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Atari 1040 STf keeps rebooting, no GEM

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I'd like to ask you a question about the video i attached at this post.
It's a Atari 1040 Stf. This machine keeps rebooting. (no GEM) but seems to try.
The Reset button is functioning ok.
I've replaced the roms but the results are exactly the same.
The old roms where originals and i've replaced them with the AM27c010 roms With TOS 1.04.

i hope someone recognizes the video and give me some directions.


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To me this look like the TV or monitor is simply losing synchronization, the ST can be connected to any CRT/LCD screen as long as that screen can handle a 15KHz horizontal refresh rate.


A new 27 inch monitor that is available and works 100%! All you need is Atari ST to VGA cables, one for color mode and one for monochrome mode.

Edited by Chri O.
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Video signal has obvious errors, and it is not just losing sync. 

What can do for start of fixing:  reseat all socketed chips, especially those square ones (Glue, MMU) . Cleaning of pins on chips and sockets recommended. And maybe something against corrosion.

Checking PSU voltages. By so old electronic it is usual that electrolyte capacitors lose capacity, so need to replace them.

If above don't fix it it may need replacing components ..


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Thanks both for the advice. 
All chips are cleaned and reseated. 
I've tried an external PSU the results are exact the same.
The square ones are soldered in this machine. I will look at the soldering of the contacts and reflow them.

@Chri The monitor and cabling I'm using is normally working with my 520ST. So a sync issue cannot be the problem, next to this it would show a sort of desktop/colors if this was the probleem 
but it keeps rebooting.

I will try to reflow the the Glue and MMU hope it wil solve the problem.


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  • 2 weeks later...

I got some other tips from an other Forum and this has helped me further with trouble shooting. And solving the problem. (i'm a king at soldering now :))
Maybe this Topic can help someone else so i quote the solution here for the future.


Power up the computer. Using an oscilloscope, verify the 8MHz clock to the 68000 CPU (pin 15). Replace oscillator if necessary. Then check pin 17 (HALT) of the 68000 CPU. It should be a TTL high. If so, go on to 3 below. If not, the CPU is halted. The reasons may be: (1) bad reset circuit, (2) double bus error, (3) bad CPU.
I've done these tests with mij new Handtek 6022BL.
The Clock is visible.
The Pin 17 goes from low to high and stays high wen i turn it on.
When i push the reset button is goes from low to high and stays high.
Seems a bad CPU then ?


Well, i''ve soldered the 68000 out of a working 520 ST (France version) (AZERTY keyboard insane to work with ;)
Next i desoldered the 68000 out of the 1040 STF
Putted a socket in de 1040 board and tested.
It was not functioning but there was some change (only white screen and rebooting)
I replaced the original Rom's back on the board and it's now functioning completely (so there must be something wrong with the
programmed rom's. I wil take a look at this later.
The 1040 has a 1.02 TOS, i'd like to replace this with 1.04.

So the problem was a dead cpu !! ... Thanks you all for the advice.

Kind regards.


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