Display fading to black

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pixeladdict
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Display fading to black

Post by pixeladdict »

Hi all,

I’m new here but have been an ST user since the late 80s. I’m currently hoping to repair an STFM with a C070789 revF board I’ve acquired and looking for some advice. I can solder and have done a few minor projects but still consider myself a beginner with electronics.

The issue is a bit odd. The display fades away to black over a few seconds when the ST outputs a white (or generally bright) video output. It does this via both RGB and composite from the monitor port on a known working CRT I use with my other machines. If I run a game that is primarily on a darker background e.g. Barbarian or IK+ it works just fine and is stable, but the white screen generated on switch-on or a program like YAART or even just the desktop sometimes makes the image fade away to black.

I’m testing it with a PSU from another ST I have that works fine (but hasn’t been recapped).

Does this sound like a capacitor issue? It’s like video signal just can’t be sustained when the screen is white. Is it likely one of the video caps at fault?

Thanks for taking the time to read,

Si
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exxos
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Re: Display fading to black

Post by exxos »

It sort of sounds like the output level is to high for your monitor and its saturating it causing it to go black.. With a RGB scart cable, it should have some series resistors in the RGB lines, something like 100R is about normal.. So you could try that assuming you not got such a cable already.
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pixeladdict
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Re: Display fading to black

Post by pixeladdict »

Thanks for the quick reply! I’m using a RGB scart lead made by Ian Priddey from Retro Computer Shack. Pretty sure the resistors are in there but I’ll check. It works fine with 2 other STs I have. I use a PVM and can switch to take the picture from the composite on pin 2, which also has the fading issue as with RGB.
pixeladdict
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Re: Display fading to black

Post by pixeladdict »

I’ve resolved this now thankfully with the STFM video re-cap kit from the store. Must have been one of those capacitors causing the issue.
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exxos
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Re: Display fading to black

Post by exxos »

:bravo:
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android927
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Re: Display fading to black

Post by android927 »

I'm having a similar problem with my 1040STF, but my screen fades to black regardless of the intensity of the output. I already tried removing and re-seating the Shifter, Glue, and MMU chips and gave their sockets a spray of contact cleaner, as well as pushing down on the ROM chips to make sure they are all properly seated, but nothing seemed to have helped. Could i also be dealing with a capacitor issue?
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exxos
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Re: Display fading to black

Post by exxos »

android927 wrote: Fri Dec 10, 2021 7:29 pm Could i also be dealing with a capacitor issue?
Possibly. Faults can have multiple causes, but that's one which no doubt isn't helping matters.
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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android927
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Re: Display fading to black

Post by android927 »

exxos wrote: Fri Dec 10, 2021 7:33 pm
android927 wrote: Fri Dec 10, 2021 7:29 pm Could i also be dealing with a capacitor issue?
Possibly. Faults can have multiple causes, but that's one which no doubt isn't helping matters.
Originally, the issue was that the red signal would randomly drop out causing all the colors to appear wrong (white appearing as cyan, etc.) I tried pushing down on the shifter and spraying some contact cleaner onto the pins (initially without removing the chip from its socket), and that seemed to resolve the issue with the red signal, but then over the course of about 5–10 minutes the screen slowly faded to black. Every time I've started the computer since then, the fade to black has occured progressively faster, now taking only about 10 seconds.

I ordered a new PSU because i initially suspected the issue was power related, but now I'm not sure. I don't recall seeing any capacitors in the video circuit since my machine isn't the FM model, but perhaps I overlooked something?
android927
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Re: Display fading to black

Post by android927 »

I should also add that the reason i suspect a PSU issue is because while the computer was initially able to boot from a disk even with the display fading out, it has since stopped being able to boot from a disk and just tries to boot over and over again unsuccessfully (similar to what happens when trying to boot a low-res game in high-res mode). I suspect that the PSU is dying and is no longer able to provide the disk drive with enough power to successfully read a boot disk.
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Re: Display fading to black

Post by exxos »

What motherboard do you have exactly ? Sounds like you may have multiple faults going on there not just display related. So a new PSU will rule that out at least.
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https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
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