SW-40 & SW-20 repair

2009/11/08                  日本語は下にあります。    ve3cgc@hotmail.com


“ SW-40 and SW-20 intermittent cracking noise in keying“ 



I built the SW-40 and SW-20 QRP transceivers and they have been working well.  In the spring, 2009, the two transceivers generated the intermittent cracking noise in keying through the earphones and it became annoying.   I e-mailed Steve, KD1JV, the designer of the rigs, for how to fix it and he responded quickly with proper explanations and suggestions.  Steve, VRY TNX for your effort.  The problem has been fixed and I would like to share my experience with you.  The following e-mails describe the problem and how to fix it.  Some pictures are up-loaded in the album of this blog under SW-40 & SW-20, 2009/11/08.  Click the image for enlargement.



Re: Cracking noise

Hi, Steve, and good morning.  I have built a SW-20 and SW-40 and been using them for nearly two years.  Since the last spring, a problem developed.  I need your help.

The problem:  On both rigs, no cracking noise when in receiving but intermittently I hear a big cracking noise I hear on the BOTH transceivers though the earphones when the key is in use for QSO.

It is very annoying and I decide to fix it.   Please e-mail me why this cracking noise are developed and generated on my both rigs.  And importantly please let me know how I can fix it.

Otherwise the rigs have been working nicely and I love it.

73 and TNX in advance.

VE3CGC  Hiro Hayashi




If the sidetone when transmitting becomes distorted, it usually means the driver stage is breaking into self oscillation and can be noted by an increase in SWR. In sever cases, this can cause the zener across the PA transistor to short out. The solution is the turn the power level down a little. Or you can increase the value of the emitter resistor in the driver stage by about 10 ohms and this will usually let you turn the power level control all the way up without the oscillation occuring. You can also increase power output by compressing the turns on the toroid in the low pass filter connected to the PA output (move the turns close together instead of spaced evenly around the core) Power output of about 2.5 watts on 20 and 3.5 watts on 40 are typical when using a 13.8V supply. Above that problems can develop.


73, Steve KD1JV




Re: Up-date for the intermittent noise

Hi, Steve.   Here is the up-date of the problem I have run into.  The problem has been fixed as I followed your instruction and information given and VERY TNX.

I took the steps shown in your e-mail and now my SW-20 and SW-40 are perfectly working.

Steps I have gone through:

(1)   I reduced the output pwr but still I had the problem on both units. 

(2)   So, I carefully monitored the behavior of the SW-20 and SW-40 and found that both units run into the problem when I was using my homebrew straight mini key connected.  I checked the resistance in keying and the resistance in keying at the contact point was not consistent when I pressed down every time and it was as high as 10 ohms in measurement.  So I connected a brass key which has much less resistance in keying at the contact point.  The resistance was measured on this key being almost nil (less than one ohm).  Then the problem was disappeared and fixed.

(3)   I re-adjusted the power level on both units because I do not want the continuous oscillation.  SW-20 for 2 watts and SW-40 for 2.5 watts and these are more than enough power for my operation.  I am very happy now.


TNX, Steve for your help.


P.S. I became curious for the resistance measurement for my straight keys I have.  I measured all.  It varies and the worst one was about 400 ohms.  I knew the contact point for that key was not clean and I will clean up the point sometime later, HI.  Generally the measurement was up to 10 ohms.  But good ones were small amount of resistance like a few ohms or less.  Brass keys show better measurements.  It is worth I remember Hi.


What I learned:

(A)   The resistance at the contact point on the straight keys caused the problem and the keys with less resistance at the contact point are recommended for use.  Now I am using a brass key so I have NO problem, giving me a good feeling and the enjoyment for QRP QSO.  I will do something to reduce this problem for all homebrew keys I have and being in use perhaps brass material for the contact point.

(B)   The contact point needs to be clean up periodically for any keys.  This is a good practice for not only for the SW-40 and SW-20 but also for any rigs. The brass straight key I am using for the two was designed and homebrewed by W9TFC, John and he kindly gave it to me as a gift. 


If you have questions, please e-mail me.

Hiro Hayashi  VE3CGC




In Japanese:



QRPの組み立てキットでSmall Wonder Labs 社(スモール ワンダー ラブス社)よりSW-40(7Mhz)とSW-20(14Mhz)CW QRPトランシーバーを買って、2年程前に組みました。 調子よく作動していましたが、2009年の春、送信中にサイドトーンに時々、ガリガリという雑音が両方とも入るようになり、設計者のKD1JVスティーブにメールを送り、修理方法を尋ねました。 直ぐに返事が帰って来て、考えられる問題と、その対策を教えてもらいました。 日本にもこのトランシーバーを使っている人があり、もし私と同じ問題がある人のため、教えられた対策方法と私のコメントを書きます。 SW-40 & SW-20, 20091108日の私のこのブログのアルバムに参考になる写真と載せました。 写真をクリックすると大きくなりますので、細部まで見られます。




送信中に、サイドトーンの音がゆがめられたら、増幅部が自己発振を起こしていると考えられ、この場合、SWRが高くなる。 最悪の時は、PWR増幅部のツェナーダイオードが壊れるときがある。 対策は出力を下げてやる。(私の場合はR24の可変抵抗器でPWRを下げた。)

または、PWR増幅部のトランジスターのエミッタ-にある抵抗を約10オーム増やすと、R24を最大出力にしても、自己発振がなくなる。 または、この時、PWRを上げるには、PWR増幅部のトランジスターのローパス フィルターのトロイダル コアの巻き線を指で寄せるとPWRが上がる。  普通、13.8Vの電源で、SW-20では2.5W 、SW-40では3.5W出るが、これ以上のPWRになると、PWR増幅部が自己発振を起こす。


以上のメールが届き、まず、2台のSW-40SW-20PWRを下げてみたが、依然としてガリガリ音は時々聞こえた。 この時、私は2台ともPWRを下げ、調整したが、まだ同じ問題が起こっていたので、トランシーバーを動かしている自作の縦ふりミニ キーとデスクトップ用の改造した電源12V DCを疑って見た。 自作縦ふりミニ キーはゼムピンを使ったキーでテスターでキーを押した時、抵抗が一定値ではなく、不安定な値を示した(0オームから10オーム)。  そこで、キーを押した時の手持ちのキーの抵抗値を全部計ってみた。 なんと計測の抵抗値はまちまちで、すこし驚き、一番安定し抵抗値が最も少ないブラス(真鍮)製のキーを使ってみた。 なんと、SW-40SW-20も全く問題なく、作動し、トラブルが解決した。 12V DCの電源で出力をSW-402.5WSW202Wに調整し、動作も全く快適で、以前のように気分良く オン エアーしています。



(A)     私は自作の縦ふりキーを持っていますが、コンタクト ポイントは将来、真鍮か銅に変えるつもりです。 コンタクト ポイントの抵抗は出来るだけ低いほうが問題は起こらない、他のリグにも同じようにいえると考えます。 この2台のQRPトランシーバーに使っている真鍮の縦ふりキー(brass key)はW9TFCジョーンが設計し、自宅にある旋盤で加工し、私に贈り物としていただいたものです。 組み立てや加工も本格的で、使い心地も良く、素晴らしいキーです。

(B)       手持ちのキーのコンタクト ポイントは時々、磨き、汚れていると抵抗が増え、送信機に悪いと考えます。 特に、真空管式送信機では以前に問題が起きたのを記憶しています。 時間のあるときに、コンタクト ポイントは磨くようにしましょう。



はやし ひろ  VE3CGC


This entry was posted in ハム ラジオ、アマチュア無線、ham radio, amateur radio. Bookmark the permalink.

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