Atari STFM 520 Black Screen via monitor out

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nick_p
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Atari STFM 520 Black Screen via monitor out

Post by nick_p »

Hi all,

I'm waiting on some parts to arrive for essential maintenance but thought I'd drop a quick post in here describing the symptoms and stuff I've done so far to see if it is indeed a Noob caused problem that could be resolved easily.

Symptom :-

The system is connected to my old CRT TV via a Monitor to Scart lead designed for an ST.
  • When powered on, the small green light bottom left of keyboard comes on.
  • The TV recognises that the Atari is turned on and switches to the right SCART socket.
  • The screen is black. Nothing else happens.
  • If I stick a disc in the drive, the drive spins but nothing further.
  • No flashing yellow LED on the keyboard.
  • The C025913-38 chip (DMA) gets HOT. Too hot to hold my finger on after a minute or so.
What I've done so far:-
  • Reseated all socketed chips and given the legs and socket a wipe with Isopropyl Alcohol.
  • Replaced all electrolytics on the main board (apart from the modulator section.. I assumed as I'm not using it, it won't matter but maybe it is still live and causing an issue).
  • Replaced the caps in my power supply. When plugged in and powered on, the PS is giving 4.9V and 12.1V and looks stable on a multimeter, but not scoped it yet.
I have new ROM and a 4MB memory upgrade ready to fit, but don't want to start that in case my current issue damages these new parts.

It's a c070789-001 rev c1 main board.

Any tips or suggested tests will be gratefully received :D

Cheers

Nick
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exxos
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Re: Atari STFM 520 Black Screen via monitor out

Post by exxos »

DMA should not get that hot.. Is it in a socket ? Maybe someone's put it in backwards...
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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rubber_jonnie
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Re: Atari STFM 520 Black Screen via monitor out

Post by rubber_jonnie »

Nope, DMA definitely shouldn't get too hot to touch, as exxos said check orientation if it's in a socket, the chip notch should match the silk screen markings on the board. Also if it is socketed check all the pins are inserted and not bent.

If it is soldered in it will need some work I'm afraid.

If you can, please add photos to this sort of question, as it helps to answer a lot of these sorts of questions.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
nick_p
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Re: Atari STFM 520 Black Screen via monitor out

Post by nick_p »

No, not socketed and doesn't look messed with. Pin1 is at the capacitor end.

Should have said that yes this is an "untested" purchase from an auction site so anything's possible, but it doesn't look like anything has been done to it. The power supply that came with it wasn't giving out any volts on the output side. Not sure if that could have caused or been a symptom of the problem I have now.

I've added some photos below.

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20210407_132129.jpg
20210407_132129.jpg (282.65 KiB) Viewed 3147 times

Many thanks

Nick
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exxos
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Re: Atari STFM 520 Black Screen via monitor out

Post by exxos »

Sounds like the DMA is bad... Also those older floppy drives are better being replaced with a more modern drive. Likely the belt has gone and it may be only single sided.

Not trying to discourage but it might just be easier to get a different machine than spend the time and cash trying to fix it all.
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
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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Re: Atari STFM 520 Black Screen via monitor out

Post by rubber_jonnie »

I'd also go with failed DMA, but it's soldered in so that will need a bit of rework to remove and fit a socket.

If you have a multimeter, I'd check the resistance between the ground pin and the 5v pin. If it's a dead short, or really high resistance, that's potentially a bad sign.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
nick_p
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Re: Atari STFM 520 Black Screen via monitor out

Post by nick_p »

cheers.

Yeah, short between 5V and GND with the chip in place. I'll unsolder it with the desoldering station later to check if it's the chip or a short between 5V and GND somewhere else.

Thanks guys

Nick
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Re: Atari STFM 520 Black Screen via monitor out

Post by rubber_jonnie »

nick_p wrote: Wed Apr 07, 2021 2:54 pm cheers.

Yeah, short between 5V and GND with the chip in place. I'll unsolder it with the desoldering station later to check if it's the chip or a short between 5V and GND somewhere else.

Thanks guys

Nick
Yeah, will be good to see if the short is internal. I kind of expect it is.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
nick_p
Posts: 10
Joined: Wed Apr 07, 2021 7:20 am

Re: Atari STFM 520 Black Screen via monitor out

Post by nick_p »

DMA chip removed.... still got just 43 Ohm resistance between 5V and GND on the board which I'm guessing isn't good.

345 Ohm between pin 20 and 40 on the unsoldered DMA chip so not a dead short or really high....

I think exxos might be right... move on to known working purchase.
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derkom
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Re: Atari STFM 520 Black Screen via monitor out

Post by derkom »

rubber_jonnie wrote: Wed Apr 07, 2021 3:09 pm Yeah, will be good to see if the short is internal. I kind of expect it is.
Seems like it almost has to be. Anywhere else and there wouldn't be any motivation for the current to flow through the DMA chip fast enough to make it burn.
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