SPECIFICATIONS Power Input: ................... 90 watt peak carrier controlled phone and CW. Output Impedance: .............. 50 - 72 ohm. Output Coupling: ............... Pi network (coaxial). Band Coverage: 80 Meter Band: .............. 3.5 - 4.0 mc. 40 Meter Band: .............. 7.0 - 7.3 mc. 20 Meter Band: .............. 14.0 - 14.35 mc. 15 Meter Band: .............. 21.0 - 21.5 mc. 10 Meter Band: .............. 28.0 - 29.7 mc. Panel Controls: ................ Meter Switch. Spotting Switch. Final Tuning. Drive Tuning. Band Switch. VFO Tuning. Audio (gain). Loading. Function Switch. Tube Complement: ............... 12AX7 Speech Amplifier. 6DE7 Carrier Control Modulator. 6AU6 VFO. 6CL6 Buffer. 5763 Driver. 6146 Final Amplifier. OA2 Voltage Regulator. Power Requirements: Filaments: ................. 6.3 Volts at 4.7 amperes AC or DC. 12.6 Volts at 2.35 amperes AC or DC. B+: ........................ 500-600 Volts DC at 150 ma. 300 Volts DC at 100 ma. Cabinet Size: .................. 6 1/8" high x 12 1/8" wide x 9 15/16" deep. Net Weight: .................... 15 1/2 lbs. Shipping Weight: ............... 18 lbs. INTRODUCTION The Heathkit MT- 1 "Cheyenne" Transmitter was designed to provide maximum power capabilities in mobile operation consistent with minimum battery drain. This has been accomplished through the use of carrier control modulation and low drain circuitry. Power levels up to 90 watts input on modulation peaks are obtained. This is ample output to drive larger transmitters if used in fixed station operation. Other features include a stable, voltage-regulated VFO, VFO spotting switch and provision for CW operation. Designed as a companion unit for the MR-1 "Comanche" Receiver, the "Cheyenne" has an identical front panel layout and tuning mechanism. The MT-1 consists of a 6AU8 VFO, a 6CL6 buffer, a 5763 driver and a 6148 final amplifier. The modulator utilizes two dual triodes: a 12AX7 and a 6DE7. The following block diagram and circuit description will serve to better aquaint a builder with the operation of the Transmitter. This knowledge is an invaluable aid to construction and, as such, is well worth reading thoroughly. Lethal voltages are present at many points above and below the chassis, consequently, great care must be exercised when any tests or adjustments are made. VFO The VFO circuit consists of a 6AU8 tube operating as a Clapp oscillator in the frequency ranges of 1750 to 2000 kc, 7000 to 7175 kc, and 7000 to 7425 kc. The tube is mounted on to of the rigid enclosed chassis partition, thus placing all heat generating components outside the VFO enclosure. A double bearing ceramic insulated tuning capacitor is used as a frequency control. The VFO tuning capacitor, consisting of two stator assemblies of different capacities, permits a large bandspread at both high and low frequencies. The coils are wound on heavy ceramic slug-tuned coil forms, heavily doped and baked. The result is a high Q coil upon which varying ambient conditions have a minimum effect. Careful placement of temperature compensating capacitors near the coils tends to cancel drift due to coil heating. In addition, a temperature compensating capacitor across the grid circuit of the tube, carefully positioned physically, provides additional compensation for other varying inductive parameters. The VFO switch is operated by an interrupted switching mechanism on the band switch. VFO output frequency is correlated with the band in use as follows: 80 meters - 1750 to 2000 kc; 40 meters - 7000 to 7425 kc; 20 meters - 7000 to 7175 kc; 15 meters - 7000 to 7175 kc; and 10 meters - 7000 to 7425 kc. This unique switching system, coupled with the vernier slide rule full gear dial drive mechanism, provides more than adequate frequency spread on all bands. The Clapp or series tuned Colpitts oscillator circuit presents a very low impedance to the tube grid at resonance. This minimizes the effect of shift in tube capacitance upon the output frequency. The capacitive voltage divider, necessary for operation of the Colpitts circuit, also lessens the effect of tube capacitance upon frequency. Both screen and plate voltages are stabilized by an OA2 regulator tube. The untuned output circuit of the VFO operates at 80 meters when the 80-meter band is used and at 40 meters when all other bands are used. This circuit consists of the output coaxial cable capacitance, plus the RF choke in the VFO plate circuit. The VFO output thus obtained insures more than adequate drive on all bands. The output is fed to the 6CL6 buffer stage. A 6CL6 tube is employed as a buffer stage to further isolate the oscillator and, at the same time, insure adequate drive even under low battery conditions. The plate circuit of the 6CL6 is untuned when operating 80 meters, slug-tuned to 40 meters for operation at 40, 20 and 15 meters, and slug-tuned to 20 meters when operating 10 meters. An untuned RF choke and the two slug-tuned coils are in series with the B+ lead to the 6CL6 plate. One section of the exciter band switch shorts out the coils not being used for a given band. The RF ground is provided by a large capacitor, since a direct ground would short the B+ lead. The output of the 6CL6 is capacitively coupled to the 5763 driver stage. Driver A 5763 tube is employed as a driver for the 6146 final amplifier. This stage utilizes series plate feed, and is capacitively coupled to the grid of the final amplifier. The plate circuit consists of a tapped coil which is ganged with the VFO and buffer band switch and is tuned by the front panel controlled variable capacitor. The driver stage is keyed in the cathode circuit along with the final amplifier for CW operation. Final Amplifier The plate circuit of the final amplifier is shunt fed with a 2.5 mh RF choke and is capacity coupled into the pi network tank circuit. A tapped inductance is used for tuning all bands and the tap is selected by the bandswitch. In the 80-meter position, a 68 mmf 4 KV capacitor is automatically paralleled with the plate tuning capacitor. The loading capacitor consists of a three-gang, 450 mmf per section, variable capacitor with the sections in parallel. Modulator The 12AX7 tube is used as a high-gain two stage resistance-coupled speech amplifier. The output of the speech amplifier is coupled to the 6DE7 modulator tube through a low capacity coupling capacitor. This low coupling capacity serves in shaping the response to favor the voice frequencies, thus allowing a higher average level to be maintained at frequencies where it will be most effective. The audio energy from the speech amplifier is coupled to the grid of one triode section of a 6DE7. This tube contains two dissimilar triode sections: one triode section is rated at 1.5 watts dissipation, and the other at 7 watts dissipation. The lower rated triode is used as a direct coupled driver, its plate being tied to the control grid of the heavier duty triode, which forms the modulator. The heavy duty triode is biased sufficiently to limit its conduction, and therefore the screen voltage on the final 6146 amplifier. This results in a low resting carrier and, consequently, very low battery drain. With modulation, the conduction of the heavy duty triode section is varied in accordance with the average voice level. This gives a controlled carrier effect by varying the screen voltage on the 6146 tube, and at the same time, the audio signal is superimposed. The net result is to produce a carrier output which increases with the percentage of modulation applied. Microphones Since mobile operation demands that properly shaped audio response be employed, a vary carefully designed ceramic microphone is included with the MT-1 "Cheyenne" Mobile Transmitter. This is to insure very effective modulation with plenty of "punch". The microphone serves to suppress all but the upper middle range of the voice frequencies and the audio system, as described above, is designed around this response. If other microphones are used, it may be necessary to alter the circuit components and the modulator for best results. In any case, the microphone should be a high impedance type and preferably ceramic, since crystal or carbon microphones can be damaged by the hot sun to which they are often subjected in mobile operation. NOTE: IT SHOULD BE NOTED THAT AN AMATEUR RADIO OPERATOR AND STATlON LICENSE IS REQUIRED TO PLACE THIS TRANSMITTER ON THE AIR. Information regarding licensing and amateur frequency allocations may be had from publications of the Federal Communications Commission or the American Radio Relay League. ( ) Supply the connection between W-4 and W-6 on the power socket as explained in the note. This connection is needed only when the power supply is not running continuously. If the power supply is controlled by its own switch, then turn the B+ voltage on and ignore the previously mentioned connection. CAUTION: Remember that potentially lethal voltages are present during the following steps. ( ) Plug the microphone into its receptacle at this time. ( ) Turn the function switch to the STANDBY position and apply the B+ voltage to power plug W on the rear apron of the chassis. NOTE: This voltage will not be applied any further than the power plug until the microphone button is depressed. The pilot lights and all tubes should be lit. If nothing unusual is noted, then proceed with the following steps. ( ) Turn the function switch and the meter switch to the GRID position. Depress the microphone button and adjust the driver tuning control until a maximum reading is obtained. It will be noted that there is more than one peak on the 10 and 15 meter bands, in which case the maximum peak is used. When this peak has been found, drive may be reduced by slightly detuning off this peak. Typical drive after loading should be near 3 ma. Repeat this operation on all bands, 80 through 10 meters. The 10 and 15 meter drive may be slightly low until the buffer plate coils are peaked in a later operation. ( ) Connect a dummy load, such as a 60 watt light bulb, to the coaxial output jack at V. The band switch should be in the 80 meter position for this check and the VFO set to some frequency known to be in the phone portion of the band. Since the VFO has not yet been calibrated, it will be necessary to check the signal on a receiver to determine that it is within the phone portion of the band. Check for and obtain grid drive as described above by depressing the microphone button and adjusting the driver tuning, with the function switch and the meter switch in the GRID position. Now, with the meter switch in the PLATE position, set the loading control fully counterclockwise and the function switch to the PHONE position. Depress the microphone button and tune the final tuning control for a dip. The dip is first obtained in this position since less current will be drawn here than in the CW position. Now that the resonant point has been determined, move the function switch to the CW position. Advance the loading control until a reading of 150 ma is obtained on the meter in the PLATE position. Redip the final tuning control. Adjust the grid drive, final tuning and the loading control until the Transmitter is loaded to 150 ma. Try to reach this point in a minimum amount of time. Now move the function switch back to the PHONE position. Advance the audio gain control until peaks of 150 ma can be read on the meter when speaking into the microphone. Do not exceed this point. A red line is provided on the meter for quick observation of peaks. In actual operatian, you will find some output indicating device such as the Heathkit Mobile Tuning Meter an invaluable aid. If such a unit is available to you, then the Transmitter may best be adjusted for maximum output in the PHONE position by sustaining a tone and adjusting the final tuning, drive, and the loading control. The point of maximum output may differ slightly in this method than in the CW position and is considered as the best method of tuning, since the Transmitter output power is being observed rather than the input power. Peaks of 150 ma should not be exceeded in normal operation. ( ) Remove the microphone from its receptacle for the following VFO calibration. Very small amounts of grid drive will be observed on the meter under this condition, since only the oscillator is involved in this operation. VFO CALIBRATION If the kit builder has access to a commercial frequency standard, an electronic counter of good quality, a surplus frequency standard of the LM or BC series, or a high quality communications receiver with a built-in crystal calibrator, such as the Heathkit "Mohawk", these are excellent for calibration. If a frequency meter is used, the frequency meter and the VFO signals can be beat against each other in the receiver. It will be necessary to calibrate only the 80, 20 and 10-meter bands. Since the 20 and 15-meter bands of the "Cheyenne" both use a common VFO switch position, as do the 40 and 10-meter bands, when the 20 and l0-meter bands are calibrated, the 40 and 15-meter bands are also calibrated automatically. Before beginning calibration, allow the Transmitter, frequency meter and the receiver to warm up for one half hour or more. During the calibration procedure, the drive control should be set at maximum grid current reading with the function and meter switch in GRID position. The spotting switch should be set to its ON position. If a frequency meter is used for calibration, the frequency meter signal and the VFO signal should be zero beat against each other in a receiver, with the BFO off. Under these conditions, the VFO is at the same frequency as the frequency meter. Aside from slight differences in measurement technique, the following procedure may be used for either method of calibration. Refer to Figure 41 on page 45 for location and identification of the various calibrating adjustments. If you should be unable to hear the VFO signal in the receiver, a piece of hookup wire may be run near the oscillator tube over to the receiver input terminal for better signal pickup. NOTE: It should be noted that the trimmer capacitors R, Q and JJ are used to determine the frequency spread covered on the dial, while the coil slugs A, B and C in Figure 41 are used to set a definite frequency point in the dial. The coil slugs are set to the calibration frequency at the low end of the dial; the trimmers are adjusted so that the calibration frequency at the high end of the band coincides with the dial reading. These two adjustments interact with each other and, therefore, will have to be adjusted alternately until optimum overall calibration is obtained. All the trimmer capacitors should be set to HALF MESH, before starting calibration. Make sure the steps regarding the setting of the main tuning capacitor and dial pointer, in relation to the dial mechanism, were not overlooked or done improperly. For 80-meter calibration, set the MT-1 dial at 3500 kc and set the frequency meter without modulation, to the same frequency. Make sure the band switch is in the 80-meter position. Adjust slug C for zero beat in the receiver. Note that zero beat will result when the VFO frequency is the same as the 3500 kc output of the frequency meter. Now, retune the VFO to the high end af the band and move the frequency meter up to a frequency of 4000 kc. Reset the receiver as in the previous step. Tune the VFO to zero beat at Lhe high end of the band and note the reading. Starting with the trimmer R plates at HALF MESH, adjust trimmer R until the VFO is zero beat on 4000 kc. Readjust slug C for zero beat once again at the lower calibration frequency of 3500 kc. Move the VFO up and check the high end, and it should be found that the dial reading is closer to the actual frequency than before, Adjust trimmer R again and repeat this process until calibration is achieved at both ends of the band. Repeat the procedure outlined above for the 20 and 10-meter bands, moving the band switch to the appropriate band for each calibration procedure. The receiver and frequency meter, or signal sources, must operate between 14000 and 14350 kc for the 20-meter calibration, and between 28000 and 29700 kc for the 10-meter calibration. Use trimmer Q and slug A for the 20-meter calibration and trimmer JJ and slug B for the 10-meter calibration. This will complete VFO calibration. Be sure to remove any wire serving to couple the oscillator to the receiver before proceeding. ( ) Now again insert the microphone plug into its receptacle, setting the function switch to its PHONE position. After VFO calibration is complete, the adjustment of the buffer coils may be accomplished. Set the band switch to the 10-meter position and the VFO dial to the middle of the band, approximately 28.8 mc. Depress the microphone button, tuning the final to resonance and adjusting the loading to maximum plate current. Advance the drive control until a small amount of grid current reading can be obtained. Adjust slug D, as shown in Figure 41, for a maximum grid current reading. Now switch the band switch to the 15-meter position and set the VFO dial to the middle of the band or approximately 21.3 mc. Peak the drive once again at a low reading and adjust slug E, Figure 41, for a maximum reading on the meter. If 15 and 10-meter drive seems low, slightly spreading the turns on the 10/15-meter drive coil will provide optimum drive. This completes testing, adjustment and calibration of your MT-1 Mobile Transmitter. ( ) If CW operation is contemplated it is suggested a two wire cable be brought out to an external SPST transmit-standby switch. The internal connections of this cable are as follows: One wire to pin 4 on female microphone socket Z, the other wire to pin 1. The function switch should, of course, be in CW position for CW operation. ( ) Insert a phone plug into the key jack on the rear apron. B+ is turned ON by throwing the transmit-standby switch to ON. The oscillator and buffer will run continuously with the driver and final stages keyed in their respective cathode leads. CAUTION: Approximately 90 volts will be present at the key terminals under "key-up" conditions. A low voltage keying relay could be used for greater safety if desired.