[Elecraft] K3 - 100 watt amp dead?

[David Olean]

Could this also be a hot pin problem that is starving the PA for power?  
The pins on the PA were changed to gold on earlier K3 sets. A contest 
would sure stress the pins if they had some resistance.

Dave K1WHS

On 11/29/2022 1:19 PM, Michael Carter wrote:
> Hi George,
>
> Your test confirmed that the low-power amp (K3 LPA) survives.
> To clarify, do you have the K3 or K3s?
>
> The original 100 watt PA (KPA3) used RF BJTs as the finals,
> whereas the KPA3A used RF MOSFETs (LDMOS).
>
> Apart from failure of one of the PA final transistors,
> the loss of TX output power when the 100 watt
> amp is switched in can be due to failure in the
> PIN diode T/R switch around the 100 watt PA.
> The fact that you can generate 12 watts indicates
> that the '100 watt module bypass' path through
> the PIN diode switch is OK.  Failure of the PIN
> diode switches connecting the LPA to the 100 watt PA
> can arise from failure not only in the PIN diodes
> themselves, but also from the bias circuitry that
> switches the diode states.  A recent post with a
> similar problem revealed the fault to be in one
> of the RF chokes (the red toroids on the 100 watt
> PA board) that feed DC to the PIN diode switches.
> The problem was a broken wire on one RF choke.
>
> I suggest opening the top cover, do a visual
> inspection of the top of the 100 watt PA module,
> then remove the 100 watt PA module to inspect
> the underside.  Barring any obvious sign of
> burned components or board traces, the next
> step would be to check the PA transistors, but
> the actual test depends on whether your K3
> has the older BJT finals or the more recent
> MOSFET finals.
>
>   Let us know what you find....., and 73,
>
> Mike, K8CN
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