Hi Guys
If I may give some input here, i have some experience with Rotel amps.
The pop you hear at turn off is inherent of this circuit, nearly all rotel amps of that time do this, yes even when bought as new. The circuit topology doesnt use current sources on LTP and is the cause. Not much you can do about this, Rotel slipped up with their delayed power on circuit, maybe this could be looked at and changed to give a shorter delay on switch off.
As for the hum, this is very tricky, these amps are very sensitive to hum pickup and the construction of the frame, the enclosure plays a part here. I think this amp might have been to a repair shop before and then the outside frame not put back together as by the factory. Yes i know it sounds crazy but trust me on this. Ill try explain.
The frame was designed so that three sides are in continuity *electrically connected* while one the remaining side doesnt make contact with the three other parts. Then the two parts if you may call it that are seperatly grounded to the star ground of the amp. In some units the lone side is not grounded at all. There should be white plastic washers on the left hand frame both at the back and front to isolate this side from the other three. The theory here is that eddy currents picked up must not form a loop which it would if the three sides were to be connected. Ground loops cause hum. I have solved this problem in many of these amps because repair shops overlook this importance and replace the isolating plastic washers with steel ones.
Secondely have a look at the pic. The wires from the pcboard to the drivers of the output stage must be threaded as shown *factory built*. Rotel slipped up here too, the drivers should not have long interconnect wires to the heatsink, this causes stray capacitance. By threading the wires in this way they get away with it. It also helps to clean all solder residue from the tiny board where the drivers are connected to. Consult the pic to see what and where the power drivers are.
Hope this helps
Alex
Ps i cant seem to be able to attach the pick, go to this thread and see the second pic. http://www.diyaudio.com/forums/showthread.php?s=&threadid=123699&perpage=25&pagenumber=1
If I may give some input here, i have some experience with Rotel amps.
The pop you hear at turn off is inherent of this circuit, nearly all rotel amps of that time do this, yes even when bought as new. The circuit topology doesnt use current sources on LTP and is the cause. Not much you can do about this, Rotel slipped up with their delayed power on circuit, maybe this could be looked at and changed to give a shorter delay on switch off.
As for the hum, this is very tricky, these amps are very sensitive to hum pickup and the construction of the frame, the enclosure plays a part here. I think this amp might have been to a repair shop before and then the outside frame not put back together as by the factory. Yes i know it sounds crazy but trust me on this. Ill try explain.
The frame was designed so that three sides are in continuity *electrically connected* while one the remaining side doesnt make contact with the three other parts. Then the two parts if you may call it that are seperatly grounded to the star ground of the amp. In some units the lone side is not grounded at all. There should be white plastic washers on the left hand frame both at the back and front to isolate this side from the other three. The theory here is that eddy currents picked up must not form a loop which it would if the three sides were to be connected. Ground loops cause hum. I have solved this problem in many of these amps because repair shops overlook this importance and replace the isolating plastic washers with steel ones.
Secondely have a look at the pic. The wires from the pcboard to the drivers of the output stage must be threaded as shown *factory built*. Rotel slipped up here too, the drivers should not have long interconnect wires to the heatsink, this causes stray capacitance. By threading the wires in this way they get away with it. It also helps to clean all solder residue from the tiny board where the drivers are connected to. Consult the pic to see what and where the power drivers are.
Hope this helps
Alex
Ps i cant seem to be able to attach the pick, go to this thread and see the second pic. http://www.diyaudio.com/forums/showthread.php?s=&threadid=123699&perpage=25&pagenumber=1
homemodder said:
Sorry to say it, but the RB 980 BX uses constant current sources in both LTP
See pst #2 at:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=40432&highlight=
Sorry again, but obviously you have no clue how the 980 BX is built. The drivers are connected directly to the PCB without any connecting wires.
Thanks for your input. With the switch on pop I am worried for my tweeters! Is there no fix for this?
With the casing - on the 980BX the top and two sides are one folded piece and there are no washers at all. Also the front, bottom and back are all screwed together without washers. So if I leave the top and sides off it may stop the hum? Will try it!
The wires from the drivers seem OK - they are very short and can only go in one way. They do not snake around or touch anything.
With the casing - on the 980BX the top and two sides are one folded piece and there are no washers at all. Also the front, bottom and back are all screwed together without washers. So if I leave the top and sides off it may stop the hum? Will try it!
The wires from the drivers seem OK - they are very short and can only go in one way. They do not snake around or touch anything.
Tube dude youre right there are ccs i should have said they are not referenced to ground and i think having electronic knowledge you would know what the some of the pitfalls are but lets not discuss electronic topologies here. I happen to like these amps, one can make them sound very good indeed.
I have never opened this model *rb980bx* so im not sure on the construction specifics regarding the drivers. Thanks for informing me that at least in the higher power models they have done away with that problematic layout. I wonder why metako says those connecting wires are there ??
Metako the frame im talking about is the frame you see after the lid has come off. It screws on to the front and the back plate. It can also be seen on the rotel 1062 thread, the first pic. As i have said i havent opened this model and it may be totally different. Look at the L shaped steel coloured parts screwed on to the front plate. Have a look at that pic.
Any chance of you supplying such a pic of the insides??
Alex
I have never opened this model *rb980bx* so im not sure on the construction specifics regarding the drivers. Thanks for informing me that at least in the higher power models they have done away with that problematic layout. I wonder why metako says those connecting wires are there ??
Metako the frame im talking about is the frame you see after the lid has come off. It screws on to the front and the back plate. It can also be seen on the rotel 1062 thread, the first pic. As i have said i havent opened this model and it may be totally different. Look at the L shaped steel coloured parts screwed on to the front plate. Have a look at that pic.
Any chance of you supplying such a pic of the insides??
Alex
Im having problems putting up pics too.
About 9 years ago i had same problem with a rb971mk2 and spent hours searching for the source of the hum. Then by luck i met 2 guys from rotel that worked in their china based manufacturing plant at a pub in London and managed to persuade one to come to my home for a peak at it. I was disgusted and dumbfounded when he pointed the source of my hum and solved it within minutes. Disgusted because it was me that removed those insulating washers while doing a modification and never thought about construction details. Eye opener regarding construction and layout. I might still have his email, gonna have a look, might save some headaches here.
Alex
About 9 years ago i had same problem with a rb971mk2 and spent hours searching for the source of the hum. Then by luck i met 2 guys from rotel that worked in their china based manufacturing plant at a pub in London and managed to persuade one to come to my home for a peak at it. I was disgusted and dumbfounded when he pointed the source of my hum and solved it within minutes. Disgusted because it was me that removed those insulating washers while doing a modification and never thought about construction details. Eye opener regarding construction and layout. I might still have his email, gonna have a look, might save some headaches here.
Alex
Well I can hardly believe it but the hum is 99.9% gone!!! I turned the amp upside down and looked carefully at all the casework screws on the bottom. One screw was loose but I thought nothing of it and removed all the screws and removed the base cover.
Everything seemed OK and I noticed that the loose screw went into a metal "riser" or spacer coming out of the PCB. There are 3 of these spacers but only 2 had screws going into them from the case, and one of those was loose. The 3rd spacer comes out in between a couple of the ventilation slots in the bottom cover and looks like it has never had a screw in there (no screwhead marks) and doesnt even properly line up.
Could these little screws to the risers be critical? I replaced the cover and screwed all the screws down tight, connected the speakers up again and bingo! the hum was much much fainter! I was amazed at the function of those little screws! Then I plugged in the interconnect of the preamp (still switched off) and the hum became even quieter, so much so that I have to strain with my ear very very close to the speaker and I can JUST still hear it. I can live with that. Maybe if I put a screw in the 3rd spacer the hum will disappear completely!
This has been a very strange experience. I have never known a small screw through the casework to be so critical. These Rotel amps are weird.
Thanks very much for your ideas Alex and Jorge - you were both in the right ballpark - it was something loose Jorge and was to do with the casework Alex.
The switch off pop is still a nuisance - it is so loud sometimes that I'm sure it will blow my tweeters soon. Will switch off at the wall switch till I find a solution.
Everything seemed OK and I noticed that the loose screw went into a metal "riser" or spacer coming out of the PCB. There are 3 of these spacers but only 2 had screws going into them from the case, and one of those was loose. The 3rd spacer comes out in between a couple of the ventilation slots in the bottom cover and looks like it has never had a screw in there (no screwhead marks) and doesnt even properly line up.
Could these little screws to the risers be critical? I replaced the cover and screwed all the screws down tight, connected the speakers up again and bingo! the hum was much much fainter! I was amazed at the function of those little screws! Then I plugged in the interconnect of the preamp (still switched off) and the hum became even quieter, so much so that I have to strain with my ear very very close to the speaker and I can JUST still hear it. I can live with that. Maybe if I put a screw in the 3rd spacer the hum will disappear completely!
This has been a very strange experience. I have never known a small screw through the casework to be so critical. These Rotel amps are weird.
Thanks very much for your ideas Alex and Jorge - you were both in the right ballpark - it was something loose Jorge and was to do with the casework Alex.
The switch off pop is still a nuisance - it is so loud sometimes that I'm sure it will blow my tweeters soon. Will switch off at the wall switch till I find a solution.
Hi metako
Glad to hear you have been able to sort it out. Just as i thought it was the framework, thats why i told you in the beginning not to think im crazy. I was unable to contact the guy from rotel, pity he was a good contact to have. That screw most probably pulls the pc board in to contact with the frame to ground it. Missing this ground connection caused the hum.
As for the pop the only thing i can think of is using soft recovery diodes as the rectifiers, this not only helps to reduce the pop but will also give you clearer midrange and treble. You will need electronic nowhow to this kind of mod
Alex
Glad to hear you have been able to sort it out. Just as i thought it was the framework, thats why i told you in the beginning not to think im crazy. I was unable to contact the guy from rotel, pity he was a good contact to have. That screw most probably pulls the pc board in to contact with the frame to ground it. Missing this ground connection caused the hum.
As for the pop the only thing i can think of is using soft recovery diodes as the rectifiers, this not only helps to reduce the pop but will also give you clearer midrange and treble. You will need electronic nowhow to this kind of mod
Alex
Are you saying Jorge that new diodes wont make any difference? If so, the only other way I can think of to protect the tweeters will be to remove the tweeter fuses whenever I switch the amp off and try to leave the amp on most of the time. I have blown these tweeters before and really dont wont another repair bill. Switching off at the wall sometimes gives a big pop too. Do you have the 980 BX Jorge? If so how do you work around this?
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