Jerry's TV
Active Member
Hi all - this is a repost to start a new thread on the SA9900 ... "sa-9900-2sc1451-ksc3503-causes-dc-on-volume-potentiometer".
http://audiokarma.org/forums/index....on-volume-potentiometer.805352/#post-11288903
I used LesE's component list (thanks a bunch!!) and have worked through the power supply cards AWR-070B; AWR-077, the input equalizer amp AWK-038, switch card AWS-080, control amp AWG-031 and protection circuit AWM-0701. No changes to Tone Switch AWR-079. Rebuilding the Control Amp eliminated a “pop” with the S9 Tone on/off and clicking through the volume control pot. AWR-070 power rails are +-58V; +- 49.1; Pin 15 -12.85; Pin 17 + 12.45 (a little high).
I have not changed the main power caps or worked on the power amp yet. It seems to be performing well. I’ll do this next once the pre-amp is fully sorted.
Audio sounds good and things are working as expected… except for one annoyance. I have a subtle 60 hz background hum on both channels. It is not effected by the volume control, input selection or any other front panel controls. If I set preamp bypass switch to “bypass” for power amp only, the hum goes away. (so it is coming from the pre-amp). When I connect a multi-meter to measure the + 58V or -58 supply on power supply AWR-070B the hum will stop. Coupling issues?
Control Amp AWG-031 - If I ground the audio pins 4 and 19 input to S9 Tone Switch, I can turn the hum on/ off with tone switch; Tone Off – no hum; Tone On = hum. If I ground pins 3 and 20 after S9 the hum stops and is not switched by S9. Grounding pins 2 and 21 output of Control Amp stops the hum as well.
If the problem was Tone Switch Board AWS-079, which S9 activates, I should be able to turn the hum on/off with S9 at any time. Re-grounding pin 12 of AWS-079 does not help.
So, this leads me to think S9 Tone on/off switch is the issue? It is mounted on Switch Circuit Assembly AWS-080, schematic shows no other circuity for S9, just the switch.
Thoughts and suggestions? I am away for the week so thought I’d put this out to the smart minds on AK for discussion, then come back to it next weekend.
Brian T
http://audiokarma.org/forums/index....on-volume-potentiometer.805352/#post-11288903
I used LesE's component list (thanks a bunch!!) and have worked through the power supply cards AWR-070B; AWR-077, the input equalizer amp AWK-038, switch card AWS-080, control amp AWG-031 and protection circuit AWM-0701. No changes to Tone Switch AWR-079. Rebuilding the Control Amp eliminated a “pop” with the S9 Tone on/off and clicking through the volume control pot. AWR-070 power rails are +-58V; +- 49.1; Pin 15 -12.85; Pin 17 + 12.45 (a little high).
I have not changed the main power caps or worked on the power amp yet. It seems to be performing well. I’ll do this next once the pre-amp is fully sorted.
Audio sounds good and things are working as expected… except for one annoyance. I have a subtle 60 hz background hum on both channels. It is not effected by the volume control, input selection or any other front panel controls. If I set preamp bypass switch to “bypass” for power amp only, the hum goes away. (so it is coming from the pre-amp). When I connect a multi-meter to measure the + 58V or -58 supply on power supply AWR-070B the hum will stop. Coupling issues?
Control Amp AWG-031 - If I ground the audio pins 4 and 19 input to S9 Tone Switch, I can turn the hum on/ off with tone switch; Tone Off – no hum; Tone On = hum. If I ground pins 3 and 20 after S9 the hum stops and is not switched by S9. Grounding pins 2 and 21 output of Control Amp stops the hum as well.
If the problem was Tone Switch Board AWS-079, which S9 activates, I should be able to turn the hum on/off with S9 at any time. Re-grounding pin 12 of AWS-079 does not help.
So, this leads me to think S9 Tone on/off switch is the issue? It is mounted on Switch Circuit Assembly AWS-080, schematic shows no other circuity for S9, just the switch.
Thoughts and suggestions? I am away for the week so thought I’d put this out to the smart minds on AK for discussion, then come back to it next weekend.
Brian T