Re: Help with 7A18 problem

 

Those inserts into which the attenuators plug could be miniserts or related to them. If they are, the outside is tin/lead plate, but inside there's a gold plated spring BeCu contact.

See: <https://groups.io/g/TekScopes/files/Minisert.pdf>

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 03 March 2019 18:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

That is good news, Bill, well done. Now that you have achieved that, why not sort the other channel, then you will have a working 7A18?

I wasn't aware that the attenuator modules had gold-plated contact pins, but it doesn't surprise me. What *does* surprise me is that the pcb sockets are tin-plated. I would have thought that they would be gold-plated, as is most of that board. Considering that the board is located, circuit-wise, right at the front of the scope, one would expect that the greatest care would have been made to ensure the best possible contacts. There is a lot of care there, what with the complex and fiddly-to-put-back-together screening and Tek could have done better.

I think I did warn you about the care needed in re-seating the attenuator modules, didn't I?

The pictures that Peter linked to on TekWiki were excellent info. I was wondering how to get some pictures, but completely overlooked that TekWiki might have just the ticket!

Something that occurred to me today, after what Albert pointed out about the 5mV/DIV setting being possibly compromised by dirty switch contacts, was that it is possible to see if the attenuator assembly has any problems by putting an appropriate signal into the input and a scope probe on to the output, which is the junction of R130, C210 and R210 or the equivalent components on the other channel. This of course requires either an 7000-series extender or some other way to access the 7A18 plug-in with a probe, plus another fully-functioning scope. Turning the VOLTS/DIV switch will allow examination of the signal-path, to some extent. This could be achieved without an extender, with one scope of the 7000-series having two vertical amplifiers, by removing both the side-panel of the DUT amplifier and the left side-panel of the mainframe. If the DUT amplifier plug-in is in the left bay, then I think it would be possible to connect the scope probe from the working vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting the signal from the DUT amplifier, though this may be of use. Is there any sense in my suggestion, or have I got it completely wrong?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 17:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Good news all. I went out and first inspected and learned more, then I removed (one at a time) the attenuator modules and reseated them. That is the short version of a longer story. The pins are very thin and bend easily if the modules are not removed straight up. That was a learning lesson that I leave in the thread here. The PC board holes have almost no taper and you have to have those pins really straight to get them back in.

In stating a little about my background before, I did not mention that I worked for almost half my career in the semiconductor industry, first in process modeling and device development, then running a RF Power design group, then running a manufacturing and business operation. From that perspective, I now have another observation. Three of the modules showed good gold plating on the pins, The PCB female sockets are tin plated. This was not the best thing TEK could have done. One of the modules showed very little remaining gold on the pins. This is typical of a Gold tin contact system after many years. Gold migrates easily and eventually forms a eutectic with tin. The result can be depleting of the Gold on the pins and worsening contacts that are much more prone to oxidation.. I am guessing that is what happened here.

Finally, in retrospect, my recommendation would be that those attenuator modules not be pulled all the way out to "reseat" them.

The reseating fixed the channel I worked on. I had not removed the cover of the other channel. I think I am going to save this plugin that had two bad channels (now just one) for parts and I am ready to attempt to fix (hopefully just reseat) the one better plugin that had just one bad channel.

Boy am I glad I apparently do not have to tilt with that contact cleaning windmill.

Thanks and I will come back with the final chapter after I clean up the other one (or not).. :-)

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 5:48 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Bill,

The attenuator modules are the little rectangular things a bit like Chiclets - there are four of them. They will have something like "307-10xx-0x" (the part number) written on them as well as "2X", "4X", "10X" or "100X", i.e. the attenuation value. As I said, they just pull out using your fingernails; there shouldn't be anything stopping that, once the thin ally cover is off. I'm not sure what you are calling the "scaling shaft", but I don't see anything like a shaft on mine. EDIT: I think I have worked it out - is this shaft going to a pot at the rear of the plug-in and is for the "POSITION" and "IDENTIFY" controls? If that is so, then yes, it will need to be removed to get the cover off, probably. My 7A18 has its "POSITION" and "IDENTIFY" controls mounted on the front and the four-way ribbon cables with the ferrite rings connect them to the circuit-board.

No! don't get Deoxit anywhere near that board. It is made from polyphenylene oxide and it has "excellent electrical characteristics" as stated in the manual. It also says, regarding cleaning it "use only water-soluble detergents, ethyl, methyl or isopropyl alcohol". One assumes that any water-soluble detergents used would need to be thoroughly removed, too (probably with deionized water).

I sort of wish I hadn't poked around in mine, because that cover over the module is a swine to get back on properly. To make sure that the "springy fingers" all go back to the right place, the top of the frame pretty much has to come off, unless you have really tiny fingers with lights on the ends. As I said, that entails taking all of the knobs off the front panel and popping it off. A small screwdriver is useful to prise at top and bottom. I neglected to say that there are also two countersunk screws which fix brackets for a circuit-board which have to be removed, plus another countersunk screw which fastens the earth-strap and an earth tag. These are easily removed, but really fiddly to put back together again (I still haven't got that fixed yet). Why did Tek make some things so easy to maintain and others such a pain?

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Bill Carns
Sent: 03 March 2019 11:06
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Quick comment. No smoking at all here. I have had that scope and plug ins for over 30 years. No Diesel engines and very very clean environment (Go to K0CXX.com) so not sure what caused this issue to rear its head. I am now wondering if just a good reseat will work - with perhaps just a very carefully applied tiny bit of Deoxit on the pins. What do you think of that?

I will read more but I take it the scaling shaft will have to come out before that attenuator module is pulled and reseated. All material and plugins are in shop a couple hundred feet away and it is middle of night here. :-)

B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: Sunday, March 03, 2019 4:40 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

Albert,
I think you are right about the 5mV/DIV setting, thinking about it. However, I believe I remember something about detecting which attenuator module(s) are the culprit(s) by finding which VOLT/DIV settings show problems, referring to the info in the manual. It seems that they are all bad, as Bill hasn't indicated that they differ very much. This seem pretty improbable to me, especially since some channels are fine and some not. I wonder if this scope has been used near to a badly-maintained diesel engine or by a chain-smoker?
Anyway, the problem might be getting solved by having a good 7A18 from Jim.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 02 March 2019 21:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

On Sat, Mar 2, 2019 at 07:58 PM, Colin Herbert wrote:


---
On the subject of the input attenuators, they are the type which are
operated by cam-switches. They have springy gold-plated contacts that
get moved up-and-down by cams on the controls shaft. The circuit board
that they are on has gold-plating, too, and the board itself is made
of an easily-damaged material. You need to use thin strips of paper
moistened in IPA and trapped between the moving and stationary
contacts, they *gently* pulling the paper strip out. You may have to
do this a few times. Don't use any other solvents or contact-cleaner
as you will probably ruin the board. Of course it may only be one or
two of these contacts that is dirty, as has already been suggested;
there are only four attenuators there - they get switched in-and-out
to get the different VOLTS/DIV settings. In the most sensitive
VOLTS/DIV setting, they are all switched out, so if your 5mV/div
setting shows lousy bandwidth, then it is likely to be caused by some
other problem, not just the attenuators and their switching . Of course, there might be a problem there, too, but it won't be the only one.
Hi Colin,

You probably were too quick in your statement about the 5 mV/div setting. In that setting all attenuators are bypassed by means of a series of closed contacts, so any dirty contacts there destroy the signal path.
I'm not sure about the 7A18 construction. My impression is that including AC/DC there are 10 contact pairs above the drum and 10 below the drum. (Each contact pair forming one switch).

It would still be nice to hear from Bill which V/div settings are good and which are bad.

Albert

Join TekScopes@groups.io to automatically receive all group messages.