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Old 02-19-2023, 06:45 PM   #1
iMic
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Default Vintage AT Power Supplies - Selecting Capacitors

Hi,

Back after a long hiatus with some old power supplies that need repairing. I've recently picked up a couple of 386SX systems, two 486DX systems, a SiliconGraphics Indy and an iMac G3 that either have verified leaking capacitors in their power supplies, or are likely to in the near future. For the sake of simplicity, instead of tackling all at once, I'm starting with the 386's and one of the 486's.


The 386's power supplies are Seasonic SS4040A's, with caps that are visibly leaking, and 9 replacement caps per unit are required. Most appear to be from the Rubycon YXB series, although due to the placement of some others it's difficult to tell if all of them are.

Code:
10v 1000uF x3
16v 47uF x2
16v 470uF x2
25v 100uF x1
50v 22uF x1

Mains: "Omats K112" 400v 100uF x1
The 486 uses an ASTEC power supply, with caps that aren't visibly leaking, but while I have an order open and the power supply disassembled for cleaning, it's a good time to get some to have on hand. This unit uses a mix of Chemi-con capacitors -

Code:
Chemicon LXF

6.3v 1500uF x1
6.3v 2200uF x1
10v 270uF x1
16v 2200uF x2

Chemicon SME

25v 47uF x2

Mains: Nichicon CE 200v 470uF x2

For the 386's I'm leaning toward the Panasonic FC series. That's based on another post I read around here a while back that suggested they would be a fitting replacement for the YXB series, they're reasonably cost-effective, and they seem to be more readily available - more so than the other brands - from the various online electronic suppliers that operate in Australia. Other brands are available of course, but either not a complete range (so some mixing and matching is required) or in lower quantities / back-ordered to a future date.

As for the Chemicon LXF and SME series, I'm not sure what their ideal equivalents are.

These are relatively light-duty supplies, running infrequently and in not particularly hot environments (except maybe an Australian summer sometimes). I'd still opt for 105C rated components as that seems to be what was specified from factory, but other than that, I don't suspect they need anything too fancy or exotic.

And if anyone wants to know why I don't just replace them with a more modern power supply - they're AT by spec, but far from the standard AT form factor.


Cheers,
Mic.

Last edited by iMic; 02-19-2023 at 07:00 PM..
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Old 02-19-2023, 07:55 PM   #2
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Default Re: Vintage AT Power Supplies - Selecting Capacitors

Just curious, are the Rubycon YXB caps leaking electrolyte from the bottom of their bungs?

I know Nichicon PR and PL in old PSUs do have this problem from time to time (especially the PR series.) United Chemicon LXF might also be affected, but not as badly. Basically, all of the manufacturers that used Quaternary ammonium salts in their electrolyte are susceptible to leakage from their bungs... though some more than others, it seems.

UCC SME is an old discontinued series, I believe. Modern equivalent is SMG seires. You can also use Nichicon VR or Rubycon PX. Panasonic FC is entry-level low ESR. However, those will work fine for the SME's too.

Like you said, these PSU don't need anything too fancy. So anything like Panasonic FC will work fine.

As for the input high-voltage caps, just check them and feel free to leave them as-is, since they rarely fail, even if very old. But if you do replace them, you can go with either 105C or 85C general purpose series. Save the old ones too, just in case you need them for a future project / repair. They should still be good for re-use.
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Old 02-19-2023, 10:20 PM   #3
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Default Re: Vintage AT Power Supplies - Selecting Capacitors

Quote:
Originally Posted by momaka View Post
Just curious, are the Rubycon YXB caps leaking electrolyte from the bottom of their bungs?
They are - I haven't inspected mine yet, but these machines came in a lot of 7, five of which went off to different owners. Of the other 5, all of them had considerable leakage from the bottom of the caps, so I fully expect to see the same on these two.


I'll have a look at the Nichicon and Rubycon equivalents too. I don't mind spending a bit extra to get a better cap, as long as I can actually source them and they're correct for this application. Would also like to stick to one or two manufacturer series per supply so I don't end up with Rubycon, one Nichicon because it wasn't available in the Ruby, one Panasonic, etc throughout the unit.

The mains capacitors I figured would be fine. All except one unit currently powers up (although I don't run them for long, a half minute at most) so the AC side is working. Given that replacing those caps would add a substantial overhead to the cost of each repair, I'll leave those alone unless I have a solid reason to replace them.
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Old 02-19-2023, 10:52 PM   #4
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Default Re: Vintage AT Power Supplies - Selecting Capacitors

Quote:
Originally Posted by iMic View Post
They are - I haven't inspected mine yet, but these machines came in a lot of 7, five of which went off to different owners. Of the other 5, all of them had considerable leakage from the bottom of the caps, so I fully expect to see the same on these two.
Ah OK, thank you for confirming.
I do have one old-ish (mid 90's) AT PSU with those and Lelon caps. So will keep in mind to check them out before assembling back that machine when I get to it.

Quote:
Originally Posted by iMic View Post
I'll have a look at the Nichicon and Rubycon equivalents too. I don't mind spending a bit extra to get a better cap, as long as I can actually source them and they're correct for this application. Would also like to stick to one or two manufacturer series per supply so I don't end up with Rubycon, one Nichicon because it wasn't available in the Ruby, one Panasonic, etc throughout the unit.
The Nichicon equivalent to Panasonic FC and Rubycon YXB are the PW series. PS, PM, and PJ series should also be relatively close. And I think YXJ series from Rubycon are the nowadays modern replacements (more or less) for their YXB series. YXF also, but they are only available for select few sizes.
With UCC, you can still get LXZ series, which will cover OK for the LXF series. KYB might be getting a little on the low side of ESR, but should also be OK.

I tend not to "discriminate" when it comes to recapping PSUs and usually mix and match whatever are the cheapest (quality) caps at the time. So more often than not, my recapped PSUs have a mix of various Japanese brands in them.

Quote:
Originally Posted by iMic View Post
The mains capacitors I figured would be fine. All except one unit currently powers up (although I don't run them for long, a half minute at most) so the AC side is working. Given that replacing those caps would add a substantial overhead to the cost of each repair, I'll leave those alone unless I have a solid reason to replace them.
Indeed.
Large caps are expensive, so I leave them alone.

For modern PSUs with APFC circuits, though, they are always worth checking, because APFC tends to stress them extra and sometimes kill even good Japanese brands. Of course, when it comes to AT PSUs, those haven't even heard of such circuit, so nothing to worry about.
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Old 02-20-2023, 12:23 PM   #5
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Default Re: Vintage AT Power Supplies - Selecting Capacitors

For output caps, older series that are still available pose less risk of stability issues.

From Nichicon, the PS and PM series are RoHS-compliant versions of the 90s vintage PR and PL series, with basically identical specs. The PJ series is a little newer but probably won't cause problems, and the PW series a bit newer still. The PA series night be risky.

From Nippon Chemicon, the LXV series is approximately equivalent to Nichicon's PJ series, and the LXZ series is approximately equivalent to the PW series. The LXY series is a somewhat limited intermediate between LXV and LXZ.

Panasonic's FC series is approximately equivalent to the PW and LXZ series.
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Old 02-21-2023, 05:12 PM   #6
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Default Re: Vintage AT Power Supplies - Selecting Capacitors

Cheers for the advice. I opened the second 386 power supply and saw it used Panasonic caps across the board, so figured based on looking at the two that Panasonic FC's were probably a safe bet.

The ASTEC I'd planned to go a mix of UCC and Nichicon, but none of the recommended equivalents were available - different specs were either "on order" or "end of life" at Element14 and Mouser. Given the UCC LXZ series would cover for the LXF, Panasonic FC is approximately equivalent to the LXZ, and the FCs would also be a suitable substitute for the SME series - I just went with the Panasonics across the board for this supply too.

It'll probably be a couple of weeks before the order comes in, but when they do, I'll get them fitted and see how it goes.
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