Thanks. I'll try to disassemble the unit, then I hope I'll see the
reason of the failure.
--- In hp_agilent_equipment@..., lothar baier
<microwaveengineer1968@...> wrote:
most likely the cam is worn out or the little ball that locks the
cam in place is missing
g47g <g47g@...> wrote: Hi Lothar,
I have a 8495A manual step attenuator, and though it has the correct
attenuations on every step(?), but it needs force to switch the
attenuator in different positions, and also the well known "click"
cannot be heard, when turning (with some force) the knob.
Could you help me with some idea and procedure, as you did with the
electronic controlled equivalent ?
Thanks
George Galambos
--- In hp_agilent_equipment@..., "microwaveengineer1968"
<microwaveengineer1968@> wrote:
Since Agilent continiously ups the spare part prices and discontinues
parts it has become more and more important to go past normal repair
practices.
Step Attenuators for example can be repaired without any special
tools or cleanroom enviroment !
Lets take a look at the failure modes first, there are a few:
1.) Blown pads
Usually common with spectrum analyzers if a level past the damage
level of the attenuator was applied
2.) Cracked pads, pad has developed a hairline crack due to
mechanical stress !
3.) Improper contact, the contact in between the attenuator and
contact fingers is disturbed.
4.) Stuck solenoid
Quite common if attenuator hasnt been used in awhile
After we remove the attenuator from the instrument the first step is
to obtain the pinout and build us a little test bench consisting of a
powermeter and a signal generator, the good old 8481D or 8484A will
do nicely, if you dont have a sig gen use the 50MHz calibrator output
of the 435 or 436 or similar model, connect the cal output to the
30dB pad connect the sensor and zero and calibrate meter, the meter
will read -30dBm.
Disconnect the sensor from the attenuator and insert the Step
Attenuator, set step attenuator to 0dB, note the level on the
powermeter, now switch in the attenuator sections one by one measure
and note the level, 70dB attenuators are comprised of a 10 20 and
40dB Pad while 110dB Attenuators normally comprise of a 10 20 and two
40dB pads !
check every section individually, you should hear a click as you
enable each section, if you find a bad section note the value it
should have if all sections appear to be good repeat the test at a
higher frequency using a sig gen, if everything appears to be ok at
low frequency then most likely you have attenuator that just needs
cleaning.
Now lets take the attenuator apart, looking at the top you will see a
nut on each SMA connector, remove this nut this will allow for you to
remove the nameplate, under the plate you will see a wealth of allen
screws, remove those but note that the screws close to the SMA
connector are shorter than the other ones.
Now look at the small sideplate, not the one with the ribbon cable
but the other side - there are 4 small allen screws, remove those and
then remove the sideplate, careful not to lose the little wire mesh
here ! now slide out the u shaped cover and remove the top side.
You now have two parts, one beein the bottom side containing the
solenoids and the other one beein the top part containing the
attenuator pads.
First lets inspect the pads, you can measure them with a ohm meter,
they should measure in the vicinity of 50ohms , except of the 10dB
pad which can measure up to 70ohms or more.
Inspect the pads visually, are there any burned or cracked pads ? if
all pads look ok and check out at DC take a surgical swab and
isopropilic alcohol and carefully clean the contacts pads on the
attenuators.
Next look at the contacts, enable all solenoids and observe the
position of the contact tongues, they should be slightly above the
surface of the block, carefully clean the contact tongues with
alcohol, be careful not to bend them.
next put the solenoids back into the 0 position and look at the top
of the solenoids, you will see two little white plastic rods where
those meet the solenoid you should see little rubber rings , there
are 4 of those on each segment, usually over the years they go
briddle and then disappear, with those rings gone the attenuator wont
make contact anymore, you can buy the rings from watchmakers supply
stores as they are beein used for sprocket gaskets !
Now if your gaskets are there put the attenuator back together after
you cleaned everything and it should work now.
If you have a broken or burned pad dont stress over it the pads are
used in all HP attenuators so just find a attenuator on ebay and take
the pad you need out of it, you can also buy pads from agilent
however be prepared to pay $100-150 per
piece
---------------------------------
Get easy, one-click access to your favorites. Make Yahoo! your
homepage.
