Re: Help with 7A18 problem

Bill Carns
 

Now think I will, when I get time today, go out and study some more, reseat those attenuator modules, then try and jigger those attenuator covers back on and try some careful testing - now that I know more about what I am looking for.

Then I will read up in the manual and make a decision on what I am going to try and do. You guys have all been really helpful. Nice group.

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.