On 24/02/2022 04:17, Ed Breya via groups.io wrote:
Yes, it was just a coincidence. The probe ground clip is the same potential as the chassis. Now, if you happened to be poking around in a line voltage or power circuit, it would be a different story.
Since the scope had been sitting unused for a long time, a number of things could have happened some time after powering it up. I think what the others referred to sounds like the infamous Rifa or Wima brand epoxy-potted paper X-caps often used in the AC line EMC filtering of this vintage equipment. These are well known to be a problem, shorting out and melting down, making noxious fumes, and quite a mess sometimes. The good news is that although they tend to self-destruct, they usually do no further damage except for blowing the line fuse, and making a mess of the surrounding area. If you open it up and do careful inspection of the line side of the PS (with power cord unplugged), you should be able to spot the problem - you'll know it when you see it.
Having equipment with these caps sitting for a long time, especially in damp conditions, or being used in a "high voltage (line 240 V) country" tends to aggravate the situation.
I'm just replacing the RIFA caps in a 2445 PSU
It's tempting to do a re-cap on the basis that "I might as well do it while it's out of the 'scope", especially as these PSU's are not the easiest to extract from the chassis and disassemble to work on but I am not sure I would rush into a blanket replacement of the existing caps unless you know that there is a problem.
Be careful that you get the right caps - the originals are (or should be, according to the service manual) Sprague (now Vishay) 672D, low ESR, 4000h@105¡ãC
But you might not need to replace them - partly out of curiosity I pulled the 10?F caps in the 87V circuit as those are under most voltage stress and they measure just fine, capacitance in tolerance and with an ESR of ~ 2.2¦¸ @ 120Hz compared with a spec of 10¦¸ - the only one I might replace, if I can find a suitable candidate is C1130 (the 87V voltage doubler) as it looks like a previous repair swapped it with a general purpose 85¡ãC Nichicon.
--
Paul
