Incidentally, for those who do not want to take the risk of opening
their SA-21 up and making a permanent solder link modification, there is
an alternative temporary fix, albeit a little less reliable.
The two contacts of the link in question connect directly to pins 16 and
17 of the blue rear connector on the SA-21. Looking into the rear of
the SA-21 with the adapter the correct way up, pin 13 is on the middle
row on the left of the connector, so just count 4 and 5 pins across and
you get to the two pins in question. These can be shorted together
using some very fine wire - thin enough so that the operation of the
connector contact is not interfered with. Then hold the link wire in
place with some adhesive tape - Scotch Magic tape works quite well for
this.
Before pushing the SA-21 back into the scanner, make sure that you
pierce the adhesive tape over each contact in the connector block with a
pin. This will prevent the corresponding pins in the scanner being
subjected to additional pressure as the adapter is pushed home in the
scanner.