Shorting Issues Between Ground And Common

In the Platinum XL there is a potential problem in which some of the screen's screws could short improperly. This paper details the issue and solution.  In general, Rotem Controllers have high levels of electrical isolation. They come equipped with an isolated transformer, ensuring that the DIG COMM and shaft GND are disconnected (see Figure 1).

The procedure described here was developed for situations when there is more than 2 Ohm resistance. This issue came up when we started using a second source for the XL LCD; the LCD supplied enabled a connection (short) between the GND and Dig COM.

Issue Description

Rotem Controllers require a good GND connection (0.5 – 2 Ohm).  When

• the GND is more than 2 Ohm and
• there is a shorting between the Dig COM and the GND connections

electrical noises could pass from the GND to the Dig COM and causes unwanted interference in the Platinum.

The Dig COM connection usually comes from the low voltage regulation circuit.  Figure 1 illustrates the separation between GND and Dig COM in a standard situation.

Figure 1: Wiring Schematic

In the Platinum XL, two LCD metal screws may cause a short between a Dig COM port and the Platinum's GND.  To prevent this short, a technician needs to replace these metal screws with the substitutes provided (a plastic screw and a plastic nut) (Figure 2).

Figure 2: Plastic Screws

Testing

Test the unit to see if there a short between COMM and GND.

1. Put a Voltmeter in short mode.

2. Check the short between a body of a Transistor and the metal protector as shown in Figure 3.

Figure 3: Voltmeter

3. In the event that there is a short, proceed to the following section.

CAUTION                  If there is no short, do not replace the screws.

Procedure

Figure 4: Location of Screws

Figure 4 illustrates the screws' location.

1. Loosen two screws on the CPU labeled "Loosen".

2. Using your finger gently lift the CPU slightly (just enough to provide easy access to the Platinum XL screen).

3. Remove the two screws labeled "Remove".

4. Place the two plastic nuts between the screen and the metal sheeting (Figure 5).

5. Insert the screws into the nuts and tighten.

6. Retighten the screws loosened in Step 1.

Figure 5: Placing the Plastic Nut

Placing the nut between the screen and the metal sheet prevents the COMM connector from shorting with the GND.