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Author Topic: Changing PSK-40 Frequency  (Read 2110 times)
AD6KA
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Posts: 2232




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« on: March 02, 2013, 07:58:46 AM »

Greets!

I have an older Small Wonder Labs PSK-40, the higher parts count version
with the DB-9 connector. (20m version was on the June 2000
cover of QST magazine) It is functionally a crystal controlled
SSB transceiver with slightly wider-than normal IF passband characteristics.
It covers the range of 7.070-7.074 MHz,
which was then the 40m PSK PSK calling area. Now
I notice that most, if not all, of the 40m PSK activity, especially
DX, is heard at or around 7.035.

What is the best way to change the frequency of this unit?
Switch out the LO crystal itself (11.981 MHz), or see if I can change
the LO or offset (4.915 MHz) frequency by changing the inductors in these
circuits? If I changed out the LO crystal would I have to change
anything else?

Any and all advice appreciated.
This is a terrific little unit and I don't want to butcher it.
73, Ken  AD6KA
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KB1GMX
Member

Posts: 700




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« Reply #1 on: March 07, 2013, 03:44:39 AM »

OK I have one of those.

The IF and BFO are 9 Mhz and the LO is 5.0688 for  9.070 and up.

To move that unit down the band you can try adding an inductor in
series with the 5.0688 and pulling it down. But you need to move
down 35 khz and that might be asking much.  You need a rock around
5.040.

The alternate is to change the IF crystals (all of them Y1-Y4 and Y7-Y10) to a new IF
and match that with the right BFO (for upper sideband Y6) plus a new LO.  Note the
caps around the IF crystals would likely have to be changed to get the right filter behavior

IN my book changing the 5.0688 crystal is the easiest.  My solution was a initially to
use a SI570 based oscillator (outboard) and later a 5.04 mhz crystal was found (luck).

A good high stability VFO might be tried but it does have to be well made and stable.
The circuit used in the WM20 (white mountain 20 and earlier SSB rig from SWL) would work
as it was a 5mhz VFO.


Allison
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AD6KA
Member

Posts: 2232




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« Reply #2 on: March 08, 2013, 10:49:01 AM »

Thanks Allison:

I also have the White Mountain 20 and WM-40
so I can check out those circuits. Will look for a new
rock for the PSK-40.
TNX AGN ES 73, Ken  AD6KA
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K4LJA
Member

Posts: 21




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« Reply #3 on: March 09, 2013, 06:42:09 AM »

Ken  AD6KA --

As Allison indicated --- a stable vfo would also work.  Consider getting one of the N3ZI DDS vfo's --- probably as stable as a crystal --- and it also has an LCD readout --- with offsets --- so you can read your PSK frequency directly.

Just a thought --- and thanks.

Randy  K4LJA
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