++++++++++++++++ From: "Sherman Banks" To: , Subject: RE: [Elecraft] K2/100: PA Oscillation Problem: Help! Date: Sun, 25 May 2003 07:12:15 -0400 I had the same problem with my K2/100 PA and Gary sent me some pictures of a grounding strap which cured my problem. I will forward the email and attachment to you directly. -----Original Message----- From: elecraft-admin at mailman.qth.net [mailto:elecraft-admin at mailman.qth.net]On Behalf Of JimWMiller at aol.com Sent: Saturday, May 24, 2003 9:47 PM To: elecraft at mailman.qth.net Subject: [Elecraft] K2/100: PA Oscillation Problem: Help! 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 shooting all day. Please review the following and let me know if you can help. 1. Occurs on 20, 15, 12 and 10 meters i.e. higher frequencies. 2. Oscillation causes current surge (blows fuses) and has high amplitude with varying frequency. 3. This one is interesting! Occurs in SSB when talking/huming loudly for several seconds. Also sometimes occurs when the ATU is in tune. In other words, only modulated signals cause the oscillation to start. Oddly enough works 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 occurs when power is between 11 and 100 watts. Seems to occur just as often at 11 watts as at 100 watts. 5. It's difficult to trouble shoot since oscillation occurs ONLY when PA is in cabinet. If removed or ½ removed but still connected by cables, all works fine! 6. Often get "PA HOT" message even though PA is cool. (What's this one about??) 7. Once an "info 80" message was seen. I'm hoping that someone dealt with a similar problem. Thanks in advance & 73, Jim WA2UMP K2/100:2679 +++++++++++++++ Date: Sun, 25 May 2003 07:23:09 -0700 From: Vic Rosenthal Organization: Transparent Software To: Lee Buller Cc: Elecraft Reflector Subject: Re: [Elecraft] Grounding Strap for KPA 100 Lee Buller wrote: > The thread on the KPA 100 going into oscillation is an interesting > one. The cure is a grounding strap, but I have not seen any > information on this modification. Has it been published? Does this > happen will al KPA 100s? Can the modification be posted? Lee - K0WA I'm sure it doesn't happen on all of them. I didn't have this problem. Then what is there about some of them that causes it? How does the strap fix it? I remember an oscillation problem caused by locating the RF choke that's across the output connector too close to the slot in the shield, but that happened only on one band (I thought) and didn't occur as soon as the 11 watt point was passed. Come on, Elecraft builders want to know EVERYTHING! Vic K2VCO ++++++++++++++ From: "John, KI6WX" To: "Elecraft Reflector" Subject: Re: [Elecraft] Grounding Strap for KPA 100 Date: Sun, 25 May 2003 22:07:41 -0700 The grounding strap is connected between one of the SO-239 connector screws and the nearest screw on the removable shield. Gary has some fairly elaborate straps that connects between these two points. I just used two ring lug connectors, one on each screw, and connected them with a #14 copper wire. You want to use a larger wire to minimize the inductance. However, something like solder braid should also work well. The oscillation seems to occur most often on 40 meters and is due to RF feedback between the amp and the K2 when the KPA100 is attached directly to the K2. If removing the KPA100 from the K2 eliminates this oscillation, then the grounding strap will help. It certainly won't hurt to add it on other KPA100's that don't show this problem. I have a picture I can directly mail to anyone who is interested in this change. -John KI6WX ----- Original Message ----- From: "Lee Buller" To: "Elecraft Reflector" Sent: Sunday, May 25, 2003 5:23 AM Subject: [Elecraft] Grounding Strap for KPA 100 > The thread on the KPA 100 going into oscillation is an interesting one. The cure is a grounding strap, but I have not seen any information on this modification. Has it been published? Does this happen will al KPA 100s? Can the modification be posted? Lee - K0WA > ++++++++++++++ From: Marteinn Sverrisson Date: Mon, 26 May 2003 08:28:24 +0000 To: elecraft at mailman.qth.net Subject: Re: [Elecraft] Grounding Strap for KPA 100 Hi I cured the KPA100 oscillation by inserting the 270 Ohm 2W metalfilm resistor described in WI6WX article on how to put KPA100 in an EC2 enclosure see John's article: http://home.pacbell.net/johngreb/KI6WX.html The resistor is connected from the junction of C67,C82,R21 to the ground of J1( the coax ground). This mod cured all unstability in my KPA100, I had tried ground straps but it did not work. 73 de Matti, TF3MA +++++++++++++++ From: JimWMiller at aol.com Date: Mon, 26 May 2003 11:12:10 EDT Subject: Re: [Elecraft] Grounding Strap for KPA 100 To: tf3ma at raunvis.hi.is, elecraft at mailman.qth.net Reducing Zin to prevent the oscillation makes sense since the RF out is finding its way back to the input. KK7P recommends this mod when the KPA100 is in a separate enclosure as a proactive means of preventing oscillations. I'm going to try both the ground strap and the 270 ohm resistor to fix my oscillation problem. If your KPA100 is blowing fusing from time to time, it may be breaking into oscillation. 73, Jim WA2UMP K2:2679 ---- In a message dated 5/26/2003 4:32:49 AM Eastern Daylight Time, tf3ma at raunvis.hi.is writes: > I cured the KPA100 oscillation by inserting the 270 Ohm 2W metalfilm > resistor described in WI6WX article on how to put KPA100 in an EC2 enclosure > see John's article: -- snip -- ++++++++++++++++ From: JimWMiller at aol.com Date: Sat, 31 May 2003 23:12:08 EDT Subject: Re: [Elecraft] K2/100: PA Oscillation Problem: !!! FIXED !!! To: elecraft at mailman.qth.net Cc: tfonstad at vbe.com, gary at elecraft.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 at 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 ++++++++++++++++