[Boatanchors] Suggestions needed

[rayfri]

Rig:   sb-104a

Problem and symptoms:     Rig works fine, transmit and receive, on all 
bands EXCEPT 80 meters.    Digital readout reads 6604.3
and does not change with tuning the tuning knob.

Reading the manual, it mentions that if 6604.3 is shown on the readout, 
there may be no premix signal available.   If this is the case
why would there be no premix signal on 80 meters, but all the other 
bands work okay?   OR, is there something else or somewhere
else I should be looking?

Have checked bandswitch contacts and they appear to be okay, but havent 
squirted cleaner on them yet...

Suggestions, comments from you 104a experts would be much appreciated.