Omega, no video.

By MrSiggler

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08-12-2022, 06:08

Hey all, I recently got my Omega assembled, and initially I got no video signal, so I checked the clocks, looked for life on address lines, etc.. and did see activity. Went over the board, and realized the Q2 transistor wasn't required for the 9958, so I removed it. And now I get a video signal, but only a black screen.

Could having Q2 populated have damaged the 9958? Is there any way to test for that?
If there's a chance it's okay, any suggestions on what else I could try?

Thanks!

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By msxmakers

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08-12-2022, 12:47

MrSiggler wrote:

Hey all, I recently got my Omega assembled, and initially I got no video signal, so I checked the clocks, looked for life on address lines, etc.. and did see activity. Went over the board, and realized the Q2 transistor wasn't required for the 9958, so I removed it. And now I get a video signal, but only a black screen.

Could having Q2 populated have damaged the 9958? Is there any way to test for that?
If there's a chance it's okay, any suggestions on what else I could try?

What I know is you must not populate the Q2 transistor using a V9958 because it will damage the VDP.
The only way to test if that chip is still working is using a working MSX2+ to test it.
Did you selected the VDP jumpers at V9938 or at v9958 position?
I will check everything once again, this chip is always hottest than the rest so if you do not burn your finger when its working it could be still in a good condition.
Double check jumpers, try more than one video connection and verify all sockets are doing a good connection.
Double check the problem is not at CXA chip because all video go through it.
If V9958 is burning in dangerous way when you power on or absolutely cold you maybe broke it.
I hope this helps.
I will suggest you to follow the step by step instructions here in the meanwhile you get another V9958.

msxmakers.design.blog/proyectos/omega-home-computer/omega-paso-a-paso/

By MrSiggler

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10-12-2022, 05:22

Thanks for the reply!

I've already ordered another 9958, but fingers crossed this one isn't damaged.
When the unit is powered on, the 9958 does get warm, but not what I'd call hot. The board was hard wired for NTSC/9958, the only 9938 thing set was having Q2 installed for a bit. I've verified that the cpuclk pin on the 9958 has a 3.4xMhz pulse, looking at the csync pin, I see a 60hz signal there. So there's some life, maybe?
However, I saw on the page you linked, that the board when fully populated should not test under 100ohms? I tested the resistance of the board at the white power connector near the rear DC barrel. I get 98ohms, but if I remove the 9958 I get 230ohms. Is that a clear sign of a bad 9958?

Thanks again!

By msxmakers

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11-12-2022, 17:48

There are several tests to do documented on my blog and also in Sergey Kiselev's instructions at:
https://github.com/skiselev/omega/blob/master/Mainboard_Building_Instructions.md
I am especially surprised that you say 3.4Mhz but maybe you did not write the correct value, the frequency measurement should be 3.59Mhz.
Try measuring all the frequencies specified in Sergey's instructions and comparing the oscilloscope image on each output (RGB) with the ones shown on my blog, at the end of:
msxmakers.design.blog/proyectos/omega-home-computer/omega-paso-a-paso/
If the signals don't look alike, trace it inward, that is, check the image on the output pins of the CXA1640, then compare its outputs with its inputs, one by one, and finally check the signals on the RGB pins ( and Csync) of the V9958.
If the CPUCLOCK frequency is really lower, check the frequency in its clock inputs. The crystal must generate 21.7Mhz if I remember correctly, check your capacitors to see that they are of the correct value, etc.
If everything works except the VDP, I'm afraid it's fried. Unfortunately, if the VDP does not generate the system CLOCK, nothing else will work either.
If SYSTEM CLOCK works there may still be no video, so check all other signals to find out more.
About the 100ohm thing, well, it seems indicative.
If I had to bet I'd say your VDP is blown, but I'm not sure it can be conclusive.
Cheers

By MrSiggler

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23-12-2022, 05:53

I got the second 9958 in, and still had no change. Still black screen. Buuut...
Turns out, it was the Z80. While I did see life on the bus when I tested early on, and it did match the contents of the ROM, lots of 76 when running the halt rom, and the halt test did turn out as expected.. so I assumed the cpu was fine, but after trying another one, it now loads up fine. It starts up my carnivore2 cartridge and loads sofarun/etc. Was very cool to see the Omega finally display something.. and then run the carnivore after i finished assembling it.

I did get lots of awful sounds and static/screaming on the audio though, hopefully a side effect of the carnivore's audio circuits not having the -12/12v supplied yet. I'll be adding one of those voltage step up things tomorrow, and hope for the best.

It looks like the Z80 was reading the ROM data correctly, but not actually executing instructions properly, in particular, jumps didn't actually change the address.

By msxmakers

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23-12-2022, 11:17

Thanks for sharing it, good job!