Re: Help with 7A18 problem

Phillip Potter
 

Please disregard!!!

Sorry,

Phil

On 3/3/2019 1:38 PM, Phillip Potter wrote:
Hi Bill,

I have the same amplifiers and when I first got them, had the same or at least similar issues.

I tore them down, one at a time, and soaked 24lb bond with IPA, to clean those switches.  Yes, they are a bear to get to!  IIRC, I had to disassemble them down a ways to get into them.  There were shields on the inner sides and on the outer sides, too.  Once I had them broken down, it was a simple thing to get the soaked paper into the switch fingers and carefully pull it through, in the same way that a turntable moves a record under the stylus... never backward!

I recommend doing this somewhere where you can leave it laying, disassembled, so that you can see how you got it apart, making it easier to reassemble it.  I took pictures on my phone at each step of the way.  It was a "trick" of a deal getting it broken down to the point where it could be cleaned, however.  Pictures helped me to stay on track and get it all back together!  Tektronix did it's homework... they were made to be repaired... gotta love 'em!

I hope I've inspired you and not the opposite.  I did have to work up my nerve to tear into them, but the result was two operable amplifiers, I am glad that I took my time and worked carefully.

Phil

On 3/2/2019 2:26 PM, Bill Carns wrote:
I'll have to go put the unit back in the mainframe and investigate but my memory says that most were bad.

I have investigated more and remove the upper capacitor cover and, boy are those things hard to get at.  Gonna be real hard to get cleaning paper in there on all the switches.  Some are not bad, others really bad to get to.  One set has an additional little cover over them for some reason.

Working on this thing is not for sissies.


B

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Saturday, March 02, 2019 3:53 PM
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.