[Elecraft] NG0R - K2 Build/Alignment Issue

[Don Wilhelm]

John,

I just looked back at your prior Q7/Q8 measurements and noticed that you 
have less than 6 volts on the collectors - sorry that I did not notice 
that before.  That may be the sole reason for your low power output.  
When the voltage is low, a Hi Cur message will often appear, and the 
output power may be low.

The reason -- in the way that the K2 controls power, it operates in a 
closed loop that is looking for the requested power output - if the 
actual power is low, the K2 increases the drive to attempt to develop 
more power.  At the same time, if the voltage to the PA transistors is 
low, more current must be drawn to produce the requested power (P = V x 
I -- reduce the voltage and the current must increase to maintain equal 
power).

OK, here is my way of getting to the bottom of a problem like yours 
(part thoughts/questions, part things to measure and do):

For some reason, the Q7/Q8 collector voltage is low during transmit.  
Are you using a good power supply capable of more than 3 Amps output 
while maintaining a voltage of 12 to 14 volts?  Are you using a power 
cord with an adequate size wire?  I recommend #16 gauge, but #18 will 
likely be OK (but no smaller).  Are the connections to your power supply 
tight?  How about the soldering of the plug that goes into the K2?  If 
there are any doubts, measure the end to end resistance of the power 
cord (measure both the positive and negative leads - both are involved 
in conducting the current).

Check the voltage at the power supply output during transmit - if the 
voltage drops substantially there, you can blame the power supply (and 
there may be nothing wrong with  the K2).  Should you find yourself in a 
situation with a bad power supply and no suitable replacement, borrow 
the battery from your lawn tractor or something of that sort and run the 
K2 from that to test.

If the power supply voltage is holding up (at least 11.5 volts, prefer 
more) during transmit, you will have to find the place in the K2 where 
the large voltage drop is occurring.  During transmit, measure the 
voltage at these points in order:
1) The center of the input power jack (voltage should be the same as the 
supply less any drop in the wire).
2) The Anode of D10 (should be the same as above)
3) The Cathode of D10 (should be 0.1 to 0.2 volts less than above)
4) The Cathode of D12 (same as cathode of D10, but this checks the 
switch too).
5) The 'C' and 'B' pads of T3 (same as the cathode of D12)
6) The 'A' and 'D' pads of T3 (more than 0.2 volt less here indicates 
improper soldering of T3).
7) The collectors of Q7 and Q8 (should be the same as #6 above).

If all is working correctly, you should have a voltage on the Q7/Q8 
collectors that is only 0.3 to 0.5 volts less than at your power supply 
output.  Find out where the severe voltage drop is occurring and correct it.

73,
Don W3FPR


John Hoaglun wrote:
> Hi Don & the group,
>  
> I replaced Q7, Q8, and Q11, and Q13 this morning.  The initial smoke test is
> showing the same results.   I am probably going to take the radio to a
> friend's house that has an o-scope and he is better at electrical theory
> that I am. 
>
> Your T5 is an interesting comment. Are there artifacts that we can check for
> around T5?  
>
>
> Plan for today:
> 1. I am going to go repeak the band pass filters on receive and transmit.
> 2. Retest band by band. (I will probably still have an issue)
> 3. Remove the W1 jumper, solder a small piece of coax jumper into place and
> test independent of the Low Pass Filters.
>
> I really need to determine if the problem is in one of three areas: Band
> Pass, RF & bias, or LowPass.
>
> Keep your thoughts coming... so far this is kicking my butt.
>
> Thanks, JH
>
> -----------------------------
> John Hoaglun
> NG0R - EN25
> http://www.hoaglun.com
>
>
> -----Original Message-----
> From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
>
> John,
>
> It is difficult to say because the 2 transistors are tightly coupled.
>
> One thing I can say is that the emitter voltage is suspect.  Both emitters
> are tied to the ground plane and should be at the same RF potential.  OTOH,
> the difference between 24mv and 0.4mv could be expained away based on RF
> pickup by the ground lead on the RF Probe - I don't know how long you probe
> ground lead is, but if it is much more than 3 inches, it is suspect too.
>
> The best method of analysis is substitution, but that means changing the PA
> transistors.  Wait until you have the replacement transistors in hand and
> just change them.  If that does not fix the situation, look more carefully
> at T5, T4, and the low pass filters.
>
> I can explain the HI Cur messages by going into detail about how the K2
> controls power output, but for now, 'just believe' that one failing PA
> transistor can cause that as well as power output at approximately half the
> expected value.
>
> 73,
> Don W3FPR
>
> John Hoaglun wrote:
>   
>> K2 Gurus... still looking for your magic.  :-)
>>  
>> Based upon some feedback from Gary I went poking around the radio with 
>> the RF probe. I looked at the low pass filters and relays to see if I 
>> stray RF floating around because maybe a relay was stuck.  I only saw 
>> RF where I expected it.
>>  
>> I tried to retune the 80m bandpass filters with the same result.
>>  
>> I decided to poke around the Q7/Q8 finals at low power with the probe:
>> 14.100mhz @ ~1w
>>  
>> Q7 Base 154mv
>> Q7 Collector 5.56v
>> Q7 Emitter 24mv
>>  
>> Q8 Base 114mv
>> Q8 Collector 5.56v
>> Q8 Emitter .4mv
>>  
>> I am not electronics genius but it sure appears that I am seeing RF 
>> from both transistors.
>>  
>> Is that much variance in the Base/Emitter between Q7 and Q8 normal?  
>> That is the closet thing I can come with for a discovery on my own 
>> tonight. (I ordered another set of Q7/Q8 earlier today as a CYA.. but 
>> I think that was an unneeded exercise.)
>>  
>>  
>> Last nights testing:
>>
>> --Freq-----1 Test----1 Msg----2 Test----2 Msg
>>  3.750mhz    2.5w    Hi Cur   2.3w      Hi Cur
>>  7.100mhz    6.1w    Hi Cur   6.6w      Hi Cur
>> 10.100mhz    0.3w    Hi Cur   4.9w      Hi Cur
>> 14.100mhz    0.3w    Hi Cur   5.7w
>> 18.100mhz    9.5w             7.3w  
>> 21.100mhz    4.1w             5.0w
>> 24.900mhz    0.5w             4.9w
>> 28.200mhz    6.0w             6.3w
>>     
>