Radio Shack TRS-80 PT-210 Service Manual page 60

Remarks:
Audible sound of 1,070 Hz is determined by time constants of VR2, R26, and C17. When adjust-
ment is required, set TxD terminal to "H: level and turn VR2.
Identically, audible sound of 1,270 Hz is determined by time constants of YR2, R26, VR3, R27, and
C17. For adjustment, set TxD terminal to "L" level and turn VR3.
Note:
In order to make an adjustment in audible sound of 1,070 Hz and 1,270 Hz, make a setting of
1,070 Hz by turning VR2.
If an adjustment of 1,270 Hz by VR3 is made first and an adjustment of 1,070 Hz by VR2 is made
next, the setting of 1,270 Hz will be deviated.