Checking ESR on old caps before recapping

[pugix]

I recapped three Power One supplies before I got an ESR meter.

https://pugix.com/synth/recapping-power-one-supplies/

When I got to the fourth, I tested the ESR with the in-circuit tester, a Signstek MESR-100 V2. All the caps tested good for ESR. So for the time being, I've held off on recapping this one. I suppose it doesn't hurt to replace good caps, but I'm confused about how to decide to recap a power supply. I've watched a number of videos on reforming old electrolytic caps in vintage equipment, but I'm not sure those are relevant. How do you decide to recap? Just age of the supply?

An aside: I tested a bunch of new electrolytics in my stock and found one bag of them to all have too high ESR. Thought that was kind of weird, but I had bought them a long time ago (10 years maybe).

Thanks,

[MikeDB]

Try biasing the high ESR caps with about 50% of their rated voltage for a week. Doesn't always work as electrolytics can do this in various ways, but saves throwing them.

As for recapping, I'd say most of the time it isn't necessary, but if you are taking a mixer or complex synth apart it saves having to do it later on.

[KSS]

how to decide to recap a power supply.

Oscope and load. Variable load preferred. <--Can be several power resistors and a switch to introduce different load and transients.
Above assumes you don't have dedicated electronic variable load*. <--One might press a synth into such duty.
*There are articles oline to build them.
But a scope and a load is the first part of the answer. Followed by testing under its intended load. Initial in circuit testing is good also, as long as the PSU is confirmed as not creating issues for whatever it's attached to. The difference between I've got this thing that's working but I wonjder if it could be working better vs I just got this thing that's been sitting around -or has unknown history. <--Test this one's PSU separate from its circuit first!

I've watched a number of videos on reforming old electrolytic caps in vintage equipment, but I'm not sure those are relevant.

Sure they are. Details matter though. The online 'recap wars' have led to MUCH needless recapping and replacement of caps known to be really good and with great long life. AKA don't throw out the baby with the bath water. And if it aint broke.. But reforming is a real thing and can extend the life of some NOS and long idle caps.*

How do you decide to recap? Just age of the supply?

See above. I've never used it's old as a primary reason. There needs to be something else bsides that driving the decision. I'm in there already -as MikedB said- *may* be worth it. Depends greatly on how hard whatever you're inside of is to get inside again later!
The *best* decider is a scope and a load, followed by its actual load in working configuration and scoped.

An aside: I tested a bunch of new electrolytics in my stock and found one bag of them to all have too high ESR. Thought that was kind of weird, but I had bought them a long time ago (10 years maybe).

Yes. Leakage isn't -solely- dependent on time in circuit. But again -and as you found- they don't all age the same. I tend to avoid the surplus avenue for elcaps.

*But -and it's a big one- look carefully into what you're putting in new. This isn't the place for no-name caps. 105 used mean something, now it can just be a marking intended to help sales. You can find many photos online of big caps with different little caps inside them. And I'm not talking about those in old tube circuits. Buyer beware -and aware- is more important than ever. If the caps you're replacing are intended as power filters, then be sure to use caps RATED FOR THAT USAGE. It's not the same as any other elcap.

Edit: Anyone working regularly on synths should have at least a pair of 12 and a pair of 15 ohm power resistors, and a few 5ohm too. Parallel sand types can be fine, but nothing really beats the aluminum integrated heatsink types. Mount them on a plate with fan intended to keep CPUs cool and you've got a good means to test PSU basics.

[EATyourGUITAR]

If you have the ESR meter you still need to know how to use it. The ESR readings that you get fall into 3 categories. 4 if you include the "it's dead jim" category. The super low ESR caps are designed for use in computer motherboards. If you follow an online guide for repairing a computer you may have unrealistic expectations of what could be normal acceptable levels of ESR for a synthesizer. If you get good at identification of the make, model, age of a capacitor then you also have real data from datasheets so that you can also know exactly when a cap is out of spec, in spec, almost out of spec (near end of life).