Amstrad Mega PC with OSSC

[io-error]

Hi All

I’ve been struggling with getting sync lock using the VGA input on my OSSC running 0.82a with a mega PC, specifically using it as a megadrive. It will lock sync for short periods and shows:

AV3: RGBHV 313p
15.55kHz 49.69Hz

When it does lock it shows a picture on my monitor but it flickers in an out, if I mess with some of the sync options I can get it too hold a lock for a bit longer but it always drops the sync after a few seconds sometimes up to 30. This is a link to the technical manual https://web.archive.org/web/20110719154701/http://www.amstradcg.nl/mega.pdf.

The OSSC is connected directly via VGA cable and I’ve tried a couple of cables, no difference.

Does anyone know if this configuration should/can work? Are there any other things I can try, I’m not familiar with the intricacies of VGA sync issues so quite out of my depth here.

Regards

[nmalinoski]

What kind of display are you using? Is it your display that’s not keeping sync, or is it the OSSC?

If it’s your display, 49.69Hz might be just out of acceptable range for its video processor.

[io-error]

It’s the OSSC dropping sync, it flashes red and the display says no sync. It works fine with other consoles.

[marqs]

The digitizer chip is a bit picky on HSYNC/VSYNC alignment which might be the issue here. You could try enabling “AV3 interlacefix” on the compatibility opts. Another option would be a sync combiner (dedicated or as part of VGA->SCART RGB converter) if you happen to have one.

[io-error]

Thanks for the reply, sorry for the lag getting back to you. I tried the “AV3 interlacefix” didn’t make any difference. I then tried an older OSSC I had in an arcade machine. This worked fine running firmware 0.76, so I flashed the newer OSSC from 0.82a to 0.76a. It works out of the box with no configuration changes required.

So I assume it is be firmware related and something has changed re the HV sync on AV3?

The display is still showing:

AV3: RGBHV 313p
15.55kHz 49.69Hz

Regards

P.S
Before I flashed it to 0.76a I tried again with 0.82a, just in case there was a weirdness with the mega PC that had been fixed with moving it around a bit. It still wouldn’t lock on the sync.

[marqs]

Could you also try other firmware versions between those? It could help pinpointing the change that affects this.

[io-error]

Of course.

So downloading from https://www.niksula.hut.fi/~mhiienka/ossc/fw/

.76a Works
.77a Works
.78a Works
.79a Works
.80a Works
.81a Fails to sync

Let me know if I can test anything else.

[marqs]

I checked the changes and there are a few possible causes. Could you try if the following helps with the newest fw: when you have sync locked, press profile load hotkey (TV/AV) on remote but don’t select a profile. That rules out the CPU detecting sync/mode change which could cause erratic behaviour if bad status is received from the digitizer.

[io-error]

Yep, .82a press the hotkey button, holds synch no problem.

[io-error]

And interestingly now I can’t get it to misbehave again, even re-flashing the firmware doesn’t bring back the behaviour of not synching.

[obsoletemachines]

hi,
i’m having similar difficulties trying to get it to work with a 15khz RGB monitor – the Sony KX14CP1

So i’m taking the signals direct off the MegaPC board RGBHV

My monitor expects TTL level and i suspect this is the issue.

Looking at the MegaPc Service manual the RGBHV goes to the monitor where there is a Toshiba TA8631 RGB Chip but before the inputs there are 75ohm pull down resistors and an ‘in-line’ capacitor 47uf.

on the TA8631 datasheet it recommmends 51ohm pull downs.

I think i need to pull down the lines and also connect the grounds.

This may help you

[Author]

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