[Elecraft] [K2] Odd AGC problem with K60XV installed

[Bill Coleman]

Don, 

I set the K2 up this weekend to try to reproduce this problem. Once again I opened it up and re-installed the K60XV board. I put a scope probe on P1 pin 6. 

I did not see the AGC failure mode right away, but I did find an intermittent oscillation on pin 6 at about 15 MHz. It would come and go, but no AGC activation.

I removed the board and re-soldered all the components around the LM358. Then I inspected the board and scraped away any residual solder flux in that area of the board between any pins.

I also re-soldered the 4.0 MHz resonator, although I did not hear or detect any relay activation when it failed before.

After that, I put the board back in with the scope probe and ran it on the workbench for a day. I saw no AGC failure mode and no more 15 MHz oscillation.

So, maybe it is fixed? Fingers crossed. 

> On Nov 28, 2020, at 6:19 PM, Don Wilhelm <donwilh at embarqmail.com> wrote:
> 
> Bill,
> 
> It is also a mystery to me that it is intermittent rather than a solid failure.  That might imply that something is breaking into oscillation and overwhelming the VRFDET line.  That is why I suggested you check the component placement and the diode orientation.
> That intermittent condition can also be caused by a soldering problem that takes some time to appear.
> 
> For the non-transverter bands, it is only necessary that the 4.7pF capacitor be in the circuit and that relay K1 be in the reset position.
> 
> If you hear a relay click when this problem starts, then look to the firmware on the K60XV to make sure all its related components are in place - there are not many.  If you do hear the relay click, one thing you can do is to arbitrarily replace the resonator.  While I have never seen an intermittent resonator (dead one = yes), it is one possibility.
> 
> 73,
> Don W3FPR
> 
> On 11/28/2020 5:59 PM, Bill Coleman wrote:
>> Thanks, Don, I’ll check it out when I have a chance next week. I appreciate your suggestions. At the moment, the K60XV and the K2 are about 35 miles apart….
>> 
>> 
> 

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Web: http://boringhamradiopart.blogspot.com
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