I've worked on a lot of these in the past 25 years and have found the step attenuator to be one of the weak areas, especially since it's an electro-mechanical part. The internal O-rings age and get gummy and stick. The contacts that turn off solenoid current get intermittent. And a strange thing that I have found is the U-channel that encloses the solenoid will develop a magnetic attraction mostly on the bottom end that can stick the section. An easy fix for this is to loosen the 2 hex-head screws and carefully slide the U-shaped piece out sideways. Then affix a piece of transparent tape to the inside bottom of this piece and trim the tape edges flush.? This is also a good time to check and clean the solenoid contacts (not the attenuator contacts).
Re-assemble and test.
Steve
________________________________
From: John Miles <jmiles@...>
To: hp_agilent_equipment@...
Sent: Monday, September 28, 2009 11:43:15 PM
Subject: RE: [hp_agilent_equipment] Re: HP8568B Error Correction Routine failure
?
Sounds like the driver for the 20 dB pad is stuck on, although
I'm not sure
how that accounts for the -7.7 dBm business (Why isn't it -10?
Attenuators
don't fail in a way that yields +2.3 dB of unexpected gain.)
They do when the element that goes to ground goes open. My 494
had a similar problem with the 20dB pad. It's pretty hard to blow
the series element, but the input resistance of the element,
normally close to 50 ohms, was more like 2k, so the previous
stage did not see 50ohms and the total attenuation when the 20dB
pad was inserted was low by several dBs.
Sure, but since he sees exactly the same signal level with both 40 and 40+20
switched in, I'm not inclined to think the 20 dB section is switching at
all, open ground or not. It seems that it would have to be making a
straight-through connection for those two settings to match, wouldn't it?
Be interesting to hear what turns out to be wrong with this one.
-- john, KE5FX
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