Tube-Type Tuning Indicator
The signal diode on the indicator input (point A of the schematic diagram) should
Fig, 2. Tuning indicator displays. The no-signal state is shown in A. B shows presence of a steady L2 kHz signal; C is a 15 Hz flickering to indicate thaf the SYNC signal is in tune.
be connected to the high side of the SYNC gain control (R1 of the MacDonald circuit). High voltage for the indicator is taken from the same voltage as that supplied for the I2AX7 limiter. With a 1200 Hz test signal connected to the input of the monitor, of sufficient level to fully saturate the limiters, the gain control on the tuning indicator is adjusted until the two light bars in the display tube almost close. When a 1200 Hz SSTV SYNC signal is being received, the display on the 6FG6 tube will flicker at a 15 Hz rate. Maximum deflection of the nickering display indicates that carrier reinsertion by the SSB receiver is at the proper frequency for optimum SSTV picture reception. Typical displays as seen on the tuning indicator are shown in Fig. 2.
Figures 3 and 4 are photographs of the tube-type monitor showing the installation of the light-bar SSTV tuning indicator. The small circuit board just below ihe base of the tuning indicator tube is used to mount the gain control, bypass capacitor and rectifier. The remainder of the components are mounted on the tube socket.
LED > uning Indicator
The components for this circuit were mounted on the plug-in circuit card containing the iimiter and FM detector circuits. The 88 mil toroid and associated parts for the tuning indicator circuit are shown in the photograph of Fig, 5, When the circuit assembly is finished, power is applied to the circuit and a 1200 Hz test signal is connected to the input at point B
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on the schematic (Fig. 6), The capacitor is selected lo resonate the 88 mil inductor to 1200 Hz by monitoring the output voltage developed across the diode load resistor with a VTVM,
To install the LED tuning indicator in a solid-state monitor such as the model by W9LUO described in the March 1971 QST, connect input point B to the collector of transistor Q2. A 1200 Hz SSTV SYNC signal will cause the LED to flicker at the
15 Hz rate. The indicator will be dark if the SYNC signal is absent. When the receiver is mistuned to (he point that SSTV video or noise is appearing in (he tuning indicator, the LED will flicker at a random rate.
From a sludy of the schematic of the Model 70 Robot SSTV Monitor it looks like the input to the LED SSTV tuning indicator (point B) should be connected o pin number 6 of the integrated circuit U1 709C limiter.
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