Amprobe AT-3500 Underground-Wire-Pipe-Locator - Manual (Page 12)

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In order to replace the ten 1.5 V mignon (AA) batteries, the battery housing must be removed as described in the following
illustrations:
Changing the batteries in the T-3500 transmitter
The batteries in the transmitter have to be replaced as soon as the red LED battery indicator starts to blink.
Should this occur while the user is busy locating a line with the receiver, he/she will be informed of the weak batteries via the
reception signal:
Ty pe of signal Normal signal Signal when batteries are weak
In order to change the six 1.5 V mono (D cell) batteries, both screws on the back of the transmitter must be (e.g. with the aid of
a coin) turned through ¼ turn (1) and the battery tray must be pulled (2). Please note that all the batteries are fitted in the same
direction.