JBL EON 518S powered subwoofer blowing fuses succesful repair

[fixy88]

I'm posting this here in case someone else runs into this issue, as I'm guessing it will come up for a lot of users of this subwoofer.

I was gifted this sub as the user had replaced it and wanted it out of their storeroom. It was dead but they didn't give me any further info. I checked the mains fuse which was open. Fuse was replaced and the unit worked fine and continued to work fine with daily testing for several weeks. Then one day the fuse blew on powering up.

I opened the unit and removed the power supply PCB from the rear panel. I found the bridge rectifier on the primary side shorted and the NTC in the soft start circuit partly destroyed. The rectifier was replaced and the power supply powered up with a current limiting bulb in series with 240 connected directly to the power supply pcb. It appeared to be working ok so the bulb was removed and powered up again. One of the 2 680uf 200v caps on the primary side exploded. This was removed and testing showed no other damage. Caps were replaced and the power supply ran fine on the bench so was refitted to the rear panel and connected to the other boards. It was refitted to the sub and powered up, again one of the 680uf caps exploded and the other bulged. Unit was removed, tested, caps replaced and ran fine on the bench.

These units use a small PCB mounted to the IEC connection. This board is used to autosense the mains voltage. A relay is used to bridge a jumper across the bridge rectifier/mains caps to double the voltage for 120v use. This relay briefly engages during power up even when connected to 240v as the voltage comes up in the sense circuit. The relay was found to be stuck in the 120v position which caused the caps to explode.

Power supply was tested again, damaged caps replaced and with the 120v/240v selection relay connections removed is now working fine.

I'm no expert, but using a cheap relay in that briefly cycles to voltage doubling position on every power up, seems to me to be asking for trouble.

Schematics for the PRX 618s are attached - it uses the same boards with slightly different components in some locations.

[fzabkar]

I would remove the relay in 240VAC regions, or replace it with a jumper wire in 120VAC regions. Add a label to the ratings plate, if necessary.

(Actually, if it goes "doof, doof", I would probably take a hammer to it.)

[fixy88]

I would remove the relay in 240VAC regions....)

Yes it's a really bad design decision in my opinion. I've left it disconnected in this unit.