SVI-728 5V problem

Por teroper

Rookie (21)

Imagen del teroper

04-06-2012, 18:53

Hi all,

I recently bought SVI-728 and was very thrilled about it. But sadly, with my luck, it almost immediately broked down. I played with it few hours and suddenly screen started to whirl and get ouf of focus. Then power went off and second later power came back on, but screen was whirling and text was very hard to read. Very soon it shutted down again. I unplugged it for a couple of hours and tried it later on same evening. Everything was fine for a while but after about ten minutes it shutted down again, and this time power never came back...

I measured PSU and readings from there was fine. From cartridge port i didn't get any readings from 5V pins, +12V was excact 12V and -12V was -13V. I opened it up to take a look inside of the MSX and nothing seemed to be fried, actually it looked very good from inside. From mainboard i did not find 5V readings at all. So the problem might be in power & videoboard. I tested this with external PSU taken from PC and placed +12V, -12V, +5V and ground signals straight to mainboard and then, miracle happened, MSX turned on and seemed to work ok Big smile So maybe all hope is not lost after all.
I didn't keep power on very long time though, because i don't know is it safe/wise to use this MSX bypassing power & videoboard with external power supply? Only checked quickly to see if there is any chance to get my MSX alive again.

But now i really don't know where to search problem from power & videoboard, didn't find any schematics from internet. I have read other posts regarding same kind of problems with SVI-728 but haven't found right answer. I need to know which parts needs to be checked just for 5V.

Any help would be highly appreciated.

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Por Jipe

Paragon (1603)

Imagen del Jipe

04-06-2012, 19:27

check the power supply connector

Por RetroTechie

Paragon (1563)

Imagen del RetroTechie

05-06-2012, 01:32

Power & video encoder is combined on 1 board, which makes it a bit difficult to troubleshoot issues like this. You might want to make a schematic of sorts of the power supply part (to understand how it works), and then do measurements to work out which part(s) are failing.

In such an old machine, electrolytic capacitors are always suspect. Also the VDP since it runs very hot (most MSX1's I have, will show some video issues after the VDP has heated up for a half hour or so). And the video encoder IC (on the power/video board) is known to fail sometimes.

I guess you can better judge what's easier - try & fix power supply, or hook up external one. In the latter case, I suggest you disconnect existing power supply somehow, so that there's no voltage on its output(s) without input voltage(s).

Por teroper

Rookie (21)

Imagen del teroper

05-06-2012, 05:26

Thanks for the answers.

I have to admit that i don't know much about electronics, but a friend of mine who understands better did have a look on power board. Power supply connector was working fine but he suggested that transistor TIP32 might be the problem because it gives readings 14V and other one gives 12V. So TIP32 needs to be changed. Does this seem to be right conclusion?

Por protek

Expert (123)

Imagen del protek

14-06-2012, 20:29

You could also check the fuses from the transformer brick. There are two of them. One supplies the 9VAC and the other the 16VAC.

Por teroper

Rookie (21)

Imagen del teroper

25-06-2012, 22:14

Great news Big smile
TIP32 replaced with TIP42C and now my 728 is alive again and works just like it should to.
Couldn't be more happy with it Wink Thanks to all for your suggestions.