Final episode of the story (I think):
I've produced a pcb layout of the PIC-based MPU replacement, and had a
couple of boards made to prove the design; I used ExpressPCB in the USA
- I was impressed by the speed and quality of their work. The boards
work fine! Full details are on my re-worked web page:
http://www.karinya.net/g3txq/corsair_display/
If anyone ever wanted to use ExpressPCB to produce a board, I can make
the .pcb file available.
Once finally installed in the Corsair I noticed a small amount of
digital noise breakthrough on the audio; that was with the RF Gain right
back and the audio gain fully on. I cured it by changing choke L1 from
its original design - 12 turns on a ft50-43 core - to an air-cored
bobbin just as Ten Tec used as a "retro-fit" in the original. You can
either make use of the original TT part and mount it on the pcb, or wind
a new one which is what I did. It removed any trace of digital noise on
the audio - obviously TT knew what they were doing! In retrospect the
0.5A draw through the choke was probably enough to saturate the FT50-43
core and reduce its effective inductance.
The only other "feature" I've spotted is that in the original version of
the Keyer firmware I omitted to set the initial state of the output
keying line after switch-on; in about 20% of the chips I've tried, the
keying line starts up as "active"; a quick Dah or Dit paddle closure
resets it OK. The hex object code now available on the web page has
fixed the problem.
73,
Steve G3TXQ
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