[TenTec] Omni 6 with Acom 2000A amplifier: TX EN mod

[Paul Christensen]

> Has anybody performed the TX EN mod on their Omni 6/6+ that fixes the
qsk keying loop issue with Acom amplifiers?  The instructions for the
Paragon are up on the Acom Yahoo Group files section or viewable at
http://n1eu.com/TXENmod.doc

Barry,

Are you dead-set on using the handshake method (TX OUT/TX EN lines) between 
the 6+ and ACOM?  On the 6/6+, be careful about using the TX OUT/TX EN when 
keying a non Ten Tec amp.   I participated in a discussion here back in the 
late '90s where some folks were advocating use of an RCA "Y" adapter, 
together with a 1N914 steering diode to key an external amp.  That method 
results in hot switching of the trailing RF envelope.  I confirmed this back 
in 1999 with my Alpha and ACOM amps when observing the keyed envelope on a 
scope.  In fact, I burned out the RJ-1 vacuum relay in my 77Dx as a result 
of this configuration, des[pite the fact that the amp should protect against 
hot-switching.  No matter what solution you decide to use, absolutely 
confirm the non-presence of RF envelope hot-switching.

My solution?

On the ACOM and Alpha amps, it's not necessary to invoke a full handshake 
and yet obtain super-fast keying that is not subject to hot-switching.  To 
accomplish this, seek out Ten Tec's "T" voltage bus.  This bus carries the 
proper timing for use with fast external amps.  Next, find the base resistor 
that feeds the Omni's ext. amp relay's switching transistor.  Lift the 
base-bias resistor and connect it to the "T" bus.  Finally, cut the 
switching transistor's collector going to the relay coil and bring the open 
collector directly out to the Omni's ext. amp key jack.  You may need to go 
into the Omni's menu and deactivate the relay since the switching transistor 
is now controlled from the "T' bus.  In this configuration, the jumper is 
palced back between TX OUT and TX EN and I've never seen indication of 
hot-swithcing at any keying speed..

Paul, W9AC