[TenTec] CorsairII obsolescence - MCU copying

[Steve Hunt]

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