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Solid State Tray Tuning

Probably one of the most critical and least understood areas of the FH transmitter is the operation and set-up of the Solid State Tray (SST), more especially the diode modulator and solid state amplifiers that follow.  RCA drawing 3729110 (Tuning Procedure, Solid State Tray) is a 20 page document that covers: Power Supply, Aural Power, Visual Power, Visual Sweep Response, Differential Gain and Phase Adjustments within the SST.  Below is a brief outline of the procedure intended only as a guide for your better understanding.  Please refer to 3729110 for the complete procedure.  

Power Supply adjustments involve installing an appropriate load resistor across each of the five PS in turn then adjusting the associated over voltage trip point and the over current protection accordingly.  Not difficult to understand, but time consuming to accomplish.

Aural Power Adjustment involves adjusting a matching network and then setting the output of the SST for 18 Watts feeding into the 80 Watt aural IPA that follows.  Quite simple to understand and accomplish.

The Visual Power, Sweep Response, Diff. Gain & Phase Adjustments of the SST involves several areas.

        1.    Checking the RF drive power to the diode modulator.

        2.    Setting the bias level of the modulator.

        3.    Checking the tuning, neutralization, performance and efficiency of the diode modulator.

        4.    Establishing if the Solid State Amplifiers are operating within the correct gain window.

        5.    Setting up the current limit of PS5 to protect the 25 Watt and 40 Watt amplifiers

        6.    Adjusting the Clamp Limit control to limit before PS5 goes into current limit.

        7.    Setting the output power of the SST to the correct level.

This is a step by step process.  You cannot make measurements or adjustments at an intermediate step without knowing that the previous step was correct.  

Adjustments within the SST are critical in achieving optimum performance of the visual signal.  Overall performance of the FH-Line of TV transmitters can equal any other transmitter out there, new or old, provided these adjustments are completed correctly.

 Explanation of each:

1.   2.5 Watts from the visual exciter is fed into the SST directly  through the phase modulator, through an attenuator and matching network into a 20 Watt Solid State amplifier, followed by a circulator into the diode modulator.  The voltage control on the front panel is adjusted for 12.5 Watts and MUST be this level.  The diode modulator will only provide sufficient output (a modulated envelope) to the Solid State amplifiers that follow when the CW input drive level to the modulator is 12.5 Watts. 

2.  Initially, in order to determine if the diode modulator is functioning correctly the DC voltage that the video rides on is set to -40VDC (adjusted with the pedestal control on the video output module).  The fixed bias voltage to the diode modulator is set to -30VDC and is never changed from that point.

3.  The diode modulator must now be tuned for maximum output power by adjusting C4, C9, C10, C15, C17, C21 and C23.  This MUST be greater than 3.5 Watts.  If it is not, the modulator is NOT performing correctly or the previous two steps have not been set up correctly.

Next, the neutralization of the modulator is checked by adjusting the pedestal level control to -85VDC and adjusting either L14 and L15 or C39 and C40 for minimum power.  This is an indication of how well the modulator performs during white picture conditions.  This power level should be very, very low - in the order of 200mW or less.  Once again, if this does not happen, the modulator is not operating correctly.

Now the pedestal control is adjusted to provide 1.5 Watts out of the modulator.  This is only a reference level to help determine if the amplifiers that follow are functioning correctly.  It is NOT the operating point for the diode modulator.  Depending on the performance of the amplifiers, the actual operating power at the output of the diode modulator is usually about 1 Watt.

4.  This 1.5 Watts is now fed into the 25 Watt amplifier to determine its gain.  The output power should be between 5 and 8 Watts.  If not, the amplifier is not working correctly.  The output of the 25 Watt amplifier is now fed into either the single 40 Watt amplifier, in the case of a 17/35FH, or the parallel pair of 40 Watt amplifiers for a 25/50FH.  The output power should be greater than 20 Watts or 25 Watts respectively.

5.  Now the pedestal level is adjusted to give an output power from the SST of 24 Watts for a 17/35FH or 42 Watts for a 25/50FH.  The current limit control for PS5 (supplies the 25 Watt amp.), R118 is now adjusted so the output power drops to zero.  You are setting up protection for the solid state amplifiers in case of overdrive.

6.  If PS5 does by chance go into current limit, the only way it can be reset is to remove DC power.  This is undesirable so a clamp limit control (R5) located in the video output module is set up.  This will limit the diode modulator output from driving the solid state amplifiers beyond the point at which PS5 goes into current limit.  Adjust the pedestal control for 21 Watt output for a 17/35FH or 38 Watts for a 25/50FH - (this is just below the point at which PS5 goes into current limit).  Adjust the clamp limit control to pull the power down to 20 Watts or 35 Watts respectively.  Now, the output of the amplifiers can only reach this power level and PS5 will not current limit.

7.  Adjust the pedestal control so the output power from the SST is 15 Watts for a 17/37FH or 24 Watts for a 25/50FH.  This is the power level before the power control attenuator and will always be the same no matter whether you are channel 7 or 13 or whatever power level you are running the transmitter at.

The motor operated power control attenuator is used to adjust the transmitter output power, NOT the pedestal level control on the video output module.  Once the pedestal level is set for a SST output of either 15 or 24 Watts it is NEVER touched again unless you are going through this setup procedure.

Once again, for a more thorough setup procedure please refer to RCA 3729110