Greetings to all. I'm hoping some of you experienced MR owners could offer some help here.
Here's my situation: I have the RM-9 amp (#121, in fact) that I've owned about 2 years. A few days ago after a listening session, I heard a loud POP on the right side, which I figured meant a fuse had blown. Bottom plate comes off, I see that I was right about the fuse, and replaced both right and left side fuses. So far so good.
Decided that it would be a good time to replace the on/off switch that has given be problems for a while (wouldn't always turn off) and I undertake that replacement. With some minor wood issues that occurred because the oak frame is thicker than the threaded barrel of your standard switch, that was basically a simple fix as well.
Now we're approaching the dark forest here, folks. I reconnected everything, all tubes back in place, power it up, and let it warm up before I re-bias; after about 15 minutes, I hear that POP again. I'm thinking the fuse has blown again, which probably indicates I've got a shorted tube on the right side. When I remove the bottom cover, I discover that the right side fuse had EXPLODED! Little shards of glass and one end cap of the fuse lay on the bottom of the case.
Okay, I replace the fuse, and, thinking that I need to determine which of the power tubes was shorted, I remove the front pair of 6550's from each channel. Power on, wait for a while, no blown fuse; Power off, remove rear 6550's from each side, power on, no popping after a few minutes. Hmmmm. All fuses still good, as determined by continuity test.
So I put on a CD. No music. Swap power tubes from front to rear, (still just 2 per side) no music. All the tubes are lighted, but then I notice that the input and driver tubes, though they light up, are not at all hot. (Power tubes Definitely Hot!)
That's pretty much where I left it then, and haven't had much time to delve into it. Looked at the bottom of the board for burn marks, or signs of arcing and such, and saw nothing obvious. The tubes lighting up means the switch replacement is not the problem. I'm willing to dig in deeper, but am hoping that someone with some expertise might lead me to a starting point here.
Am grateful for any help anyone can provide.
Jeff
