Allen,
The MC68705 is a microprocessor and not an EPROM. It does two
functions. It counts the frequency and along with some other parts,
displays it. And, as you said, it is also a memory keyer. The MC68705 is
prone to failure. A while ago, Steve Hunt, G3TXQ, designed a replacement
board using two PIC devices. One is the display and the other one is the
keyer. I kitted several of these boards and sold them to Corsair II owners.
If you ask, you might still find someone on this list who is willing to part
with their kit. The kit is a drop in replacement for the TenTec board
except you do not have any memories in the keyer. If you want to see what
one of these looks like, you can peruse Steve Hunt's web page.
<http://karinya.net/g3txq/corsair_display/
Jerry W5JH
"building something without experimenting is just solder practice"
----- Original Message -----
From: "A R" <raf_3@msn.com>
To: "tentec" <TenTec@contesting.com>
Sent: Friday, February 14, 2014 6:43 PM
Subject: [TenTec] Corsair II UV EPROM failure
My recently acquired Corsair II has a failed logic board UV eprom
(MC68705P3S) that prevents me from operating CW. The eprom failure is the
keyer "dit" input stuck close enough to a "low", that about 30 minutes
after power-up, the eprom temp reaches 50C, and the "dit" input is low
enough that all I have is a continuous, uninterruptable transmitted string
of dits. Unfortunately, the architecture doesn't have provision to defeat
the keyer function in CW mode. The eprom temp reaches the 50C temp after
about 30 minutes, with an ambient temp of 22C.
I've talked to Garry Green at TenTec, but they don't have any proms for
this ancient Corsair II. But, the unprogrammed MC68705P3S's are available
elsewhere, and Garry agreed to search the TenTec operation to see if they
have the programming fixture and the Corsair II program. I'm hoping that
Garry can find the fixture and the program. But, I'm not very optimistic
he'll be successful.
So, my options are to either find a "surplus" programmed CORSAIR II
MC68705P3S (anybody have a spare or surplus one?), or modify the Logic
Board enclosure to provide forced air cooling in an attempt to keep the
processor temp below the critical 50C.......and hope the processor "dit"
input doesn't further deteriorate.
Does anyone have a spare or "surplus" programmed processor for the CORSAIR
II?
Thoughts on cooling/controlling the temp of the processor? Forced air may
get the job done (maybe in combination with a dip heatsink bonded onto the
processor). But I recall (from a former life) that ss heat exchangers for
dips may be a solution that's not as messy as shoe-horning a 25mm pc fan
onto the logic board enclosure, but would necessarily contribute a slight
? temp rise in the CORSAIR's interior.
Any help or suggestions welcome.
73,
Allen---W7GIF
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