HP8595E spectrum Analyzer: readings at -40dB and CAL SIGNAL NOT FOUND

[auato]

Thanks a lot.
I finally disassembled the RF assy and opened the attenuator. At first glance what I found is the first relay on the right side (near the RF input connector) in a different position than the other relays (as vidible in the short video I published). Could this state be normal? Also, are the o-rings the tiny ones in the photo?

https://youtube.com/shorts/Yz_BRhvwg4g?si=0_UAxk8avDcMJ8Se

[rf-loop]

Thanks a lot.
I finally disassembled the RF assy and opened the attenuator. At first glance what I found is the first relay on the right side (near the RF input connector) in a different position than the other relays (as vidible in the short video I published). Could this state be normal? Also, are the o-rings the tiny ones in the photo?

https://youtube.com/shorts/Yz_BRhvwg4g?si=0_UAxk8avDcMJ8Se

Yes it is normal. Relays are bistable and they stay in position what was last commanded.
You can move these relays manually up or down. They have two position.

In your image can see this one O ring. Each nylon(or whatever material it is) lift-push pin is attached to the metal plate of the relay with two O-rings. There are two of these pins on the every relay plate. (The metal plate of the relay has a U-shaped hole where the pin and the O-ring are. It does not fall out of the slot because the hole through which the pin passes acts as a guide.) The pins go through the thin hole into the attenuator cavity and there these move the contact strips. The structure of the cavities inside is very delicate.
Often the pins are almost white, but in your case they are brown. It is perhaps older version. In my experience, brown pins are more prone to breaking than white ones.  (what is your analyzer serial number prefix what tell manufacturing year)

Move each relay to the lower position and to the upper position so that you can see the condition of the O rings as accurately as possible. Look closely. You  can easy move these up and down... (magnetic construction keep it in each position) 
The movement of that pin must be full. A missing, crumbled, etc. missing O-ring no longer pushes/pulls the full distance and the movable strip does not make contact in the cavity. Sometimes a small chip may have detached from the O carriage, which can prevent the full range of motion.

I can't find the full documents I made a long time ago about them and I don't remember the order of the attenuators.
However, the first one after Input is the DC-block. The next one is pretty sure 10dB, but I'm not sure right now if the order after that was 20 - 40 or 40 - 20.

If all the o-rings are fine and the movement is normal, pin need move exactly same amount than relay plate... then need to think about another possibility. One possibility is that the end of the nylon needle in the attenuator cavity is damaged and it no longer closes the contact that bypasses the attenuator chip in question. However, it can still work in such a way that you can push the contact into the attenuator position. yes, at least in theory, but I've never seen such a case. 

[scopeman]

I have rebuilt these attenuators but sometimes it's worth it to consider other alternatives.

If all else fails you can send the attenuator out for a full rebuild at the company at the link below. I had them totally rebuild an attenuator
for my HP 4407B (9KHz-26.5GHz) a job that I could not easily certify in house so I sent it out.

They did an excellent job and I got certified plots of every attenuator step. I have no personal stake in this company, I am just a satisfied customer.

It's not cheap but you get a 1 year warranty. Something to keep in your back pocket when you need it.

https://www.testequipmentplus.com/agilent-hp-step-attenuator-repair.php

Sam
W3OHM

[Kean]

More info on repairing the 33321 step attenuators can be found in this post:
https://www.eevblog.com/forum/testgear/hp-8644b-attenuator-question/

[bg1wxd]

MY ENGLISH IS NOT GOOD ENOUGH, I FIXED THE ATTENUATOR OF THE HP 8595E, IT WAS THAT LITTLE BLACK RUBBER RING THAT WAS AGED AND BROKEN, AND A BOX OF RUBBER RINGS BOUGHT FROM AMAZON COULD BE REPLACED.

[auato]

very interesting your pictures. Thank you! Fortunately or unfortunately the o-rings problem doesn't seem to be my case. They look intact.

You can easily check the attenuator without fully disassemblying it, only remove the coax connections and measure resistance with the DMM between input and output.

and if I check with a DMM between input and output the resistance is always high (with both the relay positions)

[MarkL]

very interesting your pictures. Thank you! Fortunately or unfortunately the o-rings problem doesn't seem to be my case. They look intact.

You can easily check the attenuator without fully disassemblying it, only remove the coax connections and measure resistance with the DMM between input and output.

and if I check with a DMM between input and output the resistance is always high (with both the relay positions)

Don't forget the first attenuator stage, on the opposite end of the ribbon, is a DC blocking capacitor.  So, with this in circuit you won't see any DC resistance.

In combination with the faulty 40db thru position, if you flip all the actuators in the same direction at the same time (all in or all out), you won't have any DC resistance from end to end.

It seems fairly certain there is an issue in the cavity: dirty contact, broken contact, broken nylon pin, etc.  But before digging into it, I would reconnect the ribbon cable to the 8595E one more time and again exercise the actuators: 1) You should be able to identify which one is the 40dB step and 2) Verify the 40dB actuator has the *full* range of motion like the others (nothing is sticking or lopsided).

From your photos everything looks ok from the bottom.  Figuring out which one is the 40dB step will at least allow you to focus on the right set of contacts inside the cavity.

[auato]

Guys, I don't know what happened but now everything looks OK. There is probably still something mysterious. Now as you can see from the photo, the attenuator is still outside and the HP8595E only controls the relay contacts. The attenuation steps are OK and represented as in the photo. 
frankly I still don't know what to do. I was not so keen on getting my hands inside that attenuator with those tiny mechanisms. Maybe it's dirty inside and I recall that I only sprayed some compressed air on it. In any case I studied what to do watching some videos on YT on how to operate with this stuff and if the problem will appear again, I will proceed with a disassembly of it. I could wait a few more days and then proceed with the assembly. First, however, I would like to replace that 10 ohm resistor R80 in the photo that I clumsily smoked on the analog board A7. Anyway I will keep you updated. In the meantime, Merry Christmas to all of you and thanks again

[MarkL]

Well, glad it's working.  But unexplained, it will of course come back at the most inopportune time.

Thanks for documenting the attenuator dB step positions.

[rf-loop]

@auato

It is possible there have been some small piece of what ever inside attenuator relay side.
But in more worst case there can be some other problem inside attenuator cave. I do not list all what are somehow possible.

I think your some one previous images was not from your attenuator but similar (because it is partially disassembled and bit weird way, perhaps just for image...who knows). I have used this image (part of it) here.





There need always carefully look these positions are clean (marked red arrows). These metal surfaces need be free from some small garbage and also these surfaces of nylon pins, so that it can move perfectly full amount. (I have seen many times example small piece of old brittle O-ring rubber on these surfaces, sometimes these are like attached there and need carefully remove if they exist and if still do not yet want do whole renew work (there is two methods for take nylon needles out from contact strips in cave - very risky "lottery" way  or very safe but lot of more "watchmaker" work. (example I use always dust free air room and and magnifier (microscope in my case dye to my after "best before" aged eyes )

Btw, in some pictures in this thread can see some attenuators where can see bit weird contact strips and also really weird contact strips positions... without further explanations and other images can not tell what are these and why... there is one image where strips are positions I have never seen when nylon needles have removed... really weird. (but because it is not your attenuator ... no need think more these)

If you use compressed air to inside attenuator cave. Be very careful because these thin contact strips. They are fragile. Some times also old glue can brittle (they are glued in perfect positions). Also air need be real clean and oil free, and still it can do more bad than good because some particles attached to the surfaces inside may detach and then remain in contact. The low contact pressure contact is to some extent "self-cleaning", however, because there is a small slip when the contact is made... gold alloy against gold alloy.

If the cavity of the attenuator and the end flanges have never been opened, it is quite unlikely that there is debris there. The holes of the nylon needles have quite a small clearance, so normally no debris gets in there.
But what have I ever found. Rarely. Some kind of "oil" or "grease". It is put in when the witches are made. There, at the point of contact between the nylon needles and the contact strips. Usually it is a very small dose. And it must never come into contact with the surface. Sometimes the attenuators run very hot (production test use, for example, is very tough, the user's HP didn't even give them a warranty for that use).
In such a situation, the fat may heat up and drain, especially if there has been a bit too much of it there. Grease between the contact surface does not prevent contact. But then the typical problem is that during the SelfCal / Self Check phase the contact is created a little too slowly and it fails or sometimes even gives the wrong value (capasitive contact just before galvanic)

Merry Christmas!

[auato]

I still have the HP8595e disassembled on my bench.
The problem seemed to be the third relay (40dB). On some rare occasions it did not release when the instrument was turned on or when the preset button was pressed

But now that I've discovered it furtively, as you can imagine, it's been working perfectly everytime for days. 

@rfloop thanks for the detailed image, I am checking again near the red arrows with a microscope. Honestly before reassembling it I was thinking of spraying something like WD40 or isopropyl alcohol under the magnet of the third relay but I recalled the @rfloop's advice leaving them perfectly dry. I'm attaching some images taken by a microscope and as you can see they are not perfecrly clean

I was also thinking about replacing the battery because I've had the instrument for more than 7 years and I also bought it as a second hand tool and I don't know if it's worth doing this operation now. The battery is a Panasonic CR2477 and measures precisely 3.06V

[MarkL]

...
I was also thinking about replacing the battery because I've had the instrument for more than 7 years and I also bought it as a second hand tool and I don't know if it's worth doing this operation now. The battery is a Panasonic CR2477 and measures precisely 3.06V

Whether you decide to replace the battery or not, I would recommend doing the correction constants backup procedure, mentioned previously in this thread.  Some of the numbers may change slightly depending the repairs needed for the attenuator, but at least you'll have them in case the battery dies unexpectedly.  It's free and takes 10 or 15 minutes.

As far as the bad actuator is concerned, I think it would be worthwhile to compare the driving waveform on the bad actuator against a good actuator when switching in and out.  It could still be a mechanical issue, but maybe there's a problem with the driver.

[rf-loop]

I still have the HP8595e disassembled on my bench.


@rfloop thanks for the detailed image, I am checking again near the red arrows with a microscope. Honestly before reassembling it I was thinking of spraying something like WD40 or isopropyl alcohol under the magnet of the third relay but I recalled the @rfloop's advice leaving them perfectly dry. I'm attaching some images taken by a microscope and as you can see they are not perfecrly clean

Yes, do NOT use something like WD40 there. There is danger that it can also go inside attenuator cave through nylon pin actuators holes.
These low pressure contact surfaces there must be absolutely clean and dry from any kind of oil. Oil on contact surfaces may delay or even prevent contact and also contact surfaces "self cleaning" do not anymore work properly (because oily surface collects and keep all possible small dust particles). Also if open attenuator cave these contact surfaces need keep clean and NO OIL.
There is nothing what need oil after attenuator is manufactured! Some wd40 etc can do only bad things or very bad things.

It need also keep in mind that many RF circuits may have some ceramic etc materials what need keep untouched and newer ever use some WD40 or similar things... after they are absorbed to these materials then high-frequency performance values are even permanently destroyed/changed.

In your images it looks like enough clean.
I have some old day measured these nylon needles movement dimensions and how much it tolerate reduced amount of movement in both ends.
It tolerate ~0.1mm ... I know it because in some emergency case I have temporarily used some ~0.1mm too thin O-rings and check that it works well including still enough "marginal" for reliable contacts. But in this case longevity is perhaps highly reduced if attenuator is under heavy use (continuous stepping in a short period of time, such as in some test applications of product lines, etc) (these O-rings and the upper and lower surfaces of the nylon needle also act as dampers where the kinetic energy turns into heat and can at worst fry these rings - you know when you listen to the buzz of the attenuator when it is driven continuously 24/7 at maximum speed (controlled via HP-IB under test automation system)

[scopeman]

On the cal constants I made an excel spreadsheet for recording those for my HP8593E and 8593E. I have attached a copy along with one I found years ago Hopefully it will be useful to you.

Sam

[scopeman]

Missed the last two.