Date   

Re: Starting Tek 7854 repair. General guidelines needed

Verne Morrow
 

Hi,
Here is a copy ( if it doesn't fit in your mail box, try the ebama
site or search the web for 7904power supply service.
All the 7xxx units used the same theory for the power supplies, only the
details changed slightly, so this is a invaluable
manual to have around. My 7854 has a main regulator with a carbon arc
across the pass transistors' base-emitter area,
apparently from a attempted repair - I haven't gotten back to it yet
among all the other projects, but this manial is the
place to start.
Verne Morrow

@rodd [TekScopes] wrote:

Dear all.
I finally managed to get my hands on a Tek 7854 in a "decent overall
contidition".
They are rare in Brazil, where I live.
I bought a copy of the service manual and plan to start to check it soon.
The seller sold it "as is",since he tried to repair but failed, which
resulted in a 70% reduction in price.
He told me that it would turn on for a few minutes than shut down at
first, but does not turn on anymore.
I have been reading all trends about 7k and 5k scopes I could find,
but I must confess I have not figure out how to search the data in an
intelligent way. Everytime I try I get no results.
I have 3 major questions:
1 - best procedure to turn unit on
- variac?, 60W lamp?
- how much time "warming up"?
2 - What are the components most prone to failure and/or should be
replaced anyway?
- alluminum electrolytics? all? Power supply only?
- tantallums?
3 - Will I need extensors to calibrate the unit/plugins? If
affirmative which ones
- extensor number and quantity
Note( I am taking one step further because I will have to buy them @
Ebay and they will take some time to get here.


Thank you in advance for any help.
Roger



[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]


Re: Starting Tek 7854 repair. General guidelines needed

Artek Manuals <manuals@...>
 

Roger

DO NOT use a variac or light bulb in series with the AC mains this unit
like most ( all? ) 7000 series uses a "switch mode" power supply which
are not designed for this kind of Low voltage starting sequence. If
there are shorts on the LV rails the power supply automatically goes
into a limiting mode to protect itself from shorts (often you will here
a ticking sound). Once powered up you should check all the DC power
coming out of the supply +108 on the preregualtor, and +/- 50V, +/-
15V and +5V. There is a diagram towards the back of the Schematic
section showing the test points. Under normal conditions the supply
should be up to voltage in less than 30 seconds

If the power supply voltages are off then the supply can be removed for
troubleshooting outside the scope. A set of suitable loads must be
placed on the supply when operated outside the scope. The switch mode
supply will not operate correctly unloaded. There is a document floating
around that outlines the way to do this , I don't have the power supply
loading procedure but another list member will no doubt point you to it
and Google is also your likely friend

Aluminum Electrolytic and Tantalum caps are the enemy most often found.
If your personal time is worth anything an ESR meter pays for itself the
first day. If the power supply volts are on and the supply is ticking
and a rail appears to be shorted then a cap is likely died somewhere
else in the scope.

Extensions certainly make calibrating the plug-ins easier though many
can be reached with the covers off the mainframe. As I recall you do not
need any extensions to reach the CAL/Adjustment points on the of the
mainframe . The service manual discusses the use of the calibration
plug-in 067-0589-02 but some stadard plug-ins can be used for thios
prpose as well.

You do not say which plug-ins you have? It is a good idea to start with
known good plug-ins the symptoms you describe can be a function of the
plug-in and not the mainframe. If it were me and the 7854 was my only
7000 series scope I would find a friend with a 77xx or 79xx series
mainframe and test my "unknown" plug-ins in a working mainframe this is
the easiest way to eliminate the plug-ins

Enjoy and good luck

Dave

manuals@...

On 2/18/2017 10:01 AM, @rodd [TekScopes] wrote:

Dear all.
I finally managed to get my hands on a Tek 7854 in a "decent overall
contidition".
They are rare in Brazil, where I live.
I bought a copy of the service manual and plan to start to check it soon.
The seller sold it "as is",since he tried to repair but failed, which
resulted in a 70% reduction in price.
He told me that it would turn on for a few minutes than shut down at
first, but does not turn on anymore.
I have been reading all trends about 7k and 5k scopes I could find,
but I must confess I have not figure out how to search the data in an
intelligent way. Everytime I try I get no results.
I have 3 major questions:
1 - best procedure to turn unit on
- variac?, 60W lamp?
- how much time "warming up"?
2 - What are the components most prone to failure and/or should be
replaced anyway?
- alluminum electrolytics? all? Power supply only?
- tantallums?
3 - Will I need extensors to calibrate the unit/plugins? If
affirmative which ones
- extensor number and quantity
Note( I am taking one step further because I will have to buy them @
Ebay and they will take some time to get here.


Thank you in advance for any help.
Roger



[Non-text portions of this message have been removed]

--
Dave
Manuals@...
www.ArtekManuals.com


Re: Starting Tek 7854 repair. General guidelines needed

Craig Sawyers <c.sawyers@...>
 

1 - best procedure to turn unit on
- variac?, 60W lamp?
It is a switched mode supply, so you cannot run it from a variac. That is not entirely true, there
is a diagnostic procedure for the power supply that involves a variac, but you aren't close to
needing that test yet.

- how much time "warming up"?
None - it should come alive straight away.

2 - What are the components most prone to failure and/or should be replaced anyway?
- alluminum electrolytics? all? Power supply only?
- tantallums?
Tantalum bead caps used for rail decoupling (+/-15V and 5V lines are most prone)

3 - Will I need extensors to calibrate the unit/plugins? If affirmative which ones
- extensor number and quantity
Note( I am taking one step further because I will have to buy them @ Ebay and they will take some
time to get here.
Only for plugins. You won't need that for fault finding and calibrating the scope mainframe.

Craig


Starting Tek 7854 repair. General guidelines needed

 

Dear all.
I finally managed to get my hands on a Tek 7854 in a "decent overall contidition".
They are rare in Brazil, where I live.
I bought a copy of the service manual and plan to start to check it soon.
The seller sold it "as is",since he tried to repair but failed, which resulted in a 70% reduction in price.
He told me that it would turn on for a few minutes than shut down at first, but does not turn on anymore.
I have been reading all trends about 7k and 5k scopes I could find, but I must confess I have not figure out how to search the data in an intelligent way. Everytime I try I get no results.
I have 3 major questions:
1 - best procedure to turn unit on
- variac?, 60W lamp?
- how much time "warming up"?
2 - What are the components most prone to failure and/or should be replaced anyway?
- alluminum electrolytics? all? Power supply only?
- tantallums?
3 - Will I need extensors to calibrate the unit/plugins? If affirmative which ones
- extensor number and quantity
Note( I am taking one step further because I will have to buy them @ Ebay and they will take some time to get here.


Thank you in advance for any help.
Roger


Re: FIX: 475 lack of trace cathode voltage -1000V...

Mark Wendt
 

On 02/18/2017 01:42 AM, Chuck Harris cfharris@... [TekScopes] wrote:
Hi,

Here is a repair I just finished.

Symptom: Scope was working fine. The trace started winking
off after a while, only to return when power was cycled.
Eventually, it was gone all the time.

Measurement of the CRT cathode voltage showed that the HV inverter
was working, but was running at about -1000V... about 1/2 of the
normal -2450V.

The first regulator stage was showing the proper voltages, like it
was perfectly happy to be at -1000V. The subsequent stages were
looking like they were turned nearly totally off.

Fix: Tantalum capacitor C1304, a 2.7uf, 50V capacitor.

C1304 is regularly hit with 50.7V, albeit at low current, through a
240K resistor. Tektronix knew there was a problem here, and put a
diode, CR1304, to the +50V supply to clamp the voltage across C1304
to a maximum of 50.7V. Sorry, but that is over the capacitor's
rating, and way too close for comfort. (keep doing stuff like that,
and the scope might only work 45 years before failing ;-) )

The tantalum capacitor became leaky, and upset the voltage divider
comprised of the thin film resistor array R1303A, and R1303B. The
leaky nature of the capacitor pulled the 500K resistor closer to
ground than design required, and as a result made Q1306 think that
the CRT cathode was at design voltage when it was really only about
1/2 way there.

-Chuck Harris
Chuck,

Just being curious. What did you replace the cap with? Did you derate the voltage on the replacement?

Mark


Re: FIX: 475 lack of trace cathode voltage -1000V...

zerousair
 

Well done, and thanks for posting that.


FIX: 475 lack of trace cathode voltage -1000V...

Chuck Harris
 

Hi,

Here is a repair I just finished.

Symptom: Scope was working fine. The trace started winking
off after a while, only to return when power was cycled.
Eventually, it was gone all the time.

Measurement of the CRT cathode voltage showed that the HV inverter
was working, but was running at about -1000V... about 1/2 of the
normal -2450V.

The first regulator stage was showing the proper voltages, like it
was perfectly happy to be at -1000V. The subsequent stages were
looking like they were turned nearly totally off.

Fix: Tantalum capacitor C1304, a 2.7uf, 50V capacitor.

C1304 is regularly hit with 50.7V, albeit at low current, through a
240K resistor. Tektronix knew there was a problem here, and put a
diode, CR1304, to the +50V supply to clamp the voltage across C1304
to a maximum of 50.7V. Sorry, but that is over the capacitor's
rating, and way too close for comfort. (keep doing stuff like that,
and the scope might only work 45 years before failing ;-) )

The tantalum capacitor became leaky, and upset the voltage divider
comprised of the thin film resistor array R1303A, and R1303B. The
leaky nature of the capacitor pulled the 500K resistor closer to
ground than design required, and as a result made Q1306 think that
the CRT cathode was at design voltage when it was really only about
1/2 way there.

-Chuck Harris


Re: LF Compensation of a 468 scope

Chuck Harris
 

I don't have a manual on hand for the 465B, but I
thought one of you guys was emphasizing that they didn't
use the channel switch method on the 465B.

If they did, then I agree completely with you. They
decided that it offered some advantage.

-Chuck Harris

David @DWH [TekScopes] wrote:

On Fri, 17 Feb 2017 12:54:16 -0500, you wrote:

If there were other reasons than the stored waveform
display, they would have upgraded the method for the
465B too.

There is no practical reason to change things between
the two models. The 465B calibration method worked just
fine prior to the 468.

Remember the guys/gals that did the bulk calibrations of
these scopes were just wage slaves, calibrating thousands
of scopes each year. Needless changes would needlessly
cause mistakes to happen.
They *did* upgrade the method for the 465B; it uses the same method as
the 468 which is one of the reasons I do not think the newer
calibration procedure is related to the 468 stored display.

The older 464/465/466 did not use the newer calibration method and as
far as I know, they never changed it which does not surprise me simply
because Tektronix was very conservative about changing things on
existing products unless there was a significant problem. Instead
they designed a replacement product like how the 465B replaced the
465.

There has to be some other reason.

Perhaps the differences between a stored display of vectors
vs dots vs envelopes is the answer?

Or, you could simply crank the vertical output lf
compensation out of wack, and see what it did to the
stored display.

I'm betting on a big gross obvious difference.

-Chuck Harris
That is the test I was going to suggest. Mess with the vertical
amplifier compensation and see how it affects the stored display.


Re: Manuals Plus preservation efforts update (was: Re: Fwd: [TUHS] off topic: saving old manuals)

alberto.vaudagna
 

How the think has finished? Who has the manual now?

2015-08-18 12:04 GMT+02:00 Miguel Work harrimansat@... [TekScopes] <
TekScopes@...>:



My support from Spain!

De: TekScopes@... [mailto:TekScopes@...]
Enviado el: martes, 18 de agosto de 2015 7:30
Para: [TekScopes]
Asunto: Manuals Plus preservation efforts update (was: Re: [TekScopes] Fwd:
[TUHS] off topic: saving old manuals)

In Realtime: We are barely halfway done
<http://ascii.textfiles.com/archives/4711>
http://ascii.textfiles.com/archives/4711

They are still looking for volunteers tomorrow (Tuesday, August 18).

Anyone in or near Baltimore might want to go help.

On Sun, 16 Aug 2015, Soren Jorvang <mailto:@soren>
@soren [TekScopes] wrote:

Begin forwarded message:

From: Aharon Robbins < <mailto:arnold@...> arnold@...>
Date: 16 August 2015 at 20:22:17 GMT+2
To: <mailto:tuhs@...> tuhs@...
Subject: Re: [TUHS] off topic: saving old manuals

Sorry for the empty mail.

Here's the link: <http://ascii.textfiles.com/archives/4683>
http://ascii.textfiles.com/archives/4683

Arnold
_______________________________________________
TUHS mailing list
<mailto:TUHS@...> TUHS@...
<https://minnie.tuhs.org/mailman/listinfo/tuhs>
https://minnie.tuhs.org/mailman/listinfo/tuhs

[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]


Re: LF Compensation of a 468 scope

Fabio Trevisan
 

Thanks David,

I will first look into the decoupling. Although this is the rule #1 in this
forum, I confess may have overlooked it, because my first problem with the
H.V. power supply was a such a pain.

As for the RC networks you mentioned as being thermal balance... You really
got me on this one.
I would never thought of them as anything thermal... What is thermally
dependent in them that anyhow compensate the semicons here? The capacitor?
I've seen diodes that look rather purposeless in circuit that I know are
there for thermals... but capacitors...
Since I`m not desperate yet, I will keep that advice in the sleeve.

As for R478, I indeed played with it (as per insinuation in the manual
that, in case of impossibility to achieve proper frequency response, that a
compromise would be needed).
While fiddling with it, I mostly noticed a big change in the step response
adjustments... but I confess I could not be paying attention to the LF
behaviour at this point.
Since it didn't yield any good, I set it back to its optimum point as per
the manual... Higher possible amplitude of a 4 div 100MHz sine wave.
That was just after I found R474 open, which explained a lot of weird
behaviors, like a strong position effect, and even the poor astigmatism (I
reported it here, back then).

BTW, back to that event that I found R474 open, that I basically checked
every component of the board, I also did a basic check of U464
(out-of-circuit diode test between every possible pin) and found everything
right WITH ONE EXCEPTION.
My U464 reads internally short between pin 11 and pin 13!
Do you know if this is normal to this 155-0115-00?
The scope works so well other than the small LF deviation!

Rgrds,

Fabio


2017-02-17 16:40 GMT-02:00 David @DWH [TekScopes] <
TekScopes@...>:



On Fri, 17 Feb 2017 15:42:04 -0200, you wrote:

David,
Yep it's a 464.
I was looking into the schematics and agree with you, feeding the signal
through J351 will have exactly the same effect as it does on the 468 /
465B.
Initially I thought it wouldn't be the same, as the signal is
differentiated... but then I realized that what matters here are just the
transitions. As long as there are more transitions over the sweep period
it's fine.
The 74LS175 (U1705) in the 465B/468 implementation effectively
differentiates the signal at its clock input so it only detects the
rising edge.

This is the first time I see an advantage in the annoying design flaw of
the the NORM triggering mode, that is to trigger on the channel
switching's
edge, instead of triggering on the signal itself.
It is not really a design flaw; it duplicates the vertical trigger
source selection in older oscilloscopes.

It is useless when vertical chop mode is used but higher end
oscilloscopes like the 7000 series and later oscilloscopes like the
22xx series made it not quite as useless by adding the two vertical
inputs together as a trigger source when vertical CHOP mode and
vertical/normal trigger source is selected which works surprisingly
well.

On older oscilloscopes, normal/vertical triggering was the only way to
trigger on the added waveform when vertical ADD mode is used.

Talking about LF compensation, which is the reason why this topic grabbed
my attention...I have a question?

Is there any common failure mode on the 465 / 464 / 466 that causes the LF
compensation to be off to the low side (i.e, to have LF deficiency)?

On my 464's vert. output amplifier I needed to set all LF comp. trimmers
(R444, R445 and R446) at their minimum, to get an acceptable flat-top, but
still, although it's good enough to pass the 3% calibration check, it's
noticeably overshot by, maybe 1 to 2%.
So much that, also on the pre-Amplifiers, all trimmers that have bigger
time constants (C107, C133, C207 and C233) also need to be at their very
minimum to compensate for the LF lack (or conversely the excess of H.F.).
Bad power supply decoupling can cause this. As a test, I would put
some 1uF to 10uF plastic film or tantalum capacitors across C491,
C492, C493, and C494 and see if it makes any difference.

The bias control for the output stage can cause this problem or at
least something similar. Try fiddling with R478 to see what effect it
has. The service manual mentions that this may be necessary and that
R478 should not be messed with unless there is no other option or the
vertical amplifier is repaired which implies to me that Tektronix had
a special procedure to adjust it correctly which was not included in
the service manual.

No matter I`m looking at a 5 div. 100Hz signal @ 1ms/div, or 100Khz @
1us/div, the top and the bottom look more or less the same:
An inclined straight line with a 0.1 division decay at the top and 0.1
div.
rise at the bottom.
There's no clear overshoot shape resembling a differentiator waveform at
any of the frequencies.
Thermal imbalance can cause this at low frequencies but I am not sure
how it could be tracked down. The low frequency compensation
adjustments also adjust for thermal imbalance.

Does R478 affect thermal balance? I am not sure.

If I was desperate, then I might start replacing the thermal balance
RC networks; maybe one is bad. These are found in series with the
collectors like C437/R437, C438/R438, R418/C413/R413, and
R423/C423/R425.

P.S. In time. This is not about the input attenuator's L.F.
compensation...
All I`m describing above is being performed at 5mV/div (no attenuation),
using a 50mV 50Ohm source (terminated on 50Ohm), yielding a 25mVpp signal.
Run the test using the alternative procedure in the 465B/468 service
manual where the channel switch is used to generate the calibration
signal. This will at least reveal if the problem is before or after
the channel switch.


[Non-text portions of this message have been removed]


Re: LF Compensation of a 468 scope

 

On Fri, 17 Feb 2017 12:54:16 -0500, you wrote:

If there were other reasons than the stored waveform
display, they would have upgraded the method for the
465B too.

There is no practical reason to change things between
the two models. The 465B calibration method worked just
fine prior to the 468.

Remember the guys/gals that did the bulk calibrations of
these scopes were just wage slaves, calibrating thousands
of scopes each year. Needless changes would needlessly
cause mistakes to happen.
They *did* upgrade the method for the 465B; it uses the same method as
the 468 which is one of the reasons I do not think the newer
calibration procedure is related to the 468 stored display.

The older 464/465/466 did not use the newer calibration method and as
far as I know, they never changed it which does not surprise me simply
because Tektronix was very conservative about changing things on
existing products unless there was a significant problem. Instead
they designed a replacement product like how the 465B replaced the
465.

There has to be some other reason.

Perhaps the differences between a stored display of vectors
vs dots vs envelopes is the answer?

Or, you could simply crank the vertical output lf
compensation out of wack, and see what it did to the
stored display.

I'm betting on a big gross obvious difference.

-Chuck Harris
That is the test I was going to suggest. Mess with the vertical
amplifier compensation and see how it affects the stored display.


Re: LF Compensation of a 468 scope

 

On Fri, 17 Feb 2017 15:42:04 -0200, you wrote:

David,
Yep it's a 464.
I was looking into the schematics and agree with you, feeding the signal
through J351 will have exactly the same effect as it does on the 468 / 465B.
Initially I thought it wouldn't be the same, as the signal is
differentiated... but then I realized that what matters here are just the
transitions. As long as there are more transitions over the sweep period
it's fine.
The 74LS175 (U1705) in the 465B/468 implementation effectively
differentiates the signal at its clock input so it only detects the
rising edge.

This is the first time I see an advantage in the annoying design flaw of
the the NORM triggering mode, that is to trigger on the channel switching's
edge, instead of triggering on the signal itself.
It is not really a design flaw; it duplicates the vertical trigger
source selection in older oscilloscopes.

It is useless when vertical chop mode is used but higher end
oscilloscopes like the 7000 series and later oscilloscopes like the
22xx series made it not quite as useless by adding the two vertical
inputs together as a trigger source when vertical CHOP mode and
vertical/normal trigger source is selected which works surprisingly
well.

On older oscilloscopes, normal/vertical triggering was the only way to
trigger on the added waveform when vertical ADD mode is used.

Talking about LF compensation, which is the reason why this topic grabbed
my attention...I have a question?

Is there any common failure mode on the 465 / 464 / 466 that causes the LF
compensation to be off to the low side (i.e, to have LF deficiency)?

On my 464's vert. output amplifier I needed to set all LF comp. trimmers
(R444, R445 and R446) at their minimum, to get an acceptable flat-top, but
still, although it's good enough to pass the 3% calibration check, it's
noticeably overshot by, maybe 1 to 2%.
So much that, also on the pre-Amplifiers, all trimmers that have bigger
time constants (C107, C133, C207 and C233) also need to be at their very
minimum to compensate for the LF lack (or conversely the excess of H.F.).
Bad power supply decoupling can cause this. As a test, I would put
some 1uF to 10uF plastic film or tantalum capacitors across C491,
C492, C493, and C494 and see if it makes any difference.

The bias control for the output stage can cause this problem or at
least something similar. Try fiddling with R478 to see what effect it
has. The service manual mentions that this may be necessary and that
R478 should not be messed with unless there is no other option or the
vertical amplifier is repaired which implies to me that Tektronix had
a special procedure to adjust it correctly which was not included in
the service manual.

No matter I`m looking at a 5 div. 100Hz signal @ 1ms/div, or 100Khz @
1us/div, the top and the bottom look more or less the same:
An inclined straight line with a 0.1 division decay at the top and 0.1 div.
rise at the bottom.
There's no clear overshoot shape resembling a differentiator waveform at
any of the frequencies.
Thermal imbalance can cause this at low frequencies but I am not sure
how it could be tracked down. The low frequency compensation
adjustments also adjust for thermal imbalance.

Does R478 affect thermal balance? I am not sure.

If I was desperate, then I might start replacing the thermal balance
RC networks; maybe one is bad. These are found in series with the
collectors like C437/R437, C438/R438, R418/C413/R413, and
R423/C423/R425.

P.S. In time. This is not about the input attenuator's L.F. compensation...
All I`m describing above is being performed at 5mV/div (no attenuation),
using a 50mV 50Ohm source (terminated on 50Ohm), yielding a 25mVpp signal.
Run the test using the alternative procedure in the 465B/468 service
manual where the channel switch is used to generate the calibration
signal. This will at least reveal if the problem is before or after
the channel switch.


Re: LF Compensation of a 468 scope

Chuck Harris
 

If there were other reasons than the stored waveform
display, they would have upgraded the method for the
465B too.

There is no practical reason to change things between
the two models. The 465B calibration method worked just
fine prior to the 468.

Remember the guys/gals that did the bulk calibrations of
these scopes were just wage slaves, calibrating thousands
of scopes each year. Needless changes would needlessly
cause mistakes to happen.

There has to be some other reason.

Perhaps the differences between a stored display of vectors
vs dots vs envelopes is the answer?

Or, you could simply crank the vertical output lf
compensation out of wack, and see what it did to the
stored display.

I'm betting on a big gross obvious difference.

-Chuck Harris



David @DWH [TekScopes] wrote:

I do not think they did it for this reason because that gain and
offset drift have little or nothing to do with low frequency
compensation. The digital display has a very low bandwidth
requirement.

In this case they did not have to do anything except write a different
calibration procedure. The 464/465/466 all have essentially the same
setup and could be calibrated the same way.

On Fri, 17 Feb 2017 09:08:25 -0500, you wrote:

I think the assertion that the LF compensation was
to improve the performance of the digital display
channel is more correct. Tektronix never seemed
to care if there was a little interaction between
adjustments... well, not enough to do anything about
it.

One of the big calibration issues with the 468 is
it is hard to keep the size of the digital playback
on the screen true to the original waveform.

Actually, that is a problem with the 7854 too.

-Chuck Harris

------------------------------------
Posted by: David <@DWH>
------------------------------------


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

Yahoo Groups Links




Re: LF Compensation of a 468 scope

Fabio Trevisan
 

David,
Yep it's a 464.
I was looking into the schematics and agree with you, feeding the signal
through J351 will have exactly the same effect as it does on the 468 / 465B.
Initially I thought it wouldn't be the same, as the signal is
differentiated... but then I realized that what matters here are just the
transitions. As long as there are more transitions over the sweep period
it's fine.
This is the first time I see an advantage in the annoying design flaw of
the the NORM triggering mode, that is to trigger on the channel switching's
edge, instead of triggering on the signal itself.

Talking about LF compensation, which is the reason why this topic grabbed
my attention...I have a question?

Is there any common failure mode on the 465 / 464 / 466 that causes the LF
compensation to be off to the low side (i.e, to have LF deficiency)?

On my 464's vert. output amplifier I needed to set all LF comp. trimmers
(R444, R445 and R446) at their minimum, to get an acceptable flat-top, but
still, although it's good enough to pass the 3% calibration check, it's
noticeably overshot by, maybe 1 to 2%.
So much that, also on the pre-Amplifiers, all trimmers that have bigger
time constants (C107, C133, C207 and C233) also need to be at their very
minimum to compensate for the LF lack (or conversely the excess of H.F.).

No matter I`m looking at a 5 div. 100Hz signal @ 1ms/div, or 100Khz @
1us/div, the top and the bottom look more or less the same:
An inclined straight line with a 0.1 division decay at the top and 0.1 div.
rise at the bottom.
There's no clear overshoot shape resembling a differentiator waveform at
any of the frequencies.

P.S. In time. This is not about the input attenuator's L.F. compensation...
All I`m describing above is being performed at 5mV/div (no attenuation),
using a 50mV 50Ohm source (terminated on 50Ohm), yielding a 25mVpp signal.

@Colin...
Apologies for high-jacking your topic a little bit. I promise to open
another thread if it departs too much from the subject.
By the way, having cleared the issue about the "Vert Alt Sync" and how to
perform that step of your 468's LF compensation calibration... Did it
behave, or it's not getting the LF compensation right?

Rgrds,

Fabio


Re: LF Compensation of a 468 scope

 

I do not think they did it for this reason because that gain and
offset drift have little or nothing to do with low frequency
compensation. The digital display has a very low bandwidth
requirement.

In this case they did not have to do anything except write a different
calibration procedure. The 464/465/466 all have essentially the same
setup and could be calibrated the same way.

On Fri, 17 Feb 2017 09:08:25 -0500, you wrote:

I think the assertion that the LF compensation was
to improve the performance of the digital display
channel is more correct. Tektronix never seemed
to care if there was a little interaction between
adjustments... well, not enough to do anything about
it.

One of the big calibration issues with the 468 is
it is hard to keep the size of the digital playback
on the screen true to the original waveform.

Actually, that is a problem with the 7854 too.

-Chuck Harris


Re: LF Compensation of a 468 scope

 

That confused me also. When I first started reading the calibration
procedure, I expected and assumed that the calibration signal was
still hooked to the vertical input but there were problems with that
which led me to examine the situation in detail:

1. The PG506 only activates one output at a time and they were not
using the PG506's trigger output.
2. The procedure talks about using both the channel 1 and channel 2
position controls to set the level. If the PG506 is not providing 2
signals then it is also not providing 3 signals.
3. There was no purpose behind providing a separate alternate signal
if square waves were available at any input.

Finally I reached the conclusion that the channel switch was being
used to cleverly generate a clean low frequency compensation signal.
It may be worth studying how the channel switch works in this
application if you want to build your own precision pulse generator.

On Fri, 17 Feb 2017 15:59:49 -0000, you wrote:

...

It was quite clever of the guys at Tek to come up with this idea, but it would have been nice if the Service Manual could have made it a bit clearer. To my reading, I guessed that the fast-rise calibration square-wave needed to be connected to CH1, possibly also CH2 at the same time. All in all, the Manual for the 468 is not as good at many other Tek manuals that I have read. OK, the scope is complex and the Manual runs to two volumes, but still….

I will be carrying on with the calibration of my 468, as the digital storage triggering doesn’t seem to be working properly, especially in the pre-trig window. The cursor values also don’t seem too accurate.

Colin.


Re: LF Compensation of a 468 scope

Colin Herbert
 

Fabio and David,

I cannot thank you enough, you have cleared this up for me wonderfully. Although I still have a couple of BNC-to-Peltola adapters on order, I had a go at adjusting the Low-Frequency pre-sets on the Vert Amp board, using the fast-rise output of my PG506 into CH1 and CH2 (@ 5mV/div) separately using a 10X attenuator and a 50-ohm termination. I think I got some observable improvement in the waveform slope and front edge.

I will certainly give the recommended technique a go when the adapters turn up.

It was quite clever of the guys at Tek to come up with this idea, but it would have been nice if the Service Manual could have made it a bit clearer. To my reading, I guessed that the fast-rise calibration square-wave needed to be connected to CH1, possibly also CH2 at the same time. All in all, the Manual for the 468 is not as good at many other Tek manuals that I have read. OK, the scope is complex and the Manual runs to two volumes, but still….

I will be carrying on with the calibration of my 468, as the digital storage triggering doesn’t seem to be working properly, especially in the pre-trig window. The cursor values also don’t seem too accurate.

Colin.

From: TekScopes@... [mailto:TekScopes@...]
Sent: 17 February 2017 14:08
To: TekScopes@...
Subject: Re: [TekScopes] LF Compensation of a 468 scope





I think the assertion that the LF compensation was
to improve the performance of the digital display
channel is more correct. Tektronix never seemed
to care if there was a little interaction between
adjustments... well, not enough to do anything about
it.

One of the big calibration issues with the 468 is
it is hard to keep the size of the digital playback
on the screen true to the original waveform.

Actually, that is a problem with the 7854 too.

-Chuck Harris

Fabio Trevisan fabio.tr3visan@... [TekScopes] wrote:
David, I am compelled to agree with you. The timeline between the release
of both sets the tone.
Either way, using the channel switcher as a square wave generator was a
quite an idea from the folks.
It made me even think of doing it to calibrate mine.
Of course it won't be as easy as tapping directly to the input of a TTL
gate from a cable that can be conveniently removed and reinserted
afterwards, but it's still possible, applying opposite TTL signals to the
pins 2 and 4 of the harmonica connector P350 coming out of the Very Mode
switching board.
It may sound purposeless, but a lot of us DIYers lack sometimes some of the
equipment required to calibrate those scopes (as is my case).

I'll give some thoughts to it.
I'm still not sure it would work.

rgrds,

Fabio


_____

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Checked by AVG - www.avg.com <http://www.avg.com/email-signature>
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[Non-text portions of this message have been removed]


Re: LF Compensation of a 468 scope

 

You are referring to your 464, right?

I checked the 464 schematic and coaxial connection J351 shown on the
left side of schematic 3 is the Vert Alt Sync Pulse input to the
vertical channel switch. Set the vertical mode to ALT and the trigger
source to NORM and it should work fine.

The Vert Alt Sync Pulse signal on the 465B and 468 is a TTL level
signal while on the 464 it is 0 to 5 volts. In either case, the
signaling is not super critical at least for this application.
Tektronix of course used a Peltola to BNC adapter but I think
disconnecting the coax and soldering in a test point to attach the
signal source would work.

The 465B/468 vertical compensation calibration procedure applied to
the 464/465/466 should make the calibration of the later easier. At
the very least, it is a good way to diagnose obscure problems in the
circuits before and after the channel switch.

On Fri, 17 Feb 2017 09:40:34 -0200, you wrote:

Either way, using the channel switcher as a square wave generator was a
quite an idea from the folks.
It made me even think of doing it to calibrate mine.
Of course it won't be as easy as tapping directly to the input of a TTL
gate from a cable that can be conveniently removed and reinserted
afterwards, but it's still possible, applying opposite TTL signals to the
pins 2 and 4 of the harmonica connector P350 coming out of the Very Mode
switching board.
It may sound purposeless, but a lot of us DIYers lack sometimes some of the
equipment required to calibrate those scopes (as is my case).

I'll give some thoughts to it.
I'm still not sure it would work.

rgrds,

Fabio


Re: LF Compensation of a 468 scope

Chuck Harris
 

I think the assertion that the LF compensation was
to improve the performance of the digital display
channel is more correct. Tektronix never seemed
to care if there was a little interaction between
adjustments... well, not enough to do anything about
it.

One of the big calibration issues with the 468 is
it is hard to keep the size of the digital playback
on the screen true to the original waveform.

Actually, that is a problem with the 7854 too.

-Chuck Harris

Fabio Trevisan fabio.tr3visan@... [TekScopes] wrote:

David, I am compelled to agree with you. The timeline between the release
of both sets the tone.
Either way, using the channel switcher as a square wave generator was a
quite an idea from the folks.
It made me even think of doing it to calibrate mine.
Of course it won't be as easy as tapping directly to the input of a TTL
gate from a cable that can be conveniently removed and reinserted
afterwards, but it's still possible, applying opposite TTL signals to the
pins 2 and 4 of the harmonica connector P350 coming out of the Very Mode
switching board.
It may sound purposeless, but a lot of us DIYers lack sometimes some of the
equipment required to calibrate those scopes (as is my case).

I'll give some thoughts to it.
I'm still not sure it would work.

rgrds,

Fabio


Re: 2430A-2/2

Maxy Sac
 

It's my mistake. I forgot that I need to do a COLD START after I change NV-RAM. Now everything is OK.