Revival of an Spectravideo 728

protek

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I recently got a Spectravideo 728 MSX from the Swedish cousin of the other bay. When I first tried it out, it powered up but I could not get a screen, not a proper one at least. The screen was light blue but there wasn't any basic prompt on darker blue. From time to time there were errant horizontal streaks, where I could see hints of the white letters of the basic prompt. I opened it up but didn't see anything out of the ordinary.

Later on, I tried to keep it running for a longer period of time to see, if warming up would make any difference. To my surprise, after about 10 minutes, the screen went blank and the power led died. First I thought it was the PSU but trying with another unit didn't make any difference. Both PSUs appeared to be working, when I tried them in my Spectravideo 328.

Now a bit of the anatomy of the 728. It consists of two boards: the main motherboard that has the main ICs and a video decoder/power supply board. The latter rectifies and regulates power to the main board as the "PSU" is a simple transformer, supplying only AC.

After the 728 went completely blank, I first measured the voltages coming to main motherboard. There were none, so something has died on the video/power board. It has few ICs for the video decoding but mostly just capacitors, transistors, resistors and diodes. I'll post some pictures later.

Now, if we think the power supply as a single entity, what is the first thing to check or change, when PSU stops supplyin DC? I measured the regulators and I could get voltages from all three at first but two gave about 1 V and one of those stopped giving any voltage.

I could really use some help from you HW gurus to get this thing up and running again.
 

protek

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Here are a few pictures of the video decoder/power supply board:
22062010194.jpg


22062010196.jpg


The bottom picture is oriented differently from the top. The PSU socket on the bottom side picture is on the top.
 

protek

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Today I learned how to master the solder wick. It's surprisingly easy, when you get the hang of it.

I changed all the rectifier diodes. There's still about ten electrolyte capacitors and three power transistors to go. Then I'm going to give it another go.
 

protek

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Changed also the caps and power transistor. The damned thing won't still power up! :mad:

Now what can I do next? As I don't have any schematics, should I just try to backtrack my way from i.e. +5V towards the rectifier diodes until I get voltages?

:help::help::help:
 
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spacerman

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any progress? If you have changed all TR's I'll suggest a change of ZD2 and ZD3
 

protek

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Just changed the power trannies at this point and measured ZD3 to be working.
I'm currently scratching my head with the big capacitors as the 5V line isn't getting any voltage. Even the AC lines appear dead, which would suggest a fault in the transformer. This is also odd because what are the odds that both of my SVI transformers would blow at the same time and they still seem to power up my 328. I guess I have to open them up, though.
 

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May be just the thermal protector that normally is inside the transformer's coil. Bypassing it with a drop of solder may help diagnosing the problem.
 

protek

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May be just the thermal protector that normally is inside the transformer's coil. Bypassing it with a drop of solder may help diagnosing the problem.

Ok, now you got me there. Is the protector somehow accessible so that it can be bypassed? The transformer does have two glass fuses, one for each voltage. Both seem to be ok. Have to try to get voltage readings from the plain transformer.
 

rkauer

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Unplug the computer from the mains and test if the transformer have some resistance (normally something around 200Ohm or very less). Also check the fuses for continuity.
 

protek

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Well, what do you know? Both transformers had blown a fuse from the 16VAC line. I had neglected this possibility as both adaptors had powered up my 328. But when neither of my two 328s refused to produce a picture even though they powered and after I found out that the video circuit needs 12 volts, I had to take this possibility into account.
 

protek

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The PSU circuit board is back online. It seems that something has fried on the motherboard side too, as the screen stays blank and caps lock led doesn't lit. :(

I wonder if this is a lost cause?
 

protek

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Here' some pr0n of the mainboard. Have to try to find a logic probe from somewhere.
10072010321.jpg


10072010232.jpg
 

protek

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Some update on this matter. I found myself a logic probe and did some probing with invaluable advice from members of the MSX Resource Center.
It seems that some of the data lines on the Z80 CPU are on constant low, instead of clocking and they show infinite resistance. It may be that I have to replace the Z80. :|
 

rkauer

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Did you check the traces for continuity before reaching this conclusion, mate?

Also check the solder joints before a final verdict.
 

protek

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Did you check the traces for continuity before reaching this conclusion, mate?

Also check the solder joints before a final verdict.

Actually, I did. Some of those traces didn't have continuity.
 

protek

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Some additional testing and it seems that I'm going to have to replace the basic ROM. Finding an empty EPROM was piece of cake. The EPROM programmer on the other hand is another issue.. :( Also I finding a Scandinavian ROM image isn't going to be easy. UK ones can be found but that doesn't quite match my keyboard layout.
 

protek

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Time to wipe the dust off from this old thread. The story behind my SVI-728 is that I got it from Swedish auction site Tradera.com, which is an eBay affiliate. It came to me in a big cardboard box and absolutely no padding, so my first thought was:"I'll be damned, if this thing will boot up properly." Of course, it didn't. Mostly black screen but I could see some scrolling streaks, which gave away that it tried to show the basic screen. The keyboard appeared to be working as caps lock led lit and ctrl-g produced a sound. This didn't last long as the screen soon went completely black and neither the power nor caps lock led lit anymore.

It turned out that the PSU or in this case the transformer had blown both of its fuses. I repaired the transformer but the damage had already been done. Power led lit but the screen stayed black and the caps lock led didn't lit and the ctrl-g didn't have any effect. Now, it should be mentioned that before finding out that the fuses of the transformer were blown, I had replaced all the caps, rectifying diodes and power transistors of the combined PSU/video circuit board. With the help of the wonderful people at msx.org, I did some testing with a logic probe on the mainboard ICs but that didn't produce anything informative, probably due to yours truly being n00b at electronics.

In addition to the caps, diodes and power transistors, I've replaced the Z80 CPU, the video decoder in the PSU/video circuit board and the ROM. The reason I changed the ROM was that, when I read it with Willem, it seemed to contain just: "..........." and nothing else. A ROM image that I found from the Internet contained more comprehensible stuff like basic commands and such. I burned a ROM based on that image but that didn't seem to help.

Now, when I boot the machine, the composite reception on the television is inwoked, so it is receiving a signal from the video decoder circuit. It just doesn't seem to get anything to display. Don't know, whether it is the graphics IC, Basic ROM, the Z80 or memory that is to blame here.

I'm trying to think of the next steps here. I probably should go through the pins of the most important ICs with a logic probe and write down, what the probe says. Other that I was thinking is to see, if any of the ICs gets hot. I'll probably also check the VCC lines of the ICs, whether they're getting the correct voltages. Is there anything else I could try?

The keyboard puzzles me a bit as it worked in the beginning and now it's not. If I could find out what causes this, I might get it working again. I tried to find something related to MSX architecture to be able to tell, what ICs are responsible of handling the keyboard (lighting Caps lock led, etc) but I couldn't. What chip handles the keyboard in for example Spectrum?

I've been told at MSX forums that it is not really worth fixing but I'd consider this as a personal victory, if I could get it back in the land of the living.
 

protek

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I tried measuring the output voltages of the PSU circuitry. The +5V line is giving out only +4.799 - 4.8 Volts. Is that too much below the operational threshold of the Z80 and other ICs? And remember, the PSU is inside the computer on a circuit board and the "brick" outside is only a simple transformer that supplies VAC. Also the caps on the PSU circuit board have been changed.
 

protek

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A new chapter was turned in this story as I and the 728 parted ways and it began its journey on last Tuesday to the isles of Britannia into more capable hands.

I managed to find a working one as a replacement and the old one will hopefully bring joy to its new owner.
 

protek

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Dusting up this old thread.

I recently acquired another SVI-728. This is the fourth. It was otherwise in working condition but the cartridge connector was damaged.
Edited_ImageEasyImageEditor_20140729_54_jpg.jpg

The connector is a 50 pin card edge connector with 2,54 mm pitch, so I thought I'd replace it.
Edited_ImageEasyImageEditor_20140729_55_jpg.jpg

De-soldering the connector turned out to be a tedious job as usual. Try as I might I couldn't get all the solder removed with my desoldering pump. I ended up heating each pin and pulling them out one by one.
Edited_ImageEasyImageEditor_20140729_52_jpg.jpg

I was going to use the connector I had in my parts drawer but it turns out the legs are too short compared to the original.
Edited_ImageEasyImageEditor_20140729_53_jpg.jpg

Anyone know a place to source the type of connector on the left? Or is it possible to fabricate a riser?
 
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