Hey guys, so I am currently working on restoring a "barn find" SX-990 and had a few questions. I've gone and rebuilt the amplifier stage as it originally had a blown output, but noticed a few things and wanted to see if anyone with experience could weigh in.
I replaced the driver Transistors with BD139/BD140 Transistors, and the predriver? With a KSC3503. I also had a failed bias trimmer which was replaced with a 50 ohm bourns. But I've noticed a few things and wanted to see if this was normal:
1. At maximum volume, using the tuner, the output from the amp stage seems quieter than other Capacitor coupled receivers I have, I know the potentiometer taper is likely different, but wanted to check if, at max volume, from an internal source, that having the amplifier just barely clip (puts out around 11-15v RMS into an 8 ohm load) was normal. I'm used to max volume on a receiver of this size being a clipping nightmare. I tested with an external source and a test tone and was able to get it to clipping, but it seems weird to me to have a receiver clipping at almost maximum on the volume.
2. With the replacement bias potentiometers, and my DC balance pot set to 1/2 rail on my first tranistor in the push pull pair I can only get the bias to max out at around 10mv across a single emitter resistor, is this normal behavior? There's not set bias level I've seen for these amps, so I dialed it in at around 7mv but wanted to check.
I'm more concerned about the volume taper, unless that's just normal on this model.
I replaced the driver Transistors with BD139/BD140 Transistors, and the predriver? With a KSC3503. I also had a failed bias trimmer which was replaced with a 50 ohm bourns. But I've noticed a few things and wanted to see if this was normal:
1. At maximum volume, using the tuner, the output from the amp stage seems quieter than other Capacitor coupled receivers I have, I know the potentiometer taper is likely different, but wanted to check if, at max volume, from an internal source, that having the amplifier just barely clip (puts out around 11-15v RMS into an 8 ohm load) was normal. I'm used to max volume on a receiver of this size being a clipping nightmare. I tested with an external source and a test tone and was able to get it to clipping, but it seems weird to me to have a receiver clipping at almost maximum on the volume.
2. With the replacement bias potentiometers, and my DC balance pot set to 1/2 rail on my first tranistor in the push pull pair I can only get the bias to max out at around 10mv across a single emitter resistor, is this normal behavior? There's not set bias level I've seen for these amps, so I dialed it in at around 7mv but wanted to check.
I'm more concerned about the volume taper, unless that's just normal on this model.