Mega ST video output oddity

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Atarian Computing
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Mega ST video output oddity

Post by Atarian Computing » Wed May 16, 2018 5:43 pm

One of my Megas, 2MB rev 5 has a weird problem. When I have a test cart inserted it displays everything just fine in test mode. Hi and low res. However, if I take the cart out it goes out of sync in Hi res. In low res without cart I can see a few bombs but kinda garbled.

I've tried many refurbed psus, I've exchanged the shifter, glue, mmu, and blitter for known good ones. Before I go further, does anyone have an idea where to look for next? And what causes the the output to be perfect with the cart? All tests are fine too.

Edit: Oh, and tried several monitors and adapters that are known to work fine.

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exxos
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Re: Mega ST video output oddity

Post by exxos » Wed May 16, 2018 9:39 pm

Sounds like loading on the ROM cart seems to help ? Probably change the SIL's for 2.2K... Had similar problem on my STE few weeks ago, machine would fail to boot when diagnostic cart inserted due to bus loading issues. Sounds like you got almost reverse problem to that.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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Atarian Computing
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Re: Mega ST video output oddity

Post by Atarian Computing » Thu May 17, 2018 4:21 am

exxos wrote:
Wed May 16, 2018 9:39 pm
Sounds like loading on the ROM cart seems to help ? Probably change the SIL's for 2.2K... Had similar problem on my STE few weeks ago, machine would fail to boot when diagnostic cart inserted due to bus loading issues. Sounds like you got almost reverse problem to that.
Ah, good point. I'll have to order some and try. Thanks.

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Re: Mega ST video output oddity

Post by Atarian Computing » Thu May 17, 2018 9:25 am

Ok, so I ordered a good bunch of these: http://uk.farnell.com/bourns/4609x-101- ... k%20bussed

Those are correct, right?

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Re: Mega ST video output oddity

Post by exxos » Thu May 17, 2018 12:31 pm

Yep.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Atarian Computing
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Re: Mega ST video output oddity

Post by Atarian Computing » Fri May 25, 2018 4:21 pm

So I changed all SIPs and no difference I'm afraid. This is weird.

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Re: Mega ST video output oddity

Post by exxos » Fri May 25, 2018 7:24 pm

:( So either the loading on the cartridge is snubbing some noise on the bus, or maybe the cartridge has a capacitor on the power rail is solving the problem.. I don't really know what else unless there is a bad connection on the video lead / socket or something..
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Atarian Computing
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Re: Mega ST video output oddity

Post by Atarian Computing » Sat May 26, 2018 7:45 am

exxos wrote:
Fri May 25, 2018 7:24 pm
:( So either the loading on the cartridge is snubbing some noise on the bus, or maybe the cartridge has a capacitor on the power rail is solving the problem.. I don't really know what else unless there is a bad connection on the video lead / socket or something..
So what you said gave me an idea. I have Arne's multitest cart with SuperMon. As SuperMon is a GEM app, the machine boots to the desktop. Well, I would just get the same blank screen. Same cart, same conditions, different program on the cart.

It doesn't seem to be a bad connection. Does this give any ideas?

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Re: Mega ST video output oddity

Post by exxos » Sat May 26, 2018 10:03 am

slightly confused what is working and what isn't working.. you mention it was hi-res not working before, now your talking about desktop and blank screens ?
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Atarian Computing
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Re: Mega ST video output oddity

Post by Atarian Computing » Sat May 26, 2018 10:18 am

Ok, sorry, I'll try to clear it up.

The MST in question only works flawlessly with a test cart *and* the diagnostic test loaded in both hi and low resolutions.
It will only output a blank screen in hi res in any other scenario than the one above
It will output garbled bombs in low resolution in any other scenario than the one above.

You speculated that the cart therefore must have some sort of effect on it. I got the idea of trying the SuperMon program from the same cart as, unlike the test program, it was a GEM program and required it to be started from the desktop. So this is the expected behavior of this program and I've confirmed it with another ST.

However, on the MST, using the same cart where the diagnostic test works in both resolutions, the SuperMon program displays the same blank screen as any other scenario.

So the conclusion should be that the cart itself doesn't seem to affect the operation of the computer but rather a difference between the diagnostic test and normal operation that could help pinpoint the problem.

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