Output power of SVI-738 built-in power supply?

Por Mirsku

Supporter (4)

Imagen del Mirsku

26-01-2010, 04:19

Hello.
Does somebody know what are the Amperes of the each line of Spectravideo SVI-738 Express?
I´m asking because I don´t have the original power adaptor that gives 16V AC to the inner adaptor of the 738, and all testing before that I did using 12V DC 2A power adaptor because I didn´t find any power supply that gives the correct output.

Getting 16V AC 2A powersupply is completely impossible in Finland, so I thought if I could use power supply from broken Amiga 500, make nice connectors for both side and connect it directly into mainboard and remove the original that is inside.
Old homecomputers should always donate spare parts to others when they die Smile

A500 power supply is the heavier model and gives: 5V 2.5A, 12V 1A, -12V 0.1A, and ground, so it gives the same voltages as the original that is inside.

SVI power supply has two 5V and also two 12V cables but they are connected in both power supply and mainboard so it is physically possible to connect it, but the Amperes of the 5V line seems to be a little too much if compared to missing original that gives total 2A to the whole inner power adaptor board.

Does somebody know if the Amperes will be fine or are they too much?
If using A500 power supply works then it leaves nice empty place to add board with capasitors for RGB output board.

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Por RetroTechie

Paragon (1563)

Imagen del RetroTechie

26-01-2010, 08:08

Getting 16V AC 2A powersupply is completely impossible in Finland
No it's not... AC is simply what you get out of a transformer before rectifying into DC. Read: any transformer that has around 16V secondary voltage and rated 2A or more could (in principle) do the job. No doubt a suitable transformer could be found where you live. Ofcourse to use a plain transformer, you'd have to build it into a case, add at least a primary fuse, cabling and (perhaps) power switch. A ready-made powersupply is easier because the 'electrical safety' part has been done for you.

Does somebody know if the Amperes will be fine or are they too much?
Too much? The powersupply doesn't pump a set amount of current into your machine... Smile The Ampere ratings of a powersupply indicate the maximum current you can draw from each voltage.

In MSX machines, the bulk of the energy use is through the +5V supply. Meaning: that A500 supply would be fine unless the X'Press uses more than 2.5A on the +5V line. Not sure, but I doubt it. You can feed a lot of logic circuitry with +5V, 2.5A and the X'Press isn't that big a machine.

But in your case I'd look around for a leftover laptop adapter. Many of those have output voltage just above what you need, and plenty reserve. If your Spectravideo expects 16V AC input, then it's okay to put in either AC or DC. Schematic says there's a switching regulator inside (L296 or L4964, can you confirm that?), so a higher input voltage won't generate much more waste heat. And I think the average laptop adapter isn't as bulky as an A500 supply (more modern tech). And easy to replace.

Por mohai

Paladin (999)

Imagen del mohai

27-01-2010, 10:24

I had the same problem as you.
I have been using an old 12V DC power supply with the DC board removed. It provides arrond 16 V AC. I do not know apmeres either, but it works fine.

I also tried one of the modern p¡ower supply from an ADSL router. This one delivers 16 V AC and 1 A. It works fine too, it is small and connector suits perfect for this model.