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I've got an attenuator for an 8672A that suffers from failed O-rings. In
this case it appears someone used something nasty to try to clean the
spring contacts and attenuator pads and whatever was used dripped down
the nylon plungers and turned all the O-rings to a mushy black paste.
Not a single survivor. Agilent no longer stocks them but provides the
dimensions and material type so I just need to research a source.

Steve


On 12/30/2012 10:16 PM, pianovt wrote:

Hi Jim,

Do not unscrew the solenoids on the top, at least not for now. To see
what's happening, you will have to open the lower part where the RF
action is.

The two SMA connectors have hex-nuts. Remove them. There is thin name
plate that you can now remove. It covers the whole area between the
SMA connectors. It probably has a few glue spots on its other side, so
carefully pry it off without bending the it. Now, you will see a bunch
of socket cap screws. When you remove them, you will be able to
carefully separate the parts and gain access to the attenuator pads
and plungers. The rubber O-rings are on the tiny nylon plungers that
are attached to the gold plated spring contacts. I think there are two
rings on each plunger, one on the RF side and one on the solenoid side.

Vladan

--- In hp_agilent_equipment@...
<mailto:hp_agilent_equipment%40yahoogroups.com>, Jim Schatzman
<james.schatzman@> wrote:

Vladan-

I got the cover off without trouble. By testing I note that when the

middle (40 dB) switch is triggered, the output turns off completely.
As far as I can determine, the plunger is actuated correctly. Moving
it manually also results in the loss of output. The other two switches
(10 and 20 dB) work fine.

I suppose that either the switch contacts are failing or the 40 dB

resistor is open. Apparently both switches and resistors are inside
the two-piece machined steel block. All the RF path seems to be
inside. It would appear that I need to get this apart, but I am not
sure how to. There are quite a few screws attaching the solenoids to
the steel block, but I am not seeing what is holding the steel block
together.

This attenuator has a PC board with several chips on it. I have

noticed before that HP seems to have adopted the philosophy that if 2
chips are good, then 20 must be better. It is a surprisingly
complicated device.

Any advice??

Thanks!

Jim



At 08:45 PM 12/30/2012, you wrote:

Jim,

You will have to pull out the front panel. There are a few

flat-head screws along the front frame (top and bottom, maybe even
sides - don't remember any more). Once you remove them, the front
panel pulls out of the front frame, though it still has a couple of
umbilicals attached. From there, you will see how to get the
attenuator out.

The most common failure has to do with the tiny rubber O-rings on

the plungers. This is all delicate magnifier work. Do not succumb to
the "magnetized housing" myth of repair. The attenuator depends on
magnets for latching. It's just that when the O-ring fails and either
splits or falls off, the mechanism shifts closer to the magnet and the
force of magnetic attraction increases. At that point, the solenoid
(which has nothing wrong with it) no longer can pull the parts away
from the magnet (which also has nothing wrong with it). This is when
some people decide that demagnetizing everything will fix things. If
you have the patience, study the design, it's interesting. The goal
was to use current only when switching a stage is required and then
interrupt all current to the coils as soon as the task is accomplished.

You open the attenuator by removing one of the end caps and sliding

the U-shaped steel cover off. It comes off relatively easily. Don't
try to remove it by prying with screwdrivers.

There are two versions of the attenuators. The older ones are all

electro-mechanical, the newer ones have a p.c. board with some ICs on
them.

Good luck, they can be fixed, but the work requires some patience

and you may have to order no O-rings from Agilent.

Vladan

--- In

<mailto:hp_agilent_equipment%40yahoogroups.com>hp_agilent_equipment@...
<mailto:hp_agilent_equipment%40yahoogroups.com>, Jim Schatzman
<james.schatzman@> wrote:

All-

O.k., I have an 8568B with a bad attenuator (40dB and below are

dead).

My big question is - how do you get access to the attenuator to

take it apart and clean/repair? It seems to be really buried under a
maze of stuff...

Thanks!

Jim



At 01:44 PM 12/27/2012, you wrote:

Thanks a lot of this.

I just managed to get a 8568B that attenuator has some problems

with the 20dB step.

These instructions encourage me to open the attenuator and clean

the pads with isopropyl alcohol. Now it works fine.

I didn't made a testped for it i just cold moved carefully the

solenoids and checked with dg8saq vnwa the attenuator pads. I'm not
sure is the solenoid stuck or is it actually a contact failure.

The instructions how to disaasemble the attenuator was very

accurate and easy to follow.

BR, Jarmo

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