Overheating chip on Atari STE plugged into socket U405--is it normal?

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jmprice314159
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Overheating chip on Atari STE plugged into socket U405--is it normal?

Postby jmprice314159 » Tue Aug 13, 2019 9:37 pm

Hello all,

Chip on Atari STE plugged into socket U405--is it normal for it to get quite warm? About the same temp as the 68K CPU. Not too hot to touch, but warmer than the other chips. The reason I ask is I replaced a dead PSU with a Centurion PSU, and one of the capacitors on the PSU gets quite hot to the touch, and it cannot be a faulty PSU as I tried it in my STF and the capacitor does not get hot. I bought the STE on Ebay and it had a dead PSU.

The 4MB ram banks also are warm.

Any thoughts on the temp for U405? Should it be as cool as the other chips, or around the same temp as the 68K CPU? Not so hot as scorching or anything close, but def hotter than other chips. The main concern I have is whether the PSU is going to burn out. Perhaps that is what caused the original Atari PSU to die.

Thanks,

Jason

jmprice314159
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Re: Overheating chip on Atari STE plugged into socket U405--is it normal?

Postby jmprice314159 » Tue Aug 13, 2019 10:10 pm

Couple of other questions:
What does the chip in socket U405 do?
Can it be replaced with a newer part?
If yes, what is the part number?

Thanks,

Jason

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Re: Overheating chip on Atari STE plugged into socket U405--is it normal?

Postby neanderthal » Wed Aug 14, 2019 2:41 pm

Well U405 handles main clock switch(32Mhz) and mono mon detection and is a standard scottky (S) so might get warmer than the other around TTL stuff.CPU and Dram sim banks generates some heat so dont think you have anything to worry about if everything works as supposed to.About the PSU cap, you have to ask centurion about that(I never tested one).Must admit cant remember if U405 gets hotter or not?..My exprimental STE had that one replaced(busted it due to testing) and using some low power chip (Ls,HCT or something),,btw folks dont do that for one has to add a pullup if using other versions.(mono mon detect gets wonky)

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Re: Overheating chip on Atari STE plugged into socket U405--is it normal?

Postby dhedberg » Wed Aug 14, 2019 6:53 pm

What capacitor on the PSU runs hot? Capacitors should not run hot. Electrolytic polarized caps can if the polarity is reversed or if the specs of a cap do not meet the requirements.
Daniel, New Beat - http://newbeat.atari.org. Like demos? Have a look at our new Falcon030 demo and feel the JOY.

jmprice314159
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Re: Overheating chip on Atari STE plugged into socket U405--is it normal?

Postby jmprice314159 » Wed Aug 14, 2019 10:51 pm

atari_psu.jpg
It is the capacitor that I've placed some pennies on. See picture.
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Re: Overheating chip on Atari STE plugged into socket U405--is it normal?

Postby dhedberg » Thu Aug 15, 2019 7:06 am

Doesn't seem right. That's an electrolytic capacitor and I see no reason why it should become hot unless its damaged, the polarity is incorrect or it's exposed to ripple currents that's outside of its specifications, etc.

In general, electrolytic capacitors should not be exposed to high temperatures and they should not generate a lot of heat. When their temperature increases, their ESR value (resistance) increases as well, resulting in more heat generation. It affects the life span of the capacitor. You could try and replace the capacitor with one that has a lower ESR value. It will handle ripple current better. The Panasonic FR series of electrolytic capacitors are excellent for this purpose.
Daniel, New Beat - http://newbeat.atari.org. Like demos? Have a look at our new Falcon030 demo and feel the JOY.


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