MSX dual voltage PSU - making one from scratch - Page 1
Forum section: electronics / general

author: hexbus
author_rank: Contributor
timestamp: December 13, 2024, 09:15:25 pm
content: Longtime reader, second-time poster.I have a Japanese MSX2 without a PSU, requiring 9VDC and 18VAC. The 18V is converted to ±12V (low current) for the cartridge port, while the 9V is regulated down to 5V. I’ve included a picture of an example PSU.Previously, I bypassed the multi-voltage setup by providing 5V directly and using a ±12V DC-DC converter. (And deleting a lot of components in the onboard power circuit.) This time, I want to use the original PSU port and have a compatible connector from an old HP power supply. The system requires 9VDC, ground, and 18VAC on three pins.Can I simply connect a regulated 9VDC PSU and an 18VAC PSU, tie the DC ground to one of the AC conductors, and fuse the other two connections (0.2A for 18VAC, 1A for 9VDC) to their respective pins? Or are there additional considerations for combining these power supplies to make it work?I found a nice Jameco regulated DC PSU and another nice Jameco 18VAC PSU that look suited to the task.Thanks, all!

author: Andy Chee
author_rank: Super Contributor
timestamp: December 15, 2024, 05:40:59 am
content: A schematic would be required for definitive confirmation.But it sounds like the original 18VAC may have been a centre tapped 9V-0-9V supply, feeding a bridge & 7812 & 7912 regulators.If your replacement 18VAC supply is not centre tapped, you will have to convert your existing bridge to a voltage doubler configuration (easy modification).

author: hexbus
author_rank: Contributor
timestamp: December 15, 2024, 07:39:35 am
content: [ Specified attachment is not available ]As far as the PSU itself (wall wart) is concerned, it was simply three pins: AC, DC, and ground. 9V DC out of one pin, 18V AC out of another pin, and a ground. No idea if there’s a schematic of the actual adapter.The closest A1-FS model to mine has a schematic listed and is pretty close, eyeballing it. Attaching it from here:http://niga2.sytes.net/msx/FS-A1_PWB-REV02_schematic_REV01a.pdfAttaching some close up pics of the bottom area of the board - including the one I made the 5V mods on and the unmodified one.

author: hexbus
author_rank: Contributor
timestamp: December 15, 2024, 07:54:52 am
content: Additional pics

author: tooki
author_rank: Super Contributor
timestamp: December 15, 2024, 09:31:15 am
content: Quote from: Andy Chee on December 15, 2024, 05:40:59 amWhat gives you the idea that it’s center tapped? The example power supply (which I interpret to be an example of an actual MSX power supply, based on the connector and input voltage) doesn’t. And I don’t see how you could have both 9V DC and center-tapped AC on a 3-pin plug.But moreover, just look at the PCB in the very first picture: it clearly labels the 3 pins as AC, DC, and GND. And that matches perfectly with the AC adapter shown.The schematic posted later agrees with this.

author: hexbus
author_rank: Contributor
timestamp: December 15, 2024, 09:31:43 pm
content: Here’s the schematics for the PSU itself.Back to my original question…. Two isolated linear wall wart power supplies, 9V DC and 18V AC. Can I create a “ground” between the 9V DC and 18V AC PSUs and just use two wall PSUs to create my one “PSU”?The MSX itself is doing much of the regulation here after the original PSU does rudimentary filtering with that large cap. I can likewise install a large capacitor too externally for filtering if needed :-)

author: Andy Chee
author_rank: Super Contributor
timestamp: December 16, 2024, 06:51:06 am
content: From your provided schematic, it looks like you're good to "common GND" your separate 9VDC and 18VAC wallwarts

author: hexbus
author_rank: Contributor
timestamp: April 05, 2025, 04:29:45 pm
content: A bit of a follow up....I constructed this circuit, and it works - but I'm getting a carrier of 18VAC on the 9V side. The 9VDC adapter is a switched PSU like this:https://www.jameco.com/Jameco/Products/ProdDS/2247082.pdf. The 18VAC is ahttps://www.jameco.com/Jameco/Products/ProdDS/2179622.pdfJameco linear based PSU.I did add a decoupling capacitor to the circuit (6800uf), but it doesn't seem to do anything to negate the AC. The oscilloscope (which is on a nice isolation transformer) showed a perfect 60Hz sine wave at 20V (not under load.)Thoughts on what I need to change to mitigate or change the circuit? Thanks!

author: Andy Chee
author_rank: Super Contributor
timestamp: April 06, 2025, 12:50:04 am
content: Quote from: hexbus on April 05, 2025, 04:29:45 pmTry again, but with the isolation transformer powering the MSX, not the scope.

author: hexbus
author_rank: Contributor
timestamp: April 08, 2025, 01:01:26 pm
content: Tried with the MSX on the isolation transformer and the oscilloscope on house power.Same issue. Around 9V but 60Hz wave is riding on it from the 18VAC PSU. (Sorry for the upside down pic - no idea why my iPhone did that.)