This article discusses repair and calibration of the Signal Corp I-177 tube tester. Serious electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted by Bob Putnak, all rights reserved.
Model I-177 is a very old US Military dynamic mutual conductance tube tester that was used during World War II years. The War Department Technical Manual is dated August 1944, and the tester calibrated in this article is dated April 1945. The unit is extremely well-built. Due to its age and high signal voltage, the tester is only suited to test the “antique tubes” and military tubes of its era, and does not even have a 9-pin miniature socket.
The MX-949A/U socket adapter kit (an extra accessory that expands the capability of I-177 models) provides sockets to test “newer” tubes. That said, the 5.0vac signal voltage of the I-177 tester is not suited for testing many of these newer tubes and can damage small signal tubes or provide less-than-ideal readings. That said, the I-177 does an excellent job of testing the “antique” tubes of its era.
The fundamentals for starting the I-177 project: all knobs and the meter itself are indexed at zero, check all resistors and potentiometers for accuracy and replace where necessary , replace the 0.1 mfd capacitor, clean all sockets/switches/leaf-switches/pots/rheostat with Deoxit to the extent possible. I would specifically note that the I-177 has 7 carbon resistors. Expect that most (if not all) of these carbon resistors will have increased in resistance and will need replaced. The remaining resistors are either wirewound or precision types, and while they should be tested, it is unlikely that any would be defective. Inspect all wiring (AC power cord, and also each wire connection at every tube socket pin). Remove both bulbs (#81 fuse bulb and neon shorts lamp), clean bulb connections and sockets, reinstall. The #81 fuse bulb must be only a #81 bulb (no substitutes).