SVI-738 not booting

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By protek

Expert (109)

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20-08-2021, 20:40

No worries! I already ordered a new VDP to rule it out.

By sdsnatcher73

Prophet (2237)

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21-08-2021, 19:11

Do you see oscillation on pin 63/64? If not VDP cannot work nor generate CPUCLK.

By protek

Expert (109)

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24-08-2021, 19:34

Not at least with my logic probe. One is high and the other is low.

By sdsnatcher73

Prophet (2237)

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24-08-2021, 21:49

I think you will need an oscilloscope too pick up the XTAL frequency.

By protek

Expert (109)

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24-08-2021, 22:59

As it happens I have an old analog HP 1740A scope.

By protek

Expert (109)

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31-08-2021, 20:43

Well, swapping the V9938 didn’t make any difference, so I’m going to try to measure the frequencies with the scope.

By protek

Expert (109)

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24-09-2021, 20:01

Finally some signs of life! I tweaked the variable capacitor while the new V9938 was installed and I got greeted by the MSX Basic screen. The only thing is that the picture begins to deteriorate soon after power-up. After cool down the picture is sharp again. I thought the MC1377P might have some issues when it warms up, but trying another one from an A520 modulator didn’t help, it actually turned the picture black and white, and that symptom seemed to carry over to the original MC1377P too.

By sdsnatcher73

Prophet (2237)

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25-09-2021, 08:40

So black and white screen indicates the VDP is working now but the color encoder (possibly IC64?) is dying (or is that the MC1377P you swapped? The photo of the mainboard is kind of low res). If so maybe the capacitors around that chip are not working optimal anymore.

By protek

Expert (109)

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25-09-2021, 14:36

IC64 is the MC1377P PAL encoder chip. I ordered a set of MC1377P to try out.

By protek

Expert (109)

protek's picture

21-10-2021, 14:58

A question regarding the six 10 uF capcitors that are in a row next to the RF modulator. The silkscreen shows the + to be on the same side for all six, but first three are oriented so that the negative lead is on the + side. Is that so on purpose, i.o.w. the silkscreen is wrong?

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