K2 - No TX

[djmd]

Hello,

My K2 was working previously on all bands with no problem. I been been away from it for several weeks doing other things, but decided to get on tonight and try to make a couple contacts.

I'm getting no TX at all, even connected to a dummy load. Display shows .1w on all bands regardless of RF power setting.

Voltage display -
idle 12.4v  240ma
tx   12.2v  560ma

I don't have my DMM right now and it will be a couple days before I have it back, so I wanted to post to see if there's something obvious before I start tracing. I can't imagine a part failing with the unit just sitting there. Occasionally I have turned it on to listen, but haven't tried tuning it up in at least a month.

Thanks a bunch for any assistance!

Dave
K8DJW

[djmd]

I'm going to do the transmit signal tracing tonight, but I'm starting to think I may know the source of the problem. Not long ago my K2 got a little wet - it was sitting near an open window when an unusually windy rain came through. At the time, I opened it up and let it dry, powered it up and it came on ok. But I'm starting to think that I never checked TX. The holes in the back for the antenna tuner jacks were still open at the time. Power was not hooked up to the unit when this happened IIRC, but who knows. We'll find out tonight.

djmd wrote

Hello,

My K2 was working previously on all bands with no problem. I been been away from it for several weeks doing other things, but decided to get on tonight and try to make a couple contacts.

[djmd]

Just wanted to update anyone on here who might have been curious. I believe I found the intermittant. In the process of doing the tranmitter signal tracing I found that the problem appeared to be between the pre-driver output and the driver input. With the RF probe I was seeing 6v (!) on the case of Q5 (should have been less than a quarter vrms) and only about 3mv on the base of Q6. So I reheated and added a smidgen of solder to those components, as well as T1 and C121. It seems to work consistently now.

djmd wrote

I'm going to do the transmit signal tracing tonight, but I'm starting to think I may know the source of the problem.

[Don Wilhelm-4]

 From those measurements, you definitely had a problem between Q5 and
Q6.  Congratulations on finding it.

The much higher than expected RF Voltages seems to mystify some builders
- that is *not* a problem nor should it be cause for any alarm, but is
normal behavior for the K2 power control circuits.  When there is no (or
lower than requested) power output, the K2 increases the drive in an
attempt to generate more power.

The result is that the RF voltage in the *working* stages will increase
above the normal (expected) value - when you progress just beyond the
failing point, the RF Voltage will drop substantially below the expected
value - that should tell you that you just passed the failure point.  It
makes troubleshooting by transmit signal tracing a relatively easy task.

73,
Don W3FPR

djmd wrote:

> Just wanted to update anyone on here who might have been curious. I believe I
> found the intermittant. In the process of doing the tranmitter signal
> tracing I found that the problem appeared to be between the pre-driver
> output and the driver input. With the RF probe I was seeing 6v (!) on the
> case of Q5 (should have been less than a quarter vrms) and only about 3mv on
> the base of Q6. So I reheated and added a smidgen of solder to those
> components, as well as T1 and C121. It seems to work consistently now.
>
>
>
>
>
> djmd wrote:
>  
>> I'm going to do the transmit signal tracing tonight, but I'm starting to
>> think I may know the source of the problem.
>>    
>
>  
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>
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