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
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
