K2 won't shut up.

The used Elecraft K2 I got my hands on needed some work and after using it for a while I found out that the K2 wouldn't dead silence when the AF-GAIN was turned completely CCW. When I had my summer holiday and the last day the children didn't want do holiday stuff, I sat down in the shack, took my K2 apart and start looking into it. 

When I bought the K2 I got a printed manual supplied with it, and I checked the revision of the RFBoard (Rev B3R 2002). The revision of the RFBoard and the Manual version corresponded and I didn't want to take the K2 apart for know reason, so I didn't check the Front Panel Board and Control Board revision (a big mistake which will be explained soon).

Getting started

Because the problem seemed to be in the audio section, I looked at the schematics and traced the signals going to and from the AF-GAIN potentiometer on the front panel.  

And at that moment things got confusing. 

 

Step 1: Checking the Front Panel Board 

Note : all schematics are (c) Elecraft.

k2_front_sch_rev_B

Looking at the schematic and the Front Panel Board, CW and the center of the AF-Gain potentiometer are connected. Meaning, we are Rev B.

k2_audio_04

I check the connections, and all seems to work as described in the schematic. 

Step 2 : Checking the Control Board 

k2_control_sch_rev_B

Looking at the schematic, the way the K2 would decrease AF, is by pulling IN2 and IN1 of the LM380N together.
Which might make sense, but looking at Control Board, there is a difference. 

Pin 5 of the connector is connected to Pin 3 of U9  (the AF amp). So this PCB is not a Rev B. 

k2_audio_02

It looks more like a Rev F Control Board which has a different approach on this (VOL 3 is the center of the AF-GAIN potentiometer).

k2_control_sch_rev_F

With VOL3 not connected at all, it  might explain why the AF-GAIN control doesn't kill the audio when turned competly CCW. 

At that point I knew I was on to something, but first I checked the RF Board.

Step 3 : Checking the RF Board 

 k2_rfboard_sch_rev_B

Looking at the schematic and the RF Board, again there is a difference. Pin 3 isn't connected to ground, but connected to Pin 5 of the Control Board. 

k2_audio_05

Which is more like the Rev F Control Board. 

 k2_rfboard_sch_rev_F

Know we know this, it's no surprise that the AF can't be completely turned off. It doesn't seem to me that Elecraft would sell a kit with board revisions which are not 100% compatible with each other. But then again, a small logistics error is easily made. So I won't rule it out. But know that the previous owner only used it for listening and the owner from who he bought it had multiple K2 (the supplied manual has notes about at least 2 K2's), it could also be a previous owner error.

But enough, we now know what the problem is, lets fix it.

How to fix this ?

Because the RF Board and the Control Board of my K2 seem to be post Rev B, the shortest way is to "update" the Rev B Front Panel Board. The big difference between them is the way the AF-GAIN potentiometer is connected. 

k2_front_sch_rev_B

VS

k2_front_sch_rev_C

That can be done by removing the connection between the CW pin and the center pin of the AF GAIN potentiometer with a X-Acto knife or scalpel knife. After I cut it, I used my DVM to check if the connection was really cut (it did).

 k2_audio_01

Then I had to "Un-Ground" Pin 3 of the Front Panel Board connector J1. Again I used a X-Acto knife or scalpel knife to cut the pad free from the GND plane. Again I used my DVM to check if Pin 3 now didn't connect to GND anymore. (it didn't). 

To make the connection between the center pin of the AF GAIN potentiometer, I soldered a short piece of isolated massive wire between the center pin of the AF GAIN potentiometer and Pin 3 of the Front Panel Board connector J1 as shown below.

k2_audio_03

I connected the Front Panel Board by inserting it to the RF Board and after checking everything again, I powered up my K2. 

And it worked. Now a full CCW AF GAIN shut's up the AF. 


Conclusion 

It seems that my K2 has different revision boards and the mix causing the impossibility to dead silence the AF. 
And again its it's key to check, double check and tripple check the manual vs board revisions.  

But I'm happy with the result. I can now dead silence the AF when I'm out in the field and people ask me questions like "what are you doing?".

Update 

When I looked at the Elecraft K2 manuals section at https://elecraft.com/pages/k2-high-performance-transceiver-manuals they pointed to a k2-mods-and-notes section at https://ftp.elecraft.com/K2/Mod%20Notes%20Alerts/ 

There I found the document "Application Note: Alternative Wiring for the K2's AF GAIN Control" by Wayne Burdick, N6KR, with illustrations by Tom Hammond, N0SS dated Oct. 25, 2001

This optional modification greatly reduces the level of noise or other artifacts that might be heard on a small number of K2s when
rotating the AF GAIN potentiometer. Pots judged as "noisy" are, in fact, probably good; the original circuit is simply sensitive to
the pots' inherent non-linearities. In addition to eliminating most pot noise, the new circuit provides nearly dead silence when the
control is fully CCW, while the old circuit allowed a small amount of leak-through.

It seems that it was a know issue on the old Rev B boards that fully CCW doens't dead silence the AF. 

The also explain in the Technical Notes section of that document why they use a balanced audio amplifier.