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@AOL.com
In a message dated 7/17/2015 12:11:39 P.M. Eastern Daylight Time,
amps-request@contesting.com writes:
Message: 3
Date: Fri, 17 Jul 2015 06:55:55 +0800
From: Alek Petkovic <vk6apk@bigpond.com>
To: Amplifier Mailing List <amps@contesting.com>
Subject: Re: [Amps] SB-220 6 meter conversion parasites
Message-ID: <55A8367B.6080100@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
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