[Elecraft] NG0R - K2 Build/Alignment Issue

[John Hoaglun]

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