5205

Sears 5205 VM cal

This is a nice meter. I would expect 2% accuracy on DC Volts, which is quite decent. Be sure to leave the switch in the OFF position when not in use, to protect the meter movement.

Let’s take a WAG at the variable resistors:

• VR1 - ACV calibration; can be done on any range but I’d vote for the 10V range if you have a suitable source
• VR2 - Lo DCV calibration; set for proper reading on the 0.25V range
• VR3 - Hi DCV calibration; do it on the 250V range
• VR4 - DO NOT TOUCH - used to standardize the resistance of the meter circuit, affects all ranges
• VR5 - OHMS adjust on front panel

VR4 is the only pot that controls all ranges; leave it alone unless you’re absolutely sure it’s mis-set. If so, leave it alone. :D

DCV cal should be done in the following order: VR3 first, then VR2, with values as shown above. ACV cal can be done before or after DCV. There is no interaction.

It’s highly desirable to use variable voltage sources with values as close to full-scale as possible. DC must be clean (no ripple). The actual voltages suggested above should yield maximum accuracy, but degradation if other voltages are used is very slight.

You can use any 3-1/2 or 4-1/2 digital multimeter to set your voltage sources for calibration. Any DMM will have accuracy sufficiently greater than the VOMs to serve this function properly.

Note that this meter has a battery check function that puts a load on the battery under test. Resistors R28, R29, and R30 (schematic lower right) are used for this function. Check their values and replace if damaged.

WARNING: The DC Amps function is unfused (because instrument fuses are very expensive). Be VERY careful that you do not touch a probe to a voltage source while the function switch is in the DCA position. you’ll blow the socks off the meter.