[SI-LIST] Spectrum analyzer problem, noise

Hi all,

I operate a spectrum analyzer model HP 853A with a 8558B RF unit, which
has a strange problem around the first LO, a YIG oscillator. As some of
you will use these also, I think it is OK to ask this question.

When switching on the unit, the noise floor on the screen is 20 dB above
the usual value, from 0 thru 700 MHz, then declining to normal at 900
MHz and higher. The effect will vanish if I tune to a CF of 1100 MHz or
higher. It will appear only if the unit has been in daily use for my
usual 8-10 hours, for several weeks continuously. Some months ago, I did
a lot of paperwork (and my vacation!) and didn't use it, the effect was
gone during the following several weeks of heavy usage, only to appear
again after some time.

I would like to know if anyone can give me a tip as to the particaular
cause of this failure mode, or perhaps point me in a direction where I
could start to dig for a new component.
Is it indeed the YIG osc, is it the first mixer or some circuitry around
this assembly (e. g. heater, driver, voltages)?

Thanks,

Jochen  Feldhaar DH6FAZ
Telejet GmbH
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