Re: Help with 7A18 problem

Bill Carns
 

What ?? Now confused. Was there something wrong with your first reply?

Bill

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Phillip Potter
Sent: Sunday, March 03, 2019 3:46 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Help with 7A18 problem

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








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