cleaned, but I'll replace others (including the large one I haven't removed yet).

 

[(*steve*)]

I spent some time contemplating replacement parts, but in the end I've decided to use the same rectifier diodes again as they were not subject to an overload during this fault. The particular diodes are not easily obtainable, and whilst I can't see the point in using something so highly specced, I'm not going to second-guess them.

The replacement capacitors are proving to be a larger issue. The problem is finding something that will fit. The originals have three leads, and the two active leads are not of a standard spacing. That means I'll have to bend the leads and kinda leave them dangling. I'm not too happy about that, but the scope is not subject to a lot of vibration, so tit should be fine. 68uF x 2 seems a very low filter value, but it is combined with a number of inductors, so again, I'm not going to second guess them.

One component has me stumped though.



That, does not look like this:



Unfortunately that is the best photo I can find anywhere on the net and it's too small to make out the markings.

This component is not shown on the board overlay in the service manual I have.

 

[(*steve*)]

This looks like a reasonable cap replacement: http://australia.rs-online.com/web/p/products/6997559/

 

[(*steve*)]

I think that mystery component is a 1.5nF capacitor. But what voltage rating?

 

[(*steve*)]

I can't get a 1.5nF capacitor with an appropriately high voltage rating (easily). I'm thinking one of the following:

1nF	3kV
2.2nF	3kV
1nF in parallel with 0.47nF	3kV
3n3 in series with 3n3	3kV

This capacitor bridges the incoming neutral line to the ground rail of the power supply.

 

[(*steve*)]

After a bit more of a thorough cleanup, the board looks better. But the more I clean up, the more I see that needs to be cleaned up. I think it's good enough (but I will have to pull out the solder sucker)

 

[(*steve*)]

Due to the timing of arrival of parts and a think about the things I had on hand, I decided to replace the 1.5nF 3kV cap with a 1.5nF 2kV cap. This will only be stressed if someone wants to disconnect the earth so that they can have the scope's ground floating.

That someone is not likely to be me.

I also decided to replace the diodes with a bridge rectifier I have.

And I thought I may as well replace the two 0.1uF X2 rated caps that had been affected by the electrolyte.



I was considering placing some high temperature tape over the exposed board, but the replacement 0.22uF cap has slightly wider spaced leads, so it sits slightly proud of the board anyway.

And the bang test... Total anticlimax. It took me a while to get over the lack of a bang or smoke and notice the scope was actually working.

 

[rhaugen]

Good job Steve. I enjoyed following your progress. And I, like you, would rather fix than replace parts when possible. It is amazing how many electronic items are just discarded into land fills etc just because of a few dollars worth of parts. Well done.

 

[kpatz]

Nice repair job!

And the bang test... Total anticlimax. It took me a while to get over the lack of a bang or smoke and notice the scope was actually working.

That made me think of Marvin the Martian: "Where's the kaboom? There was supposed to be an earth-shattering kaboom!"

 

[(*steve*)]

Good job Steve. I enjoyed following your progress. And I, like you, would rather fix than replace parts when possible. It is amazing how many electronic items are just discarded into land fills etc just because of a few dollars worth of parts. Well done.

Thanks. In this case I fixed it because I had to. But yeah, I hate to see good devices being thrown away for the want of a cheap repair.

I have a workshop full of them waiting for me to have time to fix them.

Nice repair job!That made me think of Marvin the Martian: "Where's the kaboom? There was supposed to be an earth-shattering kaboom!"

You know, I was fearing it so much I was actually expecting it to happen. My first thought when it powered on silently was "what's wrong now?"