Author Topic: GenRadmodel 722 Mystery Capacitor (Read 3021 times)

KK6IL · « **on:** October 11, 2022, 05:31:23 am »

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 · « **Reply #1 on:** October 12, 2022, 02:46:53 am »

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 · « **Reply #2 on:** October 12, 2022, 03:51:41 am »

Quote from: KK6IL on October 11, 2022, 05:31:23 am

What's the best way to clean the brass rotor contact area? Brasso?

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 · « **Reply #3 on:** October 12, 2022, 08:31:24 pm »

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 · « **Reply #4 on:** October 12, 2022, 10:50:46 pm »

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

RolandK · « **Reply #5 on:** October 14, 2022, 08:29:12 pm »

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.

trobbins · « **Reply #6 on:** October 15, 2022, 05:43:26 am »

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.