Seacom SEA 245 HF Transceiver

SEA 245 Notes

Scanning
- Program Scan - Select frequency, mode, etc. [Radio], 7, bin number, [enter], 1 (overwrite existing bin), [enter], alphanumeric name (default is userfreq), [enter], 1 (scanning mode voice), [enter]. If first three characters of name is different from that of the previous bin, this is the start of a new scan group.
- Scan Mode - Determines whether a scan leaves an active channel after the hang period or, instead, stays on the channel until it is inactive.
  Continuous (stay on active channel) - [Radio], 1,3,1
  Stop and Go - [Radio], 1, 3, 2
- Start Scan - Select a bin number in a scan group by keying in bin number followed by [enter] (such as 1, [enter]). Key in [Radio], 8 to start scan. PTT stops a scan.
- Current Scan
  1. 3.586 MHz LSB, RTTY
  2. 3.800 MHz LSB, Voice
  3. 7.086 MHz LSB, RTTY
  4. 7.200 MHZ LSB, Voice
  5. 14.086 MHz LSB, RTTY
  6. 14.200 MHz USB, Voice
  7. 21.086 MHz LSB, RTTY
  8. 21.300 MHz USB, Voice
  9. 28.086 MHZ LSB, RTTY
  10. 29.000 MHz USB, Voice
- Manual Channel Select - Key in channel, hit Enter
Frequency Response - It was noted that there is substantial high frequency rolloff, especially with 850 Hz shift RTTY. Frequency response measurements were made by measuring the transceiver power into a dummy load while driving the balanced audio SEABUS with different levels of audio at varying frequencies. Audio above 0 dBm does not yield higher power at any frequency. All measurements were made at 14.086 MHz LSB.
- Graph of output power in watts versus input level and frequency
- Graph of output power in dB reference to output power with 1 kHz audio input
Fan Control - I have been looking for a simple way to sequence the Dentron Clipperton L amplifier with the transceiver to avoid hot switching in the amplifier bypass relay. The transceiver PA Board schematic shows K11 as the antenna relay. PERHAPS the amplifier relay could be switched in sync with K11. The Bill of Materials shows K11 has a 10 ms operate time. I don't know what the operate time of the RF bypass relay in the Dentron Clipperton L is. Further, K11 is on the power amplifier board which is under another board (see photo). It would be difficult to access. Another possibility is using the fan control since the heat sink fans are external. This scope photo shows the timing between RF output and the fan being activated. The yellow trace is the low side of the fan. It is pulled down by transistors in the SEA245. Apparently the transistor is not saturated. The blue trace is the RF envelope. It appears the fan is activated about 5 ms before the RF output. This is probably not sufficient advance timing to avoid hot switching in the amplifier RF bypass relay. This scope image shows the carrier drop (blue), and the fan disable (yellow). The fan is disabled about 900 ms after the carrier drops. That would be fine, but the carrier up sequence is probably too short.

SEA 245 Documentation

Seacom SEA 245. This runs 150W CW or SSB between 1.6 MHz and 30 MHz. It is a dual conversion system with the first IF at 45 MHz and the second at 40 kHz. The 40 kHz IF is run through an A/D and then into a DSP where final filtering and demodulation are handled in a DSP. On the transmit side, the DSP generates the SSB (or other) signal at 40 kHz. It is then upconverted to 45 MHz, and then to the final transmit frequency. The high first IF provides good image rejection with broad band RF filters. In fact, the same band pass filters that are used on the transmit side are used on the receive side.