Saturday, January 9, 2010

Ramsey QRP80C considerations

My first awareness of that kit was in 1995, when there was a German supplier for this sort of stuff. I looked at it, and since is was a long way from being spectacular, I forgot about it again. Until recently the search for QRSSable kits started. The manual, which contains all vital information, besides the schematics diagram, can be downloaded from Ramsey electrics' webpage. With all parts listed, block and layout diagrams, it is no problem at all to reverse engineer the schematics; which I did. For reasons of copyrights I will not publish the result however.

As I said, the kit is not really spectacular, that what my understanding of the circuit is:

  • Colpitts xtal oscillator (2N3904)
  • common emitter buffer (2N3053)
  • class C power amplifier (2N3053)
  • crystal pulled by two varactor diodes
  • comes with color burst crystal (3.579545MHz)
In the manual it reads that the oscillator can be pulled 7kHz about the nominal frequency. For 80m I feel that is somewhat ambitious, but can't tell until I have built the kit (just ordered).

There is a switch that toggles between two crystal positions.

Now let's see what can/should be changed:
  1. There is a jumper JMP1 which supplies the tuning potentiometer R1 with 12V. I my view this jumper must be replaced by a voltage stabilizer, such as a 78L08. OK, will provide a little less pull, but should be much more stable.
  2. The oscillator is keyed, OK-ish for QRP, not acceptable for QRSS. Here there are a couple of options, supply the oscillator from the voltage stabilizer and key the PA instead (heavier keying transistor required!), key the buffer only or used FSK only.
    Running the oscillator from the stabilizer:
    Cut the PCB between R5 and R7, run a wire from R1 (tuning pot.) hot end to R7.
    For keying the buffer, nothing will change, since the base of the buffer is pulled up by the keying voltage via R8. If the load change on the oscillator is not too great, this should not be creating too much chirp (to be tested!).
  3. Add some resistor/transistor/what not stuff to supply a tiny bit of FSK voltage to the point where R1, R2 and C1 are joint.
  4. A second crystal could be 3.500MHz or, as soon as there is one, 3.600MHz.
  5. For QRP, the place for the second crystal could be populated with a 3.58MHz ceramic oscillator which will have a much wider pulling range. With a chance of 99.9% I will do this! So there will be one QRSS and one QRP mode for this transmitter.
  6. Also for QRP, two identical crystals could be installed in parallel, this would be a so called "superVXO". The PCB offers sufficient space so that two crystals could easily be installed where only one was once planned.
  7. When no receiver is going to be attached to J2, or RX/TX-toggle is done externally, J2 could serve as an input for the FSK keying. In this case the diode D1 can be replaced by a jumper. The parts L5, R11, L4, D2, R12, R13 and C19 must/can be omitted when D1 is replaced by a jumper.
    I consider that a keying circuit can be build in place of the RX/TX-switching stuff:
    The place of C19 could be used for a 10k resistor connected to the base of a NPN transistor (somewhere close to the tuning pot.). Said transistor could influence in one way or the other the tuning voltage....
  8. The resistor R9, which feeds the buffer, should be increased in order to not overheat the PA transistor in a 100% duty cycle. A trimmer could be added in series to R9 to allow for variable output power.
    Alternatively, a resistor could be wired in series with the choke L3 which feeds the PA transistor.
Those are the mods that came to my mind. There may be more....

Another more general idea: When looking at the parts layout diagram, two pulling inductors are foreseen for the 80m version. For less pull, one could certainly omit one by shorting it (temporarily?), which will allow smoother tuning for QRSS.

Looking forward to receiving the kit!