HP 8341B VCO Board Problem

Started by Hawaii596, 05-11-2016 -- 08:54:28

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Hawaii596

05-11-2016 -- 08:54:28
We have an HP 8341B that has sporadic very high residual FM on the carrier at all amplitudes and frequencies.  When you look at a narrow BW view on a spec an., it will intermittently look like either a nice clean carrier, or have near band spikes or FM.  It looks pretty strongly like the VCO board as we go through the alignment.  Nice clean 20 MHz and 100 MHz mixing signals coming in, and not so nice signal coming out.  If anyone has experienced this symptom and would like to opine, please do.  We are about to buy a used replacement VCO board off ebay; but thought I would pose this question first.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

Top Gun Test

#1
05-15-2016 -- 08:52:06
Hi Hawai
If its the M/N loop its going to be the feedtru caps on the main M/N mother board causing the issue .
These break down and cause bad phase noise effects . So If the M/N loop out of the VCO gives you
bad Phase Noise and looks bad , then take out  the M/N mother board and replace the feedthru screw in
feedthru caps . Its a very common problem with these 8340/41 units .  Its something ive seen on a few of these units , not saying your MN VCO could be also bad . Ive seen that also .
             John

Hawaii596

#2
05-25-2016 -- 17:32:51
Thanks John.  I passed along to the person working on it.  Just got back from Haiti, so trying to get back into the swing of things.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

briansalomon

#3
05-26-2016 -- 09:25:43
The output of one of my 8340B sweepers goes unleveled above 8db at 8 Ghz and above.

I've gone through the leveled RF output adjustments as per the service manual and the condition improved but the unit still will not meet spec.

Does anyone have any idea where to look for the defective component(s)?
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