[Elecraft] SB-221 for K3 need some help.

[Mel Farrer]

Hi all,  

I am from the old school of thinking that sometime I am going to need to key an AC relay in an amplifier and prefer to make a VERY simple interface and generate a set of dry contacts.  That way regardless of the keying gender, you are safe.  It keeps ground current loops out of the picture too.

Mel K6KBE

--- On Sun, 8/31/08, Tom Hammond <n0ss at embarqmail.com> wrote:
From: Tom Hammond <n0ss at embarqmail.com>
Subject: RE: [Elecraft] SB-221 for K3 need some help.
To: microham at microham-usa.com, "'Don Rasmussen'" <wb8yqj at yahoo.com>, elecraft at mailman.qth.net
Date: Sunday, August 31, 2008, 10:10 AM

Hi Joe & Don:

> > I found out this week that K3 is good with Heathkit
> > SB-220 amps, all you need to do is connect a shielded
> > RCA cable between the units - how easy is that? !!!
>
>Are you sure?  Most of those amplifiers had high voltage PTT
>circuits.  If I recall correctly the TR relay is a 120V 3PDT
>unit.  You may need a buffer or soft-key circuit in your
>amplifier to make it safe with the K3.

The T-R output MOSFET in the K3 is rated at 200VDC at around 5A, so it
should not be much of a concern for switching. Additionally, the MOSFET
is diode-protected against reverse spikes from the relay it's keying.

HOWEVER... this having been said, were it me, I'd add an (internal)
Lo-voltage keying interface, like that offered by Harbach, to change
the T-R keying voltage presented by the SB-221 from 120VDC to around
2VDC and maybe a couple mA of current. Pretty economical and easy to
install, and worth the effort to install.

Any T-R keying interface regardless of what you use (OEM or 3rd party)
will not, as Joe has pointed out, change the keying speed of the open-
frame T-R relay. That is a separate problem which CAN be addressed
with replacing the OEM relay with a high-speed power reed relay (drive
input) and a vacuum relay (RF output). But you CAN significantly improve
the transition speed of the relays IF you NEED the feature.

73,

Tom   N0SS


73,

Tom  N0SS

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