[Elecraft] KX1 RX and TX Problem

[Matt Stumbo]

As Don indicated, U7 was the problem. After I replaced U7 my KX1 is able to hear again. And after I replaced Q6 I am able to transmit. I’m happy I can use my KX1 again!

Thanks and 73!
Matt KS6PKY

> On Dec 28, 2021, at 10:19, Matt Stumbo <ks6pky at icloud.com> wrote:
> 
> Thank you Don,
> 
> I ordered a TPS76427 and a couple spares from Digi-Key and will replace U7. 
> 
> Any thoughts on what may have caused U7 to fail? ESD comes to mind, though I used an ESD mat and wrist strap anytime I worked on the KX1. Part infant mortality also comes to mind. My primary concern is the U7 failure is a symptom of some other failure I have yet to discover. My worst fear is I put a new U7 in and it fails the same way. Have you seen U7 fail in other KX1s?
> 
> I ordered a replacement 2SC2166 MOSFET for Q6 from RFParts. I’ll replace it once I repair the U7 issue.
> 
> 73!
> Matt KS6PKY
> 
>> On Dec 27, 2021, at 11:20, Don Wilhelm <don at w3fpr.com> wrote:
>> Matt,
>> 
>> You likely have a problem with U7.  At least, that is the thing that you need to resolve first.
>> 
>> Sorry about U6, it usually makes a loud pop when the base and collector are shorted.  You will have to replace it before you get any power out.
>> 
>> 73,
>> Don W3FPR
>> 
>> 
>>> On 12/25/2021 2:51 AM, Matt Stumbo via Elecraft wrote:
>>> I had the pleasure of recently assembling a KX1 and KXAT1 kit. I had the KX1 working on both 20m and 40m, which last worked on Sunday. Unfortunately I now have the misfortune of debugging an issue with it.
>>> 
>>> Today I found the RX and TX no longer work. The KX1 powers on, the menus behave as I expect, the AF gain changes the audio and Filter knob changes the audio, tuning results in the ATU relays clicking but without any power out the tuner doesn’t find a solution and displays 0 W and 9.9 SWR. The RF Gain knob doesn’t have any noticeable affect.
>>> 
>>> I’ve done the following:
>>> * Inspected the KX1 for poor solder joints and shorts, found none
>>> * Verified R4 is fully clockwise
>>> * Verified Q6 collector is isolated from ground
>>> * Removed the KXAT1 and jumped J7 pin 1 to pin 3
>>> * Verified TX is 0 W with an external wattmeter
>>> * Completed the voltage tables
>>> 
>>> The voltage tables indicated the following issues:
>>> * U1 pin 14, expected 0V measured 5V
>>> * Most U2 pins do not measure expected. I attribute this to U7 2.7V power output measuring 1V.
>>> * U7 pin 4, expected 1.17V measured 0V
>>> * U7 pin 5, expected 2.69V measured 1.0V
>>> * Q5 Emitter on Tune expected 2.24V measured 1.48V
>>> * All other measurements were in family with the expected values
>>> 
>>> The issue that sticks out to me the most is the U7 pin 5 output is too low. This results in the DDS U2 not receiving the power it needs to operate, hence most of the U2 voltage measurements do not match predicts. Is there any reason this 2.7V output could be low besides U7 failure? Are there other areas I should probe not included in the voltage table? The U7 input voltage is good, 5V. Do I need to replace U7?
>>> 
>>> To make matters worse, while I was measuring the Q6 collector voltage, I briefly shorted the base to the collector with the DMM probe. I have not noticed any new issues with the KX1. The KX1 still turns on, the buttons work, etc. Are there parts I should check to make sure I didn’t break or stress anything?