(this is a compilation of information from various sources including N1LQ's post to the Ten-Tec reflector plus my personal experience)
Evolutionary changes in no particular order:
- The 564 has a different font silk-screened on the input keys from the 563 keypad.
- The later 564's replaced the crystal oven with a 2ppm TCXO on the logic board.
- The later 564's use slightly different BFO crystal frequencies (9.0003 USB and 9.0033LSB/CW) from earlier units, apparently to improve the alleged CW key click problem
- The later 564's have certain changes to top and bottom foil grounding on certain boards to improve 10M birdies which are a major problem with this rig.
- The BPF filter on later 564's has a broadcast band mod to improve intermod in the presence of strong local AM BCB or SW signals, which may make a huge improvement.
- Later 564's use significant amounts of SMT construction (N1EU note: the logic board, PBT board, Control board and IF/AF board became SMT - the 9Mhz Xtal Filter board was SMT from the start)
- The last 564's lost the orange dot in the Ten-Tec insignia
- Later 564's have improved keying loop circuitry to drive non Ten-Tec QSK amplifiers.
Omni 6 upgrades:
There were three options released all at about the same time that the Omni
VI+ was released.
Option 1 - adds the VI+ DSP chips, labels for front panel keys that have
Option 2 - adds the VI+ DSP chips, new keycaps for front panel keys that
have changed function instead of the labels (looks nicer)
Option 3 - adds the VI+ DSP chips, adds the VI+ 9Mhz mixer/IF board with the
extra filter slot, includes all wiring changes to change functionality to be identical to a VI+
Option 1 was user installed, while Options 2 and 3 were factory installed.
I've owned all vintages from 563 option 3 to early production 564 to the latest production 564. Differences in performance are subtle.