520 STFM motherboard issue, seems out of sync? scrambled screen

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MarkA88
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520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

I have this 520 STFM which when switched on the yellow floppy LED comes on and it goes to desktop i assume but as can be seen the picture is scrambled / out of sync though you can just make out the green desktop colour

Could anyone point me in the right direction of what to look at?

Thanks :shrug:
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Kairos
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by Kairos »

How do you connect to the monitor/TV?
Faulty cable? Wrong type of cable?

The cable is providing CS while the monitor needs separate HS & VS? Or the reverse?
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

The monitor is an SC 1224

There is nothing wrong with the monitor or cable as it works perfectly on other ST's i have

It's the mobo that has the issue
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by 2tonBC »

start by reseating the video shifter. it is in the can in the middle of the motherboard.
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

yes i changed that chip first, same issue
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by 2tonBC »

use the RF and see if you have the same issue.
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

OK so finally got back to this

High resolution is the same as above pic but in black and white

TV output is just a dark black screen

So basically sounds like they are linked?
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

Anyone?
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by mlynn1974 »

As MarkA88 suggested it would be wise to reseat the Video Shifter chip. Even better - try it with another video shifter if one is available.
Does the screen contrast change or fade in and out?

Inspect motherboard for signs of bulging, leaking or otherwise damaged capacitors especially near the video shifter or video output circuitry.
If in doubt get a qualified repair person to replace them.
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by dlfrsilver »

the screen is like this on startup or after loading up to the GEM desktop ?
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MasterOfGizmo »

It seems to me that he has prinarily problems with the sync signals. These are not generated by the shifter but by the glue chip. So I'd assume that reseating the GLUE is more promising.
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by jd »

Get a diag cart and you can see the output on another ST or PC via serial connection and terminal app. Should help narrow down things.
MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

OK thanks for the replies, i did also think the Glue chip but on this board the Glue is soldered directly to the mobo so can't reseat it! it will need to be removed which i can do, just more work

Will report back when i have eventually done this
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

Well i changed the GLUE chip and.... the same!

So it wasn't that

Something else i did forget to mention , there is also NO sound

So i'm wondering if it could be the Yamaha or MPF?
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by dlfrsilver »

MarkA88 wrote: Mon Feb 26, 2024 7:39 pm Well i changed the GLUE chip and.... the same!

So it wasn't that

Something else i did forget to mention , there is also NO sound

So i'm wondering if it could be the Yamaha or MPF?
Not the glue chip
Not the Cable (working on another ST)

Then have you checked for dry solders on the video connector on the Atari Side ?

If the vsync pin is no more connected, you can have this problem.
PS : regarding the sound that is NO/MUTE, you mean no sound output on the SC1224 ?

Second check : do you have a TV on which you can connect this ST and see if the vsync problem / no sound arise ?

Third check : have you changed the capacitors on this machine or are you running it in its "own juice" ?
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MarkA88
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by MarkA88 »

There is no sound via any channels, not on TV via RF and not on any SM 124 or SC 1224 monitor

So there is no sound

I mention as it could be related to the sync issue?

All original caps
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by mpattonm »

Composite video signal is generated by TV modulator. TV modulator requires both +12V and +5V power supply, so does audio circuit.
Check your power supply outputs.
EDIT: Sorry I just realized you are on RGB monitor, Composite video sync is not relevant on it. However, missing sound is much likely caused by missing 12V.
Last edited by mpattonm on Wed Feb 28, 2024 6:31 pm, edited 1 time in total.
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dlfrsilver
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by dlfrsilver »

MarkA88 wrote: Wed Feb 28, 2024 10:20 am There is no sound via any channels, not on TV via RF and not on any SM 124 or SC 1224 monitor

So there is no sound

I mention as it could be related to the sync issue?

All original caps
They are dead (caps). Change them for good quality ones, and see if a change occurs on sync and sound.
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Re: 520 STFM motherboard issue, seems out of sync? scrambled screen

Post by mlynn1974 »

Have you checked the Motorola MC1377P PAL / NTSC encoder IC chip?
This chip sits behind the RF modulator on regular STFM's / STE's and sometimes fails.
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