Atari 1040 STE screen troubleshooting

[ZeroG1972]

Hello,

I recently encountered the following problem with my Atari 1040 STE (TOS 1.62 - CA4003290): 30 seconds after startup, when the desktop was displayed correctly, the image gradually blurs and the screen goes blank and power off.

I performed the following tests: changing the SCART cable, changing the screen (SC1425 -> SM124), replacing the power supply, without success.
I examined the motherboard: no capacitor seems damaged, no tracks cut.

Here is the video of the problem:


Thank you in advance for your help.

Thierry

[exxos]

Are any chips getting hot ? Is the floppy drive unplugged ?

[ZeroG1972]

Are any chips getting hot ? Is the floppy drive unplugged ?

Yes, floppy drive is unplugged, but with floppy, it's same.
I don't know if any chip is hot, it's short (< 30 sec.) between boot and issue.

[exxos]

Just put your finger on all the main chips.. They pretty much should be still cool after 30 seconds.

Also there is a large chip under the PSU.. Take it out, look for damage and clean it.

There is a small regulator to the right of the ic. Check voltages both sides before and after the screen goes blank.

[ZeroG1972]

Just put your finger on all the main chips.. They pretty much should be still cool after 30 seconds.

Also there is a large chip under the PSU.. Take it out, look for damage and clean it.

There is a small regulator to the right of the ic. Check voltages both sides before and after the screen goes blank.

The large chip under PSU (shifter) is cold when screen crash. The other chips (68000), Glue/mmu, etc... are not hot.

Where is the little regulator ? I have take a picture : https://ibb.co/wyqK0Nb
Else, the 32 mhz clock could be defective ?

[exxos]

Its here..

1040Ste.jpg

[ZeroG1972]

I have used my multimeter and I obtain :

Before troubleshooting : 8,4 v (left) and 12,5 v (right)
After : 8,4 (left) and near zero (right)

I don't understand : 8,4 v after troubleshooting but in datasheet of LM78L82ACZ, it's output value ?

I'm noob in electronic...

[tzok]

So check 12V line (blue) from PSU...

[exxos]

Before troubleshooting : 8,4 v (left) and 12,5 v (right)
After : 8,4 (left) and near zero (right)

I don't understand : 8,4 v after troubleshooting but in datasheet of LM78L82ACZ, it's output value ?

about 8V is normal.. you must have not metered 12V both times, if 12V was missing you wouldn't get 8V output.

So if you disconnect monitor, power up machine for 60 seconds, connect monitor, does the video instantly not work, or take another 30 seconds to malfunction ?

[ZeroG1972]

So check 12V line (blue) from PSU...

Sorry for bad indication : after crash I have 12.3 v on PSU blue line and same value on regulator.
And I have made test with an other PSU, it's same problem...