General Radio Precision Condensor Type 722-H
Variable Air Capacitor 50pF-1400pF Serial no. 126, date of manufactoring? approx. 1947 up to 1953 - unknown  |
Dial reading
four screws to remove from housing
massive wooden box, inside with cooper foil fixed with nails.
| Wooden box should be repaired, separate all four sides and re-glue. Cooper foil fixed with nails. Diese Holzbox ist vergleichsweise einfach zu restaurieren, alle vier Seiten sind schon aus dem Leim, auseinandernehmen, verleimen und spannen. Der alte Lack ist zu enfernen, alles schleifen und neu lackieren oder ölen, je nach Vorstellung. Eine schöne Sache wäre auch eine Schellack Lackierung aufzutragen, richtig schön glänzend sähe das gut aus, alles nur eine Frage der Zeit. Das Holz jedenfalls hat eine uniforme schöne Maserung, ist feinporig und wäre ideal für Schallack. Die Kupferfolie kann man entnehmen reinigen, polieren und wieder mit den alten runden Nägeln fixieren. |
A beautiful variable air condensor. Unit cleaned with petroleum, old grease and dust removed.
old grease
| Nice
mechanical construction, everything cleaned. Brass wheel, ball
bearing. Solid Aluminium plates. Metall frame seems to be alumium too.
Most parts of the capacitor are made of the same or similar materials,
this results in an low temperature coefficient, due similar thermal
expansion. Petroleum oder Diesel entfernt altes Fett und Schmutz leicht, Reste von verbleibendem Petroleum konservieren und wirken schmierend. Die Kondensator Platten waren sauber, man sollte darauf achten, dass dort kein Reinigungsmaterial hinläuft, da die Reste dort als Dieelektrikum wirken, Effekt wäre jedoch minimal. Ein geeignetes Konservierungsöl wäre nach Abschluss der Reinigung auch Ballistol auf den Metallflächen. |
| Connector made of brass, cleaned and polished. Later protected with a little silicone grease. This electrical conduction quality has direct influence on ESR, the electrical series resistance of the condensor. ESR should be low for all dial reading positions. There are two wipers on each side of the brass. |
| Isolalor on the high side - high side is the fix mounted capacitor plate. Made of Porzellan, Ceramic or an similar material. Highly isolating and less dieelectic losses. Choice of the used material one key for the HF quality of the condensor. Should be cleaned good, remove all liquid petroleum with alcohol, no liquid should remain. |
| After
cleaning, grease all ball bearings, brass wheel and shaft. Don`t use
too much grease, put nothing on the capacitor plates. For the
brass conctact use a little silicone grease, silicone oil or leave it
dry. Dial reading wheel in excellent mechanical condition, shaft runs very soft and precise - what a nice condensor design. These condensors a still expensive on the secondary market, but it is worth to have one - should be a device in every labority. |
Measurements
Measuring the Capacitance vs. Dial reading with an 4279A 1MHz C-V Meter
Two measurements:
Left side done by the seller (using unknown C-Meter) - Right side done by me (using 4279A C-V Meter) |
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| seller data (unknown Meter) | data with 4279A C-V Meter |
 |
 |
| a0= -1.3739pF a1= 28.95pF/major reading Sigma= 0.47pF linear capacitance vs. dial reading, nonlinear end of scale. linear fit function applied for dial X=2-46 |
a0= -1.5296pF a1= 29.528pF/major reading Sigma= 0.28pF linear capacitance vs. dial reading nonlinear end of scale. linear fit function applied for dial X=2-46 |
 |
 |
| linearity, difference in pF from lin-fit | linearity, difference in pF from lin-fit |
| Both measurements showing a similar result. Same linearity waveform and magnitude. Both capacitance meter working correct. Waveform shows the nonlinearity of the condensor. Keep in mind the nonlinearity is very low, only some pF compared to hundred of picofarads (for example 3pF at dial 1000pF = 0.3%), it is a excellent mechanical design. Highest % nonlinearity appears at lower capacitance values. |
This sticker show old calibration data
old measurement shows also an similar waveform character
| old measurement | seller measurement | own measurement |
|
| a0 | -1,972pF | -1,3739pF | -1,5296pF |
| a1 | 28,93pF | 28,95pF | 29,528pF |
| Sigma | 0,5232pF | 0,47pF | 0,28pF |
Air Capacitor ready for use - refreshing the wooden box, a future task.
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