3. During the tuning procedure the operator is actually
compensating for mineralization or conductivity of the
earth at that particular point. Different areas have different
amounts of mineralization and must be “tuned out” at
each locating site. If this were not so, then the instru-
ments could be tuned and locked at the factory. On some
rare cases the conductive mineral content of the earth is
so high that the sensitivity control must almost be turned
off to find the dead spot for proper tuning. This, of course,
will reduce sensitivity.
4. Check the battery supply frequently. The test can be done
quickly and easily while the instrument is being used.
Even though tuning is usually possible, low battery volt-
ages will result in low sensitivity.
D) Maintenance:
1. Both the 720 and 820-S employ solid-state circuitry and
they use no adjustable tuning coils so routine mainte-
nance is limited to occasional battery replacement. Two
transistor-type 9-Volt batteries are used and
Figure 3
shows where they are located and how they are mounted.
Since the cost is very low it is our suggestion that both
batteries be replaced at the same time. Remove the small
screw holding the plate in place at the bottom of the
pistol grip. Disconnect the snaps, discard the old batter-
ies, install the new ones and reassemble.
Model 720/820-S Valve Locators
7
