[Elecraft] RS232/ U1/ failure

[Viggo Magnus Nilsen]

Hello Geert., 

See that LT1039 ACN, has different pins'' lay out'' than
SN75.......Looks for me that LT1039 is for socket installation....? 

73'Viggo   LA9NEA 

Den 25.02.2023 16:23, skrev 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