- Scope keeps blowing fuses - 16 Updates
| Charles Lucas <charlesandmilly@gmail.com>: Oct 24 10:12AM -0700 Hi, everyone (including the original poster). When the power supply unit fails, there can be a basic myriad of possible problems and/or failed components. Here are some things to look for: Current limiting resistors, rectifier diodes, transformers, transistors, caps., filters, coils, etc... Shorts and opens are common problems. Look for bad connections, cold solders, etc... Lots to check for to "pin in down". Especially with older devices. Since it is an analog device (as was said), it is assumed the equipment is old, so we have to go to the old school way of checking everything, looking for key things, in order to come up with some accurate diagnosis. Good luck and have a great day. Charles Lucas |
| three_jeeps <jjhudak@gmail.com>: Oct 24 10:29AM -0700 On Sunday, October 23, 2022 at 8:14:05 PM UTC-4, Jeff Liebermann wrote: > PO Box 272 http://www.LearnByDestroying.com > Ben Lomond CA 95005-0272 > Skype: JeffLiebermann AE6KS 831-336-2558 I know about the RIFA caps exploding - thanks for the interesting video tho. What is the recommended replacement type? (assuming they can physically fit into the same space on the board) I have a few vintage Tek scopes that I should do some preventative maintenance work on.... J |
| Jeff Liebermann <jeffl@cruzio.com>: Oct 24 10:35AM -0700 >while seals lasted. The outer sealing body tended to shrink and crack >with age, allowing ingress of air and moisture, compromising >performance. They are no longer made. When I test RIFA capacitors with the Megger, I sometimes see capacitors the show some leakage for maybe 250 msec and then recover. I've been assuming that this demonstrates that self healing is function. Known good capacitors don't do that. Since testing required removing the capacitor from the PCB, I just replace any capacitor that shows temporary leakage. (i.e. better safe than sorry). This video shows what might be two self healing RIFA capacitors in action. Watch the current on the display: "40 Year Old Rifa Capacitors High Voltage Leakage Testing" <https://www.youtube.com/watch?v=m1nDrunJ1SE> (3:09) The effect is more visible with a panel meter than with a digital readout (except when the meter is moving because I'm turning the crank on the Megger). >of internal faults, but in extreme cases, you still get thermal >runaway, as the faulting area expands and eats away at succesive >layers of the wrapped structure. I haven't bothered to perform a post mortem on a RIFA capacitor. I have several suitable microscopes and would expect to find holed in the dielectric (paper or plastic) and the metalization. The RIVA caps that failed have warped, cracked, melted, or crumbled outer plastic cases. For meaningful microscopy, I would need an old RIFA capacitor in an intact case. -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558 |
| Jeff Liebermann <jeffl@cruzio.com>: Oct 24 11:10AM -0700 On Mon, 24 Oct 2022 10:29:52 -0700 (PDT), three_jeeps >What is the recommended replacement type? (assuming they can physically fit into the same space on the board) >I have a few vintage Tek scopes that I should do some preventative maintenance work on.... >J "RIFA - Replacement Choices" <https://www.eevblog.com/forum/beginners/rifa-replacement-choices/> Kemet bought RIFA. My older Tek scopes survive 30 to 40 years and usually blow electroltyics before RIFA caps. Rather than experiment, I simply bought the same value/voltage X2 "safety" caps from Kemet (via Mouser or Digikey): <https://www.kemet.com/en/us/capacitors/film/safety.html?40=133> I wasn't very careful when I ordered replacement caps many years ago. I think I ordered "metalized paper", which in retrospect, was not the best choice. Going down the list of dielectrics, only "metalized paper" and "metalized polypropylene" are available for "safety" caps. Kemet is recommending their R53 X2 caps with "metalized polypropylene film" dielectric. <https://www.mouser.com/new/kemet-electronics/kemet-miniature-emi-suppression-capacitors/> <https://www.digikey.com/en/product-highlight/k/kemet/r53-film-capacitors> X2 is for "line to line" filtering while Y2 is for "line to ground" filtering. -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558 |
| "Peter W." <peterwieck33@gmail.com>: Oct 24 11:12AM -0700 OK. What is known: a) Scope is blowing mains fuses. b) RIFA -branded caps blew up. c) The power-supply board was affected. Suggestion: 1) Remove the power-supply board. 2) Replace all, repeat: ALL the capacitors on that board of any nature, type or style with the correct (but modern) replacement type. 3) Check each other component on the board. Diodes, regulators, transistors, whatever. 4) Replace any that are either bad or the slightest bit questionable. 5) Replace board - carefully (now might be an opportunity for a DBT to 'do its thing' - but a metered Variac would be better). I kinda-sorta think that will do it. KISS and all that. Peter Wieck Melrose Park, PA |
| Jeff Liebermann <jeffl@cruzio.com>: Oct 24 12:04PM -0700 On Sun, 23 Oct 2022 11:16:19 -0700, Jeff Liebermann <jeffl@cruzio.com> wrote: (chomp...) When in doubt, replace everything. I was wondering if someone sells a recap kit for the Tek 2465. Yep: <https://www.ebay.com/itm/313637931679> It includes some RIFA -> Kemet replacement caps. -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558 |
| Cursitor Doom <cd@notformail.com>: Oct 24 10:10PM +0100 On Mon, 24 Oct 2022 11:10:12 -0700, Jeff Liebermann <jeffl@cruzio.com> wrote: ><https://www.digikey.com/en/product-highlight/k/kemet/r53-film-capacitors> >X2 is for "line to line" filtering while Y2 is for "line to ground" >filtering. Jeff, remind me to post a picture of the cap replacement I've carried out. You'll be both horrified and astounded. If I had more time I'd post it now, but it'll have to wait for the time being.. |
| Cursitor Doom <cd@notformail.com>: Oct 24 10:17PM +0100 On Mon, 24 Oct 2022 11:12:09 -0700 (PDT), "Peter W." >4) Replace any that are either bad or the slightest bit questionable. >5) Replace board - carefully (now might be an opportunity for a DBT to 'do its thing' - but a metered Variac would be better). >I kinda-sorta think that will do it. KISS and all that. I do have a metered variac, so plan to use that in series with DBT current limiting, as the bulb can react to sudden over-current much more quickly than I can. I'll slowly bring the variac up whilst checking for anything getting warm on the board. That's the best I can manage as I'm no tech repairer; haven't got the necessary fine motor skills, I'm afraid. |
| Jeff Liebermann <jeffl@cruzio.com>: Oct 24 04:52PM -0700 On Mon, 24 Oct 2022 22:17:57 +0100, Cursitor Doom <cd@notformail.com> wrote: >I'm no tech repairer; haven't got the necessary fine motor >skills, I'm afraid. Not a problem. Let a hand soldering robot do the precision work. Something like this soldering robot: <https://www.promationusa.com/9101> More: <https://www.promationusa.com/robotic-soldering-glance> and even more: <https://www.google.com/search?q=hand+soldering+robot&tbm=isch> -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558 |
| Phil Allison <pallison49@gmail.com>: Oct 24 05:21PM -0700 legg wrote: ------------------ > >>Plenty are found open or with low values after long times in service. > >>X2 dropping caps used in smoke alarms regularly fail open. > >>But those Rifa ones love to smoke and burn. ================================================== > while seals lasted. The outer sealing body tended to shrink and crack > with age, allowing ingress of air and moisture, compromising > performance. They are no longer made. ** Rifa PME271 X2 caps are still made and widely available. Strangely they are also the most expensive X2 caps on offer for no good reason. Described as being of "metalised , impregnated paper " construction. Rifa also claim to be the only supplier . Other makers, dozens of them, use metalised polyester or polypropylene films as used in DC rated caps - but with one big difference. X2 types are double wound = a technique that creates two caps in series. Such caps are immune from *corona discharge* occurring in tiny air pockets trapped inside the cap as it it being wound. They are still vulnerable to spike voltages of a few kV which eat away at metalisation and reduce cap value over time. IME Rifa X2s are notorious for catching fire and filling the room with acrid smoke. Happened twice, right in front of me, first case was a room heater and the second with a portable TV. When used for suppression in triac dimming equipment, they invariably fail early too. Bad news. ..... Phil |
| Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>: Oct 24 08:38PM -0400 Cursitor Doom wrote: > Thanks, > CD. > https://disk.yandex.com/i/UhxATIy-6Y0LIA Yikes. When a fuse blows, you replace it *once*. If it blows again, it ain't the fuse or a line transient. Start with a DMM and check for shorted diodes in the input bridge and then for a shorted filter cap at its output. (The schematic is more or less completely illegible, or I'd give the ref des.) Next most likely is a switch FET. Cheers Phil Hobbs (proud owner of a 2467 that he almost never uses) -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com |
| Cursitor Doom <cd@notformail.com>: Oct 25 08:33AM +0100 On Mon, 24 Oct 2022 20:38:01 -0400, Phil Hobbs >> https://disk.yandex.com/i/UhxATIy-6Y0LIA >Yikes. When a fuse blows, you replace it *once*. If it blows again, it >ain't the fuse or a line transient. Yeah, well twice in this case. Replacing the RIFAs *usually* does the trick (I have to say, in a half-arsed defence of my appalling conduct). >Start with a DMM and check for shorted diodes in the input bridge and >then for a shorted filter cap at its output. (The schematic is more or >less completely illegible, or I'd give the ref des.) Not sure what you're viewing it with, Phil. Others don't seem to have that problem and have been able to see the part numbers okay. >Next most likely is a switch FET. Yes, the chopper is always a prime suspect with SMPSs |
| Cursitor Doom <cd@notformail.com>: Oct 25 08:34AM +0100 On Mon, 24 Oct 2022 16:52:59 -0700, Jeff Liebermann <jeffl@cruzio.com> wrote: ><https://www.promationusa.com/robotic-soldering-glance> >and even more: ><https://www.google.com/search?q=hand+soldering+robot&tbm=isch> Maybe one day I can get myself one of those, Jeff! :-D |
| legg <legg@nospam.magma.ca>: Oct 25 09:36AM -0400 On Mon, 24 Oct 2022 10:35:23 -0700, Jeff Liebermann <jeffl@cruzio.com> wrote: >The effect is more visible with a panel meter than with a digital >readout (except when the meter is moving because I'm turning the crank >on the Megger). The video shows testing of two Y caps. These were originally factory tested at 1500VAC/2700VDC, and have no reputation for embarrassing field failures after 20+yrs. X2 caps are desiged for 'impedance-limited' applications and are tested at around 1KVAC/1500VDC. >that failed have warped, cracked, melted, or crumbled outer plastic >cases. For meaningful microscopy, I would need an old RIFA capacitor >in an intact case. Case degradation is a significant factor in aging of early production. Prior to single-board assembly mfring philosophy, these parts were shipped with long leads, loose, in bulk containers, knocking heads all the way. They were then subjected to short and long term lead stress in manual lead dressing and final physical attachment. Rifa was first to use that distinctive clear lacquer. I'm not sure how many times that formula was 'improved', but parts would arrive with visible cracks and would crumble around lead wires during lead-forming. That being said, there were sufficient 'incidents' in the first 20yrs of use to ensure that nobody in their right minds would use them in new product, even in a list of alternates, in the last 25 years. TEK scopes that employ these parts are antiques, maintained by enthusiasts, who will tell you all about the effects of heat, humidity and just plain age on plastics, epoxies and other components/materials used in them. RL |
| legg <legg@nospam.magma.ca>: Oct 25 09:53AM -0400 On Mon, 24 Oct 2022 17:21:00 -0700 (PDT), Phil Allison > Strangely they are also the most expensive X2 caps on offer for no good reason. > Described as being of "metalised , impregnated paper " construction. > Rifa also claim to be the only supplier . Phil, You will not be able to purchase the 'exact' replacement, as the series number no longer has the same operating voltage markings, safety approval listing, pert numbering or material construction. After all the merging and take-overs are done, it's unlikely to be manufactured at the same physical plant, or using similar equipment or materials, though safety docs used to be pretty inflexible in that regard - one of the main trade effects of VDE/IEC safety regime. Costs reflect a low volume mfring environment due to buyer reluctance to bother with 'improved' parts. RL |
| Jeff Liebermann <jeffl@cruzio.com>: Oct 25 09:23AM -0700 On Mon, 24 Oct 2022 20:38:01 -0400, Phil Hobbs >(The schematic is more or >less completely illegible, or I'd give the ref des.) https://disk.yandex.com/i/UhxATIy-6Y0LIA The Yandex viewer is blurry, even when viewed as "original size". Try downloading the PNG file and viewing it with a better viewer at 100% magnification. The details are quite readable in Irfanview (Windoze) and Nomacs (Linux). In Irfanview, the image can be "Sharpened" by hitting "S" on the keyboard. -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558 |
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