Tektronix 475 scope: vertical position or var v/div problem

[Thomas P. Gootee]

Tektronix 475 scope: vertical position or var v/div problem
-------------

I have a Tek 475 that has a problem with Channel 1 only:

The Ch 1 variable v/div knob moves the trace just about as well as the
vertical position knob does.

With the var v/div knob locked in cal'd position, the trace is off the
top of the screen and can not be brought all the way down onto the
screen with the vertical position knob.

Also, when the var v/div knob is used to bring the trace down into the
viewable area, the vertical position knob can also move the trace, but
can't move it below wherever it's positioned by the variable v/div
knob.

Does anyone happen to know where I should start looking?

I do have the service manual. But I'm not extremely familiar with the
475's circuitry (yet). So any pointers and ideas will be greatly
appreciated.

Thanks!

Tom Gootee

----------

 

[Jim Yanik]

[email protected] (Thomas P. Gootee) wrote in

Tektronix 475 scope: vertical position or var v/div problem
-------------

I have a Tek 475 that has a problem with Channel 1 only:

The Ch 1 variable v/div knob moves the trace just about as well as the
vertical position knob does.

With the var v/div knob locked in cal'd position, the trace is off the
top of the screen and can not be brought all the way down onto the
screen with the vertical position knob.

Also, when the var v/div knob is used to bring the trace down into the
viewable area, the vertical position knob can also move the trace, but
can't move it below wherever it's positioned by the variable v/div
knob.

Does anyone happen to know where I should start looking?

I do have the service manual. But I'm not extremely familiar with the
475's circuitry (yet). So any pointers and ideas will be greatly
appreciated.

Thanks!

Tom Gootee

Sounds like a DC balance problem.
Could be a bad input FET pair,or farther down the chain.
When you use the VarV/div knob,you're just reducing the gain to lessen the
offset,I believe.

 

[Van Gardner]

Tektronix 475 scope: vertical position or var v/div problem
-------------

I have a Tek 475 that has a problem with Channel 1 only:

The Ch 1 variable v/div knob moves the trace just about as well as the
vertical position knob does.

With the var v/div knob locked in cal'd position, the trace is off the
top of the screen and can not be brought all the way down onto the
screen with the vertical position knob.

Also, when the var v/div knob is used to bring the trace down into the
viewable area, the vertical position knob can also move the trace, but
can't move it below wherever it's positioned by the variable v/div
knob.

Does anyone happen to know where I should start looking?

I do have the service manual. But I'm not extremely familiar with the
475's circuitry (yet). So any pointers and ideas will be greatly
appreciated.

Thanks!

Tom Gootee

----------

If I had your problem I would set the controls for both channel 1 & 2
to the same position and compare the voltage at corresponding point
through out their circuits. When you find a difference this sould put
you in the ball park. At least the working channel 2 will let you
know what the voltage should be.

Van Gardner

 

[Thomas P. Gootee]

[email protected] (Thomas P. Gootee) wrote in


Sounds like a DC balance problem.
Could be a bad input FET pair,or farther down the chain.
When you use the VarV/div knob,you're just reducing the gain to lessen the
offset,I believe.

Thanks, Jim.

- Tom Gootee

 

[Thomas P. Gootee]

If I had your problem I would set the controls for both channel 1 & 2
to the same position and compare the voltage at corresponding point
through out their circuits. When you find a difference this sould put
you in the ball park. At least the working channel 2 will let you
know what the voltage should be.

Van Gardner

Van,

Good thinking! And, actually, since channel 1 itself is differential,
there are two more-or-less identical curcuit paths through IT, too,
that can be compared similarly.

While I was doing that, I got the following email that NAILED it,
posted here for others who may experience a similar problem, in the
future (so it can be found by searches of the newsgroups' archive,
currently at http://groups.google.com [formerly at
http://www.deja.com, formerly http://www.dejanews.com]).

After that, it turned out that as I was varying R110 (the Ch1 Var DC
Balance adjustment pot), while measuring voltages at various points,
the scope suddenly started working perfectly.

Thanks for the response, Van.

Regards,

Tom Gootee

http://www.fullnet.com/u/tomg

---------- Copy of the email I received:

Variable volts balance is a calibration adjustment on the 475.

R110 is the CH1 Var Bal control. The wiper goes to ground, while the
ends go
to decoupled +5 and -8.

The voltage at U120-1 should be close to 0V, according to the
schematic.

Given the age of 475s (I worked on them when I started at Tek) my
first guess
would be that the wiper of R110 has opened and is letting the bias
level
drift, or that one of the LV supplies has drifted and has forced the
bias off
center, though I would expect that to show up in other places, too.
If it's R110 wiper, just whipping it back and forth a few times may be
enough
to make contact again, at least temporarily.

 

[TekMan]

...snipped for clarity..

---------- Copy of the email I received:

Variable volts balance is a calibration adjustment on the 475.

R110 is the CH1 Var Bal control. The wiper goes to ground, while the
ends go
to decoupled +5 and -8.

The voltage at U120-1 should be close to 0V, according to the
schematic.

Given the age of 475s (I worked on them when I started at Tek) my
first guess
would be that the wiper of R110 has opened and is letting the bias
level
drift, or that one of the LV supplies has drifted and has forced the
bias off
center, though I would expect that to show up in other places, too.
If it's R110 wiper, just whipping it back and forth a few times may be
enough
to make contact again, at least temporarily.

Yes, that's quite commmon in the older tektronix 400 series scopes.
Since the 475 series scoeps you find nowadays are approx 20 to 25
years old (some even older), it's always good to have a service manual
handy.

Free download from : www.logsa.army.mil (search in the TM section,
tektronix, 475 type with Option 4 ).

hth,
Andreas

 

[Thomas P. Gootee]

Yes, that's quite commmon in the older tektronix 400 series scopes.
Since the 475 series scoeps you find nowadays are approx 20 to 25
years old (some even older), it's always good to have a service manual
handy.

Free download from : www.logsa.army.mil (search in the TM section,
tektronix, 475 type with Option 4 ).

hth,
Andreas

----------

Andreas,

Actually, I have at least FIVE DIFFERENT TEK 475 MANUALS, available in
PDF format, at:

http://www.fullnet.com/u/tomg/manuals.htm

Regards,

Tom

----------

 

[TekMan]

...snipped tom's ad ...

Tom

Tom,

nobody is interested to know about this.
Have your fun fun with your treasures.

And more free downloads from:

http://www.datasouthservices.com


scnr,
Andreas

....let's feed the trolls..

 

[TekMan]

..snipp..

Andreas,

Actually, I have at least FIVE DIFFERENT TEK 475 MANUALS, available in
PDF format, at:

Well, suppose you read them to find the circuitry described. This
would have already answerded your question about the dc balance.

rtfm, tom!

Regards,
Tom

before you ask what the abbrev. rtfm means: read the fine manual...


scnr,
Andreas

 

[Sam Goldwasser]

And more free downloads from:

http://www.datasouthservices.com

It seems you make people jump through some hoops before even any indication
of what you have available or have I missed something?

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the feedback form on the S.E.R FAQ Web sites.

 

[Thomas P. Gootee]

..snipped tom's ad ...

For those who might need or want a PDF service manual, I will
re-insert the URL:

http://www.fullnet.com/u/tomg/manuals.htm

*** Disclaimer: "Not EVERYTHING on my website is FREE." ***

Tom,

nobody is interested to know about this.



scnr,
Andreas

...let's feed the trolls..

Andreas,

Your statement is just silly. *YOU* may not be interested in knowing
about it. But you shouldn't claim to speak for EVERYONE. For myself, I
have proof to the contrary, anyway: According to my records, WELL over
a thousand people have purchased manuals from me, in just the last 18
months or so. ALSO, according to the webpage hit-counters that I used
to use, I would typically get AT LEAST several hundred more website
visits per day, for a while, whenever I posted the URL in a newsgroup.
(Silly me, for not doing it more often.)

Thanks for reminding me about that other site. They had been down for
months and I had given up trying to use it.

Thanks for the "entertainment".

Tom

-----------------------------

 

[TekMan]

Your statement is just silly. *YOU* may not be interested in knowing
about it. But you shouldn't claim to speak for EVERYONE. For myself, I
have proof to the contrary, anyway: According to my records, WELL over
a thousand people have purchased manuals from me, in just the last 18
months or so.

Yes, as the advertisement says: Shit is real could. Proof: A million
flies cannot be wrong.



go home and play with your manuals, Tom.

Thanks for reminding me about that other site. They had been down for
months and I had given up trying to use it.

Nothing to thank for. I always help such a fine guy like you are. So
use your time better (instead of responding in the group to topic)
and download there as much as you can. And charge the poor buddies who
buy from you the stuff later.


/* end */

 

[TekMan]

It seems you make people jump through some hoops before even any indication
of what you have available or have I missed something?

Hi Sam,

well, for the first time one has to do this longhaul procedure (folow
instruction on the left of the webpage).

Once you have a userid and password, the procedure is much shorter.

hth,
Andreas

 

[Thomas P. Gootee]

[email protected] (TekMan) wrote in message

Yes, as the advertisement says: Shit is real could. Proof: A million
flies cannot be wrong.

The POINT was whether anyone was interested or not. They are.

go home and play with your manuals, Tom.

I always am.

Nothing to thank for. I always help such a fine guy like you are. So
Right.

use your time better (instead of responding in the group to topic)

What about you?

and download there as much as you can. And charge the poor buddies who
buy from you the stuff later.

No need to download from there, for manuals to sell. But maybe I'll
take another look.

/* end */

Finally.