[Ham-Mac] Need help with PK-88 TNC

[Carl Miller]

Hello All,

I am new to the list and I already need help with a
problem.

I have a PK-88 TNC that I am trying to use with my old
Macintosh Classic II. 

The problem I am having is that the PK-88 will not
accept commands from the terminal. That is, until I
touch a probe or other metallic object to pin 12 of
IC-4, a 75189 quad line receiver. Once I power the
thing up and touch a metallic probe to pin 12 the
thing works great, I can send it commands, I can do
the loopback test with no problem. But once I remove
my probe, the thing goes deaf again. I also noticed
that if I remove the probe, then touch pin 12 again it
gives me the cmd: prompt. Then I can communicate with
the tnc again.
According to the Texas Instruments data sheet on this
particular chip, pin 12 is the threshold control pin.
But, since I am not an EE, I don't have a clue what
that means. The funny thing about pin 12 is that it
isn't connected to anything from what I can tell. It
certainly isn't noted on the schematic. 

What gives here? Is this thing posessed or what?

I would certainly appreciate any help or advice.
Thanks in advance,
Carl KA5KAB

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