Hi,
I've been a lurker on this forum for a while.. I recently acquired an EDC 521 DC calibrator which a known fault on the 100mV range. All the other ranges are stable and look to be very close against my best ability to test on a Keithley 6.5 digit DMM.
I've diagnosed the fault as a failure on the 100:1 voltage divider (a KRL branded 4-wire module about 2" x 3/4" x 3/4"). It is designated A-3600 on the schematic. I have contacted KRL Bantry to see if they can help, but not holding my breath on that one..
The 100mV range configures the output to a 10V range and places this across this divider, the output of which is presented to the output terminals. There is a small trim network around the divider, but it is not able to bring the voltage back into cal. In this mode, the input voltage to the divider is feed back to the output amplifier so after the divider, the output is "open loop" and specified as 20ohm output impedance, and EMF into 1Mohm compliance.
I was measuring about 88mV out for 10V in, about 12% down. After removing the resistor and verifying out of the unit, I can see the output leg is measuring ~17 ohms, not 20. Guessing it has a few shorted turns or some other damage from a "transient event"
So, what can I use to replace this? Does anyone have a parts unit from this range?
Alternatively, does anyone have a recommendation on where to go to get a set of matched resistors?
I assume I need TC matched resistor pair, 1980 + 20 ohms. I don't know the tolerance, but given the cal pot allows me to adjust the voltage approx +/- 0.3% (+/-0.3mV for 10V in) I'm guessing an initial tolerance of 0.1% or better.
Thanks,
Ash.