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 Post subject: Tek 7xxx Mainframe - 7A26 input problems
PostPosted: Sep Tue 28, 2021 6:29 pm 
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Joined: Oct Tue 29, 2013 9:42 pm
Posts: 453
Location: Fountain Valley, CA 92708
I have a 7A26 plugin for my 7704A that is misbehaving and wanted some help finding the starting point to the debugging process. The problems are as follows:

(1) The top channel "IDENTIFY" button does not work. I've been through the manual and schematics -- admittedly, not thoroughly -- and am unable to find the "secret workings" of that device. I pick up the push button at the CH 1 AMP schematic where is says "probe identify from R13" but cannot seem to locate R13. et seq.

Also, if anyone knows the theory of how this works, I would be much obliged.

(2) The top channel (except for the "IDENTIFY" problem) works fine. The bottom channel does not. Plain and simple the display is faulty. The two channels are shown below for comparison and taken from the 4V 7044A frame square wave standard. Both are measured with a stock Tek P6131 probe. Any thoughts on this would be welcome.

Thanks in advance.

Pictures of the good/bad channels are below.


Attachments:
Teh 7A26 top input.jpg
Teh 7A26 top input.jpg [ 152.66 KiB | Viewed 144 times ]
Teh 7A26 bottom input.jpg
Teh 7A26 bottom input.jpg [ 183.03 KiB | Viewed 144 times ]

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 Post subject: Re: Tek 7xxx Mainframe - 7A26 input problems
PostPosted: Sep Tue 28, 2021 10:22 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11285
Location: Sunbury, Ohio 43074
Are you using the same probe for both channels? If not, the second channel probe needs its compensation adjusted. If so, the input amp for channel two probably needs its switches cleaned.

Switches are responsible for a great many "failures" in this type of Tek gear ... and some of those switches are devilishly difficult to clean, particularly the push buttons. I usually drill a small hole in them with a hand pin vise, and squirt some contact cleaner in there.

BUT WAIT ... before you do this, exercise each and every switch, button, and control a few hundred dozen times till your hands fall off from the effort. That alone might do the trick.

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