[Elecraft] K3 Intermittently loses gain

[Brian Alsop]

On 11/1/2010 23:36, KF4VTT wrote:
>
> Did this problem happen after you connected the amp?
>
> I ran into a similar problem.  My external Ameritron T/R relay failed and
> held the amp in transmit.  The only way I realized that the amp was stuck in
> transmit besides the diminished receive was the heat I noticed coming off
> the amp.  I could see the heat rising against the backdrop of the wall. :-)
> Thank goodness I did not have drive going to the amp that amount of time.  I
> would be buying a brand new pair of 3-500's.  I removed the external T/R
> relay and started using the internal ability of the K3 to trigger the amp.
>
> Besides checking the ideas from the other guys, check your T/R circuitry for
> your amp.
>
> 73,
> Doug
> KF4VTT
> K3 #1289
> P3 #371

Any relay in the RF chain can exhibit signal drop out on RX but work 
fine on xmit.  The problem is usually caused by corroded/burned 
contacts.  Higher transmit voltage can punch through the corrosion while 
weaker signals cannot.  This is especially true for relays which are 
outdoors.  Relay replacement is the only real solution.  Contact cleaner 
is iffy.  Filing the contacts for some relays destroys the plated 
contact material and can result in even worse performance.

Some designs use "double contacts" to almost eliminate the problem.  Use 
paralleled contacts (dpdt relay) or find a relay with "bifurcated" 
contacts.  These relays have a pole with a "tuning fork" type design. 
Each arm has a separate contact.  If one contact doesn't work, maybe the 
other will.

The same goes for corroded antenna connectors and connections to the 
antenna elements.

The real work is in finding which of the above is the culprit.

If one has a low voltage portable RF source, e.g. the XG1, and a buddy 
in the shack,  one can connect it at various points of the system and 
find where the dropout occurs.

73 de Brian/K3KO




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