Hello Ken,
I also have a Tek 454. I bought that model
because I read on groups like this that it is "easy"
to work on. I must respectively disagree with you.
I came across this on the 'net, which may or may not be useful in your
particular case, John...
How to FIX the Fuzzy/Wide Trace on Your Tek 465 or 475 Scope!
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Cleaning the Volts/Div Attenuator Switch Contacts in Tektronix 4XX
Oscilloscopes
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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
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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.
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--------- Original message --------
From: "Mike D"
To: "
[email protected]"
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
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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
[email protected]
http://www.fullnet.com/u/tomg