[Elecraft] K2/100: PA Oscillation Problem: !!! FIXED !!!

[JimWMiller@aol.com]

My K2/100 is now working well again and I'm enjoying the 100 watt capability=
.=20
=20

My problem with the PA oscillation on the higher bands has been solved. This=
=20
oscillation only occurs in some PAs depending physical construction variatio=
ns=20
and other factors.  If your k2/100 is blowing fuses, then you may have an=20
oscillation but relax and read on... it's very fixable!!!

Here's what was done to solve the problem:
1. Added a small copper plate over the slot by the SO-239 connector. This=20
plate is also grounded at the connector.  A picture of the plate was supplie=
d by=20
Gary (tnx Gray).  Basically this plate increased shielding by covering that=20
hole and improved grounding.  I would be happy to email JPGs of this mod or=20
contact Gary at Elecraft.  Incidentally covering the hole was necessary and=20=
simply=20
adding a jumper did completely eliminate the problem.  I used a creative=20
variation what was recommended (connected to 2 ground screws).
2. Moved RFC-10 as far away from the shield as possible.
3. Inspected and cleaned up *all* grounds. Make sure all paint is removed=20
from ground contacts. That metal must be clean, shiny and free from paint.=20
4. Added the 270 ohm input resistor as recommend by KK7P and WI6WX when the=20
PA is configured in the EC2 enclosure.=20
      http://home.pacbell.net/johngreb/KI6WX.html
This will proactively reduce Zin and make an oscillation less likely.
5. Inspected bandpass filters. Made sure torroids were not touching each=20
other. (Not sure if this did anything.)
-----------
Well that worked. Take your time and do it neatly.  My first quick and dirty=
=20
attempts did not solve the problem. That hole must be well covered and the=20
shield well grounded.  All grounds must be clean.  It's that simple.
-----------
Now I'm looking forward to getting the K2DSP !!!!
73, Jim WA2UMP K2/100:2679
-_-_-_-_-_-_


In a message dated 5/24/2003 9:49:13 PM Eastern Daylight Time,=20
JimWMiller@aol.com writes:

> My newly constructed 100-watt PA has been going into oscillation!
> Has anyone else had this problem??  This is a tough one! I've been trouble=
=20
> shooting all day. Please review the following and let me know if you can=20
> help.
>=20
> 1.  Occurs on 20, 15, 12 and 10 meters i.e. higher frequencies.=20
> 2.  Oscillation causes current surge (blows fuses) and has high amplitude=20
> with varying frequency.
> 3.  This one is interesting! Occurs in SSB when talking/huming loudly for=20
> several seconds.  Also sometimes occurs when the ATU is in tune. In other=20
> words,=20
> only modulated signals cause the oscillation to start.  Oddly enough works=
=20
> well in CW (no oscillation) i.e. at least not manifestation of the problem=
.
> 4.  NO oscillation in when PA is off i.e. Power <11 watts. Oscillation=20
> occurs when power is between 11 and 100 watts. Seems to occur just as ofte=
n=20
> at 11=20
> watts as at 100 watts.
> 5.  It's difficult to trouble shoot since oscillation occurs ONLY when PA=20=
is=20
>=20
> in cabinet. If removed or =BD removed but still connected by cables, all w=
orks=20
>=20
> fine!
> 6.  Often get "PA HOT" message even though PA is cool. (What's this one=20
> about??) =20
> 7.  Once an "info 80" message was seen.
>=20
> I'm hoping that someone dealt with a similar problem.=20
> Thanks in advance &73,
> Jim WA2UMP K2/100:2679
>=20
>=20



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