Audio problems on VIC-20

Hi, All,

I set up a retro-gaming event at our hackerspace over the US holiday
and I brought a couple of VIC-20s (since I hadn't fired them up in a
while and didn't know if one or both had problems).  Turns out, it's
good that I did.  One was fine, the other was not.

The one with the problems - the biggest issue I had, as an example,
was while playing Gorf from cartridge, the sound cut in and out while
the game played.  I can easily swap out the 6560 VIC chip in case
that's where the problem is, but are there any other diagnostic things
I can do?

Another symptom is the colors weren't quite as "bold" as they were on
the working VIC-20.  There was still color, but on a no-cartridge
bootup to BASIC, the border wasn't quite the same color, and when
playing Gorf, the colors weren't as saturated.  Could be related,
could be two different problems.

It's been a while since I've debugged a VIC-20, so pointers to
checklists and such are most welcome.

Thanks!

-ethan

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