[Elecraft] [K3] KPA500 Build question

[Randy Moore]

Ron, you have me worried.  I had already connected the transformer 
secondary leads to the board, but I disconnected them and measured 
again.  This time, I get an open circuit (as required) no matter which 
way I orient the DMM probes.  I've checked 4-5 times - same result 
either way.  Touching the mounting hardware results in 0 ohms, as it 
should.  I've triple checked the mounting hardware and thermal pads and 
are as they should be.  I was careful to just snug-up the screws, so 
that I wouldn't crush the shoulder washer.

I removed the transformer and took the rectifier hardware apart just to 
be sure.  No damage detected to the shoulder washers.  I reinstalled all 
this and rechecked with the DMM.  Everything looks good, independent of 
the DMM probe orientation.  So I don't know what was happening earlier???

In the process of removing the transformer, the white primary lead 
pulled out of its connector.  Good thing!  The crimping was only on the 
insulation!  Got that fixed now.

If you think there's anything else I should check on the rectifier 
board, let me know.

73,
Randy, KS4L

On 9/5/2011 11:52 AM, Ron D'Eau Claire wrote:
> It sounds like there may be an intermittent short, would be a serious issue,
> or the DMM probe was touching the wrong place.
>
> The orientation of the DMM probes should not matter. That check is simply
> verifying that the insulators between the metal tab of each rectifier and
> the metal heat sink are installed correctly - the rectifiers aren't
> connected to anything yet so no electrical circuit is involved at all.
>
> Are you sure the DMM probe touched only the metal tab of the rectifier and
> not the screw or washer? (The other probe touches the bare metal of the
> Z-bracket.)
>
> If so, was all the hardware installed exactly as shown - thermal insulators
> covering the entire metal area of the tabs as shown in the previous figure
> and shoulder washers in place to ensure the screws are centered in the holes
> in the rectifier tabs so they cannot touch the tab itself?
>
> 73,
>
> Ron AC7AC
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Don Wilhelm
> Sent: Monday, September 05, 2011 8:20 AM
> To: Randy Moore
> Cc: Elecraft Discussion List
> Subject: Re: [Elecraft] [K3] KPA500 Build question
>
> Have you tried reversing the leads?  That often helps.
>
> 73,
> Don W3FPR
>
> On 9/5/2011 11:05 AM, Randy Moore wrote:
>> I'm building KPA500 #370 for a friend.  On p. 23 of the assembly manual,
>> 2nd step says DMM should read same as if the probes are not touching
>> (10.0 Mohm), but all four readings are several hundred (300 - 600)
>> Kohms.  Problem???
>>
>> 73,
>> Randy, KS4L
>>
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