On 24/02/2023 21:46, elecraft-requ...@mailman.qth.net wrote:
When the K3 is in cold room ( below 5 degrees S) my K3/0 go in TERM
mode.
Hints say ''replace'' the U1 on the main KIO3 board..........
Is the U1 SN75C1406 ( G4) ? Or can I use similar SN75 ?
Removed the top cover,and checked voltage , at gold plated test-point
close to the upper edge
of the main board +5.0 vdc. In the lower end of the SN75, test point at
the left side,seen from the front,,
start with approx + 0.35 vdc, and drops slowly to approx -0.50 vdc in 4
to 5 minutes....
My K3 need ''warm up'' also for connecting via PC to K3 utility....
Looking at the schematics, I remember that Elecraft, in history, has
used a number of different "logic to RS232 converter" chips. I think
these changed as devices became obsolete, ....
Both the SN75C1406 you mention in your other mail as well as the LT1039
that is in the K3 schematics (K3, not K3s) use a positive and a negative
voltage. I would suggest checking the VCC8A voltage on your KIO3 board
on pin 1 of your driver chip.
For the negative voltage required, at least on the K3 schematics, there
is an AF oscillator ("approx 1 kHz") and a FET and some diodes to create
a charge pump to create the negative voltage. I would suggest checking
the voltage (seems to be pin 9 for both devices) and I would expect
around -10V or so.
Given that "the problem solves when the radio warms up" my suspicion
would be in the negative voltage generator. Specifically, check the
electrolyt capacitors used in the charge pump.
I am a but surprised to see the SN75C1406 reported in your board,
because the LT1039 (listed in the K3 schematics) still has production
status and seems to be on-stock with Mouser. I did *not* fully verify
that these chips are interchangeable but I suspect a power supply
problem (sketched above) rather than a broken driver, since broken
driver chips do not tend to repair themselves with temperature.
73,
Geert Jan PE1HZG
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