Recap or not to recap? When is a good time to recap? - Page 1
Forum section: electronics / testgear

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: July 10, 2019, 10:51:46 pm
content: Almost all of my test equipment is from the 1980s and 1990s and no problems, so far.So when would it be a good time to replace the aging capacitors? Is it best to replace all electrolytic caps only or should the tantalum caps be replaced as well? I tend not to touch things if they are working, but the consequences of a shorted, open, or exploding cap on surrounding devices causing a cascade of new problems causes me some concern each time I power up the old equipment.I realize that this may be just a personal decision based on the risk, but it would be good to hear some comments, for or against. Thanks.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2019, 10:54:21 pm
content: Depends on brandsandmodels.Some have design flaws and others use components that are known to fail.Best to make a list of what you have so to seek advice for each model.

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: July 10, 2019, 11:18:32 pm
content: Quote from: tautech on July 10, 2019, 10:54:21 pmThanks, here is what I have with a best estimate on age. Presently, all fully functional.Lambda LPD-421A-FM (~1986?)Fluke 8505A Multimeter (~1983)HP 3457A Multimeter (1986)Kenwood CS-5400 oscilloscope (~1988)Keithley 197 (~1991)LeCroy 9100 AFG (1993)BK Precision 3011B Function Generator (~1989)Components appear to be original in all the units.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2019, 11:26:08 pm
content: Anything with Rifa X/Y caps should be of concern along with Schaffner mains input filters.

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: July 10, 2019, 11:32:04 pm
content: Quote from: tautech on July 10, 2019, 11:26:08 pmI shall be on the lookout for those brands. Thanks.

author: MosherIV
author_rank: Super Contributor
timestamp: July 11, 2019, 07:22:31 am
content: I volunteer at a computer museum. My colleagues advise not changing capacitors unless necessary.Tantalum caps can be problematic. Most of the time we see them used for decoupling, and they rarely cause problems if they blow. Just replace them.Rifa caps used to filter mains is a usual yes. We always replace them in BBC model B computers before using them.Sometimes, if circuits are not working correctly and the caps are 50+ years old, we may replace the caps in a section of that circuit but not every cap.Depends on the situation.(My skills at the museum are more general and software related, there are more experienced electronics engineers there than me. I do electronics as a hobby)

author: bd139
author_rank: Super Contributor
timestamp: July 11, 2019, 07:30:09 am
content: Sound advice all around.RIFA + Schaffer replace on sight.Electrolytics replace on failure.Tantalum replace on failure.A lot of the outcomes depend on if the gear has been left on for a decade or lived on a shelf for a decade.

author: nctnico
author_rank: Super Contributor
timestamp: July 11, 2019, 07:38:19 am
content: If it ain't broken then don't ruin it. A piece of equipment usually doesn't get better from a repair because there will always be some damage from the desoldering & soldering process (depending on the skill of the person doing the repair). Most of the electrolytics won't need replacing anyway. It doesn't hurt to check for leaking electrolytics though. Usually the smell of cat wee is a tell tale sign.

author: HalFoster
author_rank: Frequent Contributor
timestamp: July 11, 2019, 02:06:40 pm
content: +1 for the advice given so far.Personally, when deciding whether to replace a capacitor or not, I pretty much do the following:1) Replace *anything* that may have a RIFA.2) Study the schematic - know what purpose it is serving.3) If it is being used for anything other than decoupling, leave it alone unless it's known bad. If replaced, try for an exact replacement if possible, else as close as you can get - type, value, characteristics etc.3) With tants, I generally leave them be unless there has already been a failure - then I might think hard about replacing others that are the same type, value & supply rail *if* they are the droplet looking dipped ones.4) In relation to #3, look at the part list first and make sure that the cap is actually an Al and not a tant. Early 80's and older quality equipment (mostly HP) used a lot of cylindrical dry and wet slug tantalum capacitors - as a rule, they just usually don't go bad so one doing so is just about always an isolated event.5) Visually inspect everything you can first (a good pair of Optivisors is perfect for this) looking for leaking, bulging and other warning signs - and not just capacitors either.6) The procedure I typically follow is to turn the equipment on for about 15 to 20 seconds or so and do a quick finger and sniff test: does anything smell like it's burning or do any of the caps feel warm?7) If all is still good, turn it back on and do the same for a longer time. Personally, this is when I bring out the IR camera and look for hot spots, but using a finger works just as well, just slower. In operation, a cap should usually never get hot (or even very warm) by itself unless heated from a nearby source.8 ) Check the rails for the correct voltage and ripple as given in the service manual - high ripple especially is a sign of a bad cap somewhere.9) In general, I won't call it good until the equipment has had at least a 24 hour burn-in under working conditions. Check voltages/ripple during and at the end of the burn-in.10) When you *do* change one out, don't just blindly assume that more is better (except for, usually, the voltage rating) - on the surface, increasing the capacitance of a bulk PS filter capacitor(s) should be a good thing. More than likely the replacement has lower ESR than the original and, especially when coupled with a higher capacitance, can subject the rectifiers and transformer to much greater stresses at turn on. Usually it will be fine as long as not over done, but is it something that should be considered first. Changing the capacitance, and especially drastically lowering the ESR such as using a polymer in place of an Al on the regulator's output can cause oscillations in the output voltage. These can be - and usually are - in the 100's of MHz and not seen on lower end scopes. They are seen very well, however, by the circuits downstream since the typical PSRR at these frequencies are basically the same as my chances of winning the lottery - and I didn't buy tickets.As previously brought up, this procedure should be modified according to circumstances. No matter what, though, it is better to err on the side of caution. Again, read the manual, study the schematics and understand as much as you can on the possible effects of a change.Which is all just a very long winded way of saying "Follow the excellent advice by bd139."Hal

author: Tomorokoshi
author_rank: Super Contributor
timestamp: July 11, 2019, 02:29:22 pm
content: Mr. Carlson advocates replacing various types of paper and wax capacitors that show up in equipment from the '60's and before. These are typically found in vacuum tube equipment. He has a few videos on this topic.

author: Kosmic
author_rank: Super Contributor
timestamp: July 11, 2019, 02:55:55 pm
content: +1 to everything bd139 said.Also, having something to test ESR can come handy. And if testing in circuit do not work (give strange results) you just need to desolder 1 leg of the cap to test properly.

author: Tom45
author_rank: Frequent Contributor
timestamp: July 11, 2019, 03:29:03 pm
content: Smoke from a Tek 7A26 plugin was caused by a shorted Tantalum. A 10 ohm carbon composition resistor that had 47 turns of fine wire on it as an inductor was the source of the smoke and had to be replaced.Then a few months later the same thing happened to another 7A26. So I got the message and replaced all tantalum caps on all of my 7A26 plugins. Saved having to fabricate those 10 ohm resistors with windings in units that hadn't failed yet. There were several tantalum/resistor configurations in each 7A26.So one answer of when to recap is when you see a pattern of failure.

author: bd139
author_rank: Super Contributor
timestamp: July 11, 2019, 04:19:41 pm
content: Quote from: Tomorokoshi on July 11, 2019, 02:29:22 pmGood point.Anything that has valves/tubes in it probably needs total strip down at the moment as the carbon comp resistors drift, the caps leak and melt and the elecrical safety situation is mostly unknown.It wasn'tthatlong ago when you got a zap from the controls on your radio, you'd put beeswax in the grub screw hole on the side of the knob that zapped you and carry on as before

author: MT
author_rank: Super Contributor
timestamp: July 11, 2019, 08:24:36 pm
content: Quote from: HalFoster on July 11, 2019, 02:06:40 pmNot all RIFA's are bananas!Famous bad RIFA!Famously good RIFA!

author: bd139
author_rank: Super Contributor
timestamp: July 11, 2019, 09:00:47 pm
content: Indeed.However, bad RIFA:Satanic RIFA:

author: syau
author_rank: Frequent Contributor
timestamp: July 11, 2019, 11:44:56 pm
content: I did replacing the Tant when one or more start giving out smoke to avoid the dead short killing the power supply rail which may more difficult to repair.For the electrolytic, I just follow the repair thread in this forum (just like the MDA 53310A power supply)For the replacement Tant, I use Kemet brand.For the electrolytic, I use Rubycon ZLH 105C.All from Digi-key

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: July 11, 2019, 11:58:01 pm
content: This discussion has been most helpful as I have been debating whether to recap my audio pre amp and the mono-block power amps. Thanks for all the wisdom shared here.The second picture is clearly the result of a burning cap, but what happened in the first one. Was that a cap explosion? Can't figure out where the layer of black deposit come from.Quote from: bd139 on July 11, 2019, 09:00:47 pm

author: bd139
author_rank: Super Contributor
timestamp: July 12, 2019, 06:48:09 am
content: The RIFAs usually burst before they go nuclear spraying brown gunk over everything that you can’t clean off. Also stinks pretty bad. In the above picture it came from the top of the one next to the inlet.

author: Thomas
author_rank: Regular Contributor
timestamp: July 12, 2019, 10:33:45 am
content: Regarding electrolytic capacitors for decoupling: Can't these potentially take unobtainium components/modules with them when they fail?I'm thinking excessive ripple on supply rails. But that may not be a problem?I have a Racal Dana 1992 Universal Counter from ~1986 which works perfectly. I have bought all electrolytic caps in it, but haven't gotten around to replacing them.Would be very sad if the unobtainium module in it dies before I get around to it.

author: cvanc
author_rank: Frequent Contributor
timestamp: July 12, 2019, 11:10:35 am
content: A lot of good advice here, I'll add one more that I don't think has been mentioned yet:"When a cap of a particular brand/type fails, replace it *and* all others of the same brand/type in the unit"(I'm looking at you, blue electrolytics that say 'Made in Holland' on them...)

author: HalFoster
author_rank: Frequent Contributor
timestamp: July 12, 2019, 12:53:37 pm
content: MT: You are right - not all Rifa's are evil and I should have caught that as I was writing. AFAIK, the newer encapsulated ones are fine, it was just a period in the 80s I believe when the bad ones were made.Which makes me think that it would be nice to have a sticky topic where a list of capacitors that statistically have problems was maintained. And the ones that *don't* have problems as well - out of maybe 100 of forty to fifty year old large can screw mount Sprague Compulytics, I can only think of 3 that were bad - and two of those were both out of the same vintage HP 8640Bs. If I remember right it was the 8K ones on the 5.2V rail. Exactly the kind of info it would have posted for reference somewhere.Hal

author: rsjsouza
author_rank: Super Contributor
timestamp: July 12, 2019, 01:43:36 pm
content: badcaps forum has extensive lists of good and offending items:https://www.badcaps.net/forum/showthread.php?t=414I have a good Rifa as well

author: Jan Audio
author_rank: Frequent Contributor
timestamp: July 12, 2019, 02:03:12 pm
content: I have a question : can those old capacitors still leak ?, or are they dryd up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2019, 02:39:37 pm
content: Mostly echoing what's been said:If it's fine and working, don't worry about it.Mind, symptoms may not be externally obvious, but worsening internally; you can snoop around (e.g., measure supply ripple, or capacitor ESRs) to gauge health, and plan for repairs at some point accordingly.But if some are starting to go bad, don't try and hedge your bets, just replace them all. They'll all die sooner or later and you might as well do it all at once.Case in point I think, a good example, my Tek 475 scope. Mostly vintage 1973 or thereabouts. Worked fine for many years; don't think it had too much use before I got it, but I used it heavily for several years. In that time, the CRT faded (a typical problem on 475s), then some years after replacing that, the main supply filter caps got so dried out, the supply ripple was unavoidably apparent in trace intensity and vertical deflection. I replaced them and it's good as new in that regard. (That just leaves all the other things that are bad: scratchy switches and pots, flaky or broken attenuator modules, broken knobs, the usual...)Similarly with cheaper (consumer, prosumer) equipment, most switching supplies, all that -- the recap interval might be a year to a decade of use. Anything more than that you're just lucky, every month/year after that is a bonus, pretty much.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 12, 2019, 03:27:42 pm
content: Depending on the capacitors' technology, their failure modes can include shorts. Which may destroy much more expensive and unobtainium circuitry. So waiting for a failure is not always the best option either.AFAIK, particularly the following: film capacitors, tantalum and ceramic. Ceramic caps failures being often due to mechanical stress, especially the SMD ones.The electrolytic ones are usually more going either open, have increased leakage or increased ESR when drying up. That may also lead to other failures over time due to excessive heating/ripple, but bad electrolytics are usually easier to spot visually, as they are often bulged and/or leaking.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: July 12, 2019, 09:00:18 pm
content: bd139, thanks, now I see where all the "juice" come from. It must be one heck of a cap explosion!

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2019, 09:09:04 pm
content: Quote from: NoisyBoy on July 12, 2019, 09:00:18 pmNot so much, Rifa's generally have 2 failure types, one a fizzing spewing fail and the other more catastrophic with full mains power driving the explosion.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: July 12, 2019, 09:28:42 pm
content: Good to know, I will definitely keep an eye out, at least it is quite distinctive and easy to spot. I can deal with magic smoke, but I hate to see fire in my test gears.

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2019, 09:39:13 pm
content: Quote from: NoisyBoy on July 12, 2019, 09:28:42 pmRifa’s typically have a translucent enclosure that degrades with time evidenced by crazy cracking of the enclosure which allows electrolyte to escape and moisture to enter. If they can be inspected all around you can monitor the case and catch it before the impending failure.