CayMac IIsi Mean Well PSU Replacement

Fizzbinn

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Nov 29, 2021
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I purchased a couple of the IIsi Power Supply Replacement (Mean Well) PCB kits from CayMac Vintage a while back and wanted to post my build experience.

IMG_7949.jpg


The kit included the resistors, transistor and LED, I purchased the 4 Mean Well power supplies and the relay from Mouser:

Qty 1 - IRM-60-5 (5V 10A)

Qty 2 - IRM-10-12 (12V 0.850A)

Qty 1 - IRM-02-5 (5V 0.400A)

Qty 1 - SPDT 12A 5VDC Power PCB Relay

I used these fuse holders:

Qty 2 - Littelfuse Fuse Clips ACS PC 5MM CLIP BRASS/TIN-PLTD
https://mou.sr/48qaT8l

Here it is with logic board power cable harness and internal mains power connector (transferred from a dead original IIsi Sony PS, APS-06, 699-0567) plus the kit resistors, transistor, and LED as well as the fuse holders soldered in:

IMG_7497.jpg


And with the Mean Well power supplies and relay installed:

IMG_7500.jpg


And mostly buttoned up in the case from the dead original PS:

IMG_7502.jpg


On first test it seems to work just fine! ADB keyboard power-on, software shutdown, relay control of pass through power to monitor. This was in a fairly stock IIsi, 17MB RAM, slightly newer 80MB IBM SCSI HD and with the 25MHz clock crystal swap. I do want to test further with various PDS cards I have. Will report further in this thread when I get to that.

Another fun project! Perhaps a tiny bit tougher to build than the CayMac LC version of this but way easier than recapping a IIsi Sony PS. I've recapped 4 of those and it's a lot of work, the little daughter card is particularly fun. The IIsi Sony PS that I cannibalized for this was actually one of the four I had recapped but didn't work :(, unlike the other three that did work and were recapped identically... Not totally sure about all the relative merits but something about pretty much all new components in the original form factor has me really liking this approach.

p.s. I also love the hidden Lucy bones that get burried under the largest Mean Well power supply. I have to admit it took me a minute to get it, IIsi, Lucy. Lucy, IIsi. POWER UP LUCY indeed!
 
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JDW

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p.s. I also love the hidden Lucy bones that get burried under the largest Mean Well power supply. I have to admit it took me a minute to get it, IIsi, Lucy. Lucy, IIsi. POWER UP LUCY indeed!
I still don't get it, but I am now strangely compelled to watch I Love Lucy episodes on YouTube.

1760250370606.png
 
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egrath

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May 1, 2026
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Does anyone know if the Gerber files for this project are available? I want to build one of those for my IIsi, but the kits are no longer available from CayMac. The only one I could find online seems to be a newer revision which uses a Noyit AC/DC module, but I prefer the version pictured above with the individual Meanwell components.
With kind regards, Egon
 

Trash80toG4

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Apr 1, 2022
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BOM Mouser listing is a really nice touch @Fizzbinn! But @ $33.21 + PSU + PCB = ?

Someone at the MLA developed a PicoPSU based replacement for the IIsi. For sale or kit form, dunno at this point, but I sorta lost interest a while back when @zigzagjoe told me about their limitations when it comes to Rocket liftoff.


Now using PicoPSU in a couple of smaller projects, so reconsidering its suitability for the IIsi.
 

RetroViator

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I have built both versions of CayMac’s Power Up IIusi. The first used a PicoPSU, and the second is the model in this thread with separate Mean Well units.

I prefer the Mean Well version because it requires no modification to the mains outlet and it produces cleaner power. The PicoPSU version has noticeable ripple, but I suspect that was coming from the WX-DC2412 AC/DC converter, not necessarily the PicoPSU. I’ve tried looking for another AC/DC converter that would fit the PCB, but I haven’t found one yet. Also, both versions can have problems shutting down properly with certain logic boards.

At the end of 2025, CayMac had the original version available on his GitHub page so you could make your own boards, and he was selling the new version, but he has now taken down the original version from GitHub. I also see that he is not currently selling the Mean Well version, but that could change.

They are a bit pricey, not really CayMac’s part, but buying the collection of needed Mean Well PSUs.
 

RetroViator

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I should have added that I have a couple of the earlier PicoPSU boards, complete with parts (except with IIsi wiring harness and mains plug), available if anyone want to pay the shipping to get them.
 

JDW

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I don’t have a Mac IIsi, but I do have the Baby Face power supply that works in a Mac SE/30. I did a video about that, in fact. It uses the Meanwell modules.

The biggest issue I have with Meanwell modules is their voltage output is fixed. But the stock PSUs on the SE and SE/30 have a tiny POT inside that lets you adjust voltage. Even the Analog Board power supplies of the earliest compact Macs offered that. Sometimes, you need to tweak the voltage output a bit in Compact Macs to avoid it going so low the machine locks up.

So, is the IIsi different?

Will the IIsi’s 5V rail drop in voltage and stay low when you have the machine powered with a spinning platter hard drive and upgrades and installed?

If the voltage drops in your IIsi with Meanwell modules, how much does it drop and is that too low for the machine to technically be stable?