[Elecraft] Shutdown on 50MHz CW

[Tom Childers, N5GE]

On Mon, 26 May 2008 09:44:16 +0100, Ian White GM3SEK <gm3sek at ifwtech.co.uk>
wrote:

Please see below...

>Ian J Maude wrote:
>>Having used the radio for a while I am now starting to come up with 
>>some questions.  The first one comes from using RTTY in contests.  I 
>>generally have the display showing me the PA temp and I stated to 
>>wonder, what *is* the optimum temperature for the PA?  Also, I can see 
>>it is rising but I have no idea when it should get critical :)
>>Any clues please?
>
>No answers, but maybe a related question...
>
>The rig is a K3 with KPA100 and KAT3.
>
>Having just started to use the K3 intensively on 6m, there is a problem 
>with the PA shutting down, especially on CW. After 10-20 seconds of 
>operating, the RF power disappears. Many of the symptoms are 
>intermittent, showing no clear pattern - except that it only happens on 
>50MHz.
>
>Moments before the power dies, there are one or two warning clicks of a 
>relay. There may also be a "HIGH SWR" warning, but not always - 
>sometimes that warning flashes and it's still possible to carry on for 
>10-20 seconds. (The SWR is actually OK - tested at 100W using another 
>rig, and on a dummy load.) The problem is the same on both ANT1 and ANT2 
>outputs of the KAT3, and either with or without the auto-ATU.
>
>The problem cannot be cleared by cycling TX/RX; but it can always be 
>cleared (at least temporarily) by cycling ANT1/ANT2. That brings the 
>power back, making it possible to limp through a short QSO.
>
>However, there *may* be some relationship to the PA temperature. 50MHz 
>is the only band on which the PA temperature indication is being 
>affected by RF - on CW, it bounces upward by anything up to 5degC on 
>every key closure, and I'm wondering if these  transients are triggering 
>a PA shutdown.
>
>There is no problem on any other band. On 28MHz, I ran off a string of 
>CW QSOs in the contest, and then tested the KPA100 for about 5 minutes 
>at 120W continuous carrier. There were no RF-related effects, and the PA 
>temperature levelled out at about 60degC. It seemed quite happy at this 
>temperature.
>
>Back on 50MHz, continuous carrier caused the same kind of instant jump 
>in the indicated PA temperature. The TX did eventually shut down, but 
>the transient changes seem to have more effect than the actual PA 
>temperature. There is no problem with SSB, but the PA temperature 
>genuinely is lower on that mode. CW seems to be the worst mode because 
>the duty cycle is higher and it also creates lots of transients.
>
>
>It's all a confusing mess of symptoms... and I may not even be on the 
>right track with the PA temperature. However, I feel pretty sure that 
>the problem *is* being caused by internal stray RF.
>
>
>Since it's Memorial Day, it won't be reasonable to expect a response 
>from support at elecraft.com for at least another 30-40 hours. In the 
>meantime, I'd be grateful for any thoughts and suggestions from the 
>group.

I have experienced the same behavior on 6m, although I didn't do any extensive
testing.  Thanks the effort you put into your tests.

Tom, N5GE - SWOT 3537 - Grid EM12jq

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