The Original 706 had issues with LPF relays, mostly within the first two years of use, after that rarely. FM VCO trimmer cap failure. The cap is ceramic, just whimpy, solution is to replace it with a 20pf 3.5mm leaded ceramic. Few other issues. Poor out of band performance above 148Mhz, a couple of different mods. The best mod, is very labor intensive to do. If you need above 148, get the MKII.
706MKII Great radio, no real issues, rarely fails. FM VCO trimmer rarely. If the Vox seems to hang up, change C467 on the main unit. Apparently some caps are weak, and the T/R 8v spike isn't supressed, and it holds the VOX on.
706MKIIGWatch out. If the serial number is below 15xxxxx, Finals are gone, no longer available.
The DSP unit can get oxidized connections, resulting in garbled SSB operation/Mode mishaps/and dead modes (while other modes work fine) simply unplug the flex cable, and plug it back in, carefully.
Dead, no Rx/no Tx, check the 5v regulators on the PLL unit, there are 3 of them. Probably bad solder to one of them.
Tx power is radically different on different bands, probably the detwector diodes on the filter unit.
These two, probably others, and any manuf. who used the J7044 MosFet for a final, have a terminal issue. The finals (SR(F or K)J7044(MP) are no longer available. I have tried to contact a few sources, and a couple of parts hunters.. no Stock, No availability. The PA in the MKIIG was changed in about 2004 to a newer (totally different RD70HV)(New/Old Finals pic) device. You can swap out PA units for about $500! This includes the finals (no drivers)(706MKIIG only) A bit spendy, but at least there is a fix.
The 775(DSP) has had very few cases of Final failure. The 775(DSP) PA unit is also pre-designed to use the MRF140's, the MKIIG cannot. The 706 MKIIG has had more cases of final failure than the 775, more MK2G's sold out there too, than the 775.
Beware buying used MKIIG's. The Serial number change is OLD > 0xxxxx, Newer 15(or 16)xxxxx.
I had the unique oppurtunity to evaluate the old 706 MKIIG PA unit and the new one. The primary difference between the old final and new final, is the package. Performance criteria is actually almost exactly the same. The HF transformers and componets are the same between the two boards. The driver (old MRF1508, New MRF1518) is different on the new board, but is basically a newer design, with the same spec's.
On the VHF/UHF side I was able to grind away enough board and fit the new final in place. I needed to move the filter caps (input and output) closer to the body of the transistor, but otherwise it worked perfectly.
This will not be as easy for the HF side of things, as one finals has quite a few bottom board side lands that would be cut or damaged. If I get the chance to do this, I will report back on how it went.
Here are some pic's of the VHF/UHF operation
New PCB (note the transistor hole difference
bottom side pcb trace that gets cut
cut trace jumper, and sealed with hot glue