The TEN-TEC Model 243 Remote VFO is an accessory designed to enhance the functionality of the OMNI transceiving system, primarily by enabling dual-frequency operation. This device allows users to transmit and receive on two different frequencies, offering greater flexibility for various amateur radio operations.
Function Description:
The core function of the Model 243 is to provide a remote Variable Frequency Oscillator (VFO) capability, allowing the OMNI system to operate on frequencies determined by either the OMNI's internal VFO or the Model 243's VFO. A key feature is its MODE switch, which, along with LED indicators, offers six distinct modes of operation:
- OMNI transmit and receive
- REMOTE transmit and receive
- OMNI transmit-REMOTE receive
- REMOTE transmit-OMNI receive
- OMNI transmit-both receive
- REMOTE transmit-both receive
This comprehensive mode selection ensures that the operator can configure the system for various split-frequency operations or simultaneous monitoring. The design maintains full, instant break-in capability regardless of the selected mode. Due to the OMNI's broadband design, frequencies, even if widely separated within the same band, are processed optimally by the transmitter without requiring adjustments.
Beyond remote VFO control, the Model 243 also supports fixed-frequency operation through a four-position crystal oscillator. This is particularly useful for applications like MARS (Military Auxiliary Radio System) or net operations, where stable, predefined frequencies are required. The device accommodates both in-band and out-of-band crystals within specified limits.
A unique "REVERSE" pushbutton is included to prevent "cockpit error" during split operations (modes 3 through 6). When pressed, this button reverses the receiver functions only. For example, in "OMNI transmit, REMOTE receive" mode, pressing REVERSE will switch to "OMNI receive, REMOTE transmit" without needing to change the MODE switch. This feature helps operators spot their transmit frequency in split operation without inadvertently transmitting on their intended receive frequency.
Important Technical Specifications:
- Frequency Coverage: The Model 243 covers all amateur HF frequencies from 1.8 MHz to 30.0 MHz.
- Crystal Oscillator: Features a four-position crystal oscillator for fixed-frequency operation.
- Crystal Specifications:
- Case: HC-18/U
- Terminals: 0.041" diameter on 0.188" centers
- Mode: Fundamental only. Parallel resonance.
- Load Capacitance: 20 pF (Note: At 32 pF, frequency is slightly higher).
- Frequency Selection:
- For 160, 80, 40, and 20 meters: Crystals cut for 5 MHz ± kHz from the lower band edge of the segment.
- For 15 and 10 meters: Crystals cut for 5.01 MHz ± kHz from the lower band edge of the segment.
- Out-Of-Band Limits for Crystals:
- 160 Band: 1.7 to 2.10 MHz
- 80 Band: 3.4 to 4.10 MHz
- 40 Band: 6.9 to 7.40 MHz
- 20 Band: 13.8 to 14.55 MHz
- 15 Band: 20.8 to 21.65 MHz
- 10 Band: 27.8 to 30.00 MHz
- PTO (Permeability Tuned Oscillator): The PTO subassembly is identical to that used in the OMNI, housed in a sealed enclosure. It uses a main coil (L3) shunted by L2 and in series with L1. Adjustments of L1 (bottom slug) and L2 (top slug) determine linearity and band edge points.
- Offset Adjustment (21.0 and 28.0 MHz bands): To mitigate band edge birdies, crystal oscillator frequencies for these bands are chosen 10 kHz lower than typically required. A varactor diode (D3) compensates for this deviation, controlled by a DC voltage via potentiometer R1.
Usage Features:
- Installation:
- For early OMNI models, remove the bottom cover and desolder a jumper wire between VFO IN and VFO OUT phono jacks. For later models, remove an external jumper plug.
- Connect the 9-pin MOLEX plug from the Model 243's multi-conductor cable to the OMNI ACCESSORIES socket.
- Connect the red-banded phono plug from the Model 243 to the OMNI's VFO IN jack.
- Connect the second phono plug from the Model 243 to the OMNI's VFO OUT jack. Incorrect connection of these coax leads will render the system inoperative.
- Mode Selection: Use the MODE switch to select desired operating modes, with four small LEDs indicating the selected mode (red for TX frequency source, yellow for RX frequency source).
- Crystal Control: Position the REMOTE switch to X-1 through X-4 for crystal control. LED indicators near the main tuning dial show the shift from VFO to XTAL control.
- VFO Control: Position the REMOTE switch to VFO for VFO control.
- OMNI OFFSET Control: This control only affects the OMNI's internal VFO, not the remote VFO.
- Dial Pointer Zero Set: A serrated disc knob protruding from the bottom of the case allows for precise calibration of the slide rule dial pointer at any 100 kHz point. This adjustment is primarily for indicating the 100 kHz segment, with accurate frequency determination relying on the 0-100 dial skirt.
Maintenance Features:
- Crystal Installation: Remove the Model 243 bottom cover to access the four crystal sockets on the CRYSTAL OSCILLATOR/SWITCH Board. Each socket has a trimmer capacitor for fine-tuning the crystal frequency (nominally ±1.5 kHz deviation). The socket on the outboard side is for X-1.
- PTO Alignment:
- Connect a frequency counter (at least 100 mV sensitivity) to the VFO output plug (red-banded).
- Set the tuning shaft 25 kHz up from the full counterclockwise position.
- Adjust the L2 slug (top) for 5.000 MHz. Clockwise rotation decreases frequency.
- Check linearity by running the tuning knob up-scale and noting readings at 100 kHz intervals.
- If VFO range is compressed (e.g., 5.5 MHz reads less than expected), reset to 5.0 MHz, turn L2 clockwise, and compensate with L1 (bottom slug) clockwise for a 5.0 MHz reading. Repeat until sufficient correction.
- If VFO range is expanded, turn slugs in the opposite direction.
- A tuning tool with a reduced shank (provided with OMNI) is recommended for adjusting L1 without disturbing L2. L2 adjusts VFO range spread, L1 brings frequency to correct value.
- Offset Adjustment for 21.0 and 28.0 MHz:
- Set OMNI BAND switch to 14.0 and dial to 14.000 MHz.
- Set MODE switch to position 2 (EXT BOTH).
- Use the calibrator or frequency counter to set tuning to exactly 14.000 MHz.
- Switch BAND to 21.0 without changing tuning knobs. Adjust R1 on the XTAL/OSC board for 21.000 MHz or zero beat.
- Troubleshooting: If the Model 243 is not connected, the VFO IN and VFO OUT on the OMNI's rear apron must be jumpered to restore normal operation.
- Digital Readout Compatibility: When using the Model 244 Digital Readout with the Model 243, the two phono jacks on the rear of the Model 243 provide access to the mixed local oscillator and +12 VDC.
- General Alignment and Servicing: Refer to the OMNI Owner's Manual for detailed information on measuring voltages, removing plug-in assemblies, and other servicing procedures.
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