[Elecraft] [K2] POWER control has no effect on actual transmit power

[Ben Byer]

On Oct 27, 2012, at 6:23 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:

>> On 10/27/2012 7:44 PM, Ben Byer wrote:
>> I've had fun putting together K2 #7373, and am finally at the point of doing the 40-Meter Transmitter Alignment (manual, p.78).   I have the unit connected to a 50-ohm dummy load, and am trying to tune L1 and L2 to show 2W on the internal wattmeter.   When I press TUNE, the measured power ranges from 0.2W-0.4W.
> 
> You are in for some Transmit Signal Tracing.  Turn to appendix A in the K2 manual and do the steps indicated there.

Okay, did that.  (Lacking a signal generator, I skipped the Receive tracing, but I can try to find one and go back and do that if necessary.)

I did up through the ALC section successfully -- VPWR is 4.925.  All following values are measured with key-down.
RF probe values tend to be higher than expected -- for Xmit mixer, I get:
2. Xmit Mixer Output: Expected 0.016 Vrms, actual 0.220
3. Buffer Output: Expected 0.200, actual 1.52 (ratio is 6.9)
4. Band-Pass Filter Output: Expected 0030, actual 0.046
5. T-R Switch #1 Output: Expected 0.029, actual 0.032

Pre-driver, Driver and PA
1. Pre-Driver Output (Q5):  Expected 0.120 Vrms, actual: 3.25   (or sometimes 1.  I checked this several times, I can't explain it)
2. Driver Input (Q6) : Expected 0.026, actual 0
3. Driver Output: Expected 1.8, actual 0

It seems that the Q5 output is way higher than expected, and then the driver input is almost zero.
I tried to trace the signal between Q5 and Q6 from the top of the board -- pin one of T1 is 3.25 or 1v, pin 3 is 1/3rd of that, and then that reduced signal drops from 3.25v on one side of C121 to 0v on the other side of C121.   (Or at least, that's what it looks like from above the board -- is there any way to safely run the radio with the heat sink removed and the transmitter on?)

Cheers,
Ben (KJ6ZGJ)