Trying to understand how to find best replacement cap for old parts.

[samoelec]

Disclaimer: This is the first time I step into the realm of analog electronics and my natural language is not English, so please forgive me if I failed to use the correct terms when describing things.


Recently I purchased a Tektronix TDS1012, a 20 years old product for cheap and I thought it could be a nice opportunity for playing (i.e. recapping) and learn something in the process.
The power supply board is exactly the same from this post so I'll just steal it for now.

I'm trying to find replacements for all those Lelon electrolytic caps, but then quickly realized the datasheets for these Lelon caps are probably long gone or the information are not complete.
For example, it uses a huge 68uF 450V capacitor as output filter, but in the datasheet I found on Mouser, they didn't even list 68uF so it seems they no longer produce 68uF caps.

For now, I can only deduce from the nominal characteristics from the data of 82uF 450V LSG cap:


that a 68uF one will have ripple current less than 0.45 and ESR higher than 2.427 ohm.

One of the 68uF 450V cap I can find in my local electronics parts store is a Hitachi HU3 cap, which I also found the datasheet. (p.177 in PDF reader or P.33 on the page footer.)


This cap have ripple current higher and ESR much lower (1.3 ohm, almost half) than the Lelon cap, which makes me start to wonder: How to judge a replacement cap is "good enough" or "close enough" to the ones that are no longer in production? As finding a exact replacement can prove to be very difficult


After some Googling, I understand that higher ripple current is generally a good thing, but whether lower ESR is good or bad depends on the circuit design.
My question is, what does it depend on and how do I see it on the circuit?

Take this 68uF 450V capacitor as example, I can see that it is used to filter the output of the bridge rectifier, so higher ripple current is good. But I can't determine if using a capacitor with half of the original ESR is good in this scenario.
Also, it's more difficult for a newbie like me to identify what all other electrolytic capacitors are meant for in the case of this power supply board.

I know I should probably Google more myself, but sadly I don't even know what keywords are suitable for what I'm trying to learn.
Any suggestions and knowledge sharing are welcomed!


P.S.
I can provide much more clearer and hi-res pictures of the board if necessary.

[momaka]

By the looks of it, that 450V 68 uF capacitor is filtering just rectified mains power. So in that case, ESR ratings are pretty much irrelevant.

While I generally support and encourage people to do full recaps when dealing with a power supply, I sometimes do make an exception for the mains filter cap. In particular, if the power supply does not have Active PFC (APFC) circuit, the mains filter cap is not under that much stress. As such, even 2nd/lower-tier brands like Lelon, Teapo, OST, and etc. can still last for many many years without causing any issues. Of course if cost is not an issue (and since this is for an oscilloscope - a rather useful piece of test equipment), then go ahead and replace the mains filter cap.

In regards to your question about ESR and how far lower or higher you can go - that pretty much applies to the output of switch mode power supplies only. In general about 50% +/- deviation usually will almost never cause any issues, and some PSUs can tolerate even further deviation. +/-100% is not uncommon, and a few can work with even 2-3x lower or higher ESR.

Typically, the types of power supplies that are more likely to have an issue with large ESR deviations are the ones that use a continuous design (i.e. typically the presence of a large output toroid on the secondary side before the output capacitors.) On the other hand, discontinuous (flyback) designs are very very tolerant of ESR deviations. If you go way too low on the ESR with these, sometimes you might get a high-pitched whine from the PSU at low loads. But it's nothing dangerous for the PSU. I have recapped the 5VSBs circuit of many ATX PSUs with ultra-low motherboard-grade caps, and it's never been an issue.

[petehall347]

68uf mouser ...
https://www.mouser.co.uk/c/?q=450v%2068uf

[momaka]

By the looks of it, that 450V 68 uF capacitor is filtering just rectified mains power. So in that case, ESR ratings are pretty much irrelevant.

While I generally support and encourage people to do full recaps when dealing with a power supply, I sometimes do make an exception for the mains filter cap. In particular, if the power supply does not have Active PFC (APFC) circuit, the mains filter cap is not under that much stress. As such, even 2nd/lower-tier brands like Lelon, Teapo, OST, and etc. can still last for many many years without causing any issues. Of course if cost is not an issue (and since this is for an oscilloscope - a rather useful piece of test equipment), then go ahead and replace the mains filter cap.

In regards to your question about ESR and how far lower or higher you can go - that pretty much applies to the output of switch mode power supplies only. In general about 50% +/- deviation usually will almost never cause any issues, and some PSUs can tolerate even further deviation. +/-100% is not uncommon, and a few can work with even 2-3x lower or higher ESR.

Typically, the types of power supplies that are more likely to have an issue with large ESR deviations are the ones that use a continuous design (i.e. typically the presence of a large output toroid on the secondary side before the output capacitors.) On the other hand, discontinuous (flyback) designs are very very tolerant of ESR deviations. If you go way too low on the ESR with these, sometimes you might get a high-pitched whine from the PSU at low loads. But it's nothing dangerous for the PSU. I have recapped the 5VSBs circuit of many ATX PSUs with ultra-low motherboard-grade caps, and it's never been an issue.