Fleime
New Member
Hello,
I have acquired a Rotel RB990-BX amp which appears to have been sparsely used, compared to another which I have lying around. Being a beginner in this hobby, I first researched the amp for technical experiences. I have checked all the components individually and not in circuit (never mind the usefulness, it was fun and adding to (soldering) experience) within the capabilities of my dmm and a cheap component tester (LCR-T4). I have removed the glue on the pcb's and checked the solder joints and connections.
To my knowledge, most of these things check out, although the attributes of some components, mainly some transistors and/or capacitors, may be off or do not match up well compared to other components of the same type. I have added some files with my results which can be viewed in excel. Other file formats can be provided if necessary. Components are referred to as per the RB990-BX service manual (or the RB991-BX service manuals, as the architecture of this amp seems to be of a modified design leading up to the 991). I think these manuals are readily available on the internet.
As I could not find any defects, I reassembled the amp with its original parts. Testing it when running idle following the procedure in the "amplifier distortion, dc-offset, and you!" thread, the DC bias on both channels is set on 7 mV as per the service manual. The DC offset is 49 mV for the left channel and 45 mV for the right channel. Measuring the DC offset while shorting the inputs, adding a 100 Ohm resistor between both of the channel's speaker outputs, or both these options did not yield any significant differences (and assuming I did this correctly). The temperature of the heat sinks of both channels is about 38 degrees C or 100 degrees F. And of course more importantly, it works when playing music.
I would appreciate if someone were to go over the attributes of the components. I know the components tester's values are questionable and should be considered indicative. Also, if relevant, it appears the tester attributes a higher ESR as the value of the electrolytic capacitor decreases. Oh, and figures behind commas are decimals, European style. Sorry about that.
I would also like to lower the DC offset's somewhat, although these fall within (the end of) an acceptable range. I am aware that it may be advisable to change out the electrolytic capacitors and/or the differential transistor pairs (and perhaps more) regardless, but I would like to try a step-by-step approach to observe the results, at least concerning the DC offset (and perhaps audio quality?). If someone can advise me on a first step based on the provided information that would be great, however if a consensus is that it's better/easier to change out a bunch of components at once, I will do just that.
If more information is required, I will be happy to provide it. As stated, my experience is still limited, but I will do my best to keep with technicalities.
Cheers
I have acquired a Rotel RB990-BX amp which appears to have been sparsely used, compared to another which I have lying around. Being a beginner in this hobby, I first researched the amp for technical experiences. I have checked all the components individually and not in circuit (never mind the usefulness, it was fun and adding to (soldering) experience) within the capabilities of my dmm and a cheap component tester (LCR-T4). I have removed the glue on the pcb's and checked the solder joints and connections.
To my knowledge, most of these things check out, although the attributes of some components, mainly some transistors and/or capacitors, may be off or do not match up well compared to other components of the same type. I have added some files with my results which can be viewed in excel. Other file formats can be provided if necessary. Components are referred to as per the RB990-BX service manual (or the RB991-BX service manuals, as the architecture of this amp seems to be of a modified design leading up to the 991). I think these manuals are readily available on the internet.
As I could not find any defects, I reassembled the amp with its original parts. Testing it when running idle following the procedure in the "amplifier distortion, dc-offset, and you!" thread, the DC bias on both channels is set on 7 mV as per the service manual. The DC offset is 49 mV for the left channel and 45 mV for the right channel. Measuring the DC offset while shorting the inputs, adding a 100 Ohm resistor between both of the channel's speaker outputs, or both these options did not yield any significant differences (and assuming I did this correctly). The temperature of the heat sinks of both channels is about 38 degrees C or 100 degrees F. And of course more importantly, it works when playing music.
I would appreciate if someone were to go over the attributes of the components. I know the components tester's values are questionable and should be considered indicative. Also, if relevant, it appears the tester attributes a higher ESR as the value of the electrolytic capacitor decreases. Oh, and figures behind commas are decimals, European style. Sorry about that.
I would also like to lower the DC offset's somewhat, although these fall within (the end of) an acceptable range. I am aware that it may be advisable to change out the electrolytic capacitors and/or the differential transistor pairs (and perhaps more) regardless, but I would like to try a step-by-step approach to observe the results, at least concerning the DC offset (and perhaps audio quality?). If someone can advise me on a first step based on the provided information that would be great, however if a consensus is that it's better/easier to change out a bunch of components at once, I will do just that.
If more information is required, I will be happy to provide it. As stated, my experience is still limited, but I will do my best to keep with technicalities.
Cheers