[Test-Equipment] HP8640B Signal Generator Problem

[kb0nly]

Got a weird problem here that hopefully someone on here has already dealt with, or knows how to fix, because I have poked, prodded, and gone through the service manual without much progress.

If I connect a cable from the rf output to an external frequency counter and set it to any frequency in the 18-128Mhz ranges the frequency displayed doesn't match the frequency of the signal being generated, they are off anywhere from 5-12Mhz.

For example, 8640B set to 50Mhz shows 62.5Mhz on the external frequency counter.  Also, if I take a cable and connect the output to the external counter input and tap the int/ext button the 8640B's frequency counter will show 50Mhz as it should.

So it appears that when you select the external frequency counter input the circuitry works just fine, its the internal input that has a problem.  Can anyone suggest exactly what might be the cause, because I spent three days running through suggestions without any luck so far.  

Above or below the 18-128 Mhz ranges it operates as it should, no error in frequency.

Thanks and 73,

Scott