[Elecraft] K2 #6425 low/no sensitivity - suggestions?

[David Woolley (E.L)]

TF3KX wrote:

> the IF chain.  An interesting thing is that I hear this signal much weaker
> in the K2 audio when I touch it to the INPUT of IF-amp U12 (MC1350) than to
> its OUTPUT.  In other words, that IF amp does not amplify, but rather
> attenuate the injected signal.  I have re-checked all connections around

That suggests an AGC problem.

I wonder if the AGC mixer is out of balance and you are getting a lot of 
its local oscillator getting into the AGC chain?

I also wonder if this problem relates to two recent reports of people
being unable to find the 7MHz birdie.

> U12, re-flowed the solder, checked T7 and removed the overload diodes D40
> and D41 - with no improvement.
> 
> Problem 2:  Low 8V regulator output, 7.56V.  As a result I could only adjust
> the U2 pin 5 voltage to 3.60V using R1, in Alignment I.  Probing the

There have been a lot of reports of out of spec 8V regulators recently; 
it looks like they've received a bad batch and need to take some quality 
control action.  (Note that the U2 pin5 voltage does not allow for 
component tolerances, so a low value is not necessarily a problem.)

A low U2 pin 5 voltage, on its own, would actually increase the gain. 
On the other hand, they are not operating the IF amplifier at the supply 
voltage given in the data sheet and it may be that reducing supply 
voltage has the opposite affect on the AGC.


> voltages derived from the 8V supply, they are of course 5-10% lower than
> expected.  All DC voltages on U12 are consequently a little low, but
> otherwise not abnormal.
> 
> I plan to get a local replacement for the 8V regulator, just in case.  But
> does anyone have comments on the low sensitivity?  Would the low 8V level be
> a possible cause for this?

As it is out of specification (<7.6V), I believe Elecraft will replace 
it under warranty, and doing so will get it formally recorded as a 
quality failure.

-- 
David Woolley
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