[Elecraft] KPA3A Failure & related

[Don Wilhelm]

Bob and all,

I think this problem is not so much with the amplifier, but the K3/K3S.
Consider that the switchover from the low power amplifier to the KPA3 is 
in the 12 to 15 watt range.  That is where the LPA is generating about 
all the power it can produce.  Above that range, the KPA3 kicks in and 
everything is "loafing along".

Oh yes, aside from that, some amplifiers do fail for various reasons - 
and I think we are mixing failures of the K3/K3S with amplifier failures 
in this thread.

73,
Don W3FPR

On 2/16/2019 1:43 PM, Bob McGraw K4TAX wrote:
> This discussion has risen its nasty head earlier.  Wes and I exchanged 
> several private e-mails on the topic.  Here is what I've learned:
> 
> My purchased new from the factory my K3S -100 in August of 2015. With it 
> I drove my AL-80B with 60 to 80 watts for some two years.   Absolute no 
> issues of concern.    In April of 2018 I purchased a new KPA500 and put 
> it in place of the AL-80B.   I drove the KPA500 with my K3S at 15 watts 
> for some 2 weeks.  Then reports of "fuzzy audio" surfaced.  The 2 tone 
> test revealed excessive IMD.  I contacted Tech Support and they arranged 
> to send me a LPA3 and a KPA3A.   These were installed and all was well 
> for about one week.  Then again the reports of "fuzzy audio".  I 
> contacted Tech Support again and they said "send it in".   The service 
> report indicated they replaced the KPA3A with a rev E which is reported 
> to be more stable.  An interesting comment on the Elecraft Service 
> report; "Due to failure of the stable rev E amp, this indicates 
> something in the shack.   Possible kickback from and amplifier.   It is 
> likely to happen again if the root cause is not found. ........" 
> UGH...folks, the amp is a KPA500!     >> HOUSTON - DO WE HAVE A PROBLEM?>>
> 
> In the discussions via e-mail and here on the reflector which followed, 
> I learned of others which experienced the same type issues.  I asked 
> "what's the solution".   Interestingly the answer came back [I don't 
> recall from whom}  "stay away from 15 watts". Since then I have driven 
> my KPA500 with 20 watts .....PLUS  I have connected and engaged the ALC 
> from the KPA500 to my K3S.  This is a menu item in the K3S.  Now some 
> may not like the ALC connection and activation.
> 
> My thinking is that IF there are any spikes originated with the rise of 
> the CW signal or voice signal this may overdrive the amp.   With my P3 I 
> see this A LOT on SSB signals on the bands which exhibit initial ALC 
> overshoot.  This is both on CW and SSB.    Makes me wonder what does the 
> input impedance of the KPA500 does when hit with a millisecond or so of 
> excessive drive?   Not long enough to activate the attenuator or throw a 
> fault signal. I do know that tube amps reflect to their input the 
> effective tuning of the output.   Could this be an issue with solid 
> state amps where the input impedance goes to zero thus placing in effect 
> short on the driving transceiver?   Mind you not long enough to throw a 
> fault signal but........what does it do to the driving stage?     I've 
> not measured it and am not exactly sure how to accomplish this.  It may 
> not be the issue at all.   Perhaps some of you more technical gurus can 
> expound further on the topic.
> 
> I do know factually since the change to the KPA3A  rev E, and I 
> activated the ALC between the KPA500 and my K3S the issue has not reared 
> its ugly head.   Lets hope it doesn't.
> 
> All in all, I am VERY pleased with Elecraft Tech support and extremely 
> pleased with my K3S, P3, KPA500 and KAT500.  A winning combination. But 
> I'm still walking on eggs  and hoping none break.
> 
> 73
> 
> Bob, K4TAX