[Elecraft] KPA-500: 270v error and R17 - fixed

[Stephen Wilson, G3VMW]

Hi Keith,

Thanks for your comments and also to several others who replied
privately. I have now resolved the fault that was causing the +270v T/R
bias line to be shorted and the "270v ERR" hard fault to show.

As several others have discovered with the same fault, the problem was
C2, a faulty 0.01uF, 500v, 1206 SMD capacitor; part of the T/R circuitry
on the PA board, which was going short circuit when the 270v was
applied. The capacitor tested OK without volts applied, but on
inspection with a magnifier had a hairline crack at one end and fell
apart when I removed it.

C2 is easy to get at once the PA assembly is removed for inspection and
repair, but of course much of the KPA-500 has to be disassembled to do so.

Anyway, I have now replaced the faulty capacitor with a good one and all
is working correctly. I assume that R17 has been reduced in value from
10K to 1K on later versions of the KPA-500 and that value is correct.

Apparently, from comments received from several other folks, this is not
an unusual fault and Elecraft are well aware of the problem.

Luckily, I was able to fix the fault here in my workshop. Others have
not been so lucky and have had to return the PA assembly to Elecraft for
repair.

Now to test the newly built KPA-500 with my K3 and do the final
adjustments. I'm keeping fingers crossed there are no more nasty surprises.

Thanks everyone for their comments and help with this particular KPA-500
fault.

Best wishes

Steve Wilson, G3VMW

On 17/08/2014 18:00, KB9WMJ at wi.rr.com wrote:
> If you are positive this is not an interlock issue, then this is a TR
> Switch problem.
>  
> The resistor is probably correct, and may just be a revision to increase
> the current, but I do not know.  Check Diode D13 (S1M) to make sure it
> is not shorted, and inserted correctly.  Also, C2 (.01 uF) if it is shorted.
>  
> Next use an Ohm meter at TP2 & TP4 to ground.  Neither should
> show shorted.  TP2 points towards Q5, C16, C17, D7 and D10.  TP4 points
> at Q4, C18, D8 and D9.
>  
> Let me know what you find.
>  
> Keith
> KB9WMJ