Atari TT's LM7905CT getting too hot

Repairs & upgrades.
mikro
Posts: 207
Joined: Mon Aug 28, 2017 11:22 pm

Re: Atari TT's LM7905CT getting too hot

Post by mikro » Mon May 11, 2020 8:24 pm

@czietz, I bow to your analytical skills and knowledge. Again and again. :)

Anyway, there has been an interesting development. My "analysis" wasn't actually that different, I saw two pins going to the same place on the PCB so I was thinking "OK, whatever is coming out of those two pins, it must be in pair because one pin is perhaps not enough". So I did it, I desoldered the transformer, to check what's wrong.

When you look at the transformer, you would immediatelly think something like "Oh suuure, it is the broken pin on the right", right?
IMG_20200511_171437.jpg
IMG_20200511_171437.jpg (214.87 KiB) Viewed 136 times
Wrong. The pin on the right is the unused pin, there is no hole for it on the PCB. So yes, it is the left one which got loose. Strange, huh?

So I was looking at that hole *in plastic* seeing absolutely no sign of anything conductible whatsoever (there is only one other pin with "setup" like that, others have something coming out of the transformer). I was like YOLO, let's take the loose pin, warm it up and just smash it into the plastic, hoping that there is *something* conductible inside, some magical pudding :D.

Did it work? Hell yes it did! Well, maybe. I let the TT run without PSU cover for about two hours - I have recapped the whole MegaSTE PSU while running YAARTTT (yup, had zero faith that it would work so I was preparing this PSU for the TT), Storm, Thunder, IDE SD card reader installed. I still have to try it with the Lighting + Mach64. But it's a good sign. It's true that maybe it is a coincidence - remember I did resolder all the joints, I even fixed some joints which looked suspicious (while comparing it to the other PSU), so who knows.

I'll report when I will have it all up and running.

mikro
Posts: 207
Joined: Mon Aug 28, 2017 11:22 pm

Re: Atari TT's LM7905CT getting too hot

Post by mikro » Tue May 12, 2020 5:14 pm

mikro wrote:
Mon May 11, 2020 8:24 pm
I'll report when I will have it all up and running.
Reporting: everything put together, has been running for hours now.

Thanks everyone for hints and advice!

Maximilian
Posts: 143
Joined: Thu Feb 28, 2019 5:02 pm

Re: Atari TT's LM7905CT getting too hot

Post by Maximilian » Tue May 12, 2020 7:06 pm

The pin never connects through the plastic piece, if connected there would be a wire connected to it like the others.
The 6 other pins are all connected as seen in the schema.

Probably you solved it by soldering another loose connection.

Good to hear that it functions reliably now :dualthumbup:

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