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| GenRadmodel 722 Mystery Capacitor |
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| KK6IL:
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 at https://groups.io/g/Test-Equipment-Design-Construction/album?id=279097 The 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 |
| Conrad Hoffman:
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 |
| Wrenches of Death:
--- Quote from: KK6IL on October 11, 2022, 05:31:23 am ---What's the best way to clean the brass rotor contact area? Brasso? --- End quote --- Ammonia 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 |
| RolandK:
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. |
| KK6IL:
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 |
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