I've tested EVERYTHING! it does work but I want to get rid of this dc. the other channel is only 5mv. I've changed all the electrolytics, new bridge rectifier. DC is @ 70mv on switch on then drops slowly. switch it off and the voltage climbs then goes to negative before settling to about -60mv!!
Any ideas?
Any ideas?
This is normal. The DC offset is caused because Q601/Q605 have slight Vbe mismatch, and this causes the offset. The only way to cure this in this design is to get a batch of 2SA1016 (say 20-30), measure their Vbe with a meter, and pick two which are closest in value.
There's also other components that will affect this though, such as the tolerance of the resistors. To be honest 60mV of DC offset is nothing to worry about.
There's also other components that will affect this though, such as the tolerance of the resistors. To be honest 60mV of DC offset is nothing to worry about.
The switch off behaviour is also normal - most amplifier circuits do this, but in other amps there is a relay to disconnect the speakers during the switch off period (and the switch on "settling" period).
Hello,
The output offset is mainly created by LTP transistors Hfe mismatch.
After that, the Vbe, and also the resistor in the base DC path.
1µA in R603 is 47mV.
1µA in R635 is 8mV.
Anyway, 60mV is not too bad.
The output offset is mainly created by LTP transistors Hfe mismatch.
After that, the Vbe, and also the resistor in the base DC path.
1µA in R603 is 47mV.
1µA in R635 is 8mV.
Anyway, 60mV is not too bad.
Hi.
On my Rotel Rx 950ax, I adjust the dc offset by adjusting the resistor value to ground on the input base transistor, right now mine is around -5mv and 5mv each output.
60mV is noisy in my view.
On my Rotel Rx 950ax, I adjust the dc offset by adjusting the resistor value to ground on the input base transistor, right now mine is around -5mv and 5mv each output.
60mV is noisy in my view.
Hi i have the same amplifier ROTEL RB 930AX, and the right channel speaker has 13.7 between (-) and (+) speaker, the speaker move to strong because this voltage. The left channel is ok not DC. Please anybody can help me to repair it. I have the electronic scheme, not sure where I should test and measure , not sure if it could be a transistor or other component.....thanks
I guess you mean that there is 13.7V DC between the terminals. If so, there is likely a serious problem with semis in the output stage of the right channel power amplifier. Normally, this is job for a pro. service technician to diagnose and repair but an experienced DIY repairer can also track down faulty component(s) or perhaps bad solder joints or foil breaks and replace or repair as necessary.
After checking that the power supply voltages are correct, the best point to start from is to measure and list the voltages (with respect to circuit ground) at the C,B,E terminals of the output transistors (there are 2 pairs of 2SD1047/2SB817 types in each channel) The driver transistors 2SD688/2SB631 voltages should be similarly checked and listed. Use the good channel for reference and compare the voltages with those in the bad channel. You can use the power supply ground or any speaker ground terminal for the other meter probe. Tell us what you find but before testing, disconnect the speakers and any input leads. Shorting the input sockets with around a 1k resistor is often done to ensure there is no disturbance to measurements but probably isn't necessary.
If it seems you need replacement semis, beware that most you find in the usual on-line stores are cheap generic parts that only meet general specifications for the type. They aren't fakes, since they are not branded with original marks but they won't perform or sound like the original Sanyo parts which unfortunately, are long obsolete. Sanyo's power transistors were actually quite special and were used in most Rotel models but copies now made by several manufacturers, are mostly low grade, general purpose transistors.
Though less likely, even a bad solder joint or another type of component failure can result in a large DC offset like this type. Take care when testing with probes that you don't slip and cause more damage. It is safer to use clips, grab-hook or shielded tip probes for testing live circuits.
This site has good quality service manuals which others can download for most Rotel models, including this one: http://bwgroupsupport.com/manuals/rotel-service
After checking that the power supply voltages are correct, the best point to start from is to measure and list the voltages (with respect to circuit ground) at the C,B,E terminals of the output transistors (there are 2 pairs of 2SD1047/2SB817 types in each channel) The driver transistors 2SD688/2SB631 voltages should be similarly checked and listed. Use the good channel for reference and compare the voltages with those in the bad channel. You can use the power supply ground or any speaker ground terminal for the other meter probe. Tell us what you find but before testing, disconnect the speakers and any input leads. Shorting the input sockets with around a 1k resistor is often done to ensure there is no disturbance to measurements but probably isn't necessary.
If it seems you need replacement semis, beware that most you find in the usual on-line stores are cheap generic parts that only meet general specifications for the type. They aren't fakes, since they are not branded with original marks but they won't perform or sound like the original Sanyo parts which unfortunately, are long obsolete. Sanyo's power transistors were actually quite special and were used in most Rotel models but copies now made by several manufacturers, are mostly low grade, general purpose transistors.
Though less likely, even a bad solder joint or another type of component failure can result in a large DC offset like this type. Take care when testing with probes that you don't slip and cause more damage. It is safer to use clips, grab-hook or shielded tip probes for testing live circuits.
This site has good quality service manuals which others can download for most Rotel models, including this one: http://bwgroupsupport.com/manuals/rotel-service
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