Why does my 6514 read OHMS correctly in the unguarded mode, but shows open circuit in the Guarded mode?
Answer: Bad internal fuse!

Verify by connecting approximately 10Mohm between RED and BLK alligator clips of input triax cable. Set meter to AUTO, OHMS, open Z CHK, and the meter should read 10Mohm. Then select Guard ON. If it reads open this means the fuse is open.

The next step is to turn the power off, connect an ohmmeter between BLK alligator clip and BLK Common post on rear. This should read about 1 ohm. If much higher, then the internal fuse is open.

The simple cure is to return the 6514 to the factory for repair. However, it is fairly simple to fix with the following procedure:

1. Remove power cord and input cable from the instrument.

2. Remove two flat-head screws on bottom rear of instrument.

3. Remove rear bezel by loosening two Phillips head screws from rear of instrument.

4. Slide cover off instrument with care, and find a four-conductor cable from the rear panel to J1011 on the PCB.

5. Next to J1011 is the fuse, F1, marked 1A, 250V.

6. Solder a replacement fuse to the existing fuse. The Keithley part number is FU-100-1.