My $150
Krohn-Hite/EDC 521 DC Voltage/Current Standard arrived yesterday.
It is huge (20" deep), but not heavy. It has a tear in the red filter over the LED display that wasn't obvious in the eBay listing, and the bottom panel had a big-ass dent. Inside was covered with a layer of persistent dust.
I hammered out the dent using some hardwood blocks, cleaned the insides out with IPA, and checked for damaged components. I didn't find any, so I powered it up and it works!
The manual gives a 2 hour warm-up time. The photo was taken a few minutes after I powered it up this morning. At that time, my Keithley 2700 (powered on for ~20 min, not calibrated in years), read the output as +9.99973v, for +10.00000v after a few hours, the reading settled in at +9.99990.
A quick check of the 100v and 100mv ranges looks pretty good too, as did a quick check of the current settings. I'm logging ~24 hours of readings right now. I haven't tried the GPIB programing yet. Assuming it works, I think I write a little script to cycle it through every output setting in every range and log the measurements.
While doing the initial inspection, I noticed that there is a revision to the circuit, with some added resistors and caps, a cut trace and a bodge wire. It also looks like some power transistors were replaced at some point. The device seems to have been built in early 1997, and most of the parts are from 1996. I couldn't find/see date codes on the added parts. There are codes on the transistors, but I couldn't find the right google invocation to decode them. They are Motorola parts, marked, with a Moto logo followed by "616,” the next line is “JE350,” which is the variant/part number. If 616 means the 16th week of 1996 then it would make the transistors the same vintage as the rest. If it means something else though, it would be nice to know.
Given what Dave has reported from his teardowns of EDC gear, it wouldn't surprise me if the bodges are factory original, but I'd be curious if they were fitted later.
More photos and info in my
blog.