PURPOSE: To accurately control an electronic circuit by a simple operation and in a short period of time by synthesizing and displaying the demodulation marker signal and the demodulation signal produced by alternately switching a marker signal and a timing pulse signal and controlling the luminance signal.

CONSTITUTION: An FM deviation controller 2 alternately switches the input recording luminance signal and a marker signal and outputs a demodulation signal. Furthermore the controller 2 is provided with a dummy printed board serving as an F/V converter which converts the input frequency into the voltage. A VTR 1 to be controlled is provided with a fitting part where the printed board of an FM circuit to be controlled is inserted for connection and a CNI terminal which connects a VIN terminal which inputs the luminance signal to the controller 2. Meanwhile the luminance signal demodulated by a substrate 5 and the marker signal are alternately inputted to an oscilloscope 3. A test recording luminance signal is inputted to a substrate 4 via the controller 2 and then a volume 4a of the substrate 4 is controlled by making reference to the screen of the oscilloscope 3. In such a way, an electronic circuit is controlled in a short period of time.