Small Wonder Labs PSK-20 Mod to 20m WSPR

I bought a Small Wonder Labs PSK-20 kit a few years ago when I was dabbling with HF data modes.  The PSK-20 is, from the PSK-20 manual “[a] transceiver on a single PC-board design measuring 4.6″ x 5.27″ (12 x 13.4 cm). It is functionally a crystal-controlled SSB transceiver with slightly wider than normal IF passband characteristics. It covers the range of 14.070 to 14.074 Mhz.”. It comes in kit form and unfortunately is no longer available, however a note on the Small Wonder Labs webpage states “[there] will be a successor- schedule is uncertain”.

PSK20Parts WSPR

I built my PSK-20 then never used it.  It sat in a drawer until recently I thought I could use it for something other than PSK.  With the relatively low TX output, mine gives 2w out, it would be ideal for WSPR.  I was hoping I would be able to re-tune it from the designed 14.073MHz to 14.0956MHz but the pull on the crystals was too much and it wouldn’t do it.

Frequency Mod

The design of the PSK-20 is based on a 9MHz IF and a 5.07MHz LO giving an operating frequency of IF + LO or 9 + 5.07 = 14.07MHz.  The LO is generated from a 5.068MHz crystal and the 9MHz IF a 9MHz crystal, nice and simple.  An additional eight 9MHz crystals are used for crystal ladder filters, four in the RX path and four in the TX path.

As the design is a simple mixer it was easy to calculate that changing the LO and IF crystals would get me onto 14.0965MHz so I purchased 10 x 10MHz crystals and 1 x 4.096MHz which would mix to give me the desired 14.0965MHz ( 10 + 4.096 = 14.096MHz ) which is the frequency used for WSPR on 20m. Here are the parts you need to change to move the PSK-20 frequency from 14.070MHz to 14.096MHz.

Component

ID

Original Crystal Frequency

New Crystal Frequency

RS Part

Number

Y1

9.0 MHz

10.0 MHz

6936952

Y2

9.0 MHz

10.0 MHz

6936952

Y3

9.0 MHz

10.0 MHz

6936952

Y4

9.0 MHz

10.0 MHz

6936952

Y5

9.0 MHz

10.0 MHz

6936952

Y6

5.068 MHz

4.096 MHz

2261572

Y7

9.0 MHz

10.0 MHz

6936952

Y8

9.0 MHz

10.0 MHz

6936952

Y9

9.0 MHz

10.0 MHz

6936952

Y10

9.0 MHz

10.0 MHz

6936952

  • The RS part numbers for the crystals are 6936952 for a bag of ten 10MHz crystals (9 are needed in the mod) and 2261572 for the 4.096MHz crystal, total price for all crystals was £11.08.

I fitted the new crystals and aligned the PSK-20 onto 14.0965MHz as seen on my frequency counter and started using it with WSPR but nothing was decoding and nobody was decoding me.  I checked I had TX output and then found out the PSK-20 is natively LSB, this meant I was on the ‘wrong’ frequency and also as WSPR is an FSK type mode, the tones were inverted.  I’m not 100% sure if WSPR is tolerant of which mode it is on but PSK31 is not for which the kit was designed – in PSK you change the phase not not tone (FSK) to signal information.

So the problem I had now was to get the PSK-20 onto USB and although I found lots of people asking how to do this on various internet forums there was no answer on how to do it.  So I emailed Dave Benson, K1SWL, the kit designer and proprietor of Small Wonder Labs asking how he would suggest I mod the PSK-20 to USB.  I got an answer 40 minutes later, although I had gone to bed, fantastic support!!!

USB (Upper Side Band) Mod

Dave suggested I put a 10uH inductor in series between the 10MHz IF crystal and the C38 trimmer to pull it below 10MHz, to about 9.998MHz.

First I had to cut the track between Y6 (the 10MHz IF crystal) and C38 on the underside of the PCB, circled in Orange.

DSCF8773_Highlighted

I then fitted a 10uH inductor in series between Y6 and C38. I didn’t have a surface mount inductor so it is obvious how and where I mounted it in the picture below.

DSCF8778

Results

So I switched the PSK-20 on, connected some headphones to the AF OUT connector (watch as the volume it very high) and tuned my IC-756pro3 to 14.0965MHz USB and sent some audio, tuning both C38 and C42 (the trimmer for the LO) I was able to resolve USB! Bullseye!

IMG_20110618_235458

I then connected up the PSK-20 to WSPR and let rip.  I get about 2w output but at first I run using just 0.5w with fantastic results.  Here is a 12 hour plot of where I’ve been heard.  I use a 1/4 vertical on 20m with the feedpoint 1.5m above ground and 8 radials which act like guy ropes.

First12hours

If you would like more information or details on how I achieved this then contact me using the Feedback option on the site.

16 Comments

  1. Nick Tsakonas

    Very good mod, I wish I had one.I always thought that I could get one but they got discontinued.

    Maybe at some point… From the schematics alone it is quite straightforward…

    Good work!
    Nick, SV1DJG

    Reply
    1. m1bxf (Post author)

      Yes, there was talk of a new version under the new owner, but not seen anything. Simple design to copy or look at QRP Labs Ultimate 3 as a driver and a small PA, will only give you TX though.

      Reply
  2. w3bmnky

    I would like to mod my psk-20 for jt-65 use @ .076. Can I get there without new crystals

    Reply
    1. m1bxf (Post author)

      Hello ??? I’ve no idea, have you tried?

      Reply
  3. n8vz

    Much like you, I have a PSK-20 that I built, used a few times, and then set aside. I find your mod for using it on WSPR to be intriguing. I’m also interested in converting the serial port to USB, as another person suggested. (Yes, I realized the need to switch it to Upper Side Band, here I’m using the computer meaning of USB — hi hi.) I’m not as technically proficient as you appear to be, so any step-by-step directions you might have for making your mod would be appreciated. If you know anything about the serial port-USB mod that information would be appreciated as well. Tnxs & 73 de Carl, N8VZ (n8vz —-at—- arrl –dot– net)

    Reply
    1. m1bxf (Post author)

      Hi Carl,

      I’ve done some updates to the article to make it clearer which crystals need to be changed to what value, hopefully that might help you. There is not really a step by step procedure except
      1. remove all crystals,
      2. fit the new values listed,
      3. cut the trace indicated for the USB mod
      4. fit a 10uH inductor

      As for USB (computer USB) this has no place at all in this kit, The PSK-20 only has 12v in and audio in/out with PTT being VOX. There is no serial or USB (Universal Serial Bus) or anything at all like it in the PSK-20.

      Hope this helps.

      Gavin, M1BXF.

      Reply
      1. n8vz

        Thanks for your response. Well there certainly is a serial connector. It’s a DB-9, listed as part J4 in construction manual version that I have issued 06/09/00. I did read where someone had replaced the serial port with a USB connector, which is a little neater than using an external serial-to-USB adapter. — 73 Carl, N8VZ

        Reply
        1. m1bxf (Post author)

          Hi Carl, Yes looking at the documentation it seems the early models had a serial port for PTT, http://smallwonderlabs.com/PSK-series%20issues.htm however the version I have does not so I’m not able to advise on the best way to mod this to USB. However there are loads of USB>TTL with RTS/DTR pins presented, such as the FT232 module (http://goo.gl/6V0RRx), which can then drive the PTT circuit, FTDI will most likely have a config utility allowing you to customise the USB device name to something like ‘PSK-20’ also, so I bid you to go forth and play to make this work for you 🙂 Nothing is impossible…

          Gav.

          Reply
  4. Matthias, DF2OF

    Hi Gavin,

    from your description above I still cannot see, how the USB mod has been done. Did you replace the COM port or add an extra USB device?
    I want to fit a USB port to my PSK20

    73, Matthias DF2OF

    Reply
    1. m1bxf (Post author)

      Hi Matthias,

      LOL; USB = Upper Side Band and not Universal Serial Bus.

      Gavin.

      Reply
      1. Matthias

        LOL, yes, found that out this afternoon as well 🙂 boy oh boy…anyway I got it working with a USB-Serialadapter this evening. Might do the upper side band mod as well..
        Cheers,Matthias

        Reply
  5. Peter VK7KPC

    Hi,
    Just modified my PSK20 as per your idea.
    First contact New Zealand both ways.
    Still some fine tuning to get the best out of the filter networks (not centred on 10MHz yet, about, 500Hz off).
    This probably means my output power is down a bit also.
    Thanks for your idea and Daves fix for correct sideband

    Reply
    1. m1bxf (Post author)

      Hi Peter,

      Glad it worked for you, it’s quite a simple mod and it’s a shame the PSK-20 is not available any more and I’ve yet to find an equivalent kit to substitute it.

      I was very happy with the performance of mine, as you can see from where I was heard; http://i0.wp.com/www.geekshed.co.uk/wp-content/uploads/2011/06/First12hours1.jpg

      Gavin.

      Reply
  6. ZVI AMIT 4X4OQ

    GOOD MORNING GAVIN , PLEASE GIVE ME A INFO.I WISH TO ADAPT MY PSK 20 TRX TO WSPR. BUD NOT PEMANENTLY.( I WOULD LIKE TO WORK ALSO IN PSK AND WSPR) SO I BUILD TWO SMALL BOARDS WITH A C/O RELAY TO ADAPT THE 10 MHZ CRISTALS AND ANOTHER BOARDS FOR THE OSCILATOR CRISTALS.BEFORE I MAKE THIS ADAPTORS THE INFO I NEED IS IF THE SLUG TUNED COILS L2,L5,L6 ARE ENOGH BROAD ORE THEY HAVE TO BE RETUNED.THIS INFO IS CRITICAL FOR ME BEFORE I MAKE ALL THE ADAPTER BOARDS.THANCKS A LOT,ZVI 4X4OQ ZVIAMIT11@BEZEQINT.NET

    Reply
    1. m1bxf (Post author)

      Hi, your idea of a daughter board for allowing switching between WSPR and PSK frequencies should work. L2,L5,L6 did not need adjusting after the modification.

      Let me know how you get on.

      Reply
  7. Peter

    Looks good Jimmy!

    Reply

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