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2445A X2 mains input capacitor failures

g3xaq
 

Both the X2-rated 0.068uF capacitors on the mains input on my 2445A scope exploded today with much smoke and consequential panic. The part numbers at C1016 and C1018.

I've read of similar failures here and elsewhere. Has anyone a theory for why they die? Should I be looking for another root cause fault?

Getting the PCB out for repair seems to be a challenge. My unit doesn't match the manual I downloaded, in that there is an extra screw next to the mains switch lever that holds a piece of black plastic to the chassis that appears to need to be removed. It is not mentioned in the manual and of course I do not have a long enough Torx driver to hand to get to it. The description of removal of the Vertical Bracket doesn't seem to match mine either and I don't really see just yet why it needs to come off. Same for the fan, although I can't see propely if indeed it is in the way. My chassis is Feb 1998 and my manual is Dec 1986. Are there different revs of the 2445A that don't quite match up with the service manual?

Thanks, Alan.
 

Hi Alan,

I also had read about the AC line-filtering caps being a weak point so I replaced them as a preventive measure when recapping the LV power supplies for my 2465A and 2467.

The plastic bodies on the caps coming out of the 2467 showed substantial cracking; the 2465A caps less so. My theory is that the caps deteriorated and in your case, failed due to age.

I suggest you check the condition of R1016 and R1018 since your caps failed catastrophically. Perhaps those resistors were damaged when the caps shorted and blew up.

Since your AC mains voltage is at 230V (compared to the US at 120V) I would expect that your caps will be more stressed and have a greater failure likelihood (vs. North American units) as time passes.

Is your chassis dated Feb 1998 or Feb 1988? I suspect the latter.

My units were produced in May and June 1988. I suggest you just figure out what you need to unscrew to get the power supply out. In my case I found it is not necessary to unsolder the fan power cable but it was needed to remove and set aside the fan for access to some of the screws holding the LV power supply in place.

Regarding the black plastic piece, is that the lever that depresses the AC Power switch? If yes, that does not need to be removed.

Patrick Wong AK6C

--- In TekScopes@..., "g3xaq" <alan@...> wrote:

Both the X2-rated 0.068uF capacitors on the mains input on my 2445A scope exploded today with much smoke and consequential panic. The part numbers at C1016 and C1018.

I've read of similar failures here and elsewhere. Has anyone a theory for why they die? Should I be looking for another root cause fault?

Getting the PCB out for repair seems to be a challenge. My unit doesn't match the manual I downloaded, in that there is an extra screw next to the mains switch lever that holds a piece of black plastic to the chassis that appears to need to be removed. It is not mentioned in the manual and of course I do not have a long enough Torx driver to hand to get to it. The description of removal of the Vertical Bracket doesn't seem to match mine either and I don't really see just yet why it needs to come off. Same for the fan, although I can't see propely if indeed it is in the way. My chassis is Feb 1998 and my manual is Dec 1986. Are there different revs of the 2445A that don't quite match up with the service manual?

Thanks, Alan.
g3xaq
 

Hi Patrick

--- In TekScopes@..., "Patrick Wong" <patwong3@...> wrote:
I suggest you check the condition of R1016 and R1018 since your caps failed catastrophically. Perhaps those resistors were damaged when the caps shorted and blew up.
These resistors look OK and measure fine.

Since your AC mains voltage is at 230V (compared to the US at 120V) I would expect that your caps will be more stressed and have a greater failure likelihood (vs. North American units) as time passes.
Ah, that sounds like a reasonable theory.

Is your chassis dated Feb 1998 or Feb 1988? I suspect the latter.
Yes, 1988. Typo.

Regarding the black plastic piece, is that the lever that depresses the AC Power switch? If yes, that does not need to be removed.
Thanks for that. I've got the boards out of the case now. It was just that the LV assembly was an interference fit against the front and back chassis members. All it needed was sufficient faith to give it a good pull

I'll fit new capacitors this evening and get it all put back together. Thanks for your help.

Alan
 

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Hint - there are three screws (and maybe a post) ?that hold the rear panel to the main frame. If you loosen those screws enough to get a little play, the power supply will come out and go in much easier.
?
Regards,
Tom
?

----- Original Message -----
From: g3xaq
Sent: Tuesday, September 25, 2012 10:50 AM
Subject: [TekScopes] Re: 2445A X2 mains input capacitor failures

?

Hi Patrick

--- In TekScopes@..., "Patrick Wong" wrote:
> I suggest you check the condition of R1016 and R1018 since your caps failed catastrophically. Perhaps those resistors were damaged when the caps shorted and blew up.

These resistors look OK and measure fine.

> Since your AC mains voltage is at 230V (compared to the US at 120V) I would expect that your caps will be more stressed and have a greater failure likelihood (vs. North American units) as time passes.

Ah, that sounds like a reasonable theory.

> Is your chassis dated Feb 1998 or Feb 1988? I suspect the latter.

Yes, 1988. Typo.

> Regarding the black plastic piece, is that the lever that depresses the AC Power switch? If yes, that does not need to be removed.

Thanks for that. I've got the boards out of the case now. It was just that the LV assembly was an interference fit against the front and back chassis members. All it needed was sufficient faith to give it a good pull

I'll fit new capacitors this evening and get it all put back together. Thanks for your help.

Alan

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