Hi all,
After a bit more diagnostic work, I determined that IC5, a NE5532D, on the AJACK board was the likely culprit, so I replaced that IC this evening, which involved some surface-mount soldering work. That has mostly resolved my problem; at least the audio is clear now and I'm getting 3.6 dBu output instead of -1 dBu on the line outs during the L/R channel test. Note that the regular NE5532D is hard to get, but the NE5532AD is very common. The only difference appears to be that the latter has a guaranteed maximum noise level, whereas the former only has a typical level.
One note about the AJACK board... The schematic shows series chokes on the line outs and AUX in, but none of those three connections actually have them. They're just plain ol' jumpers on the actual AJACK board. I'm guessing they were omitted because Yamaha determined they weren't needed for EMI compliance, but this may have limited the ESD resilience of the AJACK board.
I modified my keyboard's AJACK board to add ESD protection (TVS silicon avalanche diodes) to the mic in, aux in, and line outs, and I added MOV protection on the DC 16V in. The DC in already has reverse polarity and overcurrent protection, which effectively protects the MOV from overcurrent, too. (note - I make no guarantee of the suitability or reliability of the modifications I made - my mods are rather hackish)
I may still have an issue with the mute circuit as well causing some remaining channel crosstalk, but that's a project for another day.
The mute circuit consists of three bipolar transistors.
(attached images: AJACK board with missing chokes L15 through L20; and my modified AJACK board with MOV and TVS protection)
- Greg
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