Bulk capacitor burning in

[satjanand]

Hi all,

Hope to find a solution.

I have a 1040STF that has had a lot of work done: RD-50A PSU, Marpet 4MB RAM upgrade, Gotek, drive A/B toggle switch It seemed fine, until today I noticed burning of the bulk capacitor's (C149 / 4700uF / 16V) positive side. Voltage over the capacitor is a neat 4,95V.

Swapping an old Atari PSU in and relocating Marpet's original bank disablers from C149 to C130 doesn't help, C149 keeps burning the board.

Where do I start?

I have some 4700uF / 16V's lying around. Do I swap the cap, or should I first undo the RAM upgrade?

IMG_20230207_211434.jpg

[stephen_usher]

Remove the cap. It's got a short. There should be no current going through there.

[satjanand]

Remove the cap. It's got a short. There should be no current going through there.

Okay, so went ahead and replaced the cap. Disappointingly, replacing the cap makes no difference. The second I turn on the Atari, the thru-pcb-hole which marks the + side of the cap starts smoking.

IMG_20230207_231529_edit_422536311610525.jpg

[rubber_jonnie]

I think the problem here is that there is corrosion shorting the + side of the cap to the ground plane surrounding it. You can see the negative terminal connects to the same ground plane on the left of the picture.

I suspect a multimeter in continuity mode will show a short between 5v and GND.

Simply soldering the new cap to the old lead has left the potential problem in place. You need to remove the old leads and clean up all the solder then make sure the corrosion is cleaned up, and if possible add some new solder mask to prevent more corrosion forming. If you don't have any solder mask, then clear nail varnish also works.

[Dlfrsilver]

Hi all,

Hope to find a solution.

I have a 1040STF that has had a lot of work done: RD-50A PSU, Marpet 4MB RAM upgrade, Gotek, drive A/B toggle switch It seemed fine, until today I noticed burning of the bulk capacitor's (C149 / 4700uF / 16V) positive side. Voltage over the capacitor is a neat 4,95V.

Swapping an old Atari PSU in and relocating Marpet's original bank disablers from C149 to C130 doesn't help, C149 keeps burning the board.

Where do I start?

I have some 4700uF / 16V's lying around. Do I swap the cap, or should I first undo the RAM upgrade?


IMG_20230207_211434.jpg

yes as said earlier, this is a short circuit. it requires some investigation, you probably have some bad traces (since changing the cap did not solved the problem).

[Steve]

Gotta PROPERLY de-solder that old cap, clean the area and re-solder a new cap...

[stephen_usher]

Actually, I'd desolder, clean and then use a multimeter to check the resistance between ground and +5V on the board without anything in there. It's quite possible that there's a fault on the PCB itself.

[tzok]

The electrolyte used in capacitors is conductive, if it managed to soak into the PCB it made it conductive. Also, even slightly charcoaled PCB becomes electro-conductive. The PCB needs to be flushed from the electrolyte and any burned parts of it need to be removed. Also, you should protect the cleaned area of the PCB from absorbing the moisture from the air by soaking it with an electro-insulating varnish or a cyanoacrylate glue.

[satjanand]

I think the problem here is that there is corrosion shorting the + side of the cap to the ground plane surrounding it. You can see the negative terminal connects to the same ground plane on the left of the picture.

I suspect a multimeter in continuity mode will show a short between 5v and GND.

Simply soldering the new cap to the old lead has left the potential problem in place. You need to remove the old leads and clean up all the solder then make sure the corrosion is cleaned up, and if possible add some new solder mask to prevent more corrosion forming. If you don't have any solder mask, then clear nail varnish also works.

yes as said earlier, this is a short circuit. it requires some investigation, you probably have some bad traces (since changing the cap did not solved the problem).

Gotta PROPERLY de-solder that old cap, clean the area and re-solder a new cap...

Actually, I'd desolder, clean and then use a multimeter to check the resistance between ground and +5V on the board without anything in there. It's quite possible that there's a fault on the PCB itself.

Thank you. After properly desoldering and replacing the cap, and thoroughly cleaning the PCB, the machine seems fine now. Time for some stress testing.

The electrolyte used in capacitors is conductive, if it managed to soak into the PCB it made it conductive. Also, even slightly charcoaled PCB becomes electro-conductive. The PCB needs to be flushed from the electrolyte and any burned parts of it need to be removed. Also, you should protect the cleaned area of the PCB from absorbing the moisture from the air by soaking it with an electro-insulating varnish or a cyanoacrylate glue.

Thank you, I will get into that after thoroughly testing the machine, making sure the problem has been found.

[rubber_jonnie]

Thank you. After properly desoldering and replacing the cap, and thoroughly cleaning the PCB, the machine seems fine now. Time for some stress testing.


Thank you, I will get into that after thoroughly testing the machine, making sure the problem has been found.

Good stuff, and shows the importance of good visual inspections and proper clean up of corroded areas.

Glad you're up and running.