[Amps] SB-220 6 meter conversion parasites

[K1SG at aol.com]

Hi Steve,
 
I agree with Alex...you're not describing parasitic oscillations (i.e.  
oscillations at some random frequency.)  You're describing a high power  
oscillator!  Remember the difference between an oscillator and an amplifier  is 
only a feedback path. If you can measure the frequency of the oscillation,  
and it's on 6 meters, you have output leaking back into the input circuit.   
I'd look at the shielding around the input circuit - I used some PCB stock to 
 add isolation in mine. I'd also look to use separate, shielded relays, as 
in the  2008 article,  to make sure that's not where the feedback is  
occurring.
Also, if you can post some pictures of the top and bottom views of the  
chassis, that may be revealing as well.
 
Steve  Gilbert
K1SG
K1SG at AOL.com


In a message dated 7/17/2015 12:11:39 P.M. Eastern Daylight Time,  
amps-request at contesting.com writes:


Message: 3
Date: Fri, 17 Jul 2015 06:55:55 +0800
From:  Alek Petkovic <vk6apk at bigpond.com>
To: Amplifier Mailing List  <amps at contesting.com>
Subject: Re: [Amps] SB-220 6 meter conversion  parasites
Message-ID: <55A8367B.6080100 at bigpond.com>
Content-Type:  text/plain; charset=ISO-8859-1; format=flowed

Hi Steve.

What  brings you to the conclusion that its a parasitic oscillation?

Have you  measured the frequency?

It sounds more like the amp is simply taking  off, in which case, no  
amount of fiddling with suppressors is going  to help you.

Component size, layout and lead lengths, etc, are critical  factors at 
50MHz. That's where I'd be looking to solve your  problem.

Cheers and good luck,
Alek. VK6APK

On 17/07/2015  4:38 AM, Steve London wrote:
> Finished the SB-220 6 meter conversion,  but it seems to have major 
> parasitic oscillations.
>
> A  little background - I basically followed the 2008 QST article, 
> except  I have the original relay in it. I don't have a source of 
> nichrome  ribbon, so I made parasitic suppressors recommended by 
> another avid  VHF guy - a copper strap U, and a 300 ohm resistor 
> straddling the U.  No difference with or without those parasitic 
> suppressors. The tuning  capacitor is bigger than the one in the 
> article - Surplus Sales is  sold out of the smaller one. However, the 
> tank does resonate on 6  meters.
>
> Just to be sure, I'll describe the symptoms: engage  the T/R relay with 
> no input signal. Grid current, plate current and  RF output ! I can 
> change the level of these from 0 to potential  disaster by adjusting 
> the tuning capacitor. The RF chokes in the grid  circuit have not (yet) 
> fried.
>
> Sources for small  quantities of nichrome ribbon ?
>
> Other suggestions  ?
>
> 73,
> Steve,  N2IC