A USEFUL TEST DEVICE

Evening Post, Volume CXX, Issue 112, 7 November 1935, Page 30

A USEFUL TEST DEVICE

The amateur experimenter is usually content to check the working of the LF stage or stages of a receiver by some such simple expedient as the application of a finger to the grid terminal of each valveholder. However, it is sometimes necessary, and at all times desirable, to have some more reliable indication that the full normal amplification is being obtained from each stage, and where AC mains are available a simple device can be a useful addition to one's work-bench "properties."

It consists merely of a test lead terminating in an insulated prod and connected to the "live" side of the mains through a condenser of about 0.0005 mfd. capacity.

By applying this miniature "signal generator" to the grid of each valve in turn from the detector onwards, and noting that ■ the loudness of the hum obtained from each stage, the LF amplifier portion can be quickly checked. Thus an appreciable amount of the time otherwise spent in testing a receiver can be saved.

When dealing with the initial stages of high-gain amplifiers it is likely that the artificial signal applied in this manner may be excessively strong. O£ course, an obvious way to reduce input is to use a smaller condenser, but in most cases the microscopic voltages needed are obtained .by connecting the tester to the "earthy" side of the mains.