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PSU recapping - what's the general consensus on when to do it?

My general rules towards when I should put in the time and effort to recap...

- Not working (obviously)

- Flakey functionality that is likely related to power problems (a bad power supply can affect everything on a board)

- Contains known bad capacitors (like ones from the cap. plague)

- The components or complete system are old and rare/expensive enough to justify it even if nothing is currently wrong
 
At one place where I worked (in the 80's I think), time was a premium but not so money. If a SMPS failed, we would spend, say, 10 to 15 minutes on it at most, looking for open/shorted PN junctions, etc. If we could not fix the SMPS within that time period, the SMPS would get shelved, and the faulty parent equipment get a replacement SMPS. At some point, a company was found that would fix 'any' SMPS for a fixed cost. If they could not repair the SMPS, there would be no cost. This arrangement worked out well, and for us, quite economic. We started to take a look at the soldering in the fixed SMPS' to see what was being replaced (yes, we looked at the soldering before the unit was sent out). The redone soldering indicated that the company was simply doing a replacement of all of the small to mid value aluminium electrolytics around the switch mode controller chip. The large input filter caps were not touched, neither the filter caps on the outputs.

I can imagine some non-technical person doing the cap replacement (i.e. labour not requiring technical knowledge), and then the SMPS went to someone who had the knowledge on how to test it.

Occasionally, we would see additional repair work, e.g. optocoupler swapped out.
 
We started to take a look at the soldering in the fixed SMPS' to see what was being replaced (yes, we looked at the soldering before the unit was sent out).
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That's quite interesting to hear you say - I used to work for a company in the UK which undertook similar 'repair anything' work, but we used to go one step further and try to ensure that the soldering of the replacement parts was done exactly the same way as it had been for the original parts, if the solder was sparsely applied, we did that, if the solder had originally been heaped on, we did that, if the capacitor legs had originally been bent over to keep the part in the board, we did that. In short, we went to considerable trouble to exactly reproduce the original style of soldering.

If you clean a solder job up really thoroughly with the right sort of cleaner, it gives the new joints the same 'old' dull looking finish as all of the other original joints, making it difficult to tell what has actually been resoldered. The reason we went to all this trouble was to make it more difficult for anyone to 'steal' our hard-won knowledge of what were essentially common recurring faults in equipment. We knew them, but having found them the hard way in the first example seen, we didn't want to give that knowledge away to any casual observer.

Occasionally this did cause us problems when a customer would take a look to see what we had done, and it looked as though we had replaced absolutely nothing except of course that their item was now working - this would then lead to the suspicion that we had just found a loose connection or something and charged them for more than we had done.
 
Of course you are right, if anyone was determined to know what we had changed they could have recorded the details, brands, colours, values, markings of every single component in the unit before submitting it for repair - we just aimed to make it more difficult for anyone looking for obviously disturbed or resoldered components at the after-repair stage.

Just to give you an example, there was a particular type of motor drive unit which we used to get in some numbers from a company which obviously used a lot of them. Fairly early on, we realised that essentially the same two electrolytics were failing every time so they were a good earner for us, a predictable easy fix, but would also have been an easy fix for the company we were fixing them for if they had any technically competent people like you who were able to 'read' what we had replaced. If they had been able to determine what we were doing to fix them, then that particular revenue stream might have dried up for us.

Known 'Stock' faults like this have always been the bread-and-butter of any repair technician / repair company whether they work in the domestic or industrial field, they compensate for other jobs which are long and difficult and may have to be repaired at a loss, the first time at least.
 
SiriusHardware said:
Known 'Stock' faults like this have always been the bread-and-butter of any repair technician / repair company whether they work in the domestic or industrial field, they compensate for other jobs which are long and difficult and may have to be repaired at a loss, the first time at least.
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VCR's were very complex. I remember doing a 'VCR theory of operation' type course (even though it would be rare that I encountered one). Still, it could be a lot of effort to track down the cause of certain faults. A technician where I worked discovered a VCR/TV repair technician selling a hard-copy database of field-learned fixes for many make-models of VCR. We all chipped in to buy the document.
 
I read an article on the web a few years back (which I obviously cannot find now) which had some basic rules which I adhere to.

Basically I remember the checklist as follows when dealing with some piece of equipment to restore :

* Replace if leaking, bulging, smelling.
* Replace capacitors of same type like those which were leaking, bulging, smelling.
* Replace capacitors that measure out of tolerance, both capacitance and loss-wise
* Keep the rest

So for example in a Tektronix 611 from the early seventies which I just recently worked with I only replaced one capacitor. It was a tantalum capacitor that was corroded. I checked the other capacitors and they all measured very good. Low loss and very close in capacitance.

Another project is a DEC VR241 screen made by Hitachi. I checked the capacitors. One had a slight bulging top. Replaced all of the same type (even though they measured fine out of circuit). Found one 2200 uF cap which was only 1700 uF. The rest was within spec and low loss and no leaks neither smells could be detected.

The slightly annoying thing is that the replacement cap for the 2200 uF was measuring slightly below 2000 uF, so still within 10%. It is not certain that a new capacitor is measuring better than an old. DEC often used epoxy sealed capacitors in their designs. I seldom find those to be bad.

On the other hand, in a Macintosh SE/30 supply I worked with a couple of years back most of the output filter caps were leaking so they got replaced and so did the capacitors on the motherboard since they also leaked.
 
Chuck(G) said:
If it's average consumer-grade stuff, yes, I'd get out the meter and check the electrolytics. But I'm not going to junk a bunch of oil capacitors because they're "too old". And there's a whole bunch of fairly recent stuff that's been destroyed by "shotgun replacement" of caps. Any time that soldering iron comes out, one must take into account the possibility of damage.

It's the "shotgun" advocates that I have the biggest problem with.
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I don't really recall mentioning oil capacitors, though I did replace some non-functional PCB-laden caps with modern Chinese/Grainger caps (they are for a wig wag, an antique railroad signal). I'm also not trying to shotgun replace everything, hence the purpose of this post in the first place.
 
Some caps just bulge without even being installed. I had a bag of "cheap" 105C low ESR ones I left on a sunny window sill and they bulged out within a summer.
I also have "new old stock" PSU's from the late 80's that have done the same thing on some caps while just sitting in a box.
Drying out isn't always visible, so testing is the only sure way of weeding out the potential issues.
I usually reform large linear psu caps and then do a timed voltage drop check without a load to check for leakage, and then with a resistor to check it's value.
 
Bringing back an old thread I started - I think some PSUs, that have historically known failures, like that in the SPARCstation IPC/IPX, will get a recap by default. I recently acquired three more IPXes (was going to keep a second, until I got an LX and decided one of each is fine). Two of the supplies had Elna caps that were leaking BAD. @glitch noted the same on his website, and I'm seeing a trend myself. One supply did power up, but I decided to recap it anyways. Sure enough, one or two caps were starting to leak, even though the supply still worked. I figured it's preventative maintenance at that point.

Now, I'm stuck with the PSU for my LX - it worked initially, but now no longer powers on (and confirmed it's not the cord, as that works with an IPX). The LX supply is a lot more complex, particularly with capacitors underneath transistor heatsinks that are in awkward locations. With my luck, even though the caps physically look good, something's probably leaking. Plus, I found some more Elna caps, and already, I'm NOT a fan of this brand based on prior experiences of 30+ year old Elna caps.
 
...capacitors underneath transistor heatsinks that are in awkward locations.
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And of course those will be the ones - not just because they are awkward to get to (Ref: Sod's Law) but because of their proximity to heatsinks. When I'm looking for failed (but not visually obviously failed) capacitors in something like an SMPSU the ones I look at first are small ones - which don't contain a lot of electrolyte to start with - mounted very close to heatsinks or other hot-running components like high frequency transformers.
 
SiriusHardware said:
And of course those will be the ones - not just because they are awkward to get to (Ref: Sod's Law) but because of their proximity to heatsinks. When I'm looking for failed (but not visually obviously failed) capacitors in something like an SMPSU the ones I look at first are small ones - which don't contain a lot of electrolyte to start with - mounted very close to heatsinks or other hot-running components like high frequency transformers.
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Good point, given that yes, some of the capacitors removed around said heatsinks had indeed been leaking electrolyte. It wasn't nearly as bad as the IPX PSUs, but still enough to warrant replacement (and also not show a capacitance when tested out of circuit on my Fluke multimeter).
 
Chuck(G) said:
Old DC wall wart power supplies almost universally require re-cappling. A transformer generates heat. In a closed container, this cooks the electrolytics.
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Ha, that doesn't surprise me, either. I had a supply that had one bad/leaking cap. After opening it up, I just opted to go with a new supply, that thankfully Mean Well made. I opted for new instead of trying to fix the old in that instance, as it was for a rare Sun SPARCstation prototype that was never released.
 
I have found that Sprague Twist lock capacitors are prone to have the positive terminal all corroded off just inside the rubber sealing. In a recent Tektronix 4013 restoration two out of three twist lock were bad. The same with my Tektronix 4051. A Calcomp 565 plotter had twist lock which also was open circuit. Now I replace Sprague Twist Lock when I see them. I learnt the lesson the hard way when starting up my Tek 4051 without checking the PSU a number of years ago and the non-existing filtering cooked a very hard to find laser trimmed resistor network. Not fun at all.
 
MattisLind said:
I have found that Sprague Twist lock capacitors are prone to have the positive terminal all corroded off just inside the rubber sealing. In a recent Tektronix 4013 restoration two out of three twist lock were bad. The same with my Tektronix 4051. A Calcomp 565 plotter had twist lock which also was open circuit. Now I replace Sprague Twist Lock when I see them. I learnt the lesson the hard way when starting up my Tek 4051 without checking the PSU a number of years ago and the non-existing filtering cooked a very hard to find laser trimmed resistor network. Not fun at all.
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I'm not familiar with the Sprague Twist Lock, but that said, I think we all start noticing trends with certain brands of capacitors having issues and come to the conclusion of just replacing whenever seeing them. This is how I'm starting to feel whenever I seel Elna capacitors. Same goes for brands of computers in general, like vintage DEC stuff. For whatever reason, like 95% of the DEC stuff I've seen have ALL had leaky caps, from about mid-90s downwards.
 
It's getting to the point where I recap anything vintage that's still working whenever I open the power supply for any reason -- to blow out dust or change a fan, even! I've caught so many caps juuuuust starting to leak. The biggest effort is getting the supply out and open anyway!
 
g4ugm said:
I used to take that approach but having had a perfectly good expensive MicroVax PSU that was working with no signs of leakage , leak and die in storage, I m now less conservative
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If you only have a dozen machines then you can do that, if you have hundreds, it will get expensive and time consuming. Who is to say the replacements won't be crap and leak in short order?

I order capacitors from ebay when I need replacements and for the most part they work fine (I test them before using them), but one batch that sat in a cool dry place inside their bag ended up leaking in storage which I never seen before.
 
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