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i'm posting this so that if anybody comes across this problem in the future you can find it here on a search and fix it quickly. my TS-680 was set to 4316khz to monitor the USCG weather fax transmissions of NMG in New Orleans. one morning it suddenly went to 4310, so i dialed it back to 4316, and a few minutes later, it changed to 4320. so i dialed it back to 4316, set the F.LOCK function and left for work. when i came home for work, it was still on 4316, and i had faxes from the day saved on the PC, and it would be a couple more hours before NMG began sending new ones, so i unset the F.LOCK function, and immediately the radio began bobbling back and forth from 4310 to 4320 randomly. this radio was in storage for a few years, and i finally got it out and began using it again a couple of weeks ago. it wasn't doing this before being in storage. one control that is used for setting the VFO is the M CH/ VFO CH knob. i had forgot that this control stepped the VFO 10khz when the VFO was being used. i had been using the 1MHZ UP/DOWN and the main tuning knob. as it turns out, the M CH/VFO CH knob had a dirty contact, and that's what was causing the problem. after rotating the knob a few times the problem went away. if it happens again, i'll hit it with some Deoxit, but it seems to have been cured just by working the control.