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Attenuator Repair For HP/Agilent Microwave Equipment

From the HP_Agilent_Equipment Mailing List on Yahoo:

Since Agilent continuously 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 power meter and a signal generator, the good old 8481D or 8484A will do nicely, if you don't have a signal generator, 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 0 dB, note the level on the power meter, now switch in the attenuator sections one by one, measure and note the level. 70dB attenuators are comprised of one each 10, 20 and 40dB pads while 110dB attenuators normally comprise of one each 10 and 20 dB pads 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 signal generator. If everything appears to be OK at low frequency then most likely you have an attenuator that just needs cleaning.

Now let's 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. Be 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 being the bottom side containing the solenoids and the other one being the top part containing the attenuator pads.

First lets inspect the pads, you can measure them with an ohm meter, they should measure in the vicinity of 50 ohms , except of the 10dB pad which can measure up to 70 ohms 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 isopropyl alcohol and carefully clean the contact 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 brittle 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 being 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, don't stress over it. The pads are used in all HP attenuators so just find a similar 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


test_equipment/attenuator_repair_for_hp_agilent_microwave_equipment.txt · Last modified: 2013/01/08 19:00 (external edit)
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