Panasonic FS-A1: power adapter again

By evpo

Supporter (3)

evpo's picture

12-06-2018, 07:41

Hi everybody,

I have bought Panasonic FS-A1 without the power adapter. I have seen several threads about replacement solutions. I have an unused ATX power supply unit that can give me +12VDC, -12VDC and +5VDC. Attaching this bulky PSU to the MSX computer is not a big problem for me.

I studied the schematics linked from this page:
https://www.msx.org/wiki/Panasonic_FS-A1#Fixes.2C_Repair_and...

I found four points where I can attach the power and ground.

Do I need to remove the elements on the board that convert 18VAC and +9VDC to +/-12VDC and +5VDC? I looked at the other solution with two cigarette lighter adapters (http://niga2.sytes.net/msx/mk2pw.html). The elements are removed there but I don't know if they are removed to make more space for mounting the new voltage converter or they have to be removed so they don't interfere with the directly connected power. In that solution the circuit is different from mine because it's FS-A1 mk2 but mine is FS-A1. If I need to remove the original power converter, which elements do I need to disconnect?

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By gdx

Prophet (2359)

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12-06-2018, 12:26

ATX power supply unit is not a good solution. If you do not need the FM-PAC you can use a simple DC 5V power supply. 2A or 2.5A should be enough.

The best solution is that:
https://www.msx.org/forum/msx-talk/hardware/panasonic-fsaa51...

No modification is required.

By RetroTechie

Paragon (1462)

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12-06-2018, 21:05

You can feed +5V, +12V and -12V DC directly to suitable points on the board (don't forget to wire up GND return!), BUT:

It would be wise to bridge the voltage regulators with a diode. Voltage regulators like 78xx family don't like their outputs powered with nothing on the input, especially if large capacitors are there, the reverse current can kill the regulator. A simple diode like 1N4001 will do. Marked end goes onto input DC point (like where input buffer capacitor is), the other end onto the regulated DC output (5V, 12V etc). For example see application notes in a 7805 datasheet.

Most floppy-less MSX machines only need +12V in analog sound / video circuitry, and -12V DC is often not used internally (appears to be used by some audio opamps in the FS-A1 though).

Given the low power requirements for +/-12V, I'd think most elegant is to use a single 5V DC input, and then add a 5V -> +/-12V DC converter module internally. Something like a 1 or 2 Watt type should do, shouldn't cost an arm & a leg... Smile Like how it's done in a 1chipMSX.

By gdx

Prophet (2359)

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13-06-2018, 02:37

RetroTechie wrote:

Given the low power requirements for +/-12V, I'd think most elegant is to use a single 5V DC input, and then add a 5V -> +/-12V DC converter module internally. Something like a 1 or 2 Watt type should do, shouldn't cost an arm & a leg... Smile Like how it's done in a 1chipMSX.

Yes, for example less than $6 on ebay for a 5V USB adapter + the DC-DC Converter.

https://www.ebay.com/itm/Practical-5V-3A-Power-Supply-Micro-...
https://www.ebay.com/itm/12V-Dual-Output-DC-DC-Converter-3V-...

Only the USB connector is missing.

By evpo

Supporter (3)

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13-06-2018, 13:05

Thanks for the detailed reply. I added the diodes to the picture below. Does it look OK? The +5V regulator is M5236L. Do I need the same bridge for it? Is it enough to attach the ground to GND_10 on the picture?

By RetroTechie

Paragon (1462)

RetroTechie's picture

13-06-2018, 23:40

Point is to make sure that when supply rails are powered directly, the big(ger) capacitors on inputs charge up, each diode bypasses this charge current around the semiconductors. Under normal operation it will block & act as if not there.

Diodes across IC16 (78L12) and IC17 (79L12) both look fine to me, polarity too. For the +5V: I'd place diode across the power transistor, from "+5V_MASTER" to + side of C1. So marked side to E(mitter), other side to C(ollector) of Q3. Preferably not solder directly to Q3 - transistors tend to be heat-sensitive, and die easily when overheated with a soldering iron! Shocked! Same goes for IC16/17 - transistor style TO-92 housing? TO-220 is more robust...

Didn't look it up, but IC18 will probably be some sort of shunt regulator, a glorified / adjustable zener diode (along the lines of extremely popular TL431). Probably serving to pull juuust enough current through the base of Q3 such that output stays at 5V.

By evpo

Supporter (3)

evpo's picture

17-06-2018, 05:55

I appreciate your help, RetroTechie. I followed all your instructions and made it work. I soldered three 1N4004 diodes in the places you told me.

gdx, I agree that ATX is not the best solution and thanks for your links. I used four big resistors to give ATX required load and they are very hot.

The final schematic:

The diodes are below. The ground is connected to C29:

J24 connects to -12VDC
J25 to +12VDC
J26 to +5 VDC
Picture:

The cables go out through the power button opening:

Frogger:

By Grauw

Enlighted (7545)

Grauw's picture

07-08-2018, 03:32

I just read in a Youtube comment by B. Young that the “HP SDD018-N1000” power supply meant for external HP CD-rom drives works out of the box for these Panasonic MSX2's. Supposedly they only cost $7 orso. Especially nice for US people since they take US voltages (100-120V).

Edit: Never mind, output voltages are completely different (5V / 12V vs. 9V / 18V for the FS-A1). May be a good source for the plug though.

By gdx

Prophet (2359)

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07-08-2018, 11:26

He modifed the power supply? If it is not the case, I advise against doing that. The FS-A1 will probably not support this long and -12V must probably not be there.

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