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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com