Hello, I have an old 840bx4. It suffers from two issues. The amp clips (makes loud popping sound) at half volume mostly when playing low bass. I suppose that can attributed to the big old power capacitors (in the schematic c910 c909 10000uf 50v and 923 4700uf 50v ), which have been replaced at some point by some technician with lower valued 8200uf rubycon caps?
Moreover the amp after playing looses one of the two channels, more specifically it sounds really quiet heavily distorted and you can only make out some bass notes. But By setting the tone control to MONO both outputs sound fine, so this problems only happens on stereo and only on some inputs ( cd sounds fine while aux and tuner not, swapping the sources around make no difference so its an amp problem), one way to remedy this is by applying contact cleaner in the selector switches, then the problem goes away for a bit but then returns ( thought I might be misinterpreting the symptoms, so i wouldn't consider the above 100% true). Any advise on how i should approach this ?
I have attached the service manual
Moreover the amp after playing looses one of the two channels, more specifically it sounds really quiet heavily distorted and you can only make out some bass notes. But By setting the tone control to MONO both outputs sound fine, so this problems only happens on stereo and only on some inputs ( cd sounds fine while aux and tuner not, swapping the sources around make no difference so its an amp problem), one way to remedy this is by applying contact cleaner in the selector switches, then the problem goes away for a bit but then returns ( thought I might be misinterpreting the symptoms, so i wouldn't consider the above 100% true). Any advise on how i should approach this ?
I have attached the service manual
Try re-soldering the input selector switch connections to the PCB. I had similar symptoms on an old pre-amp and initially thought the switches were worn out. Turned out to be hairline cracks in the solder. Hard to see but fairly easy to fix.
I have a similar problem with this model. I works fine with the "Tone" switch in the off position but in either "on" or "mono" the left channel goes quiet? I have used switch cleaner but it did not solve the problem.
Left channel of IC702 is along the problem path, so is suspect. Confirm about 0VDC on opamp output. If not 0V, problem is opamp or associated parts/traces.
If output bias looks promising, and you don’t have any other equipment, with Tone switch ON, try temporary jumpers between working right channel and problem left channel, like points joined one at a time. You should get equal volume in each channel, perhaps with reduced level. Candidate points are C603, R751, R713, etc. With luck, you may be able to converge on an open part/trace.
If output bias looks promising, and you don’t have any other equipment, with Tone switch ON, try temporary jumpers between working right channel and problem left channel, like points joined one at a time. You should get equal volume in each channel, perhaps with reduced level. Candidate points are C603, R751, R713, etc. With luck, you may be able to converge on an open part/trace.
It was C601, a Black Gate 10uf. On closer inspection it looks misshapen and measures around 1nf ! (although in the service manual it states C601 is 4.7uf?) Replaced it with a silmic 10uf and switch functions again. Thanks very much for the help. 👍
Hello,
Here is yet another little insight into the repair of a Rotel RA840BX4
I confirm the problems with the overheating and the PCB…and often not with parts !
My repair showed me that , after I had changed the burned 1k resistors, I had to search trough the whole preamp section to find some cold solder joint or better said cracked joint/traces on the 5532 common to both channel IC702….it looked at first like switch problems , it wasn’t it was lost solder / intermittent contacts directly on the pads…(I’ll soon release the video on the repair part)
Also when I changed resistors as per your schematic , some pads had a tendency to get off from the pcb (with little heat, I was careful)……indicating high sensitivity / aging of some of the finer traces of the pcb….
When nothing makes senses, that is where too look for, when repairing this unit 😉
Here is the first video about the repair itself for reference :
And there the second one that covers making mods that alleviate most overheating problems of such units….
Here is yet another little insight into the repair of a Rotel RA840BX4
I confirm the problems with the overheating and the PCB…and often not with parts !
My repair showed me that , after I had changed the burned 1k resistors, I had to search trough the whole preamp section to find some cold solder joint or better said cracked joint/traces on the 5532 common to both channel IC702….it looked at first like switch problems , it wasn’t it was lost solder / intermittent contacts directly on the pads…(I’ll soon release the video on the repair part)
Also when I changed resistors as per your schematic , some pads had a tendency to get off from the pcb (with little heat, I was careful)……indicating high sensitivity / aging of some of the finer traces of the pcb….
When nothing makes senses, that is where too look for, when repairing this unit 😉
Here is the first video about the repair itself for reference :
And there the second one that covers making mods that alleviate most overheating problems of such units….