when to re-cap

[JohnRoberts]

My main use for the ESR meter these days is troubleshooting switching power supplies.

Yup, I recall in the early days before they started making low ESR aluminum, tantalum caps were not the only ones that would fail short circuit.

JR

 

[Scodiddly]

Yup, I recall in the early days before they started making low ESR aluminum, tantalum caps were not the only ones that would fail short circuit.

Just about every laptop-style brick switching supply I fix has one big electrolytic that's gone very high ESR, or maybe just failed open altogether.  Figures, it's the one that smooths the incoming high voltage DC and so takes a huge amount of wear.

 

[JohnRoberts]

Just about every laptop-style brick switching supply I fix has one big electrolytic that's gone very high ESR, or maybe just failed open altogether.  Figures, it's the one that smooths the incoming high voltage DC and so takes a huge amount of wear.

Capacitors for those applications provide a ripple current spec. It is logical that higher ESR capacitors will suffer more internal heating from ripple current ( power=current squared times R). This spec is important for power supply design, not so much for audio paths, with the exception of passive crossovers.

JR

 

[emrr]

I've mentioned it some other places, tangent to this discussion.  I restore a lot of 70-80 year old broadcast tube gear, and that's the place I regularly see coupling caps with really high ESR that measure correctly for capacitance.  Even then they frequently pass full response and sound 'fine'.  A new cap will probably sound obviously better/clearer but you have to make the comparison to tell, it's not obvious.  Only once have I seen a coupling cap with very high ESR roll off treble, and it took replacing it to prove amp response was not what it should have been.  That's all vastly older and different technology, of course, but FWIW. 

Another ESR tangent:  the Siemens NOS film coupling caps people are sticking in U67 reissues and MK67's in search of 'that tone' are 5-10x higher ESR than a new cap.  All the examples I've tested/installed for people have been mid 1970's vintage NOS. 

 

[Scodiddly]

Capacitors for those applications provide a ripple current spec. It is logical that higher ESR capacitors will suffer more internal heating from ripple current ( power=current squared times R).

Makes sense - as the ESR starts to creep up with age and wear the heating increases, causing higher ESR... the bad caps that I find in those power supplies have really high ESR, but they fail rather suddenly.

 

[rackmonkey]

To Doug’s point, an observation I’ve made when inspecting older tube gear is that electrolytic caps from the tube era often fail open, particularly old Spragues. I don’t know if it’s something different about the construction, or just the higher temperatures often encountered in tube gear, but something about those older electrolytics make the rules of thumb different, as Doug points out. In short, I wouldn’t make all the same assumptions and use the all same techniques when dealing with a 1940s unit and a 1980s or a modern unit.

 

[user 37518]

What kills electrolytics is age and temperature, specially temperature, caps are rated at a certain number of hours at certain temperature, for example 1000hrs@85°C, which means that if you have the caps work at 85°C they will last 1000 hrs, as you reduce the temperature the lifespan dramatically increases. Consoles are noted for killing caps since many of them run pretty hot, my MCI JH528 is one dramatic example, the damn thing is a heat radiator, it eats up caps in no time, an off the shelf 85°C cap would not last more than say 6-8 years in those conditions, which is why I used 105°C caps, which will last much longer.

For switching PSUs, capacitor ESR is an important factor (not really important for audio thou) due to the high switching frequencies at which SMPS work, high ESR makes the caps heat up (again, temperature is the problem). Most problems with SMPS are usually worn out capacitors, specially because manufacturers rarely use high quality caps, even the big name brands use generic chinese caps with low ESR, they don't design for longevity that is for sure, my 12 year old plasma TV has problems turning on, I am 99% sure the caps are the problem, I am just too lazy to open it up and change them.

In some audio equipment ESR testers are not the way to go, leakage testers are much better in that regard, for example, specially in old tube equipment, many caps can measure ok in capacitance and the ESR might not be that bad, considering that back in the day capacitors didn't have very low ESR to begin with, but when applied with the full working voltage they leak and create all kinds of havoc, like changing the bias point of tube gear, even tiny ammounts of leakage current can be extremely problematic (for example if you have a 1Meg resistor and a cap leaks into it 10uA, the difference will be 10V, which can dramatically change the bias point) and its difficult to diagnose, old electolytics and paper caps are notorious for this behaviour. In those cases I just replace all electrolytics with new ones and paper caps with polys, I don't even test them anymore to see if they are good or bad, I just test the old Mica caps which might be ok.

If the equipment runs cold, even if its old the caps might be ok, but my philosophy is, if its 20-25 years old or older, just replace the caps, and if it is old and runs hot, even more so.

 

[emrr]

Size plays into heat also.  There were multiple 25mfd / 100v can caps on some late 1930’s RCA that were turned on 24/7 for 50+ years in a radio station, each about 8” x 2”, all looked fine for value and ESR.  I just dealt with re-stuffing some 1933ish film cap cases, all 1 and 2mfd film types half the size of a deck of playing cards.  Some gone, but others very close on value and low ESR. It can be random what’s lived and died.