Help with blown fuses on Rotel RB-1080

[truong]

Hi all,

I purchase used RC-1090 and RB-1080 last week. I've been using them with a pair of old speakers that I have. They work fine so far.
This weekend, I bought a brand new pair of DynAudio Audience 82 + Audioquest Dimondback XLR balanced interconnection + cheap 18 gauge speaker wire from RadioShack (temporary until I get better speaker cables). I hooked them up and listened for about 3 hours. Then I read somewhere that the speaker wires should be the same length, so I shut them down and trim 1 of the longer speaker wire to have the same length as the other. When I hook them up again, I got no sound to both of the speakers. I also noticed that when I hook the wire to 1 of the speakers, I heard some popping sound from the speaker. I went through all the steps that I know of to try troubleshooting the problem. Finally I pinpointed it down to the amp (RB-1080). The internal fuses were blown. Through out dozen of time trying connect/disconnect the speaker wires, I don't think I ever let the +/- part of the wires touched.
I bought some replacement fuses. But the fuses just kept blowing. At one time I was able to listen continuously for a few hours. But as soon as I turn the amp off or turning it on, the fuses blown again. I already made 2 trips to RadioShack and blow 16 of them fuses. Finally, my amp blows the fuse on the back of the amp. So as this point, I'm not sure what I should do.
So here are my questions:

1. Is there something bad with my new DynAudio speakers? How can I tell whether they are still in "perfect" condition (as they are brand new)? Have I done any damage to them? Is there a way of testing and know for sure?
2. Is there a minimum length of speaker wire required? Are the cheap 18 gauge speaker wires the culprit?
3. What should I do next?

Thanks in advance for any suggestion.

 

[TLS Guy]

Hi all,

I purchase used RC-1090 and RB-1080 last week. I've been using them with a pair of old speakers that I have. They work fine so far.
This weekend, I bought a brand new pair of DynAudio Audience 82 + Audioquest Dimondback XLR balanced interconnection + cheap 18 gauge speaker wire from RadioShack (temporary until I get better speaker cables). I hooked them up and listened for about 3 hours. Then I read somewhere that the speaker wires should be the same length, so I shut them down and trim 1 of the longer speaker wire to have the same length as the other. When I hook them up again, I got no sound to both of the speakers. I also noticed that when I hook the wire to 1 of the speakers, I heard some popping sound from the speaker. I went through all the steps that I know of to try troubleshooting the problem. Finally I pinpointed it down to the amp (RB-1080). The internal fuses were blown. Through out dozen of time trying connect/disconnect the speaker wires, I don't think I ever let the +/- part of the wires touched.
I bought some replacement fuses. But the fuses just kept blowing. At one time I was able to listen continuously for a few hours. But as soon as I turn the amp off or turning it on, the fuses blown again. I already made 2 trips to RadioShack and blow 16 of them fuses. Finally, my amp blows the fuse on the back of the amp. So as this point, I'm not sure what I should do.
So here are my questions:

1. Is there something bad with my new DynAudio speakers? How can I tell whether they are still in "perfect" condition (as they are brand new)? Have I done any damage to them? Is there a way of testing and now for sure?
2. Is there a minimum length of speaker wire required? Are the cheap 18 gauge speaker wires the culprit?
3. What should I do next?

Thanks in advance for any suggestion.

I bet you did touch a + and - speaker wire together, or short a + to chassis which is the same thing.

You have now done major damage to your Rotel power amp, which will require costly repairs.

Pleas ALWAYS switch off power amps before plugging and unplugging speakers.

 

[Adam]

Welcome to the forum!

I'd suggest trying the following. Disconnect the new speakers and either (a) connect your old speakers again using whatever speaker wire you were using originally or (b) don't connect any speakers. I'd try the option B to begin with. Then power cycle the Rotel. If you're still blowing fuses, then it's not the new speakers. If you aren't blowing fuses, then it's related to whatever you changed (speakers and/or wire).

You have now done major damage to your Rotel power amp, which will require costly repairs.

That's a pretty strong statement, Mark. What makes you so sure of that?

 

[jamie2112]

I bet you did touch a + and - speaker wire together, or short a + to chassis which is the same thing.

You have now done major damage to your Rotel power amp, which will require costly repairs.

Pleas ALWAYS switch off power amps before plugging and unplugging speakers.

He may not have blown the unit Mark. I would think that the fuses could be changed to 8a and he may be fine. This issue has occured with many of the older 1080 units and the 8a fuse has been the thing that works best as they don't use the 12volt trigger which in most of these 1080's is the issue. Rotel has an internal replacment kit you can buy for your unit..

 

[truong]

He may not have blown the unit Mark. I would think that the fuses could be changed to 8a and he may be fine. This issue has occured with many of the older 1080 units and the 8a fuse has been the thing that works best as they don't use the 12volt trigger which in most of these 1080's is the issue. Rotel has an internal replacment kit you can buy for your unit..

Thank you all for replying so far. I don't use 12V trigger right now.
Are you suggesting me to get 8A fuse instead of 6.3A for internal fuses? If so, I'll get some tomorrow when I pick up the replacement for the fuse on the back.

 

[truong]

Welcome to the forum!

I'd suggest trying the following. Disconnect the new speakers and either (a) connect your old speakers again using whatever speaker wire you were using originally or (b) don't connect any speakers. I'd try the option B to begin with. Then power cycle the Rotel. If you're still blowing fuses, then it's not the new speakers. If you aren't blowing fuses, then it's related to whatever you changed (speakers and/or wire).

Thanks. I'll try that as soon as I get some new fuses.

 

[PENG]

Thanks. I'll try that as soon as I get some new fuses.

Before you do that please note page 8 of the RB-1080 manual:

"Replace only with the
identical size and type fuse (3AG125V12A
for U.S. version or 5ST6.3 for European
version)."

So if you are in North America, you should have the 3AG125V12A that is a 12A fuse. If you use something less you risk blowing it when you turn power on even if you have NOTHING connected to the amp.

Depending on the point of the supply voltage Sine wave when you turn power on and the polarity of the residual magnetism in the amp's power transformer, the transformer inrush current can range from next to nothing to many times the full load current. Manufacturers have to choose between using a lower current rating slow acting fuse or higher current rating fast acting fuse. It looks like Rotel preferred the fast acting fuse for better short circuit protection. So make sure you check whether your amp is set to 115V or 230V and size your fuse according according to page 8 of the manual.

You reported that the fuses blew when you turned power on so I think you will be fine as soon as you have the right fuse in place. Good luck!

 

[TLS Guy]

Welcome to the forum!

I'd suggest trying the following. Disconnect the new speakers and either (a) connect your old speakers again using whatever speaker wire you were using originally or (b) don't connect any speakers. I'd try the option B to begin with. Then power cycle the Rotel. If you're still blowing fuses, then it's not the new speakers. If you aren't blowing fuses, then it's related to whatever you changed (speakers and/or wire).


That's a pretty strong statement, Mark. What makes you so sure of that?

I think the fact that he was blowing the rail fuses on the amp board and after a number of changes blew the fuse to the primary of the power transformer, is very ominous. The fact that the problem started after interfering with the speaker leads with the lower on is even more sinister.

 

[truong]

I stopped by Rat Shack today after work to get some new fuses. I brought along the blown AC fuse from my amp. What printed on the fuse was BUSS AGC 12 on one end, and 32V on the other end. The gentleman who works there helped me find the fuse. We looked up online and found the spec of the fuse. It's 32V and 12A., but we couldn't find any fuse that's 32V. Now that I just saw the message from PENG mentioned about 3AG125V12A (I assume it's 125V and 12A), I realize the previous owner had the 32 V fuse in it all a long.

What would be the effect of a lower voltage fuse (32V vs. 125V) on the equipment? Would that be the root cause? And why did many rail-fuses get blown first before the AC fuse was blown?

can someone please educate me!

 

[TLS Guy]

I stopped by Rat Shack today after work to get some new fuses. I brought along the blown AC fuse from my amp. What printed on the fuse was BUSS AGC 12 on one end, and 32V on the other end. The gentleman who works there helped me find the fuse. We looked up online and found the spec of the fuse. It's 32V and 12A., but we couldn't find any fuse that's 32V. Now that I just saw the message from PENG mentioned about 3AG125V12A (I assume it's 125V and 12A), I realize the previous owner had the 32 V fuse in it all a long.

What would be the effect of a lower voltage fuse (32V vs. 125V) on the equipment? Would that be the root cause? And why did many rail-fuses get blown first before the AC fuse was blown?

can someone please educate me!

A fuse of higher voltage will not be an issue, only of it had a lower voltage rating. The most important parameters are the current and time to blow ratings.

I still feel your amp has a serious problem. The rule with fuses is to replace once. If it blows a second time, then to only start the amp on a variac with instruments on it. To continue to replace fuses usually results in more extensive damage and expense. I'm concerned this has occurred as you have now started blowing the main fuse.

That Rotel should be able to drive your Dynaudio speakers just fine. Speaker wire will only cause this sort of problem if there is a short, which I fear there was.

 

[Lordoftherings]

I will bring my Rotel component (RB-1080) to a Rotel service center and get an estimate of the problem. So that way you know exactly what's happening and what have to be done.
Explain the problem and ask them to give you a free estimate.

That will save you worries and lost time, plus it will bring you faster on the road of recovery.

Bob

 

[TLS Guy]

I will bring my Rotel component (RB-1080) to a Rotel service center and get an estimate of the problem. So that way you know exactly what's happening and what have to be done.
Explain the problem and ask them to give you a free estimate.

That will save you worries and lost time, plus it will bring you faster on the road of recovery.

Bob

That is your wisest course of action. Good luck!

 

[PENG]

What would be the effect of a lower voltage fuse (32V vs. 125V) on the equipment? Would that be the root cause? And why did many rail-fuses get blown first before the AC fuse was blown?

can someone please educate me!

The primary voltage in the US is 115 to 125V so you should use 125V fuses for both safety reason and for proper short circuit protection. It wouldn't be the root cause for the problem you described though.

This is a link to the manual if you don't have it yet: http://www.rotel.com/content/manuals/rb1080.pdf

You said you blew the rail fuse 16 times, but what is the rating of the original fuse? At one point you mentioned a 6.5A fuse, that seems low for a 200W/ch amp. Jamie suggested you use an 8A fuse, and it sounds like he knows something about the kind of problem you are having.

As a last attempt before sending it out, you may want to contact Rotel and find out if you have the right size of rail fuse to begin with. If the previous owner had a 32V fuse in it instead of 125V one, may be he had the wrong fuse on the d.c. side as well. I recommend you take this step only because you had mentioned at one time you were able to listen continuously for a few hours. If any output devices are blown you would not be able to listen to it at all.

Other than the possibility of using undersized fuses, as TLS said, you should only replace the fuse once and if it blew the second, or the third time then it is time to send it out for repair.

 

[Lordoftherings]

That is your wisest course of action. Good luck!

My Rotel dealer does not charge me to tell me the problem, only to fix it.
I guess I got a nice dealer, but then I'm a good customer too.

 

[truong]

The primary voltage in the US is 115 to 125V so you should use 125V fuses for both safety reason and for proper short circuit protection. It wouldn't be the root cause for the problem you described though.

This is a link to the manual if you don't have it yet:

You said you blew the rail fuse 16 times, but what is the rating of the original fuse? At one point you mentioned a 6.5A fuse, that seems low for a 200W/ch amp. Jamie suggested you use an 8A fuse, and it sounds like he knows something about the kind of problem you are having.

As a last attempt before sending it out, you may want to contact Rotel and find out if you have the right size of rail fuse to begin with. If the previous owner had a 32V fuse in it instead of 125V one, may be he had the wrong fuse on the d.c. side as well. I recommend you take this step only because you had mentioned at one time you were able to listen continuously for a few hours. If any output devices are blown you would not be able to listen to it at all.

Other than the possibility of using undersized fuses, as TLS said, you should only replace the fuse once and if it blew the second, or the third time then it is time to send it out for repair.

Thanks. I won't be able to post link, but I found this h**p://bwgroup-support.com/downloads/reference/rotel/Rotel%20Fuse%20Chart.pdf

So I think the rail fuses are correct.

 

[PENG]

Now that I see there are 4 fuses, 2x6.3A per channel it makes sense. Again if the fuse ratings (current and voltage) are correct then it is time to send it out for repair. Regardless, if they call for 250V rated fuses on the rail side, you should not use the 32V ones. For the primary you also should stick with 125V rated fuses. If you send it to Rotel for repair I am sure they will have the right fuses anyway but you may want to ask them for spares if you have the habit of blowing them.

 

[TLS Guy]

The primary voltage in the US is 115 to 125V so you should use 125V fuses for both safety reason and for proper short circuit protection. It wouldn't be the root cause for the problem you described though.

This is a link to the manual if you don't have it yet: http://www.rotel.com/content/manuals/rb1080.pdf

You said you blew the rail fuse 16 times, but what is the rating of the original fuse? At one point you mentioned a 6.5A fuse, that seems low for a 200W/ch amp. Jamie suggested you use an 8A fuse, and it sounds like he knows something about the kind of problem you are having.

As a last attempt before sending it out, you may want to contact Rotel and find out if you have the right size of rail fuse to begin with. If the previous owner had a 32V fuse in it instead of 125V one, may be he had the wrong fuse on the d.c. side as well. I recommend you take this step only because you had mentioned at one time you were able to listen continuously for a few hours. If any output devices are blown you would not be able to listen to it at all.

Other than the possibility of using undersized fuses, as TLS said, you should only replace the fuse once and if it blew the second, or the third time then it is time to send it out for repair.

You can have failing output devices work for a period of time. This is how it happens. There is an arc through the semiconductor. Usually base to collector. As it arcs the line fuse blows. Then when the voltage gets to a critical point again, there is another arc and a blown fuse. Eventually a weld takes place between the plates, and the unit no longer functions at all.

The Rotel unit has thermal protection, however there will be DC offset protection also. These situations cause DC off set and will activate this protection. I doubt an amp of that power uses relay protection, and I think it is of a generation prior to opto-coupler protection. So I suspect it has some sort of DIAC/TRIAC clamp protection circuit. When these circuits are activated, they short the output stage compounding the problem and damaging the output stage further. However they effectively prevent speaker damage.

 

[PENG]

You can have failing output devices work for a period of time. This is how it happens. There is an arc through the semiconductor. Usually base to collector. As it arcs the line fuse blows. Then when the voltage gets to a critical point again, there is another arc and a blown fuse. Eventually a weld takes place between the plates, and the unit no longer functions at all.

Actually I missed the part that he said he blew 16 rail fuses. Had I caught that part earlier on I would have asked him to verify the fuses were correctly rated and if they were then the amp needed to pay the factory a visit. That being said, he did have the wrong fuses in terms of voltage rating. I think we can agree that while that would likely not be the reason he blew the fuses the way he did, but still, you don't substitute 125 and 250V fuses with 32V ones, if only for safety and equipment protection reasons.

 

[TLS Guy]

Actually I missed the part that he said he blew 16 rail fuses. Had I caught that part earlier on I would have asked him to verify the fuses were correctly rated and if they were then the amp needed to pay the factory a visit. That being said, he did have the wrong fuses in terms of voltage rating. I think we can agree that while that would likely not be the reason he blew the fuses the way he did, but still, you don't substitute 125 and 250V fuses with 32V ones, if only for safety and equipment protection reasons.

Actually I was not perturbed by the rail fuses being 32 volt. I assumed there were two rail fuses on each amp board, with each fuse supplying a rail, so -32 +32 would be 64 volt, which would be the voltage between rails you would expect to find in an amp of this power. The power fuse to the transformer primary should obviously be 125 volt for US operation.

I fear after replacing fuses that often, the damage has spread back to the driver transistors and associated diodes by now, so unfortunately this may prove to be a costly repair for the OP.

As you concurred the real take home lesson from this sad story, is to always have an amp shut down, when working on speaker leads. Lesson two, replace a blown fuse only once, with one of the correct specification. If it blows it is service time.

 

[PENG]

Actually I was not perturbed by the rail fuses being 32 volt. I assumed there were two rail fuses on each amp board, with each fuse supplying a rail, so -32 +32 would be 64 volt, which would be the voltage between rails you would expect to find in an amp of this power. The power fuse to the transformer primary should obviously be 125 volt for US operation.

I fear after replacing fuses that often, the damage has spread back to the driver transistors and associated diodes by now, so unfortunately this may prove to be a costly repair for the OP.



As you concurred the real take home lesson from this sad story, is to always have an amp shut down, when working on speaker leads. Lesson two, replace a blown fuse only once, with one of the correct specification. If it blows it is service time.

I agree, the 32V fuses for rail seems a little low but should be good enough for the +/- rails.

According to the link he posted,

http://bwgroup-support.com/downloads/reference/rotel/Rotel Fuse Chart.pdf

the OEM rail fuses are 250V 6.3A. Why do you think they call for 250V, standardization to minimize their stocks?