Thanks Paul. This seems like a fundamentally important issue that
probably gets glossed over. Your solution seems like a fundamentally
sound alternate solution.
From Acom's findings, it seems that the TX EN is the problematic
circuit of the keying loop. TX OUT is simply being generated by TX
ACK, which I believe should be fine.
Acom's diagnosis (paraphrased from the doc with reference designators
changed to match Omni 6+ control board schematic p4-17): "The problem
in the original wiring is that when you pull the TX_EN input to the
ground, the transceiver unconditionally changes to transmit mode,
regardless of having pressed PTT or CW KEY at all. Also, you cannot
return to RX after releasing the PTT or KEY until the TX_EN is
grounded. . . . . Another problem exists in the original schematic.
The base of Q14 is floating during RX mode, so any (even
insignificant) residual emitter-collector current of Q15, as well as
any reverse collector-base current of Q14, both would lead to
uncontrolled transmit mode of the transceiver."
73, Barry N1EU
On Tue, Oct 18, 2011 at 12:33 PM, Paul Christensen <w9ac@arrl.net> wrote:
>> 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
>
>
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>
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