I had an issue with my 339, where it would work in some switch positions but not others, and even depending on which direction the switches were moved (probably why I scored it so cheap!). I traced it to an alignment problem between switch wafers on the various boards, coupled with a small amount of backlash in the wafers. It was quite awkward to see, but once I knew what I was looking for I could clearly see that in some cases the quite thin moving switch contact was not quite connecting as it should.
Even more awkward to fix because of the nature of the construction - required a bit of mechanical twisting and turning, but the unit seems quite reliable now..
Don¡¯t know if this is your issue, but possibly worth a look.
Roman
