[HomeBrew] A Curtis 8044ABM keyer glitch

[Philip Atchley]

Hello all,

Later this week I'm expecting the arrival of a Bencher paddle, and so, 
needed to build a "keyer" to use with it.  A friend had sent me a 
homebrewed Curtis keyer that had been partially dis-assembled, wires 
cut, etc, but it did have the 8044ABM chip in a socket 8-)

So I decided to build another keyer on a fresh board with some "extras" 
that his didn't have.  Things like Sidetone Just in case I use it with a 
homebrewed rig that doesn't have sidetone, input for a straight key, and 
lastly, an opto coupler to sample the RS-232 port for when I want to use 
the computer for contesting (which I enjoy on occasion). Kind of an all 
in one unit.  Except for the case, it is essentially identical to one I 
built, oh, probably 7-8 years or so ago.

Construction went well.  I used the circuit from the 1993 ARRL handbook, 
making the annotated changes to use the 8044ABM version (extra pins and 
functions).  I used good construction habits etc. Everything seemed to 
work, except for one little glitch!  The Dit paddle input worked as it 
was supposed to, producing a string of dits when it was grounded.  The 
Dah input had a problem.  Grounding it would produce a string of 
alternating Dah-dits, just like I had both pins grounded.  BUT THEY 
WEREN'T.  I checked, and rechecked ALL the wiring, pulled the IC out and 
checked the isolation between the inputs (infinity) etc.  But still it 
would Di-dah when I wanted to send a Dah!

Then I remembered that I had a similar problem on the one I built so 
long ago, but couldn't remember the "fix".  So, I grounded the dah input 
to produce constant Dah's, put a .01uF capacitor on a grounded clip lead 
and touched each of the IC pins in turn (bypassing it to ground).  When 
I touched it to either pin 15 (sidetone pitch) or pin 17 sidetone output 
to the transistor's base) the unit straightened up and sent the proper 
"dah's".  Bypassing either of those pins also lowered the tone some, but 
that was acceptable as I have a pitch control on the board.  I soldered 
the .01uF bypass capacitor in permanently from pin 15 to ground and the 
unit works like it is supposed to!

I tested it with the transceiver, the "simple" RS-232 interface works 
well with my computer/transceiver, the hand-key works, the paddle input 
works.  Now I just need the paddle to arrive so that I can get some 
practice in before using it on the air.  I probably haven't used a 
paddle for any extensive work for 6 years or so now!

QUESTION:  Has anyone here ever had a problem with the Curtis 8044ABM 
keyer chip "glitching" like this?  As I said, I used good construction 
practices, but "something" was causing the sidetone to get onto the Dah 
line (no the wires weren't run adjacent to each other).  I'm thinking it 
occurs internally in the chip.


73 de Phil  KO6BB
My DXing tools (RECEIVERS):
    Hammarlund SP-600 (Circa 1952)
    Heathkit Mohican  (Circa 1962)
    Icom IC-751  (Circa 1982)
    Grundig S350 (Circa 2004)

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