++++++++++++++++ Date: Tue, 17 Sep 2002 15:48:19 -0700 From: Wayne Burdick Organization: Elecraft To: Elecraft Cc: w1hue at ida.net Subject: [Elecraft] Builder Alert #17: More accurate current metering APPLICABILITY K2 serial numbers 0001-2855 BACKGROUND The K2's current readings (in V/I DISPLAY mode) can be made both more stable and more accurate by the addition of a single capacitor. The capacitor goes from the "ISENSE" line to ground (Control board). This was brought to our attention by Larry, W1HUE/7, who discovered that RF was getting into the ISENSE line, especially on 160 meters. By adding a cap on ISENSE, he dropped his 160-meter transmit-mode current reading by over 20% and reduced the "bouncing" from one value to another. We have since confirmed this on several other K2s. It may eliminate false "HI CUR" indications on the lowest bands that were due to the artificially high readings caused by RF on the line. MAKING THE MODIFICATION You'll need a capacitor of anywhere from .01 to 0.1 uF (10 to 100 nF), ceramic or monolithic. Solder the capacitor across R7 on the Control board, keeping leads short. Make sure the capacitor doesn't contact any adjacent pads or component leads. In addition to this capacitor, we recommend increasing the size of C42 on the Control board from .01 to 0.1 uF (or larger; ceramic or monolithic, not electrolytic). If you have installed a KPA100 option in your K2, you have probably already done this, since an 0.1-uF cap is supplied with the KPA100 kit for use at C42. If you don't have any suitable capacitors in your junk box, send a request to parts at elecraft.com and we'll send you two 0.1-uF caps at no charge. NOTE: Two capacitors will be supplied for this mod in all K2 kits with s/n 2856 and higher, and their installation will be covered in the manual errata sheet. 73, Wayne, N6KR ++++++++++++++++ Date: Wed, 18 Sep 2002 13:05:37 -0700 From: Wayne Burdick Organization: Elecraft To: WA9GQT at aol.com Cc: elecraft at mailman.qth.net Subject: [Elecraft] Re: Builder Alert #17; current metering Hi Rod, The increase in C42 helps a lot with voltage/current reading stability. But as Larry East discovered, a cap across R7 solves the specific problem of inaccurate 160-meter current readings. They could be off by as much as 20% due to RF getting into the ISENSE line. I doubt that this has caused anyone serious trouble, but we feel that it's worth it to add the extra cap. 73, Wayne WA9GQT at aol.com wrote: > > Hi Wayne! > I am confused about this modification. I did the original mod on an > earlier posting about adding a 0.1uFcap between pins 4 & 8 of U3 on the > control bd. This would increase the C42 value. > I guess I need to add a 0.1uF across R7 on the control bd. The other > cap seems to have helped but I guess the one across R7 must help even more. > Please explain. Thanks. > > 73 +++++++++++++++