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A forum for the discussion of matters of a technical nature. All such activities are undertaken at the readers discretion and own risk. If you don't know what you are doing, don't blame us if it all goes wrong!
Does anyone know how to configure a Marconi 2955B for SSB radio alignment/testing? In RX Test the mode won't change from FM to AM or 0M (only in TX Test), so I'm guessing I am missing or misunderstanding something fairly key. I read somewhere else the 2955B should be set to produce a CW signal and set 1KHz off the target frequency but I have no idea how to do that either.
You set frequency, then mod, and for mod level you set either a value for deviation as x Khz, or a value for mod depth as x % - the AM/FM is auto selected. For CW, just set 0% mod.
For SSB alignment, set it to AM with no modulation (0%) and 1KHz lower for LSB. Same for USB, but 1 KHz higher.
For example, for "mid" channel 20 (27.205 MHz), set the generator to 27.204 for LSB and 27.206 for USB.
SSB just needs a carrier of a known level and frequency for alignment.
For level, it's usually done in "dBm". You should hear a clean 1KHz tone down to -110dBm or so, getting gradually noisier as you go down towards -125dBm.
For frequency on receive (i.e. to set the 12 o clock position on the clarifier) adjust the LO frequency in the radio for exactly 1KHz at the speaker. Once that's done, adjust the Tx clarifier trimmer for the correct frequency on Tx (easiest done by temporarily misadjsting the balanced modulator). You'll then have correct frequency alignment, other stations on frequency will be clarifier centered.
Thanks very much for all the suggestions and excellent advice. I didn't realise I had to set the mod value in % for AM to auto-select. I also wasn't aware of the 1KHz offset for USB and LSB. Thanks again - trying it today and will feed back.
Your respective advice worked well and I was able to complete the alignment. However, I was surprised to find that on this set, a Tristar 747 (PTBM121D4X) with XTal sub-board (PTOS011AOX), that it is not transmitting the same power across all modes. I'm getting the following:
Hi
Ch 41 - 4.1W
Ch 60 - 5.1
Ch 80 - 6W
Mid
Ch 1 - 8.8W
Ch 20 - 7.3W
Ch 40 - 6.3W
Lo
Ch 1 - 11.2W
Ch 20 - 10W
Ch 40 - 8.9W
I have used the alignment procedure from the HAM Intl Multimode II service manual, which matches the PCB layout but contains only the schematic for the PTOS011AOX board.
Wondering if anyone has come across this and can help?
