I will try to do that in the coming days. I need to pull out the preamp from the cabinet. But I still need to find an expert that really know how to fix this....
Update and more questions, opinions...
Well, there is no really update regarding the amplifier due to busy holiday, and I really exhausted my option with local technicians. However, there is some development that I think you guys can give me some ideas, opinion...
Couple months ago I borrowed a Rotel RB 1070 (2 channels amp 120w/channel, made in the same era, same designed). It worked beautiful without problem (in that same setup, same cables, connections, outlet..etc). Yesterday I found and bought another RMB1095 locally, in very good shape (not cheap, I paid $900 for it). Today I hooked it up and within 1 hour watching tv, the protection mode kicked in!!!??. I also bought new pairs of speaker (monitor audio). So right now the only common denominator is the Denon avr, and both the amp and avr plugged to the same outlet. However, that same avr and the rotel RB1070, and Emotiva amp worked perfect without issue (same outlet connection too). What do you think maybe the cause here?? It is unlikely that I bought both rmb1095 defective and broken?!. Of course the seller sweared it worked perfect for him!!.
Btw, with my older rmb (that we tried to fix), just for testing couple months ago I hooked it up with an older Denon avr 3805 (same connection, same cables), and it went into protection too.....so i doubt the problem came from the denon avr.
Lucky I called the seller today and was able to return it (minus $50). I highly doubt both units are defective. Must be something wrong from my end. I did test the power outlet couple weeks ago, and with amp, preamp and some other components plugged in, it dropped to the lowest 116V (out of 120V). The tech told me that was not enough to trigger protection mode. Now assumed if it was the power outlet issue... what can I do?? Can I change out the fuse (with higher ampere) at the breaker box?? will it fix my problem?? or what can I do?? opinion?? potential fix??
Well, there is no really update regarding the amplifier due to busy holiday, and I really exhausted my option with local technicians. However, there is some development that I think you guys can give me some ideas, opinion...
Couple months ago I borrowed a Rotel RB 1070 (2 channels amp 120w/channel, made in the same era, same designed). It worked beautiful without problem (in that same setup, same cables, connections, outlet..etc). Yesterday I found and bought another RMB1095 locally, in very good shape (not cheap, I paid $900 for it). Today I hooked it up and within 1 hour watching tv, the protection mode kicked in!!!??. I also bought new pairs of speaker (monitor audio). So right now the only common denominator is the Denon avr, and both the amp and avr plugged to the same outlet. However, that same avr and the rotel RB1070, and Emotiva amp worked perfect without issue (same outlet connection too). What do you think maybe the cause here?? It is unlikely that I bought both rmb1095 defective and broken?!. Of course the seller sweared it worked perfect for him!!.
Btw, with my older rmb (that we tried to fix), just for testing couple months ago I hooked it up with an older Denon avr 3805 (same connection, same cables), and it went into protection too.....so i doubt the problem came from the denon avr.
Lucky I called the seller today and was able to return it (minus $50). I highly doubt both units are defective. Must be something wrong from my end. I did test the power outlet couple weeks ago, and with amp, preamp and some other components plugged in, it dropped to the lowest 116V (out of 120V). The tech told me that was not enough to trigger protection mode. Now assumed if it was the power outlet issue... what can I do?? Can I change out the fuse (with higher ampere) at the breaker box?? will it fix my problem?? or what can I do?? opinion?? potential fix??
Hi, I did measure the ground resistance and posted the measurement somewhere in the previous posts. Plus, with the Rotel RB1070 amp working perfect in the same place with this Denon avr... the Denon should not be the problem. Also somebody (earlier posts) confirmed that these measurement numbers are ok....
Hi nguyenphananh,
Without being able to look at these amplifiers on the bench in front of me, I can't do more than guess.
Rotel is on average a much better designed amplifier than an NAD and some others, but in this case I feel the basic design of these amplifiers are the problem. Something is making the amplifier "unhappy".
Did you notice elevated temperatures above the amplifier after it went into protection? To solve this you would need to determine what is activating the protection circuit, and the time involved leads me to suspect temperature. Things that could cause this might be the reactive component of the speakers or the wire. Enough capacitance can cause an amplifier to oscillate after it warms up a little. A little RF energy on the input, if not properly filtered out, can drive the amplifier at high ultrasonic frequencies and that will also heat things up.
Finally, this should be obvious so I'll apologise for bringing it up. I am not suggesting you aren't observant. Some amplifiers run hotter than others (like a Threshold Stasis or a Nakamichi of similar design). If you don't have enough airflow, that could certainly do it.
Not an answer to your problem with that amplifier, but hopefully an answer to the situation.
-Chris
Without being able to look at these amplifiers on the bench in front of me, I can't do more than guess.
Rotel is on average a much better designed amplifier than an NAD and some others, but in this case I feel the basic design of these amplifiers are the problem. Something is making the amplifier "unhappy".
Did you notice elevated temperatures above the amplifier after it went into protection? To solve this you would need to determine what is activating the protection circuit, and the time involved leads me to suspect temperature. Things that could cause this might be the reactive component of the speakers or the wire. Enough capacitance can cause an amplifier to oscillate after it warms up a little. A little RF energy on the input, if not properly filtered out, can drive the amplifier at high ultrasonic frequencies and that will also heat things up.
Finally, this should be obvious so I'll apologise for bringing it up. I am not suggesting you aren't observant. Some amplifiers run hotter than others (like a Threshold Stasis or a Nakamichi of similar design). If you don't have enough airflow, that could certainly do it.
Not an answer to your problem with that amplifier, but hopefully an answer to the situation.
-Chris
Hi Chris
thank you for spending time trying to figure out my amps' issue. Infact, all your thoughts and concern are legitimated. To answer your questions:
1. I did extensive research... and yes, this series/generation of Rotel amplifier did have design defective!!. Rotel later on came out with a service bulletin recommended the tech to replace 2 groups of components. First was a set of resistors (5 in my case). These resistors that original used in these amps were under voltage, under value!!??. Second was a set of ICs, which also prone to be defective. Both of these components if affected will cause protection mode shut down, and distortion. However, these fixes, correction never completely solved the problem, so on the later year models, they completely redesigned the protection circuit trigger board. On my amp, the tech "supposed " to already replaced all these components, yet it still didnt fix.
2. As I mentioned earlier, these amps run VERY VERY HOT. If you dare place your hands on top after an hour or so, you will get burn!!.(so yes, it was hot when it shut down). I dont own these amps from new, so I dont know whether they were like that from beginning, or is it normal. Anyway, with all my electronics, I placed 2 computer fan on top blowing down to keep these amps coolers.
3. Im aware of the heat issue. These amps were placed on top of the cabinet with nothing around, plus 2 cooling fan on top... so air flow obstruction should not be a problem
4. I also stated earlier. I did an experiment: I disconnected the speakers and wires, the unit still went into protection!!. so speakers should not be a problem. From my elimination process, I realized as long as there is signal input (and playing), the unit will shut down!!. Anything else like speakers, trigger board... doesn affect the amp at all.
5. Whatever you said about RF interferrence , oscillating totally made sense.. and I highly suspect you are on the right track... I just dont know how to confirm these and how to fix it. What strange was if I replaced it with the Rotel RB1070 (2 channels), which was of the same era and same design, it run without problem... Assumed that was the cause, what simple step can I do to fix this problem?? replace the RCA cables? some other extra device to filter, eliminate the oscillation? RF interferrence??
thank you for spending time trying to figure out my amps' issue. Infact, all your thoughts and concern are legitimated. To answer your questions:
1. I did extensive research... and yes, this series/generation of Rotel amplifier did have design defective!!. Rotel later on came out with a service bulletin recommended the tech to replace 2 groups of components. First was a set of resistors (5 in my case). These resistors that original used in these amps were under voltage, under value!!??. Second was a set of ICs, which also prone to be defective. Both of these components if affected will cause protection mode shut down, and distortion. However, these fixes, correction never completely solved the problem, so on the later year models, they completely redesigned the protection circuit trigger board. On my amp, the tech "supposed " to already replaced all these components, yet it still didnt fix.
2. As I mentioned earlier, these amps run VERY VERY HOT. If you dare place your hands on top after an hour or so, you will get burn!!.(so yes, it was hot when it shut down). I dont own these amps from new, so I dont know whether they were like that from beginning, or is it normal. Anyway, with all my electronics, I placed 2 computer fan on top blowing down to keep these amps coolers.
3. Im aware of the heat issue. These amps were placed on top of the cabinet with nothing around, plus 2 cooling fan on top... so air flow obstruction should not be a problem
4. I also stated earlier. I did an experiment: I disconnected the speakers and wires, the unit still went into protection!!. so speakers should not be a problem. From my elimination process, I realized as long as there is signal input (and playing), the unit will shut down!!. Anything else like speakers, trigger board... doesn affect the amp at all.
5. Whatever you said about RF interferrence , oscillating totally made sense.. and I highly suspect you are on the right track... I just dont know how to confirm these and how to fix it. What strange was if I replaced it with the Rotel RB1070 (2 channels), which was of the same era and same design, it run without problem... Assumed that was the cause, what simple step can I do to fix this problem?? replace the RCA cables? some other extra device to filter, eliminate the oscillation? RF interferrence??
Hi nguyenphananh,
Okay, then step one would be to reduce the bias current to sane levels which will drop the amount of heat generated a great deal. To be honest, a bias current of 20 to 30 mA is more than enough. Many good designs can run at 10 mA or less and deliver their best performance.
Most amplifiers have an R-C filter at the input. 100pF or so to signal common is normal, and the series resistance can be around 2K2 or thereabouts. In severe cases you might find an input choke as well. Have a look at the RB-1070 input stage between the RCA jack and input pair, then also this amplifier to see if there are any differences. The input filter is actually pretty standard across many brands and time. So have a look at say, Marantz 170DC or 300DC and other amplifiers of good quality. "High end" brands are poor models to look at as all bets are off. These designers often do not do a good job and are guided by the internet more than engineering practices.
Between these things you may have an answer. Additionally you may have an RF transmitter close to you. There was a location where we had two AM radio stations, 50 KW each within one km! It was so bad many CD players would not work. They were at 1.01 MHz and 1.05 MHz, so that made things worse. Could be a Police or Taxi base office, or a repeater for something. Any of these things can cause havoc with a poorly protected amplifier.
-Chris
Okay, then step one would be to reduce the bias current to sane levels which will drop the amount of heat generated a great deal. To be honest, a bias current of 20 to 30 mA is more than enough. Many good designs can run at 10 mA or less and deliver their best performance.
Most amplifiers have an R-C filter at the input. 100pF or so to signal common is normal, and the series resistance can be around 2K2 or thereabouts. In severe cases you might find an input choke as well. Have a look at the RB-1070 input stage between the RCA jack and input pair, then also this amplifier to see if there are any differences. The input filter is actually pretty standard across many brands and time. So have a look at say, Marantz 170DC or 300DC and other amplifiers of good quality. "High end" brands are poor models to look at as all bets are off. These designers often do not do a good job and are guided by the internet more than engineering practices.
Between these things you may have an answer. Additionally you may have an RF transmitter close to you. There was a location where we had two AM radio stations, 50 KW each within one km! It was so bad many CD players would not work. They were at 1.01 MHz and 1.05 MHz, so that made things worse. Could be a Police or Taxi base office, or a repeater for something. Any of these things can cause havoc with a poorly protected amplifier.
-Chris
Like I said, I did not own this amp from new, so dont know how it was brand new. Since I owned this amp (and the one I bought (and returned) few days ago), these ran very very hot. Im not sure but will not be surprised if the heat has something to do with the shutdown. The "authorized dealer" tech worked on my amp stated that the only 2 components that related to the amp's shut down was the 12v trigger circuit and the protection ciruit. From my research, the protection circuit triggers shut down when certain temperature is exceeded. The tech aware that the amp ran hot, but he stated that was normal!?. Plus at home, I have 2 computer fans on top to lower down the temperature.... but so far didnt help. Also he stated he already rebuilt the protection circuit (replaced some capacitors).
Hi Chris, I highly suspected you are on the right track. From the hunch, I believed by doing what you suggested, my problem will be solved. Unfortunately, these electronics are out of my scope. While I am familiar with mechanical, I am clueless about electronic. I wished I can find a technician around here that has understanding, I will pass on these infos for him to follow and try out. To be honestly and not mean to be offending anybody, I couldnt find any proper trained, experienced tech around my area or southern los angeles!! There are few TV repair shops around here which owned by mexican technicians. While I am sure they are trained and know their stuff, they can barely speak english, let alone reading forum messages (and they dont too for sure)!!. The "rotel authorized" tech that fixed my amp (didnt turn out so well) seems to be the only hope. But with his experienced and tilte... he didnt even bother listen to my suggestion, nor looked at these advises!!.
I have a russian friend and have him checking around his community for electronic technician who is capable. We found a "potential" guy. However the guys didnt even want to work!!. We been begging him for months to take on this job without success. Talking about frustration!!
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Some how I have a feeling your suggestion of reducing the bias current will fix my issue!!. This amp run extremely hot, and the protection circuit is temperature control!!. If we can reduce the temperature, perhaps it will not trigger the protection circuit. With that said... I have no idea how to do that!!. What tool or equipment does it take to do this job?? can the person who not very familiar with this process change the bias current and CHANGE THE SOUND QUALITY, CHARATERISTIC??. If it need special tool, I maybe out of luck, but if it not, I might be able to ask some tv repair shop to do it for me... what do you think??
P.S. I remember watching youtube video clip, some guy with a Rotel RB1080 (2 channels) also have shut down issue (dont know it similar to mine or not). He adjusted something and it cured the issue (shows on oscilliating scope)
P.S. I remember watching youtube video clip, some guy with a Rotel RB1080 (2 channels) also have shut down issue (dont know it similar to mine or not). He adjusted something and it cured the issue (shows on oscilliating scope)
I emailed the Rotel tech who worked on my amp previously (couldnt fix it still) asking for opinion regarding lower bias current. This is his replied: ".....First, your amp did not run hot enough HERE for me to be concerned. Second, if the bias was to high, it would be obvious to me as the incoming AC current would have been much higher than it was while here.
Lowering the bias current will drop the operating temperature, at the sacrifice of distortion.
Also, if it was temperature shutting this down, I don't think you'd be able to turn it back on to normal operation until it had cooled down, and from what you've said in the past this is not the case...."
It is true that when the amp went into protection mode, I recycled the power switch and it worked right away until the next shut down (could be next hour, or few hours... randomly). While he stated it was not hot enough, I think the amp ran very very hot!!. If I left my hand touching for a minute or more, I could get burn!!. So what do you think??
Lowering the bias current will drop the operating temperature, at the sacrifice of distortion.
Also, if it was temperature shutting this down, I don't think you'd be able to turn it back on to normal operation until it had cooled down, and from what you've said in the past this is not the case...."
It is true that when the amp went into protection mode, I recycled the power switch and it worked right away until the next shut down (could be next hour, or few hours... randomly). While he stated it was not hot enough, I think the amp ran very very hot!!. If I left my hand touching for a minute or more, I could get burn!!. So what do you think??
Hi nguyenphananh,
It's difficult to say much without the amplifier being here. If you can't hold your hand on it comfortably, it is too hot - period.
The tech sounds very defensive. I'm not sure what the issue is, but if a client told me the amp ran that hot I would have it straight back in for a test. Honestly, any report of high temperature from a customer should ring alarm bells.
Any competent technician can set the bias current given the service manual. You can easily cause an amplifier to fail (blow outputs) by setting the current too high or slipping with a meter probe. There will be a reason for your amplifier to run at a high temperature. It is either oscillating, or just simply the bias is too high. Reducing the bias below the factory setting isn't doom and gloom either. I have seen service manuals with typos with the level of bias current. An experienced technician knows what normal bias current levels are and would not be fooled by an error in the manual.
-Chris
It's difficult to say much without the amplifier being here. If you can't hold your hand on it comfortably, it is too hot - period.
The tech sounds very defensive. I'm not sure what the issue is, but if a client told me the amp ran that hot I would have it straight back in for a test. Honestly, any report of high temperature from a customer should ring alarm bells.
Any competent technician can set the bias current given the service manual. You can easily cause an amplifier to fail (blow outputs) by setting the current too high or slipping with a meter probe. There will be a reason for your amplifier to run at a high temperature. It is either oscillating, or just simply the bias is too high. Reducing the bias below the factory setting isn't doom and gloom either. I have seen service manuals with typos with the level of bias current. An experienced technician knows what normal bias current levels are and would not be fooled by an error in the manual.
-Chris
Hi Chris, I have questions regarding "reduce bias current...". I found a service manual for this Rotel RMB1095 amp (attached picture). Under the "service", it stated (adjust to) DC mili volmeter read 5mV. You mentioned set to 20-30mA.... So assumed if I am going to readjust the bias current, what should I reduce it to?? is this document what you were talking about??
BTW, after 2 botched purchases (RMB1095 .... and lucky I was able to return the 2nd one), I couldnt risk my hard earn money with used amps (bought lots of used audio equipment before without problem), so I gifted myself a brand new RB 1582 mkii amp (2 channels). Still, after wasting almost $1500 on this used amp, I cannot justify throwing it away, so I still find way to fix it, but not as urgent as before.
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Hi Bill,
Thank you. Yes, for 0R22 emitter resistor measuring across a single one would yield about 23 mA which is entirely reasonable.
Hi nguyenphananh,
The amplifier should not run very warm at those levels. Measure the supply voltage and multiply that by 0.023 to give you the heat dissipation from each output transistor. That should give you an idea of how much heat each transistor dissipates at idle.
First measure the bias current on each output transistor (to make sure one isn't running way too hot) and record them. I would love to know what level the bias current was set to. One easy way to do this is connect one lead of your meter to the positive output terminal for that channel. Then just measure the lead of the emitter resistor nearest the emitter. You'll have two hands to do this. I do not want you to measure on the emitter itself as it might be too easy to slip with the probe and short the transistor emitter to the collector. That would cause next to instant failure of that channel.
There are times when they invert one channel and the active output is the negative terminal while the positive one is "grounded" (ie some Carvers and others). Some other amplifiers are bridged, which means both the red and black terminals are active (ie the Adcom GFA-1 and 1a). So keep this in mind when looking at other amplifiers.
-Chris
Thank you. Yes, for 0R22 emitter resistor measuring across a single one would yield about 23 mA which is entirely reasonable.
Hi nguyenphananh,
The amplifier should not run very warm at those levels. Measure the supply voltage and multiply that by 0.023 to give you the heat dissipation from each output transistor. That should give you an idea of how much heat each transistor dissipates at idle.
First measure the bias current on each output transistor (to make sure one isn't running way too hot) and record them. I would love to know what level the bias current was set to. One easy way to do this is connect one lead of your meter to the positive output terminal for that channel. Then just measure the lead of the emitter resistor nearest the emitter. You'll have two hands to do this. I do not want you to measure on the emitter itself as it might be too easy to slip with the probe and short the transistor emitter to the collector. That would cause next to instant failure of that channel.
There are times when they invert one channel and the active output is the negative terminal while the positive one is "grounded" (ie some Carvers and others). Some other amplifiers are bridged, which means both the red and black terminals are active (ie the Adcom GFA-1 and 1a). So keep this in mind when looking at other amplifiers.
-Chris