MODEL 405 kEYING MOD

The TEN-TEC Model 405 was a 50w (output) linear amplifier, originally designed for use with the Argonaut 505 - WHICH HAD ONLY 3 WATTS OUTPUT POWER.

The amp's T/R relay was Carrier Operated (COR).

When operating SSB, there was really no big problem but in CW mode, the front end of the first dit was always shortened because RF was applied to the amp before its relay could switch.  This hot switching could eventually cause relay failure.

I solved the problem by using the "T" line inside of the Argonaut to key the relay, rather than the carrier itself.  I was using a positive signal to key the amp.  It is a good idea to buffer the "T" line before extracting the signal to use for this purpose.  I will describe that mod elsewhere.

This Mod is NOT what I did back in 1975.  

This mod is to enable the 405 to be driven by other more modern transceivers which have open-collector switching - such as the Ten-Tec Argonaut 6 or FTDM-Duo, or Aerial-51 SKY-SDR.

In this mod I simply re-route the +12v directly to the relay's coil and open up the ground connection and insert an RCA phono jack to be used with the transceiver's AMP KEY line.

This enables the actual keying of the amp but does not address the timing delay necessary to avoid hot switching.  The Argonaut 6 has 17 mS of pre-dit delay before applying RF.  This is fast enough for the 405's relay.  If you use another transceiver, you should adjust its pre-dit delay to about 15 to 25 mS.

CAUTION:  DO NOT OVERDRIVE THE 405.

3 or 4 Watts is all that is required to drive it to full power.

A schematic drawing of the mod as well as the original manual are in the DOWNLOAD section below. 

DOWNLOADS: