Friday, August 28, 2009

Green Radio

A long time ago, I spent a little money and got myself a GRC9. Yep, green radio. That was to a time where you got whole sets and not just the main event. It came with all aerials, spare parts, headphones, microphone, morse key, speaker, power amplifier (LV80), inverters, two rucksacks full of stuff, all cables, service manuals, etc... It still sits in Freiburg and is waiting for me to pick it up.

Meanwhile, I decided it was time to obtain a SEM 35. In contrast to the GRC9 it came naked. Yep, it really did. Just the transceiver and the clipon power supply - no handset, no aerial, no power cable and worst of all, no circuit diagram.
Took me about four full evenings on the internet to finally locate eight image files on the circuit diagram. Ohhh boy!!! Now, I ordered a handset, speakers seem to be unavailable. Let's hope for the best!

It was claimed the SEM 35 was comparably stable, even with the PLL disabled (there is a mod on the internet that allows for continues tuning of the rig telling so).
Here come in options for modifications, i.e. expanded usability...
The rig has got a first i.f. of 11.5MHz and a second i.f. of 470kHz. One could considered doing something with that. Something like adding an SDR to the intermediate frequencies. This would give a range of about +/- 48kHz (96k-sampling rate) about the center of a channel. With a channel spacing of 50kHz, the whole range is covered with nice overlap.
For the first i.f. options are a 46.0MHz oscillator for the normal approach and 15.333MHz for the "softrock lite 30" approach. Honestly, since there are no crystals, one need to think of a more advanced solution.
The second i.f. could be dealt with using a 1.880MHz oscillator, which to me again looks like a more advanced thing to do. But there is an offset solution: 1.8432MHz, resulting in a center SDR frequency of 460.8kHz. Actually, I do prefer this, for various reasons.
The plan is, as soon as I got my act together, to add an SDR receiver, maybe a softrock, in front of the 470kHz i.f.-filter.
I am curious if the stability of this green radio from the seventies is good enough to capture 10m WSPR and QRSs.

If the above project is successfull, one could consider of doing something similar to the transmit train, allowing for CW. The PA of the SEM 35 looks linear to me, which, in a pure FM rig, does not make much sense however. Before running sideband through it, some more thinking and testing is required.