Hello Geert !
Many thanks for your reply and info,
Checked the voltage on U1: SN75...... pin 1: + 13.2 vdc, pin 8 : -
2.54 vdc, pin 9:GND, pin 16: + 5 vdc.
Voltage on pin 8 is to low, - 7 vdc is the correct voltage if all ok...
If I can get a new U1 I will change it....
My rig has serial number 2185,( buyed used from Netherland) :-)),,, I
looked at the chematic of K3 from
2010, and it is LT1039ACN.on U1.
So will try to find a new U1 replacment, try that first.....
73' Viggo LA9NEA
Den 25.02.2023 16:23, skrev Geert Jan de Groot:
> 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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