Fixing an old oscilloscope and I need your experience - Page 1
Forum section: electronics / repair

author: nitzane
author_rank: Newbie
timestamp: August 29, 2022, 07:04:09 am
content: Hey guys, my hp 54645A scope is acting up recently. I looked inside yesterday and found an old rifa cap in its power supply that went to the farm in the sky where smokeless caps go to live in.Its a 0.047 uf cap One side says PME 261 EB The other says 630-/300~ mp,im guessing this is a reference to its voltage reference dc/ac.Usually they have an x or y designation on them but this one doesnt.What capacitor would be a good modern replacement in your opinion?Also i figured i might as well recap the whole power supply,but the capacitors are glued in with white silicone like material. Any good advice on taking that off without damaging anything?maybe a way to dissolve it?pictures:https://imgur.com/a/s2NEHCgThank you!

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 29, 2022, 09:04:49 am
content: I normally go for X2 rated film caps likehttps://uk.rs-online.com/web/p/film-capacitors/1730535Dissolving seems more effort than needed. I ussually try to wiggle caps out and leave the silastic in place where possible. If you can insert the new caps in the old silastic "bed" it might still have some anti-vibration or shock effect. Maybe not, but it doesn't hurt either way.

author: strawberry
author_rank: Super Contributor
timestamp: August 29, 2022, 07:33:49 pm
content: WIMA looks better

author: nitzane
author_rank: Newbie
timestamp: August 29, 2022, 07:39:10 pm
content: Thank you guys!Ice-tea - can you elaborate please on the different ratings? what does x2 mean vs y or none at all?Nitzan

author: strawberry
author_rank: Super Contributor
timestamp: August 29, 2022, 07:48:43 pm
content: Y human safety(high failure safe. connected in series for double safety): used across primary & secondaryX mains safe(cant catch fire, voltage transient resistant, healing ...): used across L(line)&N(neutral)X&Y main purpose to suppress RF radiation

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 29, 2022, 08:14:45 pm
content: Quote from: strawberry on August 29, 2022, 07:48:43 pmY is also used from line to earth. So, in general, where a single failure would lead to a dangerous situation. Normally, you would need double isolation there (per example, you don't rely only on the varnish on transfo wire but you would also need a plastic separator) but Y-caps are a double isolation on their own.

author: AlcidePiR2
author_rank: Regular Contributor
timestamp: August 30, 2022, 05:56:21 am
content: Quote from: nitzane on August 29, 2022, 07:04:09 amWhy do you want to do that? These big caps are usually not the problem. Try to make it back to work with minimal replacement first.Did the rifa cap shorted ?If yes, you can just remove it for tests.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 30, 2022, 09:16:29 am
content: the white silicone sticks so good to caps and pcb, it is normally not possible to pull the cap out, even if correctly desoldered with a proper solder sucker station,i cut with a blade CAREFULLY while pulling the cap, so i only cut the siliconeand NOT into the pcb or traces !!Another trick, desolder one pin, be sure it is correctly isolated from the pad it was supposed to be soldered to,now measure its value, series resistance, leakage, if all looks normal, within what you normally expect for thistype and size cap, then dont waste your time or money replacing it. mark it as ok, solder it , and continue.test the caps near power diodes, power regulators, power resistors, bridge rectifiers,hot places, high ripple currents, tend to short capacitors life time,so better seek here first, if all turn out ok here, most likely no need to waste more time else where

author: nitzane
author_rank: Newbie
timestamp: August 31, 2022, 10:32:04 am
content: Thank you everyone!This was very helpful. i come from fixing game consoles and the capacitors there usually need replacing but it looks like the only one thats failed is the rifa one.Ill update here when i get my scope running againNitzan

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2022, 03:09:02 pm
content: Unfortunately if that is an X or Y capacitor, then it is not causing the problem with the oscilloscope. X and Y capacitors are for suppressing conducted EMI and it would run fine without them.