[TenTec] Corsair II keyer problem

[peter barville]

At the risk of boring those not remotely interested, here is a summary
of my findings re the (as yet unsolved) problems with the Corsair's
internal keyer.

The problem manifests itself in one of two ways, both of which are
intermittent.  It will either suddenly stop responding to the dash
contact of the paddle, and therefore stop sending any dashes, or it will
suddenly start to send an uncontrollable string of dashes. Both can be
cured by de-powering the rig, and then immediately turning it back on.

Even if the rig has been switched on for a matter of hours, the keyer
will always perform normally when first used, and will only exhibit one
of its fault conditions after a (sometimes short) period of use.

During the periods of 'non-dash activity' pin 19 of the MC68705P3 *is*
being grounded by the dash contact of the paddle normally.  Any pre-
recorded message in the keyer's memory is re-played normally (including
dashes).

When the keyer decides to send uncontrollable dashes, pin 19 is sitting
at almost zero volts (instead of the 5 volts which should be present
without the dash contact of the paddle being made).  Unplugging the
3-way dot/ground/dash connector on the board makes no difference.

I have put in a new (tantalum) 2.2uF de-coupling capacitor on the chip's
re-set line (pin 28), and new 470pF de-coupling cap on the dash keying
line.  I have carefully re-seated the chip.

Dots and digital read-out seem fine.

Nothing I've tried has made any difference to the problem, and I am
beginning to think that the chip itself must be at fault.  At the same
time, I am surprised that any chip fault is a) intermittent, and b)
manifesting itself in two different ways.  Why does the rig never power-
up with the keyer fault present?

Does this sound like a chip fault to you?
-- 
72, peter g3xjs

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