exxos's Atari, Amiga & retro forum

Among the many ST mandatory fixes I want to replace some capacitors in my STe....

First, there's 4700uF axial-capacitor replacement]the big 4700uF/16V axial electrolytic capacitor (C103) close to the power connector.
I already have one of Exxos' replacement capacitor boards, but since then I've acquired another STe which I also want to re-cap.

I'm about to order electronic components for some other projects, and it appears that the 4700uF capacitors (which were unobtainable when the above article was written) are now available, such as these from Farnell.

Unfortunately, axial leaded ones are scarce, but bending the legs of an equivalent radial leaded capacitor should do the trick: (image source: Console5 Tech wiki)

So is it just a matter of replacing said capacitor with any quality-branded and similarly valued (4700uF, 16V or higher voltage) and low ESR variant such as Panasonic's FR- or FS-series with a long lifetime (i.e. 10000 hours) and high temperature rating (85 or 105 C) capacitor?
As for low-ESR I'm not sure which values to look for (I've also heard that a low ESR value isn't always a universal solution to replacing ageing capacitors, but you should choose replacement capacitors with similar ESR values as the originals had (when they were new), so it's a little hard to know what to order).

Next is the STe reset fix which involves two electrolytic capacitors (C100 and C102). I've read the whole article, and understand that C102 (22uF/16V) is to be replaced with the same value, but when it comes to C100 (10uF/35V) I don't know what to make of the article.
Is it safe to assume that I can just get the two capacitors with the same values (and a quality brand from a reputable dealer) to avoid any second guessing and experiementation? But what kind of ESR values should they have?
And which capacitors does the Exxos reset capacitor circuit fix kit contain?

For capacitor replacement I always replace with the same value and if possible a higher voltage and temperature rating.

ESR is probably not something I'd worry about for an STE to be honest, but if you want to that's up to you, though I think they are generally more expensive. Obviously there are certain scenarios where you'd want low ESR.

As for replacing axial caps with radials it's something I've done from time to time but I'd always recommend putting a piece of heat shrink sleeve over the capacitor to hold the bent lead in place and insulate it, like this:


One thing to note, and I don't think the ST range has any, but the Atari 8 bit range does, there are non-polarised electrolytic caps known as bi-polar caps, distinguished by having BP on the sleeve and AFAIK these can't be replaced by polarised caps.

Thanks for clearing all that up!
And excellent advice about the heat-shrink sleeve.
So ordering new ones turns out not to be as complicated as I first thought :lol:

Talking about low-ESR: back in the 80s (when I first started getting into electronics and building stuff) there wasn't any talk of ESR values (in fact I only learnt about it a few years ago). For electrolytic capacitors you'd just get the correct capacitance rating, voltage and if necessary check the physical size to fit the PCB. That was it.

But these days, with ESR being a "thing" to consider....
when choosing replacement capacitors for the ST (or similar electronic devices from that era), among the many capacitors available from reputable dealers such as Farnell, Mouser, Digikey and so on; which ESR-values would be considered OK and which values would deviate too far from the capacitors that the Atari factory originally used, possibly causing problems?
(From what I've learnt reading about ESR in various forums, websites etc., the general consensus is that "lower ESR is better", but after some more in-depth research, I've found out it's more like "it depends on the circuit design").

@Pacman

TBH I don't really much bother about ESR because nobody ever seemed to on the old computers I'm usually working on. Standard caps will do just fine, and TBH I've never had to replace anything other than PSU caps in any of my ST/F/M/Es.

The only time I did was when I was fixing my audio mixer and needed specific caps.

Vishay do axial 4700uf's, I have a bunch of these in stock for when I re-cap an ST or Falcon:
https://no.farnell.com/en-NO/vishay/mal ... dp/1695261

rubber_jonnie wrote: 23 Nov 2024 12:32 TBH I don't really much bother about ESR because nobody ever seemed to on the old computers I'm usually working on. Standard caps will do just fine, and TBH I've never had to replace anything other than PSU caps in any of my ST/F/M/Es.

I see what you mean.
Are "low-ESR" caps marked as such and "standard" caps usually don't advertize anything about ESR values?
In other words, how do I avoid getting low-ESR caps? (that's why I was asking which ESR values were normal for caps from that era/quality standard that Atari ST computers were manufactured.

Steve wrote: 23 Nov 2024 13:00 Vishay do axial 4700uf's, I have a bunch of these in stock for when I re-cap an ST or Falcon:
https://no.farnell.com/en-NO/vishay/mal ... dp/1695261

Yes, I noticed those too, and should be a nice direct replacement :thumbup:
But it's quite expensive and even though its life expectancy is 8000 hours there much cheaper radial capacitors (by Panasonic, Vishay, Kemet, Epcos, Rubycon and Wurth) with 10.000 hours life expectancy. But then again I'm unsure about what are suitable ESR numbers.

Pacman wrote: 23 Nov 2024 15:11

rubber_jonnie wrote: 23 Nov 2024 12:32 TBH I don't really much bother about ESR because nobody ever seemed to on the old computers I'm usually working on. Standard caps will do just fine, and TBH I've never had to replace anything other than PSU caps in any of my ST/F/M/Es.

I see what you mean.
Are "low-ESR" caps marked as such and "standard" caps usually don't advertize anything about ESR values?
In other words, how do I avoid getting low-ESR caps? (that's why I was asking which ESR values were normal for caps from that era/quality standard that Atari ST computers were manufactured.

This is my point, don't get hung up about the ESR, just buy the caps you need. If you're buying somewhere like Digikey, Mouser, Farnell etc they may well have the datasheet and it will tell you the ESR, but it's a bit of a moveable feast because you will get different results depending on the frequency of the test and for what you're doing the ESR won't likely matter that much.

I've replaced hundreds of caps and have never worried about the ESR, which in my experience is more of an issue as the caps fail and their resistance increases/capacitance decreases.

Just get the same capacitance, the same voltage or higher and I usually go for 105C rather than 85C.

We're talking about 30-40 year old caps, so whatever you install is likely better than the originals.

ESR is only really something you wanna be selective over in power supplies. The ESR of the original caps are absolute crap compared to even average caps these days, so whatever you get will be an upgrade.

And in regards to 8000 hours ... there's about 8000 hours in an entire year. So you'd have to leave your ST on constantly for a whole year to meet that. I bet even the old caps in your ST haven't even reached 8000 hours. Gotta put this in perspective :)

rubber_jonnie wrote: 23 Nov 2024 16:04 I've replaced hundreds of caps and have never worried about the ESR, which in my experience is more of an issue as the caps fail and their resistance increases/capacitance decreases.

Just get the same capacitance, the same voltage or higher and I usually go for 105C rather than 85C.

Not overthink it :D
I understand.

Steve wrote: 23 Nov 2024 16:21 ESR is only really something you wanna be selective over in power supplies. The ESR of the original caps are absolute crap compared to even average caps these days, so whatever you get will be an upgrade.

So basically anything goes (on the motherboard)?

And in regards to 8000 hours ... there's about 8000 hours in an entire year. So you'd have to leave your ST on constantly for a whole year to meet that. I bet even the old caps in your ST haven't even reached 8000 hours. Gotta put this in perspective :)

Hmmmm... hadn't thought about it like that.

This guy advertises axial capacitors. I am assuming that mF doesn't mean millifarads. I have never bought capacitors from him, but other things. For me, the customs here are much friendlier towards packages from the USA than the UK, for example, and it only takes about on week longer, although the US postal service can be quite variable.