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Fault Between TP17 and TP20 (?)


ronmhauser@...
 

Hi everyone.  Can someone help me to understand the signal I should be seeing at TP20 during transmit?  Similarly, the signal I should see on TP17 during tx?

Background:  This is a v5 uBitx, the second that I have built.  I put it together back in June, calibrated and tested, working good.  Built it into a briefcase kit for travel and portable.  I have been using it mostly on JS8 digital mode, mostly on 40m.  The problem I am tracking down is an intermittent "weak" transmit, lack of power on the crossed needle swr meter.  I usually see 10W or a bit more.  The fault condition is 5W or even 1W.  The radio will be running JS8, sending heartbeat transmissions while I'm doing something else on the bench.  I hear the relay click, look over, and notice low power.  I can't force it to happen but it's getting frequent enough that I hooked up a scope probe to try to catch it.  As with all intermittent faults, I'm hoping it will become persistent so I can find it.

Process So Far:  I have a permanent SMA coax hooked up to TP3 with the intent of using this for a low level signal source.  So I started there.  With scope probe on TP3, I made note of the signal level when power output was good.  When I saw low power, I noted that the TP3 signal level was also low.  I believe that this means the PA is good, it's amplifying what it sees, and the fault is upstream.  I then moved the probe to the mike input and looked at the audio signal.  Signal level stays same when low power fault.  I expected this since I use this audio interface with the other radio and it doesn't show the fault.  I then move probe to TP20.  Signal level stays same when low power fault.  I move probe to TP17.  Signal level drops when low power fault.  

At this point I think I have an intermittent issue between TP20 and TP17.  I should mention that I am using the Analog Discovery 2 scope function for this.  I have really run to the edge of my understanding (possibly past the edge!).  I welcome any recommended corrections to technique, or suggestions for next steps.  What should I try next? 

Thanks!

73
-Ron
K0EIA


Evan Hand
 

Ron,
My first suggestion would be to check the mixing transformer T7, and the SSB filter matching transformers T5 and T6.  There may be a loose connection there.

You could also tap on the crystals, as one of them may be starting to fail.

With the WSJT-X set to TUNE you can measure the signal at TP17.  It should be a sinewave at approximately 11mHz.  Tapping on the components could lead you to the culprit.  My vote would be one of the transformers, as you have been moving the rig around during travel, and the toroids are not well secured other than through the soldered leads. At some point, you may want to use hot glue to hold them in place if that is the issue.

The above are just suggestions on my part.  I would suggest you proceed with care.
73
Evan
AC9TU


 

Resolder components in that area. A Plated through hole may be flakey and may be opening up
when heated a bit.

Raj

At 02/09/2020, you wrote:
Hi everyone. Can someone help me to understand the signal I should be seeing at TP20 during transmit? Similarly, the signal I should see on TP17 during tx?

Background: This is a v5 uBitx, the second that I have built. I put it together back in June, calibrated and tested, working good. Built it into a briefcase kit for travel and portable. I have been using it mostly on JS8 digital mode, mostly on 40m. The problem I am tracking down is an intermittent "weak" transmit, lack of power on the crossed needle swr meter. I usually see 10W or a bit more. The fault condition is 5W or even 1W. The radio will be running JS8, sending heartbeat transmissions while I'm doing something else on the bench. I hear the relay click, look over, and notice low power. I can't force it to happen but it's getting frequent enough that I hooked up a scope probe to try to catch it. As with all intermittent faults, I'm hoping it will become persistent so I can find it.

Process So Far: I have a permanent SMA coax hooked up to TP3 with the intent of using this for a low level signal source. So I started there. With scope probe on TP3, I made note of the signal level when power output was good. When I saw low power, I noted that the TP3 signal level was also low. I believe that this means the PA is good, it's amplifying what it sees, and the fault is upstream. I then moved the probe to the mike input and looked at the audio signal. Signal level stays same when low power fault. I expected this since I use this audio interface with the other radio and it doesn't show the fault. I then move probe to TP20. Signal level stays same when low power fault. I move probe to TP17. Signal level drops when low power fault.

At this point I think I have an intermittent issue between TP20 and TP17. I should mention that I am using the Analog Discovery 2 scope function for this. I have really run to the edge of my understanding (possibly past the edge!). I welcome any recommended corrections to technique, or suggestions for next steps. What should I try next?

Thanks!

73
-Ron
K0EIA