[HBR] My "75S-3W"

[Ian Wilson]

Though I would write up the results of my attempt to build a dual conversion
receiver modelled on the Collins 75S-3C architecture. I chose this because
it allows me to use the 1.3-3MHz PTO that is working beautifully, and
because
it uses a BPF to provide 0.2MHz of 1st IF bandwidth without requiring tricky
tracking tuning. I have a few 3.05MHz +/-175kHz BPFs here that I picked up
at some swap meet. These fit the frequency range well enough.

The receiver is working FB right now, thanks to those who helped with
suggestions about getting the IF strip working well. At present this is just
a 7MHz receiver but I plan to add a synthetic box-o-crystals using the
Si507.
The only problem at the moment is a large birdie. This may be because I
don't
have a crystal of the proper Collins frequency. The lineup is:

  - 6EJ7 RF stage
  - 6U8A xtal oscillator
  - 6DJ8 Pullen mixer
  - 3.075MHz BPF
  - PTO
  - 6U8A mixer (triode used as buffer to frequency counter)
  - Collins 455kHz mechanical filter
  - 2x 6EH7 IF
  - 7360 product detector
  - 12AT7 BFO
  - G4OEP audio-derived delayed AGC (solid state)

I left space on the IF board for an IF AGC amplifier but the audio-derived
AGC is working well enough for the moment.

The audio stage, PSU, etc, is all a lash-up at the moment.

The tuning rate of the PTO is 50kHz/turn. I have a National 6:1
reduction drive
in front of it, giving 8.3kHz/turn. This seems to be a very good compromise
and it is easy to tune SSB transmissions.

I have no mechanical skills, so none of the schemes that I considered for
driving a frequency indicator from the PTO shaft were practical. Since I
need
to use a uP to control the Si570 anyway, I may as well add an LCD display
and frequency counter to display frequency directly (can add in the required
offsets since I know which 200kHz band is being used).

I hope to offset the negative karma accrued from using sand state by using
a magic eye signal strength indicator. Currently looking at the choices
available in the boxes-o-tubes here.

Happy Holidays to all!

73, ian K3IMW