[Test-Equipment] HP 141 analyzer low gain

[Gary Schafer]

I had a few replies off list. I will add a little more info that may 
help diagnose the problem.

The connectors look very good between the IF and RF units. Bent the 
fingers in a little just to be sure. Connectors look like they are fully 
engaged when plugged together.
The signal levels were the same afterwards. Moving things around does 
not change anything. Very stable. Just down about 20 db.

The RF section is an 8554L. The IF is an 8552A. The mainframe is a 141A. 
It is the old one.

The oscillator signal at 0 HZ does go up and down with the Log reference 
level  knob. That signal level is as follows:
At maximum attenuation of the Log reference level the 0 HZ level is at 
-50 dbm on the screen scale.

With the log reference level set at -30 dbm (front end attenuater set at 
10 db) the 0 HZ signal reads zero dbm on the screen scale.

With the log reference level set at -30 dbm the reference test signal 
that should be -30 dbm, reads -50 dbm. (that would be at -20 db on the 
screen scale) -20 db below what it should.

When adjusting the gain (screw driver adjustment) on the RF unit it has 
a range of only about 5 or 6 db.

If I adjust the vertical gain on the IF unit (screw driver adjust on 
lower right corner) it has a range of 30 or so db. Where I have it set 
now gives a linear display. Changing the attenuation 10 db gives a 10 db 
change on the display.

If I increase that gain all the way I can get the cal signal up to where 
it is only 8 to 10 db low but then the display is not linear when I 
change attenuation. A 10 db change makes more than a 10 db change on screen.

If I lower that gain quite a bit you can see the signal go into limiting 
when you change the attenuation. The display will show less than 10 db 
change when changing the attenuation 10 db.

So where I have it set seems to be at the proper place but I am down 
about 20 db on gain somewhere. Linearity seems good through all the 
attenuater levels. 10 db change in attenuaters always gives 10 db change 
on screen.

I wonder if this is the kind of loss I would be seeing if one of the 
diodes were bad in the first mixer? It looks like it did get a shot of 
RF as the front end 10 db attenuater was smoked.

After repairing the attenuater I checked the level through the 
attenuater, low pass filter and the additional 3 db attenuater that go 
just before the mixer input. Levels were what they should be there. 
(-3.5 db) for filter and 3 db attenuater. So it looks like they did not 
get destroyed when the front end attenuater got smoked.

Any other ideas appreciated.

Thanks
Gary  K4FMX




John Miles wrote:
> Usually due to poorly-mating coax jacks in the D-connectors between the RF
> and IF plugins.  We're talking 9 times out of 10 here. :)
> 
> Bend the fingers toward each other and/or add an extra washer or two to help
> the connectors mate, and you will probably be good for another 30 years.
> 
> -- jm
> 
> 
>>-----Original Message-----
>>From: test-equipment-admin@mailman.qth.net
>>[mailto:test-equipment-admin@mailman.qth.net]On Behalf Of Gary Schafer
>>Sent: Friday, February 27, 2004 8:09 PM
>>To: test equipment
>>Subject: [Test-Equipment] HP 141 analyzer low gain
>>
>>
>>I finally got around to doing a little work on my old HP 141 spectrum
>>analyzer. Had to repair the input attenuater. Someone had smoked the 10
>>db front end pad. Replaced resistors and works ok now.
>>
>>The analyzer has low overall gain. Down about 20 db. If I turn the gain
>>adjustment up all the way (front panel screw driver adjustment) I can
>>get the gain up to about -10 db down. But it is not linear there. When
>>changing the step attenuaters the crt does not indicate 10 db each time.
>>
>>With the gain calibration backed off to where the analyzer is 20 db
>>below where it should be calibrated everything seems to work fine except
>>of course the calibration is off by 20 db.
>>
>>Any thoughts on where to look for the problem?
>>
>>Thanks
>>Gary  K4FMX
>>
>>
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> 
> 
>