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Technical Information

SIDEBAND SUPPRESSION ISSUES:
Central Electronics 100/200V

1.0 Introduction

The Central Electronics 100 and 200V exciters utilize the phase-shift method of single-sideband generation. Early phasing type exciters, although relatively inexpensive to construct, offered inferior sideband rejection as compared to competitive filter-type units. Typically the early sets, i.e. Central 10A, 20A, etc. used a simple Dome phase shifter and achieved suppression on the order of only 35 to 40dB. Additionally, component drift in the phase shifted audio channels caused relative audio levels to vary, and so, resulted in a deterioration of sideband suppression and the need for frequent servicing.

The 100/200V exciters perfected the phase shift method to a degree that exceeded the capabilities of filter-based exciters. A special audio phase shifter was developed, using matched precision components, whose accuracy allowed suppression in the range of 55 to 60dB. Audio feedback was used throughout the phase shifted audio channels to make the relative audio amplitude less sensitive to component aging.

Equally important is the accuracy of the RF phase shifter used to derive 90-degree offset balanced modulator carrier injection. The 100/200V use a low-Q, RL/LR network to achieve the desired 90-degree shift. The inductors of both portions of this network are slug-tuned to allow for accurate phase shift adjustment. The low Q of this network minimizes the possibility of sideband deterioration due to network component aging.

2.0 Sideband Suppression Adjustment

Before proceeding with the single sideband adjustments, the technician should familiarize himself with the illustrations contained in Appendix I. The ultimate objective in this adjustment process is to obtain an oscilloscope pattern containing a minimum amount of ripple when a pure sine wave is applied to the 100/200V audio input. That is, in a correctly adjusted single-sideband transmitter, a single audio tone should produce a single RF carrier output (within the measurement capabilities of a typical oscilloscope presentation).

A low distortion audio oscillator, having less than 0.3% distortion, is required for the alignment procedure. Oscillators having distortion higher than 0.3% will introduce distortion products which would appear as ripple in the oscilloscope presentation and would limit the ultimate sideband adjustment to that of the oscillator rather than the exciter under test.

The following adjustments determine the sideband suppression: AF Ratio, AF Balance and the 8MHz RL/LR phase-shift network (total of four adjustments). Before commencing the adjustment process, verify that the RF Power Output control is set to maximum level, that the Emission control is set to either USB or LSB and that the Speech Level control is set to approximately the 9 O'clock position. The audio oscillator should be set to approximately 1300Hz and its output level to less than 20 millivolts. The Speech Level control should be adjusted so that the RF output power developed during the test is somewhat less than 20 watts. This setting prevents excessive RF heating and assures that the test signal is below the clipping level designed for the Audio Limiter stage. Note that the clipping process of the Audio Limiter stage, if operational, would introduce high-order distortion products which could limit the accuracy of the single-sideband adjustment process.

Adjust the two carrier balance potentiometers for minimum carrier output as indicated on the oscilloscope. If a noticeable amount of ripple is present on the oscilloscope, try adjusting the AF Ratio and AF Balance controls first. Switch between USB and LSB Emission Switch positions and observe if the oscilloscope display appears equal. Usually what is observed is that the ripple is minimum on one sideband setting but different (worse) for the other. The objective in varying the two AF controls is to achieve an equal presentation for both USB/LSB settings. Therefore, try slowly adjusting the AF controls while frequently changing USB/LSB setting to achieve the best sideband rejection.

If adequate suppression is not achieved by varying the AF Ratio and Balance controls, it will be necessary to adjust the RF phase shift inductors. Each time these inductors are varied, it will be necessary to determine that the carrier is fully balanced out before attempting AF adjustments. It is often helpful to select the sideband having the worst suppression and then slowly varying one RF phase shift inductor at a time to see if there is any improvement. If an improvement is noted, retune the AF controls for best observed pattern. It may take several attempts to achieve the best suppression, so be patient!

A much more accurate method for adjusting these controls requires an RF spectrum analyzer. This device permits the technician to visually observe the ratio of Upper vs Lower Sideband suppression, in real time, as the adjustments are being made. Alternately a communications receiver having narrow bandwidth filters can be used, in much the same way, by observing the S-meter differences between USB/LSB Emission Switch settings.

3.0 Sideband Suppression Problems

Insufficient sideband suppression is a common problem with newly acquired 100/200Vs. As these sets are nearly forty years old, it is likely that some corrective action will be necessary to bring units within specification. It is recommended that the following be verified prior to any extensive troubleshooting:

A) Verify that the 6mF/50V Mod Caps are serviceable. If these are the original white ceramic electrolytics, manufactured by Chemtronics, they should be replaced by tantalum capacitors of the same (or somewhat larger) value.
B) Observe that the Carrier Balance potentiometers achieve balance within their normal range of adjustment. If one is offset to one end of its rotation, suspect that the 8 MHz Oscillator plate coil is not correctly adjusted or that one of the diodes (or socket-pin connection) is defective. If replacement is needed, a suggested replacement device is the Motorola MBD701 silicon hot-carrier device. Note that all four diodes should be replaced whenever one has failed. To adjust the Oscillator Plate Coil, using an oscilloscope (times 10 probe) measure the signal level present at pin B of T102. Adjust the coil's slug for maximum 8MHz output level.

C) Verify that the PS-2 audio output levels, as presented to V-8 (12AT7) grids are equal in level. If measured levels are found to be unequal, refer to the PS-2 Service Notes for suggested troubleshooting steps.

D) Inspect the 3900 Ohm 5% carbon composition resistors feeding the plates of V 8, 9 and 10. On the 100V, those parts are shown as R146 and 149. On 200Vs the same parts are shown as R149 and 150. If these parts show signs of temperature discoloration or measure out of the specified 5% tolerance, they should be replaced with, minimally, 2 watt 5% tolerance replacements. Although carbon composition resistors were used throughout the 100/200Vs, today's metal film resistors offer superior performance and stability in these audio circuits. Therefore consider replacement with film types in lieu of the original carbon types, if available.

E) Check all tubes in the audio stages with a high quality transconductance tube tester. Replace all weak or marginally good tubes with high quality replacements.

F) Remember that the sideband alignment process must be repeated several times to achieve optimal settings. This takes PATIENCE. But the reward for careful adjustment is that the exciter will likely require no future retuning, except in response to catastrophic component failures.

If the difference in ultimate sideband suppression between USB and LSB settings exceeds 3dB, it is likely that a small amplitude shift, as a result of component aging, has occurred in the V-8 through V-10 circuitry. Two cures are possible. One is to verify that the symmetry of resistor values in the two audio paths is within tolerances. The other involves the installation of separate AF Balance potentiometers for independent adjustment in USB and LSB modes. A modification kit for this dual AF Balance feature is presently under development.

In the majority of instances, it is possible to achieve the suppression specification on at least one of the two sideband modes using the procedures described above.



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