Well Glen, I did it. The 38S now has a 500 Hz, two crystal
bandpass filter in it and it seems to work quite well. It does
single signal if the signal isn't too strong. If I can hear it
on the other side of the passband, the r.f. gain is up too high! :-)
I'll try to describe what I have installed:
I used a transformer to get from 750 ohm push-pull output of
U1 (pins 4 and 5) down to 50 ohms to drive the filter.
The primary is 19 turns, wound as close together
as you can get them using #26 wire, and the secondary is 5 turns,
wound in the gap left by the primary. I've got a 100pF cap across
the primary to tune it to 10MHz. The secondary drives a 270pF cap
in series with the first 12MHz xtal, then another 270pF to ground,
the second 12MHz xtal in series with the first, and a 220pF cap
to ground on the output of the second xtal. So, a conventional
ladder setup with crystals and caps, except for the final cap which
is a parallel element instead of being in series. This is needed
to match the 1500 ohm load at pin 1 of U3.
Other changes: Remove C9
Remove original X2
Remove R6
Replace C10 with the 220pF output cap
Replace C17 with a jumper
Change C21 to 33pF to retune the Lo to the proper place
Traces to cut: Make a cut on the trace that connects between U1 pin 4
and X2 at the indicated trace cut area. This isolates
the primary of the new transformer so that it is connected
to U1 pin 5, only.
Make a cut somewhere in the middle of the trace going
between T401 secondary and the xtal X401 bottom pin.
This is where the first 270pF will get soldered.
The second 270pF gets soldered from the X401 upper
pin location to the ground area to the left of that pin.
Make a cut in the middle of the trace going from
xtal X2 bottom pin and U2 pin 1. Connect a jumper
from U1 pin 1 to X2 upper pin.
The transformer goes in with the primary leads in the holes that Ori
provided. They are the two holes, vertically oriented to the left
of center of the T401 outline, when looking down at the component side
of the PC board. The secondary uses the two lower holes Ori provided,
within the outline, to the right of center. This will ground one lead,
and attach the other to the 270pF capacitor on the input of the
filter.
The crystals that I used are from Mouser, part number 520-HCA1200-S,
and is a 12 MHz series unit made by ECS. I also bought 4 cyrstals
made by Fox, but they don't seem to be as high Q as the ECS units.
12MHz 12MHz
U1-4 -|----------|------||-----xtal-------xtal--------- U2-1
P | S 270pF | |
19t P = 100pF S 5t | |
P | S 270pF = 220pF =
U1-5 -|-- | | |
Gnd Gnd Gnd
Optional 38S 500 Hz filter
So that's my $02 for this weekend. BTW, I put the modifications
in for the 1000 Hz frequency response as you had done it. About
the same results as I had before, but using your method will make my
38S like others who did your version.
72 and enjoy what is left of Sunday.....Jim
Jim Kortge, K8IQY (ex NU8N) | BMHA, NorCal, QRP-L
jokortge@tir.com | __o Cascade 17/40 SSB
Fenton, MI |_`\<, Mizuho 17/40 SSB
.... .. .. . . .. . (*)/(*) . . . . . ..
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