Atari TT's LM7905CT getting too hot

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mikro
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Re: Atari TT's LM7905CT getting too hot

Post by mikro » Sat May 09, 2020 11:54 pm

As it is getting late I didn't have time for something serious so I just tried one more thing - MegaSTE PSU in the TT. And, as you would have guessed, it not only works but the regulator is freaking hot as well. But 1,5 hours later everything still works as it should.

So that makes my initial suspect not guilty and most likely something wrong is with the PSU itself.

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Re: Atari TT's LM7905CT getting too hot

Post by exxos » Sun May 10, 2020 12:25 am

I would check the caps in the PSU make sure they are soldered properly. The extra "load" of thunder could be stressing the PSU enough to have to much ripple on the 5V rail etc. The regulators can get hot if way to much ripple also.. Check power rails with a scope...

Some example ripple figures on my site.. https://www.exxoshost.co.uk/atari/last/ ... tm#PSM5341
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mikro
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Re: Atari TT's LM7905CT getting too hot

Post by mikro » Sun May 10, 2020 7:57 am

It ges more and more interesting. While I had the MegaSTE PSU in TT, I was thinking let's try the other way around - so I put the TT PSU in the MegaSTE. And to my surprise, the PSU behaves totally differently - the regulator is nowhere that hot (I would say warm at best) and of course no restarts. What is interesting, another part is getting hotter now - SBLI640CT.

In contrary to TT PSU, that MegaSTE PSU hasn't been recapped but it does have PC111 installed by default.

So to summarise:

- TT PSU in TT = super hot regulator, warm SBLI640CT, restarts
- MSTE PSU in TT = super hot regulator, warm SBLI640CT, doesn't restart
- TT PSU in MSTE = warm regulator, very hot SBLI640CT, doesn't restart
- MSTE PSU in MSTE = not sure aboure regulator and SBLI640CT (to be tested but I don't expect surprises here), doesn't restart

So I would be really interested in @czietz's observation in his TT PSU, with all the hardware loaded - whether the regulator is hot or not.

EDIT: added SBLI640CT behaviour. Since now I have something to compare to, I will check all the caps & voltages.

czietz
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Re: Atari TT's LM7905CT getting too hot

Post by czietz » Sun May 10, 2020 9:33 am

Imho, your tests already show that the hot 7905 regulator is normal in a TT and not related to your restart issue.

Still, I did the measurement first thing in the morning: about 85°C(!) on the heatsink of the 7905. I expect it to be cooler when the PSU is closed, because the position of the grille in the metal cover will ensure that there's airflow over the heatsink.

Doing some back-of-the-envelope calculation shows that the high temperature is not a sign of a fault, though. One can assume approx. 100 mA per ECL component (EDIT: TT high PLL and TT high driver), i.e., ca. 200 mA load on the -5 V rail (neglecting the audio DAC). Thus, the 7905 has to dissipate (12V-5V)*200mA = 1.4 W. This tiny piece of sheet metal that is acting as heatsink is not terribly efficient. Let's assume a thermal resistance of 40 K/W without forced airflow. This would put us 56 K over ambient temperature, not that far from my measurement.

Like exxos already suggested, you should check the ripple on the other rails (particularly +5V) and compare between the two PSUs.

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Re: Atari TT's LM7905CT getting too hot

Post by czietz » Sun May 10, 2020 10:01 am

czietz wrote:
Sun May 10, 2020 9:33 am
Still, I did the measurement first thing in the morning: about 85°C(!) on the heatsink of the 7905. I expect it to be cooler when the PSU is closed, because the position of the grille in the metal cover will ensure that there's airflow over the heatsink.
PS: With the PSU cover closed it's 50 - 55°C -- proving that I chose well not to throttle the fan as many other people do.

mikro
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Re: Atari TT's LM7905CT getting too hot

Post by mikro » Mon May 11, 2020 11:25 am

@czietz thank you for the test & confirmation, I had no idea that is normal to have such temperature on a electronics component, wow.

Yesterday I checked all the rails, I couldn't see anything suspicious. It's true I had only the poor man's tool for detecting ripples, i.e. min/max but it seemed all right.

So I decided to check soldering, before doing anything else. And while re-soldering all the pins, suddenly, one of the pins on "K9047 HI-POT" (the big yellow thing, I couldn't even identify it in the schematics, what the hell it is?) became loose:
IMG_20200510_235401.jpg
IMG_20200510_235401.jpg (187.85 KiB) Viewed 584 times
I guess this is not OK. How it became loose I have no clue, I haven't touched that component at all, ever.

Is this part replaceable? I guess I'll try to desolder it and check what went wrong there but I feel I'm not going to like it.

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Re: Atari TT's LM7905CT getting too hot

Post by czietz » Mon May 11, 2020 11:51 am

The "big yellow thing" is the switching transformer, T1 in the schematic. These are usually custom-made for a particular PSU and not off-the-shelf parts. :(

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thorsten.otto
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Re: Atari TT's LM7905CT getting too hot

Post by thorsten.otto » Mon May 11, 2020 1:23 pm

mikro wrote:
Sat May 09, 2020 9:44 pm
Just staring at empty desktop makes the computer more "stable"
Depending on what OS you are running, that should not make a difference. Original TOS, and EmuTOS without using a stop instruction, will just sit in a busy loop when AES waits for events.

czietz
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Re: Atari TT's LM7905CT getting too hot

Post by czietz » Mon May 11, 2020 4:16 pm

thorsten.otto wrote:
Mon May 11, 2020 1:23 pm
mikro wrote:
Sat May 09, 2020 9:44 pm
Just staring at empty desktop makes the computer more "stable"
Depending on what OS you are running, that should not make a difference. Original TOS, and EmuTOS without using a stop instruction, will just sit in a busy loop when AES waits for events.
Even then, there's much less activity on the bus when the OS is just spinning in a loop. Less switching signals = more constant current consumption.

czietz
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Re: Atari TT's LM7905CT getting too hot

Post by czietz » Mon May 11, 2020 4:24 pm

Correlating the photo with the schematic, I think this is the +5 V rail. Even though not explicitly shown in the schematic, from the photo this rail seems to have 2 windings in parallel, one of which now is not connected anymore (and probably wasn't even before the pin fell out). With only one remaining winding, the PSU will not be able to supply the rated current on the 5 V rail.
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