TEN-TEC CENTURY-21 (01) PDF MANUAL

POWER: Connect the line cord to a source of 105-125 VAC, 50-60 Hz.

ANTENNA: Connect a 50 to 75 ohm, unbalanced resonant antenna, such as a beam, dipole, or vertical. Balanced, long wire, and similar antennas with higher input impedances should be used with an antenna tuner to provide a reasonable match (SWR) to 50 ohms.

KEY: Connect a straight key, bug, or electronic keyer to this jack. Electronic keyers preferably should have a reed relay or a positive voltage, low saturation transistor output circuit, such as is employed in the TEN-TEC Model 670 Century Keyer.

GND: To reduce the possibility of stray RF pickup on interconnecting cables, which may cause parasitic oscillation, all station equipment should be well grounded to earth. This also reduces the possibility of electrical shock and provides some lightning protection. Connect a short, heavy lead, preferably shield braid, between the chassis lug on the rear panel and a good earth ground.

1. Select the desired band with the BAND switch. If working the 21 or 28 MHz bands, the accessory crystals must be installed.

2. Tune the dial to the desired frequency within the selected band.

3. Set the DRIVE control fully counter-clockwise (CCW).

4. Apply power by actuating the POWER ON-OFF switch located on the RF GAIN control. This is a push-pull switch. The pilot lamps should illuminate.

5. Push the SET DRIVE button and advance the DRIVE control until the power meter indicates 70 watts input power. Release the button. The transceiver is now ready for operation.

6. The power supply is equipped with an automatic current limiting shutdown circuit. If the DRIVE control is set too high, or if the antenna should suddenly become disconnected or shorted, excessive current drain will turn the unit off. To reset, make sure the cause of the overload is corrected, turn down the DRIVE control, and then cycle the POWER ON-OFF switch. Panel lights will come on and normal operation may be resumed.

GENERAL

FREQUENCY COVERAGE: 3.5 to 4.0; 7.0 to 7.5; 14.0 to 14.5; 21.0 to 21.5; 28.0 to 28.5; 28.5 to 29.0 MHz. (Crystals for the last three segments are available as accessories).

VFO FREQUENCY STABILITY: Less than 20 Hz change per degree Fahrenheit, averaged over a 40° change from 70° to 110°, after 30 minute warmup. Less than 20 Hz change from 105 to 125 VAC line voltage.

TUNING RATE: Approximately 17 kHz per revolution of main tuning knob.

POWER REQUIREMENTS: 105-125 VAC, 50-60 Hz. 10 watts receive, 100 watts transmit.

SEMICONDUCTORS: 25 transistors, 26 diodes, 5 integrated circuits.

PC BOARDS: 4 plug-in types, 6 integral.

CONSTRUCTION: Rigid aluminum chassis and sub-panels. Aluminum case. Grey front panel, black textured vinyl cover.

DIMENSIONS (HWD): 6-1/8″ x 12-1/2″ x 12″

WEIGHT: 15-1/2 lbs.

RECEIVER

SENSITIVITY: 1 µV or less for 10 dB S+N/N.

SELECTIVITY: Three position; 0.5, 1.0 and 2.5 kHz.

VFO FREQUENCY: 5.0 to 5.5 MHz. Double Direct Conversion.

ANTENNA INPUT: 50 ohms, unbalanced.

AUDIO OUTPUT: 1 watt @ 8 ohms, less than 2% harmonic distortion. Built-in speaker; PHONES jack.

OFFSET TUNING: Approximately ± 5 kHz, defeatable with ZERO BEAT switch.

TRANSMITTER

DC INPUT POWER: 70 watts.

RF OUTPUT POWER: 30-40 watts, typical.

OUTPUT IMPEDANCE: 50-75 ohms, unbalanced.

T/R SWITCHING: Full break-in CW with PIN diode switch.

CW SIDETONE: Internally generated; adjustable level.

METER: Indicates total DC power drawn from supply.

Antenna: Any unbalanced antenna presenting 35 to 75 ohms impedance to the Century/21 will work satisfactorily. This represents an SWR of 1.5 to 1 or less. When using random length antennas or open wire feed systems, a matching network should be used to obtain an equivalent impedance.

An unbalanced antenna has a feed point with one of the two terminals at ground potential, typically using co-axial cable with the outer shield connected to the ground terminal. Balanced antennas, which have both feed terminals above ground potential, can be converted to an unbalanced feed by inserting a balun or using an antenna tuner designed for balanced loads.

Ground: To reduce the possibility of stray pickup on interconnecting cables, all station equipment should be well grounded to earth. This is especially important when using high impedance antennas and matching networks where RF voltage levels are high.

For proper operation, the key line requires a very low resistance path to the chassis with no appreciable voltage across it. Therefore, electronic keyers with reed relays or transistor-switched circuits incorporating low saturation NPN transistors are recommended. Improper key line conditions may cause lower than rated power output, an improper keying envelope, and/or key clicks. The TEN-TEC Model 670 Century Keyer is designed specifically for use with the Century/21.

The key actuates circuits on the CONTROL BOARD that supply operating voltages. When the key is closed, the receiver is disabled, operating bias is applied to the transmitter, and the antenna is isolated from the receiver input. A short time delay in the audio muting circuit eliminates keying clicks without interfering with the full break-in feature.

The two AUX 12 VDC jacks on the rear panel may be used to power external equipment, such as an electronic keyer. They are connected to the +12 volt regulated supply in the Century/21, and any power drawn will show as additional WATTS INPUT on the panel meter.

The full six amperes can be drawn from these jacks with full regulation, but any power taken from these jacks reduces the power available for the transmitter. For every ampere drawn from the jacks, the actual input power to the final amplifier will be reduced by approximately 12 watts. (The Model 670 keyer requires only a small fraction of an ampere.)

POWER ON-OFF: Located on the RF GAIN control, this is a push-pull switch. In addition to turning the unit on and off, it is used to reset the power supply if it shuts down from an overload. To reset, simply recycle the switch after removing the cause of the overload (usually the DRIVE control set too high).

BAND SWITCH: Selects the desired 500 kHz band of operation. The AUX position can be used with an optional crystal for additional coverage on the ten-meter band.

MAIN TUNING: Simultaneously adjusts both receiver and transmitter frequency. The dial is marked in 5 kHz increments from 0 to 500 kHz. Clockwise rotation increases frequency on 14, 21, and 28 MHz bands and decreases frequency on 3.5 and 7 MHz.

OFFSET: This control tunes the receiver independently of the transmitter by approximately ± 5 kHz. It is useful for zeroing in on received stations without changing your transmit frequency, such as in a roundtable or when working DX stations that are listening off-frequency. It can also be used to eliminate an interfering station by tuning to the opposite side of the zero beat.

ZERO BEAT: Pushing this button defeats the OFFSET control and allows you to set your transmitter exactly on the received station’s frequency. Tune the incoming signal to zero beat (where the tone becomes inaudible) while depressing the button. After releasing the button, you can use the OFFSET control to set the most pleasing pitch.

SELECTIVITY: Three positions are available:

• 2.5 kHz for SSB or general CW listening.
• 1.0 kHz for CW or SSB with QRM.
• 0.5 kHz for CW on a crowded band.

RF GAIN: This control is a potentiometer in the receiver antenna line. It acts as an attenuator to reduce susceptibility to overload in the presence of extremely strong signals.

AF GAIN: This controls the input level to the audio amplifier. Adjust it for a comfortable audio level in the speaker or headphones.

SET DRIVE: This push-button switch is in parallel with the key line and places the transceiver in transmit mode. It is used while adjusting the DRIVE control to set the desired input power between 60 and 70 watts, as indicated on the panel meter.

DRIVE: This control sets the level of RF applied to the final amplifier. For full rated input, set it for a meter reading between 65 and 70 watts. Increasing drive beyond this point will trip the electronic circuit breaker.

The Century/21 as shipped has crystals for the 3.5, 7.0, and 14.0 MHz bands. Three additional crystals are available as accessories for other bands. To install these crystals:

1. Place the unit upside down and remove the six bottom cover screws.

2. Carefully lift the cover off, exercising caution so the leads going to the speaker are not stressed.

3. The crystals are inserted into the sockets provided on the mixer board (see Figure 2 in the manual for location).

The available accessory crystals are:

1. The speaker is most effective when the unit is on a hard surface with the two front snap-up legs extended. Adequate sound is still produced with the legs down.

2. Provide adequate ventilation for the heat sinks. Do not confine the unit to a small volume without forced ventilation.

3. The Century/21 can be operated as a QRPp (low power) transceiver simply by adjusting the DRIVE control downward.

4. The sidetone level control is accessible through a finger hole located in the bottom plate.

5. The VFO oscillator is adversely affected by AC magnetic fields. Locate any AC-operated equipment (power supplies, clocks, keyers, rotator controls) as far as possible from the transceiver to prevent frequency modulation (“dirty” CW note). The most common cause of this issue is placing such accessories on top of the Century/21.

6. Due to the possibility of high voltage transients, do not change bands while transmitting power to the load. Place the transceiver in receive mode or ensure a “key-up” condition before changing bands to avoid risk of destroying the output transistors.

7. Although improper antennas will not damage the unit, an SWR below 2 to 1 is suggested for maximum performance. If the power supply repeatedly shuts down due to over-current conditions from a high SWR, the unit can be operated at reduced power by rotating the DRIVE control counterclockwise to a position where the supply does not trip out.

1. The CENTURY/21 will give the best performance when properly loaded.

2. Even though the output transistors are resistant to damage from improper loads, they will not operate satisfactorily under all load conditions.

3. The output transistor dissipation will increase if not properly loaded.

4. Reactive impedance components in the antenna are applied to the transistors and may cause parasitic oscillations.

5. A given SWR reading does not tell you anything about the reactive components and is not accurate unless the load is a pure resistance.

6. A given SWR indicates one of two possible impedances. Each acts differently on the transceiver’s performance.

7. The most efficient operating point is when the load is 50-75 ohms, resistive.

8. The meter is not an indication of output power, but the input power.

9. If the breaker repeatedly trips, it is an indication that the load is far enough from optimum to cause high transistor dissipation.

10. It is possible for the power supply regulator to drop out of regulation just prior to tripping the breaker, causing hum modulation on the transmitted signal. With a proper load, the current drain will be considerably below the tripping point, so no hum should appear.

The Century/21 uses a broadband, solid-state (transistor) design, which behaves differently from traditional vacuum tube circuits with resonant tanks.

Broadband vs. Resonant Tanks:

With a tube circuit, if the tank is resonated and the load is light, the tube’s internal power dissipation is low. As the load is increased, power is delivered to the load. With transistors in a broadband circuit, applying drive with no load causes significant power to be poured into the circuit, which must be dissipated by the transistor, potentially tripping the power supply’s circuit breaker.

SWR – Two Kinds:

A Standing Wave Ratio (SWR) indicates a mismatch but doesn’t specify if the load impedance is higher or lower than the desired 50 ohms. For example, an SWR of 3:1 could mean a load of 150 ohms or about 17 ohms.

• If the load is 150 ohms (high impedance), the power demanded from the supply will be much lower and will not trip the supply.
• If the load is 17 ohms (low impedance), the power demanded will be very high, and the supply may repeatedly trip out, even though the SWR meter reads the same in both cases.

Efficiency:

Transistor amplifiers act as a constant voltage source. The output power varies depending on the load value and increases as the load impedance goes down. A 3:1 SWR on the low side of 50 ohms will demand much more power from the amplifier than a 3:1 SWR on the high side. Unless the load is near the design value, the transistors will heat up unnecessarily without delivering more power to the antenna.

Removal of Top:

Remove the upper two screws on each side. Lift the top off.

Removal of Bottom:

Remove the four screws holding the speaker grille. Feed the speaker down through the cutout, using the notches in the cutout to clear the outer edge of the speaker. Remove the remaining six screws: two on the bottom, two on each side.

Removal of Front Panel:

1. First remove the top and bottom covers.

2. Remove the BAND switch knob using a small flat-bladed screwdriver.

3. Remove the main tuning knob and SELECTIVITY knob using a 4-40 Allen wrench.

4. The remaining knobs pull straight off. It may be necessary to gently pry them off with a flat-bladed screwdriver. Protect the front panel from scratches by using a piece of cloth or cardboard under the driver.

5. Remove the PHONES jack using a 1/2-inch wrench.

6. Unsolder the wires to the ZERO BEAT switch, SET DRIVE switch, and METER. Note and record the polarity on the meter leads.

7. Using a 1/2-inch socket, remove the four control nuts from the OFFSET, DRIVE, RF GAIN, and AF GAIN controls.

8. Pull the panel forward and remove.

Pilot Lamp Replacement:

The panel lights are all 14-volt, bayonet type, No. 1813 or 1892. Access to these bulbs is by removing the top of the unit.

Fuse Replacement:

The fuse is located inside the bottom of the unit. Replace it with a 2A SLO-BLO fuse.

The chassis wiring is color-coded. The following table shows the function and typical voltages for each wire color.

*Depends on DRIVE setting
#Depends on OFFSET setting

The PTO determines the linearity and band edge points via two slug-tuned coils, L1 (bottom slug) and L2 (top slug). An alignment may improve linearity. You will need a frequency counter with at least 100 mV sensitivity.

1. Adjust the OFFSET control for zero offset. Connect a frequency counter to the PTO output terminal on the right rear of the enclosure, coupling as lightly as possible for a stable reading.

2. Set the BAND switch to 14.0 MHz and the main tuning knob to full CCW rotation. The dial should indicate 50 kHz below the band edge (±10 kHz). If not, push down on the spring-loaded idler gear arm with a screwdriver to disengage the dial drive gear and move the dial to indicate 50 kHz below the band edge.

3. Turn the tuning knob to indicate 14.000 MHz on the dial and observe the counter. If it is not 5.000 MHz, slightly touch up the L2 slug. The slug position should be such that a clockwise rotation decreases the frequency.

4. Run the tuning knob up the scale and observe the counter readings at each 100 kHz point. If readings are within ±5 kHz, linearity is within specification. If not, proceed to step 5.

5. If the PTO range is compressed (e.g., the reading at the 14.500 MHz dial position is less than 5.500 MHz on the counter), reset the tuning to the 14.000 MHz dial position (for a 5.000 MHz counter reading). Then, turn the L2 slug a fraction of a turn clockwise and compensate for the frequency decrease by turning the L1 slug clockwise to bring the counter back to 5.000 MHz. Repeat step 4 to check the correction.

6. If the PTO range is expanded instead of compressed, compensation in the opposite direction is indicated. This is done by turning the slugs in the opposite direction.

NOTE: To reach L1 without disturbing L2, use the tuning tool with the reduced shank. L2 is used to adjust the spread (range) of the PTO, and L1 is used to set the frequency to the correct value.

The ZERO BEAT function uses a trimpot (R12 on the Control Board) to set its frequency. This adjustment ensures that the pitch of a received signal is the same whether the ZERO BEAT button is pushed in or not.

1. Turn the OFFSET control to its center (zero) position.

2. Tune in a station with the main tuning knob and adjust to zero beat (the point where the tone is inaudible).

3. Push the ZERO BEAT button in and adjust trimpot R12 for the same pitch.

4. Check the adjustment by retuning to a slightly different pitch and pushing the ZERO BEAT knob. The pitch should stay the same.

Calibration of the meter and trip point requires an adjustable 2.5 ohm, 100-watt load resistor and a 10-ampere DC meter. This procedure is performed on the Power Supply board (80353).

1. Connect the load resistor and ammeter in series and bridge them across one of the Aux 12 VDC output jacks on the rear panel.

2. Calibrate the meter in receive mode (not transmit).

3. Adjust the load resistor for a current of 6 amperes as indicated on the external ammeter.

4. Adjust the meter calibration thumbwheel potentiometer (R7) so the front panel meter reads 70 watts.

5. Adjust the current limit trimpot (R1) so the power supply trips at the same 70-watt point on the meter (which corresponds to the 6A load).