Repairing my Monster HTS 3500

I have an older Monster HTS 3500 that died some time ago. In the interest of not putting it in the landfill, I took it apart today to see what’s wrong.

One of the MOVs is toasted. It’s a Ceramate GNR20D201K. I can’t be sure, but I’m assuming a lightning strike or the like took it out. Don’t ask me why there isn’t a gas surge suppressor in here. Cheap construction would presumably be the answer, but I don’t have a schematic and don’t feel like digging too deep. I’ll replace the MOV and see what happens from there. I probably have some that I can scavenge, we’ll see.

Feb 8, 2012
I removed the burned MOV using my desoldering gun, and was about to replace it when I decided to check the thermal fuse that’s squeezed between the first two MOVs. It is open, so I need to replace it. It appears to be an SF139U, which as near as I can tell is no longer made. But it was 10A, 250V, 142 Celsius. I checked the other thermal fuses and they’re all good. I ordered new thermal fuses from Digi-Key, part number 317-1134-ND. I don’t have the same tape that Monster used to keep the MOVs near the thermal fuse. I’ll likely just use powdercoating tape since I know it’ll survive higher temperatures than the thermal fuse and I have some on-hand.

To make sure this would fix things, I shorted across the leads of the open fuse. Sure enough, the HTS 3500 comes to life and shows no errors. Hooray for some inexpensive troubleshooting and parts saving me a couple hundred dollars.

Feb 13, 2012
I replaced two MOVs and the thermal fuse, and all is well. I put the case back on and it’s ready to go back in the family room.

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