The PCB material in the early 70's was more brittle than the material used in the early 80's, which is pretty much still used today. PCB material kind of matured by the end of the 70's. The biggest issue with the boards in the A77 is that you may have to use some excessive force to pull them out after being in place for decades. The B77 doesn't use PCB edge connectors like the A77 does, so the PCB won't see as much stress if you pull them.
The A700 is another story. A rather sad one, unfortunately. When the A700 was developed in the very early 70's (together with the Studer A67) the engineers at Studer had an attack of modernism and designed something that was 10 years ahead of it's time. The result was an unprecedented transport using a large (and disgustingly expensive in that time) custom IC that handles all the transport functions. A bit like a microprocessor, but with a fixed programming. That transport can hold it's own against everything that has been developed since. After that the attack of modernism on the Studer side stopped and the A67/B67 were populated with rather conservative but well performing electronics similar in philosophy to the Studer A80. On the Revox side, however, they went over the top and designed all the A700 electronics with opamps. Certainly not a good plan in 1972, when opamps were not very good and sounded even worse.
So the A700 is a bit like the RS1500. Very nice transport, but the stock electronics sound so bad that they are beyond salvation. The only option you have is to disconnect the stock electronics and use external circuitry.
