[Elecraft] KPA3 Rev B not output

[George Fremin III]

TLDR; the power amp is now working and putting out 100 watts.

I thought I would follow up on this post.

After I swapped a different KPA3 into the radio that was not working - that radio worked.  That proved to me that it was likely the KPA3 that was the issue.   I had inspected it and cleaned connectors and reseted several times and it never did work in the original radio.

In order to do some further tests I decided to put the defective KPA3 in the donor radio.
I powered the radio up and it was putting out 100 watts.  I did the power gain calibration and it was working.

Since this was the radio I wanted to put into service on my remote station I swapped out the K3 that was on the remote with the now working KPA3.   This radio gets used a fair amount and during the WPX KM5X made about 400+ contacts on 20m on the remote.  And it seemed to be working fine. Early Sunday morning the 100 watt amp stopped working again.  I swapped in another K3 and got the remote working.  Sunday night after the contest I decided to look at the non-working KPA3 again.  I pulled it out and got out the magnifier light and looked it over well - I decided to reflow some solder in a few places - resistors that folks had talked about and all the pins on the connectors some of which did not look great.  One pad on one of the red coils also was lacking in solder a bit.  But nothing was glaring but I decided that I might have a cold solder joint somewhere. 

I continued to look at things and it was then that I noticed that one of the tabs on one of the power transistors had no solder on top of it - it was shiny gold - and the solder between it and the board did not look all that well flowed.  This tab was hard to see as several leaded components hide it from easy view.  I worked on that joint and soldered it well.  I am kinda thinking that might have been the issue all along.  Put it back in the radio and it works fine. Calibrated the gain and put it back on the remote.  Remote users are saying things seem to be working fine now.

This KPA3 was in a K3 that belongs to K5OT that he has left here for us to use in contests etc.
It is one of two K3 radios used in all contests here for 10 years or more at this point.  As I think back on things this K3 sometimes had some issues with ALC or power out or something.  Never total loss but sometimes it seemed it was not working like the other.   So maybe that was this joint getting worse over time. 

Of course I did heat up other things so - I can not prove it but that tab was very very poorly soldered and the more likely issue. 



> On May 20, 2023, at 9:23 AM, George Fremin III <geoiii at kkn.net> wrote:
> 
> I posted here last fall when one of our K3 radios stopped putting put any power during the last few hours of the CQ WW CW contest.  For many reasons I am finally getting back to looking at this failed radio. 
> 
> I swapped in a known working KPA3 Rev C module and the radio works as expected now on all bands.
> 
> So that leads be back to wanting to fix or get fixed this KPA3 that is not putting out power. 
> 
>> From the responses last fall and doing some looking via search this is what I know so far:
> 
> I see nothing burned on the board. 
> 
> The four red toroids appear to be fully connected to the board. 
> 
> This KPA3 is a Rev B version.
> 
> It has Toshiba 2SC2782 output transistors. 
> 
> In a number of threads there seems to be mention of some diagnostic tips that WE6R or Elecraft has posted but so far I have only found some small parts of it quoted in messages.  Is there some place this exists in its full form?
> 
> Are there things I can check with the module out of the radio?  It looks like it would be very hard to do much checking with it in a radio since access to the parts / traces on the board is likely impossible. 
> 
> Any hints or tips would be very welcome. 
> 
> Thanks
> 
> George Fremin
> K5TR
> 
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