[Elecraft] [K1] Internal Noise Bursts every 12 seconds or so

[Bob]

Thanks for the ideas, Mike!

Well I can rule out the Filter Board, as the noise is still there when the
Filter Board is removed.

I hoped I could remove the CPU to see if there was an issue there, but with
the CPU gone, the audio is eliminated (probably need to pull something high
or low to get the audio back).

In looking over the Front Panel Schematic quickly, my attention was drawn
to the 6 volt power supply generated on the Front Panel ("6A") and
distributed to several high gain parts of the RF board.  Any noise on 6A is
going to be an issue.  I also noted that there is no traditional bulk
storage cap on the input of the 78L06 regulator, U4, typically .3 to 10 uF,
and while there are a few decoupling caps on the output, the closest one to
U4 is several inches away C5.  The output cap on a 78L series regulator is
typically 0.1uF mounted as close to possible.  Finally the original
specified regulator has been obsoleted and a different unit was substituted.

I've seen issues with TO92 cased 78L series regulators breaking into some
strange oscillations when the input or output caps are not sufficient.  So
I'm going to check that out too.

For the moment I will ignore it and move forward to complete the TX section.

73, Bob, WB4SON



On Mon, Dec 28, 2015 at 11:46 PM, Mike Morrow <kk5f at earthlink.net> wrote:

> Bob wrote:
>
> > Even though the second CPU on the filter board is mostly inactive, no
> > telling what internal loops are going on that might result in it turning
> > that 4 MHz resonator into a periodic signal.
>
> In addition to the relays on the filter board itself, the PIC on the KFL1
> board controls RF board relay K1 (the RF Attenuate ATTN relay).  But that
> info is not likely to help isolate the problem you report.
>
> I own K1 #175 that I built in November 2000.  It was unfortunately a
> lightning strike victim a few months ago, ending almost 15 years of
> service.  A most excellent radio for all thoses years.
>
> I have never before read a report of a problem similar to what you
> describe, except for one in 2001 in which a burst of internal noise was
> generated on 20m after going from transmit to receive.  This was caused by
> the PIC on the KNB1 noise blanker PCB, and1 was corrected by adding a 0.01
> uF capacitor on the PIC's Vdd line to ground.  It's a long shot, but you
> may want to check the installation of the similar capacitors on the Display
> PCB (C4) and KFL1-2 (C27).
>
> Mike / KK5F
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