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 say
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
Thanks, JH
-----------------------------
John Hoaglun
NG0R - EN25
http://www.hoaglun.com
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