[Elecraft] KPA500 transformer taps

[Don Wilhelm]

Ray,

As Phil has indicated, the 10 volt drop in the AC mains may be the only 
problem and should be investigated.  Tracking it down to the trouble 
spot as Phil has detailed is the 'only way to go'.
What I would look for is loose connections (screws) at the breaker box 
and at each receptacle that is connected to that circuit.

If you are capable, remove each receptacle in the circuit and check it. 
If it is wired with the push-in 'backstab' connectors, remove the wires 
and connect them securely under the screws.

There are two types of "backstab" connectors.  One type uses the screw 
to tighten a clamp around the wire.  Those are normally OK, but you may 
want to make certain the screws are tight.
The other type does not depend on the screws, but just have prongs in 
the wire insertion holes to secure the wire.  If you have that type, 
remove the wire (twist while pulling and it will come out) and secure 
the wire under the screwhead.

With that much AC voltage drop under load, you have a condition where 
the failing junction has significant resistance and could cause 
overheating and possibly a fire.

73,
Don W3FPR

On 6/1/2017 12:41 PM, Phil Kane wrote:
> On 6/1/2017 8:11 AM, Ray W2RS via Elecraft wrote:
>>
>> With no load, my AC line voltage is 121 and my power supply voltage
>> is 73.  With full load, the readings are 111 and 53, respectively.
>> That is with the yellow transformer tap in place.  Should I switch to
>> another tap?
> As Josh said earlier, be very concerned about the10 volt drop in the AC
> voltage measured at the load.  Depending on the "amp-feet" of the
> circuity feeding the load, the drop for a 1 KW AC load should be no more
> than the industry standard of 2 volts.
> 
>   If you have the opportunity, put the amplifier on a dummy load and
> measure (CAREFULLY) the AC voltage at the entrance panel, the socket if
> it's a duplex one, and the load taps.  This will tell you where the drop
> is occurring.  I can't emphasize it more - BE CAREFUL.  As an EE you
> know that.
>