[Elecraft] K2 Muting

[W3FPR - Don Wilhelm]

Folks,

I had previously suggested a 'wire-OR' off pin 17 of the MCU to accomplish 
external muting for the K2.
Jack Brindle pointed out that CMOS logic devices can be overloaded with such 
a 'wire-OR' configuration without a current limiting resistor.  I have used 
many 'wire-OR' circuits with TTL logic, and since CMOS is "TTL compatible" 
(with a 5 volt Vdd supply), I erroneously extended the wire-OR solution to 
CMOS logic (in error).

It turns out that in this particular case, a current limiting resistor is 
easy to add, and in fact, for this particular circuit, Wayne Burdick has 
previously suggested adding a series resistor of about 100k to slow down the 
K2 mute timing in order to reduce the audio pops that some have seen with 
the KDSP2 added.

So if you wish to mute the K2 from an external source, you may still use the 
'wire-OR' technique of attaching an open collector transistor to the gates 
of Q6 and Q7 on the Control Board (the 'hot' end of C23 is a good attachment 
point), but in addition, one should cut the trace from U6 pin 17 on the 
solder side of the Control Board and add a resistor between U6 pin 17 and 
C23.  A value as low as 1k ohms will limit the current from the MCU to 5 ma 
or less (a safe value), while a value of 100k has been suggested by Wayne 
Burdick as a 'mute slow-down' for reducing the KDSP2 pop.  So take your pick 
of values between these two to protect the CMOS MCU output from being 
overloaded when in its logic high state.

Sorry for any confusion that I may have caused.

73,
Don W3FPR