Got a Rotel 970BX which has been serviceable. Been using it in bridged mono mode. Recently noticed a performance degradation and pretty sure based on some bench measurements (no voltage drop across the emitter resistors) the right channel NPN output pair is dead. Switching to stereo operation and no right channel seems to confirm my initial diagnosis.
In looking closer I noticed that the left channel has the original design 2SD1047/2SB817 output devices but the right channel has NJW3281G/NJW1302G output transistors, which according to my old notes, I installed several years ago when I first acquired the amp.
The theory part of my question goes like this: when running this (AB push/pull??) topology in bridged mono how important is it for the devices on one channel to be from the same product family (2SD1047/2SB817) versus a different family (NJW3281G/NJW1302G)?
The practical part of my question goes like this: If in fact the 2 right channel NPNs are blown is it better (does it matter) if I replace them with the 2SD devices (to match all the devices on the left channel) or with the NJW devices to match the pull side of the right channel).
In looking closer I noticed that the left channel has the original design 2SD1047/2SB817 output devices but the right channel has NJW3281G/NJW1302G output transistors, which according to my old notes, I installed several years ago when I first acquired the amp.
The theory part of my question goes like this: when running this (AB push/pull??) topology in bridged mono how important is it for the devices on one channel to be from the same product family (2SD1047/2SB817) versus a different family (NJW3281G/NJW1302G)?
The practical part of my question goes like this: If in fact the 2 right channel NPNs are blown is it better (does it matter) if I replace them with the 2SD devices (to match all the devices on the left channel) or with the NJW devices to match the pull side of the right channel).
The NJWs are probably better transistors. If that channel keeps blowing though there is probably something else wrong in that circuit. Check proper bias circuit action and look at the edges of a square test signal before leaving it alone.