Rotel RB991 Humming badly, what have I done?

soiset

OK, first, I'll admit I screwed up. I am wiring some LS50's for active biamping. I had everything set up, with the RB991 running the two mids, and the RB981 running the tweets. It was sounding sweeter and sweeter as I adjusted the controls on the crossover. Then I went around to the back of one speaker, and saw that the leads from the two mid-bass drivers had come out of the bottom binding posts (I haven't got the gaskets yet, so they are wired and hanging out of the cabinets.

Being so very smart, I neglected to shut the system down before I went to put the wires back in, and, of course, I shorted the leads. A loud 60 Hz hum kicked in after the pop from the speaker, and it will not go away. When I remove the rca inputs from the back, the hum stops. Plug them in and it's right back. I played some music on top of it, and I blew a fuse (maybe two by this point). So now I have replaced all five T6.3 fuses and have hooked it back up. Hum is there, and I haven't tried playing any music over it.

The hum does not change in volume with any of the controls. I have isolated the problem to the amp, because the other amp drives the tweets and woofers fine from the same inputs.

What the hey?

OK, now I'm thinking it is some kind of noise, perhaps ground loop, in the inputs. If I unplug the inputs at the amp, no noise. If I unplug them at the other end (the pre for now), the noise depends on where the rca's are and how I am holding them. If I touch the interconnect, the noise increases. If I place them just right, the noise goes away almost entirely. The thing is though, that none of this was happening before I shorted the speaker leads. Can a noise sensitive situation be created by something like that?

ben62670

Try disconnecting your cable TV/SAT wire. I had the same problem.

soiset

All that we have in that regard is our cable modem. Do you mean unhook that somewhere?

Also, I have tried running an XLR cable from the pre to the amp, and no difference.

OK, I tried unplugging the cable at the modem, and there was no difference. I have also noticed that the hum in the right channel is quieter than the hum in the left. There is also a rhythm behind the hum of approximately 1 Hz.

billbillw

You fried something. Possibly a filter cap, a resistor, who knows. I've got the service manual for that amp if you think it will help. If you're not good with circuits, I'd recommend taking it in to a repair shop and don't mess with it any more. You could do more damage trying to play it in that state.

reeltrouble1

Sorry to hear of your loss. Shop repair or boat anchor seems like the only options.

RT1

most of us have been there.

F1nut

Agreed, you fried something in the amp. It is most definitely not a ground loop/cable TV problem.

billbillw

Soiset,
I thought your name looked familiar. Your Chris, right? Didn't know you were a Club Polkie when we were talking before.

I'm assuming this that Rotel gear that I helped you out with before on the voltage switch. Sorry to hear about this since you got a good deal on that stuff even though the seller was a jerkoff liar.

sucks2beme

Sorry to hear you killed it. Shorts like that are what always do in amps.
There went about $200 in repairs. How many of us have gotten in a hurry and did the same thing?:mad:
I don't know any good repair shops in the DFW area. There's one in Austin that I have heard of.
Maybe Russman knows one?

sucks2beme

If in doubt, get together with another audio buff and try some things.
Other than the power amp, I'd be inclined to thing the heavy draw by the PA
dead short may of damaged the powe strip/conditioner.
The money bet is still the amp. Can you easily got back to using just the single amp?
Sounds like the xovers have been pulled?
I know some docs I saw for the RB981 said they went to overtemp sensing
for overload protection for better sound. I can't find a like statement
for the RB991. Old style protection would of shut it down quickly.
I don't see where you said it ever went into protection, just POP and huuuuum.
I would thunk a ground loop or bad conditioner would hum on both amps.

soiset

Let's hope I do better than boat anchor, eh?

I took it to a local electronics shop on Saturday, with the tech manual. I had replaced all five fuses, and looked around as well as I knew how, and gave up. The evaluation is $95, which is applied to the repair. I should know something this Saturday.

I sure hope it isn't a $200 repair. The fuses are all slow-blow. I wonder if that had anything to do with the fact that another component of the amp was damaged. The amp labels all call out for slow-blow.

BillW, I had no idea you were a polkie. I really appreciate those manuals you sent me; I wouldn't have known what to do otherwise. All in all, even with the frustration of dealing with a liar, I am still happy to have gotten an RB-991, RC-995, RCD-995, and an RT-940ax for a net of $700, a little soldering, and some worry. So if I have to spend some dough to get the amp running again, I figure I'm still way ahead.

Never, ever, ever again will I be such a dumbass as to leave my equiment on and screw with the wires.

billbillw

Chris,
I doubt the amp repair will be that expensive. Luckily, most of the parts in your standard 2-ch amp are pretty inexpensive (except for the transformer, the large caps, and the output transistors). I'd say $150 or less. Good luck. Hope everything works out.

sucks2beme

I saw a H/K amp that never blew the fuses, but took out most of the power
transistors AND the input trasformer first. That's the trouble with big current design,
is it's hard to find a balance point between aways blowing fuses, and
blowing them before there's damage.
Hopefully, everything will be ok.
I say $200 only because that seems to be the number that always seems to come up.
I think they have a coin that they flip, one side says $200,
the other says "you can't fix this".
If I don't DIY, I normally don't mess with stuff any more.

soiset

sucks2beme wrote: »

Sorry to hear you killed it. Shorts like that are what always do in amps.
There went about $200 in repairs. How many of us have gotten in a hurry and did the same thing?:mad:
I don't know any good repair shops in the DFW area. There's one in Austin that I have heard of.
Maybe Russman knows one?

The shop I took it to is a little operation in the strip mall at southwest freeway and Abrams. An older Chinese man is the sole proprietor, and claims 34 years experience. I figure he must be good with a soldering iron and mulitmeter, and can follow a wiring diagram. I don't know of any audio repair specialists in the area, but would like to.

miner

The Home Theater Store has a Rotel repair technician. I believe there is one off of Centeal Exprwy and Knox-Henderson.

soiset

miner wrote: »

The Home Theater Store has a Rotel repair technician. I believe there is one off of Centeal Exprwy and Knox-Henderson.

Thanks. I'll look it up, and if my local guy can't do it, I'll haul it over there.

soiset

The amp is back from the shop, and... the guy could find nothing wrong. It sounded like he tested everything on the left output board, and finding nothing wrong, he hooked it up to some speakers. I went in this morning and although he did not have a preamp, he had a Sony cassette deck hooked directly to it, and it was running to some big crappy speakers. It played at one volume, loud, and there was no hum, even between songs in the quiet part.

The rca's he was using were little crappy unshielded ones. We unplugged the cables from the tape deck, and still no hum, where there had been hum before. Even weirder, I touched the end of the rca's, and still no hum, whereas here at the house, the hum volume doubles when I do that.

So I brought it back home (out $95) and plugged it back in. Humming. I set it up in my gym, connected to the Klipsch speakers in there. Humming, BUT, I can make the hum go away by moving the patch cords around (they aren't connected to a source in the gym). I can get it just right, and it's quiet, except that every 8 seconds or so, the hum flutters back in.

I hooked up an XLR cable to the input, and switched the amp to balanced. Hum. When I touch the metal housing at the far end of the XLR, the volume of it goes way up.

What the fudge is going on? And why did it happen to start right after I shorted the speaker leads on the right side? And why would it be louder on the left?

sucks2beme

Time for some component swaps.
Something's wrong.

soiset

I put an XLR in each input, switched to balance, and the hum dropped to an inaudible level in the right channel, and disappeared entirely in the left. I have some TRS to XLR cables arriving Tuesday, so I will be able to go all balanced from my xover to the amp. Maybe it will work.

My RB981 in the same string of components produces no hum. Goldang mystery, I tell ya!