Alan SP9BM wrote: I have just acquired a used K2 (includes noise blanker, SSB adapter, internal tuner) which seems to work fine (hears well, and I have made some 4w SSB contacts across the Caribbean islands), except for two little issues:
first, the AF gain control, when turned fully anti-clockwise (7 o'clock position), does not silence the rig, although the control does work in reducing/increasing audio volume over the rest of its range. Volume at the minimum position is about where it would be if a normal rig were at the 9 o'clock position. I pulled the knob off and tried the bare control: no change. second, if the RF gain control is turned or touched at all, the rig immediately goes silent and all the bars in the bar-graph meter are illuminated. Over a period of about three seconds the bars are extinguished, from right to left, and the sound comes back up. The control itself works in limiting RF gain normally, depending on the position to which it is turned. I have another K2 (no noise blanker, no internal tuner) which works entirely normally, so I know this behaviour is unusual. Just wondered whether anyone had any previous experience with these phenomena. My impression is that the rig worked normally in the past, but may not have been used for the last couple of years. Serial No. is 2086. I got it from the original owner; although it had not been built by him, but by one of the for-hire builders. Would replacing the pots help? Grateful for any assistance. --------------------------------- That sounds like a bad RF gain pot or a missed solder connection in the RF Gain or AGC circuits that has become 'noisy'. It could be a bad pot. Touching the pot causing it to act up is a very good clue. I'd look very closely at the connections to the pot first. The audio gain control action you're seeing is also normal for that vintage K2, but there is help available. The original AF gain control design used the pot to "ground" the audio, rather than control the gain. To be more precise, the K2 uses a "push-pull" balanced audio line between stages so the traces on the pc boards won't pick up digital noise (such noise will be 'common mode' on both of the audio circuit traces and is easily suppressed). The pot was across the balanced lines so it "shorted out" the audio when turned fully down. Unfortunately, the pots develop a little bit of resistance at the "minimum' setting over time and so won't turn the gain all the way down any longer! The gain control circuit was redesigned to provide better control that does not require such low resistance at the minimum setting. It also changes the audio control to be logarithmic in nature, like most audio controls. That happened in 2001 and was implemented at about S/N 3000, if I recall correctly. However, Elecraft, as usual, provided instructions for owners of older K2s to upgrade their rigs with the new circuit. The mod requires changing a few traces and adding a couple of wires for the new circuit, but no other parts are needed. Your present AF gain control is probably fine. You'll need a little bit of wire. I used some stranded wire scavenged from a bit of old telephone extension cable. The mod is on the Elecraft web site at: http://www.elecraft.com/Apps/K2_AF_pot/K2_AF_gain_app_note.pdf For an Adobe Acrobat PDF file or http://www.elecraft.com/Apps/K2_AF_pot/K2_AF_gain_app_note.htm For the same thing as a web page. I incorporated that mod in my K2 S/N 1289 and it works like a champ! Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
