In some instances, this problem occurs by accident. The instrument was placed on the ground while the surveyor or technician was performing another task, and someone ran over it with the truck. In other cases, we have found that the operator broke the case when he applied excessive torque; usually while in the process of exposing the target he had just detected. The electronics case consists of two custom machined parts that can only be procured as a set. The main cause of this problem is again “Operator Error” and the customer should be billed accordingly.

Other problems that can occur

In order to properly diagnose performance problems within the unit, you will need to access the instrument main PC Board. You can do that by removing the four adhesive backed rubber bumpers, and then remove the screws underneath and lift-off the cover.

Unit is dead or there is no audio output

Check that the batteries are installed properly and that the combined battery voltage is above 5.0 VDC. Check that the Power Switch is in the ON position and that all cables are intact and properly connected.

• Some of our older units had a toggle power switch. If your unit has a toggle power switch, it has probably failed. To check the power switch, short across the back two pins; if the unit then begins to operate, you can either replace the toggle switch or return the unit to Dunham & Morrow where they will perform a power switch upgrade, at no charge to you.

• If the unit is drawing current but there is no audio output, then the problem is most likely a cracked transistor (reference Figure 1). Our older units had an aluminum front panel, and Q5 can be damaged by repeated dropping of the unit on its head. You can either replace Q5 or return the unit to Dunham & Morrow where they will replace Q5 and the Aluminum Control Panel at no charge to you. In some cases the speaker itself may need to be replaced.

The unit goes into saturation at turn-on

The basic locator circuit design employs multiple high forward gain, negative-feedback circuit blocks. In order for the DML2000 magnetic locator to perform correctly, the excitation circuitry must be working properly along with all 4 magnetometer signal channels. Even marginal performance by any one of these circuit blocks can cause the unit to go into saturation when it is turned on.

To alleviate this type of problem, we have significantly upgraded the circuit design. Examine the white label on the instrument PC Board, if the designation reads “REV S” or higher, your unit has the new, improved circuit design. The magnetometer excitation frequency has been set to a fixed frequency. In addition, seven other PCB components have been changed; including the four signal channel tuning capacitors, the two excitation inductors and the main frequency setting resistor. The beauty of this fix is that it works, the new 102028 REV S circuit board, works on old or new units, without exception. It is our recommendation that whenever you encounter a unit that goes into saturation at turn-on, you replace the PCB with a new “REV S” PCB.

Figure 1, DML2000 PC Board layout

The instrument screams at all times

There are two separate potential causes for this type of problem:

• The most likely cause is C14; a surface-mounted capacitor that is no longer physically connected to its PC board solder pads. To check the physical integrity of C14, push down on the capacitor using an insulated tool or the eraser end of a #2 pencil. If the screaming stops, replace C14. We discovered that the solder bond on C14 was breaking because the PC board flexed every time the customer dropped the unit. To eliminate this type of problem, we added shock absorbing rubber bumpers to the PC board, and changed C14 from a surface mount capacitor to a leaded capacitor.

• If C14 is not the cause, then the only other likely cause is a broken wire in the sensor wiring harness. Several years ago, we redesigned the sensor wiring assembly to eliminate this type of problem.

To confirm that you have a broken wire in the sensor assembly, connect a DC voltmeter between ground and the PC board signal locations listed below.

1. Z-top signal channel output - R201 lead.

2. Z-bottom signal channel output – R301 lead.

3. X signal channel output – R501 lead.

4. Y signal channel output – R601 lead.

If the voltage on the DC voltmeter is nominally ± 4.0 V dc, then that signal channel is in saturation. In a working signal channel, the output will not exceed 3.5 V dc and the output voltage will vary linearly as the instrument sensor assembly is moved around in the Earth’s magnetic field. If the broken wire is in the excitation circuit, then two of the signal channels will be in saturation.

To fix a broken wire in the sensor assembly follow these procedures:

1. Remove the 4 screws securing the PC board to the case and then remove the PCB.

2. Cut off the rubber crutch tip.

3. Take an X-acto knife and carefully run it around the inside of the sensor tube. This will separate the clear rubber binding material from the inside of the sensor tube.

4. Push down on the sensor connector from the inside and the sensors should slip out of the tube. If they resist, repeat step 3 above until the sensor assembly slides out freely.

5. Examine the sensor wiring harness for evidence of a broken wire. Typically you will find that the broken wire is in the middle of the sensor assembly, where the top sensor is soldered to the sensor PC board.

6. Before repairing the broken wire, use a fast setting adhesive to re-glue the sensor PC board to the appropriate notches in the sensor support tube. Once the glue has hardened, continue with the repair of the wire break, making sure to allow sufficient stress relief for the wire to flex. Then reassemble the sensors.

In case you did not notice, the sensor connector goes down as you face the unit (it is on the underside of the sensor PC board). Push the sensor assembly back into position, and then hold it in the tube as you reattach the Electronics PC board to the other end. Seal the sensors with Silastic or Room Temperature Vulcanizer (RTV) and then glue a replacement crutch tip onto the end of the sensor tube.

Click here For a list of available replacement parts.

Other problems

This covers all of the typical problems. If this does not cover your problem call our service department at 703-661-2144 or email warren@magneticlocator.com.

Always remember that the ultimate responsibility for any repair is Dunham & Morrow, Inc. and you can send the unit back to us for repair at anytime.
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