[Elecraft] [K2] K2-100 Test and Alignment Issues

[William Moore]

Don,

I am finally able to get back to my KPA-100 construction project and this
email is to complete this thread with the result. 

Yes the problem was T4. I removed, rewound and replaced it. You were
correct, I had wound T4 backwards. With the correction I was able to easily
null the bridge to 12 millivolts. R26 and R27 required almost no adjustment
to set the correct values. The rest of the setup and test process went
perfectly.

So, to those who are or will be building the KPA-100, keep in mind that T4
must be wound exactly as illustrated and instructed in the manual.

Thanks again for your help, Don.

Bill, VE2WMA


-----Original Message-----
From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
Sent: Thursday, January 30, 2014 4:50 PM
To: William Moore; don at w3fpr.com; elecraft at mailman.qth.net; 'Mike Harris'
Subject: Re: [Elecraft] [K2] K2-100 Test and Alignment Issues

Bill,

Regarding T4, there is one other possibility that I did not mention before.
If the direction of the windings is not as shown in the manual, it will work
"backwards" which may be what you are observing.

As for the minimum voltage of 1.4 volts, that is WAY too high.  The bridge
should null to less than 14 millivolts with 15 or 20 watts flowing through
it.

You could do a sanity check on it by attempting to null the bridge by
monitoring the DC voltage at U5 pin 3 rather than U5 pin 1.  The voltage on
U5 pin 1 should be the same as at pin 3 if U5 is working OK.
If you still have a high voltage at U5 pin 3, then try doing the balance
(nulling) while monitoring U5 pin 5.  If you can obtain a null at U5 pin 5,
then for certain T4 is wound in the wrong direction.

73,
Don W3FPR

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