[HBR] My "75S-3W"

[Ian Wilson]

Every so often I am inspired to put more time into this receiver.
A few months ago I lashed up a Si5351A to take the place of a
mythical bank of crystals that go with a dual-conversion design.
Since I needed a PIC to control the LO device, I added an LCD
and frequency counter - no longer any need to keep doing IF
math to work out the receiving frequency.

For the first time I got to try things out on something other than
40m. Bad news - lots of noise but no discernible signals on 20m.
Played around with the front end some more but then put the
beast back on its shelf.

This week I tried a different preselector and tamed down the RF
amp stage more. No real progress.

Then I tried using LO injection on the low side (11.103 MHz)
instead of high side (17.247 MHz) (my first IF is centered on
3.072MHz). This made a noticeable difference; I was able to
find a couple of SSB QSOs on a not very lively band.

The reason: I have a triode mixer stage (6DJ8 Pullen-sort-of).
There is a common cathode resistor of 3.3k. I had stupidly used
a high wattage wire wound resistor there. I am guessing that it
was starting to look like a short at 17MHz. Anyway, I bypassed
the 3.3k to ground, added a 330 ohm carbon resistor (plenty to
develop the RF from the cathode follower). Now I have signals
with high side injection also.

Another 5 years work on this radio and it should be ready for use :)

73, ian K3IMW


On Sun, Dec 23, 2012 at 11:21 AM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

> 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
>
>