Tek 2430A Magic smoke from low voltage power supply.

[johnh]

Powered up a TEK 2430A cro that i haven't used for a while.

Self test ok.

About to connect probe and the magic smoke comes out.

A couple of shots of the damage, one whilst it was still in the case and the other I pulled the power supply out.

A shot of the blown cap and badly burnt resistor.  Pulled the choke L715  to check it, couldn't tell if it was damaged.
Cleaned it up, removed the damaged label and all the soot and crap. 
LCR meter reads 68uH.  CCT diagram says 80uH   :

Cleaned the board, ready for repair.  Don't have the right replacement capacitor.

My question should I replace the mov's and arrestors?

[rexxar]

It looks like the capacitor failed short, which created an (almost) direct path between live and neutral. Did the fuse go? Though, maybe it wasn't drawing enough current to blow it. 5A is still a lot of current, certainly enough to vaporize those resistors  It looks like the MOVs and arrestors will be okay, they would be out of the current path, assuming C706 did go short. I'd go ahead and check C816, it might have taken a hit when the other one died. Those inductors would have been passing a whole lot of current, I don't think it would have damaged them, but I'm not sure.

Did it go bang? 

[DC5AJ]

Hi, I had the same issue a month ago. I have replaced the cap and the burned resistor, scope works. But look at the other X caps as well, the .068uF has a crack on it and on the other side of the board you will find two smaller ones. In my power supply they all had cracks. From other repairs I can tell these old WIMA caps failed very often.

[johnh]

There was short bang.  The other half didn't hear it, I had the study door closed.

[Orange]

This is a common failure on the 2400 series. The caps are metalized paper types from RIFA or WIMA. Once they get cracked, they start to collect moisture.....

It's best that you replace all the RIFA caps with MKT (polyester film) X2 (Y1) types. Apart from the resistor no damage is done in most cases.

 

[johnh]

Look like like I will have to order the parts from RS or E14 as the local supplier Jaycar  doesn't stock a large range

[lowimpedance]

On a different note while you have it apart. Might be a good idea to check the switch mode power supply electro's. I did a repair some time ago on a 2430 that would not start up and pretty much all of the caps were either shot or close to it.

[johnh]

Capacitors arrived from E14 Thursday.   Soldered them in today reassembled the scope.

Crossed the fingers.  Turned it on.  You beauty it works.

[codeboy2k]

good repair... by the way, those are thermistors, not MOV's, used for inrush limiting.

Furthermore, I find their placement of the spark arrestors a little odd.  Why put them so far up the chain, instead of closer to the line / neutral input.  I would put them before the caps, not after (although I realize that would not have helped here, as the cap probably cracked and had moisture ingress which eventually led to the the internal short).

Can anyone offer pros/cons for putting the arrestors (E609, E616) in the position the designer did on the schematic vs where I think I would put them.  Like I said, I would have put them after the fuse just before the RLC network.

[GreyWoolfe]

  on the repair.  Enjoy the scope.