Revival of an Spectravideo 728

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.

Here are a few pictures of the video decoder/power supply board:
http://koti.mbnet.fi/tsaarela/TF/22062010194.jpg

http://koti.mbnet.fi/tsaarela/TF/22062010196.jpg

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

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.

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:

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

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.

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.

Quote:

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Originally Posted by rkauer

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.

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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.

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.

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.