[Johnson] Invader Problems

John Lawson jpl15 at panix.com
Tue Sep 19 10:59:23 EDT 2006



   And now, the Serious Answer:

On Tue, 19 Sep 2006, David C. Hallam wrote:

> I have just moved my Invader 2000 to the beach to start trouble shooting on
> a problem that has me puzzled.
>
> When I turn the rig on, I have to leave it on standby for 20/30 minutes
> before I can get any output.  Then after 2 or 3 hours of operation, the
> output goes away.  If I let it sit overnight, I can get output again after
> the 20/30 minute wait.


    1.  The previous response is dead-on:  much more information is needed, 
and some careful analysis by you will provide that.  What are the 
conditions and parameters, what are the voltages, currents, and output 
power levels (in volts, amps, watts..) before, during, and after these 
episodes?  Have you observed the RF on a monitor scope? Is it 'clean'?


    2. Absent the above info - this sounds like a Cranky Capacitor to me... 
a coupling or bypass cap that has gone south and is changing radically 
under load and temperature conditions...  In my case, although the 
circuits are not that similar - I have had problems somewhat like this, 
traced to faulty (and incorrectly replaced [by me!]) capacitors in the 
plate tuning and tank circuits of my Valiant.  In one case somewhat like 
yours, operating conditions changed radically during a QSO, the a few 
transmissions later the rig became unstable and untunable - the plate 
current 'ran away' and stayed there.  I had previously replaced a 'stack' 
of aging micas with newer ones, save that I wrapped the leads together 
near the bodies, and extended just one lead of the three to the rest of 
the circuit - rather than twisting them all together in a bundle (the way 
Johnson engineers did).  The currents through those capacitors heated, and 
eventually melted, that single lead, like a fuse.


    3.  See "1." above.


Cheers

John  KB6SCO
Valiant, Ranger, Elmac AF-67
R-390, R-390A, R-388





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