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How to FIX the Fuzzy/Wide Trace on Your Tek 465 or 475 Scope!



Cleaning the Volts/Div Attenuator Switch Contacts in Tektronix 4XX Oscilloscopes


It might work wonders for your classic Tektronix 400-series scope.

And you can probably do it yourself, even if you've never opened a scope, before!

(C) Copyright 2004, by Thomas P. Gootee


Here is a copy of the procedure that I sent to a guy with a Tek 475 that had a fuzzy trace, and "distorted" when he adjusted the volts/div knob. His original email is below, followed by my reply.


--------- Original message --------
From: "Mike D"
To: "tomg@fullnet.com"
Date: 01-30-03 08:41

Do you repair scopes? I have a tek 475 that powers up but is very fuzzy and distorts when adjusting the volts/division knob.

I don't know if its worth repairing or if you can reccomend a better scope from your stock for a comparable price.

Thank You

Mike

Mike,

Yes. I do repair *lots* of oscilloscopes. I do it mainly for in-house instruments that are to be resold. But, I also do it occasionally on a fee-for-service basis.

The symptoms you describe sound familiar and it sounds like a pretty easy repair job, which I could complete for $50. However, depending on your location, if you are in the continental USA, the shipping could be $20 to $30 or more, each way.

So, let me tell you what I would try first and maybe you can do it yourself. (I'd hate to see someone spend more than $100 for something that they might be able to do in less than 30 to 60 minutes.)

Without seeing the scope myself, I could easily be wrong. But, from your description, my opinion is that, most-likely, the switch contacts in the volts/div attenuators (and/or also in the trigger-view and bandwidth-limiter) are dirty or oxidized.

They are very easy to get to and to clean. And, although it might sound a little daunting to a first-timer, it really isn't difficult at all, requires only very-basic tools and supplies, and will probably work wonders for your scope's performance.

Here is the basic procedure:

Unplug the scope and place it on a large, flat, smooth surface, such as a table or floor.

Remove the scope's case: Start by removing the four screws in the rear panel's cord-wrap feet. They may be either Phillips-head or "star" (Torx) types. The feet may come off along with the screws. There are usually also two other screws, near the top and bottom of the rear panel, in the center. Remove them, too. After all six of the screws have been removed, rotate the carry-handle up and out of the way of the front panel, probably to the "straight up" position.

It is recommended that you stand the scope on its front panel end and pull the case up and off. But, if you DON'T have a front panel COVER, you can either try it anyway, hopefully on a padded surface (and carefully!), or else just leave the scope sitting on its feet.

From the rear of the scope, with your fingers on the sides of the case, push on the rear panel with your thumbs. If the scope is not standing on its front panel end, you will need to be very careful, as the scope slides out of the case, to also support the front end, keeping it up, so the bottom front edge of the case does not damage any of the internal parts of the scope by scraping against them.

Note that if the scope's CASE is DENTED, especially if it's dented on a corner-edge, it could be *VERY* difficult to remove the scope from its case! It might then require *QUITE* A BIT of brute force, pushing on the rear panel (or pulling on the edges of the front panel) and pulling on the rear edges (or pushing on the front edges) of the case. (Sometimes, in that situation, it helps to alternate between trying to move the left and right sides; and maybe even also to alternate between moving the top and bottom.)

After the scope is out (whew!), see the note in the next paragraph, and then carefully stand the scope on its SIDE, so that the vertical board, on the side of the scope, behind the v/div knobs, is facing UP.

NOTE! I usually try to lift and move the scope's internal assembly ONLY by the edges of the front panel (or CRT bezel) and rear panel, or other solid structural/steel parts. And be careful not to set the scope on any objects or uneven surfaces that might touch any of the internal components.

Just behind the volts/div knobs, inside the scope, there are two silver, metal "boxes", which house the attenuator/switch assemblies.

Remove all of the necessary screws to open the top of one of the boxes. (Do only one box at a time.)

Inside the box, there are four little rectangular, plastic "attenuator modules", each usually labeled with "somenumber X" (e.g. "4X"). Using a longnose pliers, or something similar (or whatever works), grasp one of them and pull *straight* up, to remove it. I'm not sure if they're static-sensitive, or not. But, just in case they are, I would lay it either on an all-metal part of the scope, or on a piece of aluminum foil, or something like that (i.e. on bare metal), with the pins facing down.

I usually only remove one attenuator module at a time, just so I don't get them mixed up. But you could do two at a time, if it's more convenient (which it often is).

While looking at the area of the circuit board where the attenuator module came from, rotate the corresponding v/div switch back and forth, until you can see which positions raise and lower the tiny gold U-shaped double-contact sets, up and down, from/to the circuit board.

Cut a small piece of clean, white paper, as lint-free as possible, into a thin strip, a little wider than one set of contacts, and long enough for you to handle/manipulate (probably something like three or four inches long and 3/16"-or-so wide?). Do not use cloth, which might snag, or anything abrasive. (You may need to cut new pieces, periodically, as you go through this procedure.)

Turn the volts/div knob, to raise the set of contacts that you're about to clean, up and away from the board.

Carefully slide one end of the paper under the contacts.

Apply a drop or two (or three) of isopropyl (rubbing) alcohol to the paper, under the contacts. It's OK to get it on the contacts, board, etc, too. I assume that the more concentrated the isopropyl is, the better. But I've had good results using the plain 70% drugstore type, and the 90% drugstore type. (The results would probably be even better if you use Caig's "De-Oxit" product, for the cleaning, followed by Caig's "Pro-Gold". They're available at Caig's Website.)

Immediately turn the volts/div switch so that the contacts are lowered against the paper and then (carefully!) pull the paper *straight* out from under the contacts. This should probably be repeated several times for each set of contacts.

Be careful. If you bend the contacts, you may be in trouble. And they are *quite* fragile. But, it should be fine as long as you try to pull the paper as parallel to the board as possible. Certainly, don't ever pull straight *upward* on it, and not even more upward than sideways.

Also, NOTE that a couple of the contact-pairs, the two that are nearest to the front of the scope, are raised and lowered by the AC/GND/DC lever-switch, instead of by the volts/div switch.

When all of the visible contacts have been cleaned that way, replace the attenuator module, carefully, by pushing its pins back into the holes in the circuit board and then pushing it all the way in. (It might not hurt to also clean the attenuator module's pins, although I've never needed to, nor tried to.)

[If you *DO* happen to have some of Caig's "De-Oxit" product, it might be good to apply a tiny amount to each pin of each attenuator block. I usually also apply it to the metal shield, wherever it makes contact with the screws or their mountings (and the the screw-posts/mountings themselves), and wherever the shield makes contact with any other metal surfaces, and also on those metal surfaces themselves. And the screws themselves should also be treated, as should any nearby "ground" screws (i.e. loosen, treat/clean, and re-seat). ALSO, make sure that you treat the grounding "spring" that protrudes up from one of the screws, where the crew touches it, and on its top where it touches the scope's case (and also do that part of the inside of the case). You can often SEE the difference in color of the metal, as the oxidation is removed, especially if you use the concentrated form of De-Oxit that comes in the small plastic "needle-dropper" bottle.]

Replace any attenuator modules that won't be in the way, any more. Then, remove the next attenuator module(s) and repeat the above procedures, until you've gotten to all of the switch contacts in the "box".

When one whole channel's switches are done, replace the metal cover over the attenuator/switch assembly and do the second channel in exactly the same way (or, what the heck, maybe *better*, if that was your first time...). [NOTE that the "fingers" that may be on one side of the shield/box need to go back into the same position that they were in before the top was removed.]

Now that you know what the tiny, gold switch contacts LOOK like, you will be able to much-more-easily find the SIMILAR ones that are on the SAME circuit board, but are toward the REAR of the scope, near the middle of the board. There is no cover over them. So they're MUCH easier to get to. There should be one group of four of them and another group of two of them. Moving the Bandwidth-Limiter and Trigger-View controls, on the front panel, will raise and lower them. Clean them in the same way as described above, for the attenuator switch contacts.

One OTHER thing to check: (I'm not sure, at the moment, if this is only for the 465 model, or if it also applies to the 475:) JUST behind the metal attenuator/switch shield "boxes", there may be a screw that connects a metal piece of the box/shield to the circuit board. If that screw is loose (which I have seen on about a third of the 465 scopes I've worked on!), or if it doesn't make good contact, you may see very noticeable effects in the trace(s), such as fuzziness or distortion, and may see trace-distortion effects when adjusting the trigger-level control (possibly especially-so at the highest frequencies), and possibly other triggering problems.

If cleaning the switch contacts doesn't fix the problems you're having with the "fuzzy" or wide trace, and the attenuator switch anomalies, further diagnosis and repair may be necessary.

Let me know if you try this, and how it works. If you would still like me to work on your scope, just let me know.

And let me know if I can be of more help.

Thanks again!

Best regards,

Tom

Tom Gootee
tomg@fullnet.com
http://www.fullnet.com/u/tomg


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