[Elecraft] K2 receiver going temporarily deaf

[Don Wilhelm]

Mike,

I have been pouring over the schematic and can see very few components that
would create a failure as you describe - and be OK when the antenna shield
is disconnected.

If your antenna/balun presents a DC path to ground, I can see one possible
failure mechanism.  OTOH, if your antenna has a DC open circuit, you can
ignore all I say here!
The ONLY component I can see that would cause that behavior is C114 - if it
is breaking down (temporarily shorting) it could drag the signal to near
ground and may appear to work just fine if the antenna shield is removed.
If this capacitor is shorted, the antenna could provide a low resistance DC
path for the voltage at the anode of D4 and block its conduction path.

If you agree with my analysis, try removing C114 and substituting another
.01 capacitor.
The other 'test' might be to insert a capacitor in series with your antenna
feedline center conductor - if that 'cures' the problem (still works with
the antenna connector shield connected) you will know that you have a DC
path out of the K2 - and a shorted C114 is the only DC path that I can find.

Other things to be suspicious of are the diodes in the TR switch, but I fail
to understand how removing a ground path from the antenna would 'cure' the
problem if the diodes were at fault.

73,
Don W3FPR

----- Original Message ----- 

> "My troubleshooting technique on this scenario, seeing as he says it only
> happens when the radio has warmed up, may be temperature related."
>
> Actually I didn't say it only happened when the radio has warmed up. Of
> course the K2 was on and had been for a while but I think that is just
> circumstantial.
>
> I really don't think it's a temp related problem.  When the receiver goes
> 'deaf' it does so very suddenly and and just as suddenly 'clears' itself
> when it begins working again. It just doesn't have the 'feel' of a
component
> degrading due to heat over time (or vice versa).
>
> Once a component quits due to heat (or cold) it quits. While it can indeed
> recover due to the absense of heat (or cold) in the interim it wouldnt
> explain how in the interim the K2 receives just fine if I disconnect the
> coax shield AND that the K2 will suddenly start receiving again while it
was
> still warm (as it did the first time).
>
> Someone mentioned a possible problem with the latching relays. This is
more
> in tune with the behavior I'm experiencing... it 'feels' more something
like
> that, i.e. a 'set/reset' type of condition. That's why I first thought of
F1
> (until I looked at the schematic)
>
> But if it were a latching relay on the RF board wouldn't it be limited to
> one (or two) bands? When the receiver is 'deaf' it's deaf on ALL bands.
> Well again, except when I disconnect the (ground) shield. Then it receives
> just fine on all bands.
>
> I just pulled the Dipole down to check the center balun/insulator. That's
> the only piece of the puzzle I don't feel confident saying is 'solid'.
>
> Mike K5PU
>