NOS 520STFM Motherboard/PSU care

[polbit]

I got my hands on a brand new, never used 520STFM. Before I fire her up, I just wanted to understand few things. As far as electrolytic caps, do I have to do any conditioning at this point, or just try and replace if needed? Should I recap the PSU anyway, or wait for issues? Anything else?

[Tomswork]

My experience has been do not trust old electrolytic capacitors. If they were stored or used in circuits they leak or dry out.

Tom

[frank.lukas]

Leaking electrolytic capacitors are rare. My experience are that the Atari PowerSupply mostly good quality even after 30 years. But recap is not a bad idea but your have to use good material ...

[exxos]

PSU's get pretty complicated as some common ones like the SR98 I have been seeing 100s of failures on these from over 25 years ago due to bad caps. Other power supplies like the DVE types stand the test of time much better for some reason.

Just for initial testing should be fine either way, but really this is why you need a scope to see what ripple there is on the power supply. Just because capacitors look okay does not mean they are.Usage and storage temperatures all play part as well. If you really want to look after your machine and do some basic servicing!

Of course I wrote a huge page on all this... https://www.exxosforum.co.uk/atari/last/psu/index.htm

[PhilC]

@polbit its a difficult one. You'd think not used at all for 30 years, so its going to be like new.

I would suggest you get it serviced. A lot of things could happen over the years, electrolytic capacitors can deteriorate due to age but also its possible that your new looking AT has been exposed to damp over the years.

I'd not just plug it in and hope for the best unless you are able to cope with the possible consequences. So, in short, play it safe and service it, then hopefully you'll have many years of good service from your new old ST.

P.s. if getting it serviced then check out the Mandatory fix list for a few other tweaks to make it more reliable.

Edit: knew @exxos would jump in here before i finished typing :lol:

[polbit]

PSU's get pretty complicated as some common ones like the SR98 I have been seeing 100s of failures on these from over 25 years ago due to bad caps. Other power supplies like the DVE types stand the test of time much better for some reason.

Just for initial testing should be fine either way, but really this is why you need a scope to see what ripple there is on the power supply. Just because capacitors look okay does not mean they are.Usage and storage temperatures all play part as well. If you really want to look after your machine and do some basic servicing!

Of course I wrote a huge page on all this... https://www.exxosforum.co.uk/atari/last/psu/index.htm

Ideally I would love to get one of your power supplies, eagerly awaiting stock :)

I have read your guide, or should I say guides, and have learned a lot. In this case I'm just trying to understand if there is any difference between unused caps for 30 years vs used then stored, and it sounds like there really isn't. I have one known good PS that I currently use, Mean Well, and I used that to try it. Everything worked perfectly, fortunately the disk drive doesn't use the belt...

I will test the original PS (it is the SR98...) before I use it, been meaning to get a scope anyway.

[tzok]

Don't replace parts without checking them, especially if it really is a NOS unit. Would you like to have kidney transplanted without any diagnostics, just because you are getting old, but otherwise in a perfect health? I bet - no. If it is an original part and fully working - keep it.

This is Mitsumi PSU from my Atari 1040 STe. It is now 29 years old, definitely not a NOS:

mtm.jpg

Ripple under 1A load is under 50mV.

This is brand new MaenWell RD-50A:

mwl.jpg

Ripple is about 30mV under same 1A load.

Both are perfectly fine for ST.

[rubber_jonnie]

Ok, so I'm going to stick my oar in here and add my opinion.

If you're going to power it on and test, then do so with a load, such as the load resistors like @exxos has in store, rather than use the ST as a load.

This gives you a bit of a safety net in that if a component in the PSU does fail under load, the worst that will happen is that the PSU dies.

I've had a quite a few PSUs come through my little workshop, and as a rule of thumb, I will replace the capacitors, because despite them appearing to be good as other people have posted, it is all too easy to forget that these items have mechanical properties as well as electronic properties.

Electrolytic capacitors have a rubber bung which acts as a seal around the base, and over time, that rubber can shrink and leak. I know this because I've recapped 2 PSUs recently, one from an Amstrad portable and the other the analog board on a Mac Classic.

The bungs had failed on both sets of capacitors, and whilst there was no sign of electrolyte on the PCB, it wasn't until I removed them that I found the bases of the capacitors to be wet with electrolyte, and obviously leaking and not working as advertised.

To be fair to Atari PSUs, I have never seen any leak, but all have benefited from cap replacement.

My point? Don't assume that because they look fine they will stay fine. At the end of the day they are still 25-30 years old and the rubber bungs are still subject to an aging process.

I don't go along with the kidney analogy because with a PSU the parts are cheap and easily replaceable. It's called preventative maintenance and means you have taken pro-active action to ensure that components that are known to suffer from an aging process have been replaced with known good. You have your car serviced don't you? It's the same thing, trying to pre-empt and prevent failures.

It's an approach I wished I'd taken with both the Mac Classic and the Amstrad, because if I'd done so I wouldn't be trying to repair both of them right now.

My 2 cents done.

[Steve]

I prefer this analogy to the kidney one:
Even if I managed to find a 30 year old NOS classic car, I would still change its oil :)

[rubber_jonnie]

I prefer this analogy to the kidney one:
Even if I managed to find a 30 year old NOS classic car, I would still change its oil :)

Exactly.