Broken SVI728. Schematics?

Por skumlerud

Resident (55)

Imagen del skumlerud

07-12-2009, 21:03

My SVI728 is broken. +12V and +5V is dead on the PSU/video board. I have been googling for the schematics but with no success. Does anybody know where I can find the schematics?

An alternative is to replace it with a small AT PSU I have, but I would then need to find a suitable video signal on the motherboard. The VDP is covered by a heatsink, does anybody know exactly which version of the VDP is fitted?

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Por Eugeny_Brychkov

Paragon (1232)

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08-12-2009, 20:39

It seems this model features external AC power supply which should have ~16v and ~9v at its output. Did you check that?

Por skumlerud

Resident (55)

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08-12-2009, 22:15

Yes, the external PSU is OK. It works fine with my other SVI.

I've checked the PSU board. It's dead. I've successfully started the 728 by hooking it up to an AT PSU, but there are some issues with this.

1. I can't just remove the PSU/video-board because there's no composite video on my VDP (a 9129). There's sync and luminance on pin 36, so I can get a b/w picture, but I need some way to generate a composite signal.
2. The PSU/video board has two leads to the motherboard that I'm not 100% sure what do (SW1 and SW2). SW1 appears to be GND, but I don't know about SW2. If I disconnect it, the SVI will still boot but freeze as soon as I touch a key. When it's connected, everything works but games are a bit faster than I'm used to. Could this be some sort of 50/60Hz detection?

Btw in addition to a broken PSU board, the keyboard is also not working properly (abused by it's previous Nemesis2-playing owner). Are there are PS/2-adapters available?

Por RetroTechie

Paragon (1563)

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09-12-2009, 00:24

Yup it's a pity that video & powersupply share the same circuitboard, makes replacement a PITA.

But you say that the external AC power brick is okay? IIRC, there's some powertransistor-circuitry on the board to regulate 5V / 12 / -12V? If so, I'd just -temporarily- disconnect the DC lines to motherboard. Then remove all that powertransistor-crap. Then fix AC -> DC part (diodes / buffer elco's) if necessary. And then throw in plain 7805 / 7812 / 7912 regulators (with a bit of re-wiring), using the original cooling plate. Easier than rebuilding the video output, I'd say.

Btw. it's not simply a blown fuse on that power/video board, is it?

Por flyguille

Prophet (3031)

Imagen del flyguille

09-12-2009, 03:02

SW1 & SW2 is a switch, it goes to the cartridge door switch if it has one, when you inserts a cartridge the switch of the door TURN off the PSU, so inserting the cartridge with the MSX powered ON will not results in damage.

IIRC it is an "normal close circuit" it is to say that on normal condition SW1 shorts with SW2, then when you inserts the cartridge the switch gets OPEN, so it turn off the PSU, when the cartridge is fully inserted the switch gets CLOSE and the PSU turns ON.

(or maybe the logic is inverted) "normal open circuit"

in both cases it is just the connector for a switch....

Por skumlerud

Resident (55)

Imagen del skumlerud

09-12-2009, 12:23

But you say that the external AC power brick is okay? IIRC, there's some powertransistor-circuitry on the board to regulate 5V / 12 / -12V? If so, I'd just -temporarily- disconnect the DC lines to motherboard. Then remove all that powertransistor-crap. Then fix AC -> DC part (diodes / buffer elco's) if necessary. And then throw in plain 7805 / 7812 / 7912 regulators (with a bit of re-wiring), using the original cooling plate. Easier than rebuilding the video output, I'd say.

Yes, I could solve it this way. I do have a small AT PSU that fits nicely in the case though, so I'd rather use that one if I can solve the video issue.

Btw. it's not simply a blown fuse on that power/video board, is it?

There are no fuses. +5V and +12V fluctuates. I think it's a problem with the rectifier.

Por skumlerud

Resident (55)

Imagen del skumlerud

09-12-2009, 12:24

SW1 & SW2 is a switch, it goes to the cartridge door switch if it has one, when you inserts a cartridge the switch of the door TURN off the PSU, so inserting the cartridge with the MSX powered ON will not results in damage.

Thank's, very interesting :-) This makes it a lot easier to replace the PSU board if necessary.

Por skumlerud

Resident (55)

Imagen del skumlerud

10-12-2009, 22:34

So I successfully replaced the PSU/videoboard with an AT PSU. It turns out that my VDP outputs YPbPr, but my TV does not accept this signal so I guess it's not up to today's standard. I will have to either repair/reuse the videoboard or find some other way to convert YPbPr to composite or RGB.

Another question though: Does anybody know what the 728 use -12V for? It seems to work just fine without it.

Por Eugeny_Brychkov

Paragon (1232)

Imagen del Eugeny_Brychkov

11-12-2009, 07:03

Sound mixing and amplification

Por skumlerud

Resident (55)

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12-12-2009, 19:31

Sound seems to work fine without -12V too, but I'll take your word for it ;-)