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
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