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8591E(M) 2nd Converter Repair Help

Question asked by cellularDNA on Sep 13, 2011
Latest reply on Jan 29, 2012 by euchuma
I'm a newcomer to the forum, seeking help with repair of an 8591E 2nd converter,
which seems to display 18-20 dB less output than expected.  Please forgive me
if I've posted in an incorrect topic space.

The instrument would no longer self-calibrate. Thinking that someone had blown
the first converter, I tested it first connected in circuit, then as a module.
To my surprise, it appeared fine using the levels shown in the block diagram
of the 08590-80316 document.

Checking the RF FE plumbing, I also found no problems.  However when I checked
the 2nd converter/tripler by itself, I found its output clearly not as prescribed.
Likewise coupling a signal generator to the 3rd converter input at 321.4 MHz with
zero-span delivered very nearly the expected level on the screen of the analyzer
using the input quoted in the block diagram (I reverted to the factory calibration
values).

Suspecting the tripler, I measured the test point and the 1.8 GHz level was correct.  Although
the 8590L/8591E block diagram doesn't specify what the 600 MHz input to the
tripler should be, I guessed it should be -1dBm like the 8592 diagram since the
architecture seemed similar in that area.  Indeed, that turned out OK too.

I disassembled the 2nd converter assembly, found the mixer diode and traced
the relatively simple circuit.  I lifted the diode to free it from the circuit and measured
what appeared to be a typical schottky Vf, about .347 volts.

It seems counter-intuitive that the mixer could operate normally at DC but be so
compromised at 2GHz, but that seems the only conclusion.  I've squinted at the
little numbers on the diode and can't find any reference to it in any HP or Agilent
sources.

Might anyone have any insights on what can cause a 2nd converter to act this way
and/or what diode would consititute an acceptable replacement?  Any help would
be greatly apprecited.  

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