[Heathkit] question on keying a SB-221

[William Houghton]

Yes, the Harbach kit will allow just about any rig to key the amp. Advantages 
are that their kit is less than $30.

Another solution is to "roll your own" interface. 

Some other vendors make an external interface box. I think Ameritron made one. I 
also think The Heathkit Shop had one. Don't know if they still do. An advantage 
of an external box is that you can move it to a different amp. Some of the 
external boxes that I have seen had connections and switches so that you could 
easily switch between different amps and different radios. 





________________________________
From: rbig <rbig at cox.net>
To: heathkit at mailman.qth.net
Sent: Wed, April 17, 2013 10:48:18 PM
Subject: [Heathkit] question on keying a SB-221

Thanks, Bill.  Great answer.    

I appreciate the response.  

It seems that using the Harbach mod will make any rig key it OK



TS-430S: no, not directly.

FT-100EE, yes.

Here is the reason. The Heathkit amplifiers, like many of their era, used a 
relay with a coil voltage of about 120VDC.  Unkeyed, the relay input of the amp 
will sit at this 120V. To key the amp you switch this voltage to ground. 


Chances are that your nice solid state TS-430 is not designed to switch 120V to 
ground.

In the old days, rigs typically used a mechanical relay to key amplifiers. In 
some radios one of the contacts was connected to chassis ground. In some rigs, 
both contacts "floated". Either of these relay methods can be used with the 
SB-2xx amps. The FT-101 series has a relay with one contact connected to the 
chassis if I recall correctly.

A common method to interface the sold state rigs to these amplifiers is to 
install an interface circuit so that your radio sees a low voltage low current 
keying signal and the interface switches the 120V signal. The interface can be 
as simple as installing a low voltage relay powered with a small transformer. 
The Harbach SoftKey circuit performs this interface (I don't know if theirs uses 
a relay). 

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