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

[Ben Byer]

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.

I can adjust the POWER knob to show 0.1W - 15.2W while the unit is idle, so the knob reading circuit works -- but adjusting the knob doesn't change the reported transmit power even slightly (remains 0.2W-0.4W), and neither does adjusting L1 or L2.

I suspect that the measurement is correct, because the overall unit power consumption is measured as 11.9V 0.20A when idle, and 0.42-0.44A when in TUNE mode (regardless of what I set POWER to before enabling TUNE -- same result with 0.1W vs 15.2W)

I already checked the voltages on RF Q5, Q6, Q7, and all seem reasonable.  (Actually, initially I measured 0v on the Q6/Q7 collector during transmit, and discovered that I had swapped two wires on T3.   Fixing that brought the voltage up to almost 12v, but did not change the transmit power).

Any suggestions?

Thanks,
Ben (KJ6ZGJ)

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[Don Wilhelm-4]

Ben,

You are in for some Transmit Signal Tracing.  Turn to appendix A in the
K2 manual and do the steps indicated there.

73,
Don W3FPR

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.
>

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[David Dietrich]

Along with what Don Wilhelm suggests, I would also look at all the RF board SMT IC's on daughterboards and see if any of the pins on the IC's themselves are not 100% soldered.  I had a similiar problem on my K2, and the problem turned out to be an unsoldered pin.  Once it was soldered, and a connection made, the K2 was up and running.

73,

David
KC9EHQ
K2/10 #7164




________________________________
 From: Ben Byer <[hidden email]>
To: [hidden email]
Sent: Saturday, October 27, 2012 6:44 PM
Subject: [Elecraft] [K2] POWER control has no effect on actual transmit power
 
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.

I can adjust the POWER knob to show 0.1W - 15.2W while the unit is idle, so the knob reading circuit works -- but adjusting the knob doesn't change the reported transmit power even slightly (remains 0.2W-0.4W), and neither does adjusting L1 or L2.

I suspect that the measurement is correct, because the overall unit power consumption is measured as 11.9V 0.20A when idle, and 0.42-0.44A when in TUNE mode (regardless of what I set POWER to before enabling TUNE -- same result with 0.1W vs 15.2W)

I already checked the voltages on RF Q5, Q6, Q7, and all seem reasonable.  (Actually, initially I measured 0v on the Q6/Q7 collector during transmit, and discovered that I had swapped two wires on T3.   Fixing that brought the voltage up to almost 12v, but did not change the transmit power).

Any suggestions?

Thanks,
Ben (KJ6ZGJ)

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[Ben Byer]

On Oct 27, 2012, at 6:23 PM, Don Wilhelm <[hidden email]> 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)

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[Mike K8CN]

Ben,

It sounds like re-heating of the solder connections on both sides of C121 is in order.  The fact that your RF voltage on terminal 3 of T1 has the proper ratio to that on terminal 1 implies that T1's solder joints are OK.  The absence of RF on the driver (Q6 base circuit) side of C121 could be due to a very low impedance being presented there, but this would have been reflected in a much lower voltage reading at terminal 3 of T1 ** if ** C121 was properly connected.

I would hesitate to operate the transmitter without the heat sink attached even for brief periods of time.

Good luck, and 73,

Mike, K8CN

[Don Wilhelm-4]

Ben,

When you have low output, it is normal for the RF voltages at the
working stages to be much greater than the listed "expected" values.

Since the Q5 output is higher than expected, but the Q6 input is zero
(or close to it), you have a problem between Q5 and Q6.  Refer to the
schematic and check all components between those 2 stages. Normally I
would say that toroid T1 is the most likely problem, but since you said
the output RF voltage is 1/3 of the input RF voltage, I suspect
something else - check for solder bridges.

The only way you can do a TUNE or transmit without the heat sink
installed is to remove T3.

73,
Don W3FPR

On 10/28/2012 12:09 AM, Ben Byer wrote:

> On Oct 27, 2012, at 6:23 PM, Don Wilhelm <[hidden email]> 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)
>

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[Ben Byer]

On Oct 28, 2012, at 5:51 AM, Mike K8CN <[hidden email]> wrote:

> It sounds like re-heating of the solder connections on both sides of C121 is
> in order.  The fact that your RF voltage on terminal 3 of T1 has the proper
> ratio to that on terminal 1 implies that T1's solder joints are OK.  The
> absence of RF on the driver (Q6 base circuit) side of C121 could be due to a
> very low impedance being presented there, but this would have been reflected
> in a much lower voltage reading at terminal 3 of T1 ** if ** C121 was
> properly connected.

Thank you (and Don) very much for your help!   Reflowing C121 fixed it, and my K2 is now complete.  Now I just have to come up with an antenna, and I'll be on the air.

Cheers,
Ben (KJ6ZGJ)

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