[Elecraft] RS232/ U1/ failure

[Geert Jan de Groot]

On 24/02/2023 21:46, elecraft-request at 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