GenRadmodel 722 Mystery Capacitor - Page 1
Forum section: electronics / testgear

author: KK6IL
author_rank: Regular Contributor
timestamp: October 11, 2022, 05:31:23 am
content: I recently bought a GR 722 variable cap on eBay with a front panel marked 722-ME, which would be a delta C range of 0-105 pF and 0-10.5 pF. Instead the dials are marked 200 to 1100 in steps of 50 and 0-50 on the other dial. The closest catalog item would be a 722-D, but the only catalog listings are for dual ranges, 25-110 uuF and 100-1100 uuF. Photos are athttps://groups.io/g/Test-Equipment-Design-Construction/album?id=279097The dial readings are accurate from 200 to 1200 rather than 100 to 1100/The front panel has 3 terminals but one unused, and the wire between the capacitor stator and the terminal is a piece of coax with the shield not connected at either end. A mention is made in the GR Experimenter that bare wires are used as insulation would have a higher dialectic constant and thus add stray C.Any idea exactly what I have here?What's the best way to clean the brass rotor contact area? Brasso?John KK6IL

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 12, 2022, 02:46:53 am
content: Hi,Give me a day or so to do some research. Couldn't find the photos on groups.io for some reason. Lots of Q-meter pages!Best,Conrad

author: Wrenches of Death
author_rank: Regular Contributor
timestamp: October 12, 2022, 03:51:41 am
content: Quote from: KK6IL on October 11, 2022, 05:31:23 amAmmonia weakens brass. It makes it brittle. Unless something has changed recently, Brasso contains ammonia.And, NEVER use products containing ammonia to clean brass cartridges cases for reloading. Seriously.WoD

author: RolandK
author_rank: Regular Contributor
timestamp: October 12, 2022, 08:31:24 pm
content: The M of the MD means Minus. So the capacitance at value 0 should be about 1104 and 135 pf.You connect your 722-MD tuned to zero to your not as accurate, but reproducible measurement-gear and read the value.Than you add your unknown capacitor parallel to the 722-MD and dial the 722-MD to the same value on your measurement-gear as before. Then you can read the exact value of your unknown, which you had subracted from the 722-MD.If you have the lowest capacitance for the 722-MD at scale reading zero, there is something else in the box.ps: can't open your pictures, link goes to login-site.

author: KK6IL
author_rank: Regular Contributor
timestamp: October 12, 2022, 10:50:46 pm
content: It's clearly not an 722-ME as the front panel is labeled or an -MD. 200 on the dials is about 200 pF and 1100 is about 1100. Dial not marked but usable to 1200 pF. Clearly not in the -M Minus family. The current 722-D goes from 25-110 and 100-1100, so doesn't match 100-1100, although the 722-D two terminal I saw mentioned might have been different. Catalog J lists 222's; K1 lists 722-D dual range.OK, not Brasso, thanks for the warning, Wrenches. Silver polish? Deoxit? Fader Lube?Hope I got the photos attached properly - have trouble with that every time.John

author: RolandK
author_rank: Regular Contributor
timestamp: October 14, 2022, 08:29:12 pm
content: Your 722 seems to be a -D with 2 extra plates for the extra 100pF.Some pictures just to compare:The 722-N is connected in the middle, it has lower series resistance (8 mOhm) and inductance (24 nH)The 1422-ME has the variable plates mounted 180° rotated for the minus behavior. You can see it on the picture if you compare the two capacitors (The scale of the 1422-ME starts with "0").The1422- ME is calibrated by srews which adjust sectorwise the distance between the last two plates.The 722-N is from 1954, so over 68 Years old and it is spot on. It is only missing the handle.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 15, 2022, 05:43:26 am
content: Thanks for the photos - certainly an improvement on the adjustment resolution and internal trimming than my mid-1920's GR239E which spans 30pF to 1025pF with a simple linear 0 to 100 dial. As far as I can measure the original 1025pF FS cal has changed to 1033pF +/- about 2pF.

author: Whitefoot
author_rank: Contributor
timestamp: February 07, 2024, 03:06:41 am
content: A bit late: