So, hi everyone, long time no see.

I moved from Canada to the USA, and after I unpacked my C64, i found that the 6th channel in MSSIAH didn't work. That is to say, SID2 in a sid2sid had 1 muted channel.  I swapped sids. Same. Swapped again. Same. Removed SID2SId, checked all the sids, all work perfectly directly socketed.  All work perfectly in SID1.  SID2, only the first 2 channels work.

What might be causing only 1 single voice in 1 single sid not to function?  Could it be the mssiah cartridge? Could the problem be with the sid2sid board? I can't fathom how it's possible...

Please help?

Double check that the CS connection (the wire soldered between the chips) goes to the right place on the C64.

Also check that pin 13 (number 2 counting up from the bottom left corner) has a connection to pin 13 on the other chip/the long legs. You might have a broken trace on the PCB, bad solder joint, missing solder joint etc.

Thanks Nitro. The CS is on the right pin (7th from the left) and the trace is still there between the legs. It's a real mystery.  Not sure why 2 of the 3 channels would work just fine on SID2.  It's driving me crazy. I checked every leg of the two sockets actually. 

I'll try building a new SID2SID and see if that works.  Otherwise, who knows... This is just one of those "this tech is 35 years old so deal with it" scenarios.

Have you checked continuity between the two pin 13's with a meter or only through visual inspection? Something like micro-crack in the trace, or a dry solder joint that has separated from the surface can be hard to detect visually.

If you look at the SID datasheet page 3, you'll see that voice 1 and 2 are in the lower half of the memory map, where A4=0. So it seems like for whatever reason, the second chip is not reacting to writes to the top half of its memory map. This could be because the A4 line is broken on the SID2SID. (If it was broken elsewhere, the same problem would affect both chips equally, even without the SID2SID.) It could also be because the IO1 pin doesn't decode correctly when A4=1 for whatever reason, which could be an issue with the machine itself.

I only was able to do a visual inspection because my multimeter is in Toronto and I am in Los Angeles, however, I have a multimeter arriving in a couple of days so I'll doublecheck it again. Thanks again for the help.  I envy all you "smart people" who know how things work.

i just checked every trace, they are all good, all the pins are connected in the right way.  I just don't get it.   Hopefully my new SID2SID will work right...