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GenRadmodel 722 Mystery Capacitor
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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