LF Compensation of a 468 scope


Colin Herbert
 

Hi,

Does no-one have any observations on this? Perhaps I am being too impatient.

I have ordered two of these BNC-to-Peltola adapters from Qservice, so I have
to wait a while for the delivery before I can do much more. I remain puzzled
that this scope and the 465B and no others in the 46x series (or the 475A)
use this method.

Colin.



From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 11 February 2017 20:45
To: TekScopes@yahoogroups.com
Subject: RE: [TekScopes] Gain calibration of a 468 scope





Continuing..

I have now got to the point of trying to adjust the Low-Frequency
Compensation. I believe that I have a little too much overshoot. The manual
states that on the Vertical Output Board there are five pre-sets which
affect the compensation at 1, 10 and 100KHz. That is all very well, but
there is also a requirement set the 486 vertical mode to CH1 & CH2 alternate
and disconnect P686 from J686 on the interface board. This J686 is the "Vert
Alt Sync". Now, either a 1KHz square-wave from a low-frequency generator (an
SG502?), or the fast-rise negative output from the PG506 is connected to
J686 with a BNC-to-Peltola adaptor. I don't understand why this is
considered to be necessary or what it is actually doing.

Neither the 464 nor the 465 scopes use this protocol, but the 465B does. Is
it really necessary, or could one simply leave P686/J686 connected and just
adjust the pre-set on the Vertical Output Board?

Any help and information appreciated. I am learning a lot fiddling with this
beast.

Colin.





_____

No virus found in this message.
Checked by AVG - www.avg.com <http://www.avg.com/email-signature>
Version: 2016.0.7998 / Virus Database: 4756/13926 - Release Date: 02/10/17


Fabio Trevisan
 

Hello Colin,
You may get disappointed that my answer is more like a validation of your
problem than an answer properly said.
I own a 464 which Is slightly off in the low freq. compensation as well.
No matter I do, even setting all those trimpots to their minimum, the
square wave tops and bottoms remain still lightly inclined (the tops begin
higher and then fall a little and the bottoms the opposite).
I never noticed any instruction to disconnect some peltola internal cable
on my service manual, but I may have skipped something accidentally.
I fail to understand what in the world this signal of alternate trace sync
has anything to do with the subject of adjusting the lo-freq compensation.
Now you triggered my curiosity and I will look into the 468's SM to see if
I can see any explanation to your issue and perhaps I'll find some
explanation to my problem as well.
I was holding this problem (of mine) for a later time when I'd have solved
other minor issues that are also pending, but now it's itching!
Brgrds,
Fabio

On Feb 14, 2017 4:24 PM, "'Colin Herbert' colingherbert@blueyonder.co.uk
[TekScopes]" <TekScopes@yahoogroups.com> wrote:



Hi,

Does no-one have any observations on this? Perhaps I am being too impatient.

I have ordered two of these BNC-to-Peltola adapters from Qservice, so I have
to wait a while for the delivery before I can do much more. I remain puzzled
that this scope and the 465B and no others in the 46x series (or the 475A)
use this method.

Colin.

From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 11 February 2017 20:45
To: TekScopes@yahoogroups.com
Subject: RE: [TekScopes] Gain calibration of a 468 scope

Continuing..

I have now got to the point of trying to adjust the Low-Frequency
Compensation. I believe that I have a little too much overshoot. The manual
states that on the Vertical Output Board there are five pre-sets which
affect the compensation at 1, 10 and 100KHz. That is all very well, but
there is also a requirement set the 486 vertical mode to CH1 & CH2 alternate
and disconnect P686 from J686 on the interface board. This J686 is the "Vert
Alt Sync". Now, either a 1KHz square-wave from a low-frequency generator (an
SG502?), or the fast-rise negative output from the PG506 is connected to
J686 with a BNC-to-Peltola adaptor. I don't understand why this is
considered to be necessary or what it is actually doing.

Neither the 464 nor the 465 scopes use this protocol, but the 465B does. Is
it really necessary, or could one simply leave P686/J686 connected and just
adjust the pre-set on the Vertical Output Board?

Any help and information appreciated. I am learning a lot fiddling with this
beast.

Colin.

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

_____

No virus found in this message.
Checked by AVG - www.avg.com <http://www.avg.com/email-signature>
Version: 2016.0.7998 / Virus Database: 4756/13926 - Release Date: 02/10/17






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


Colin Herbert
 

Hi Fabio,

I too have a 464, which was giving me problems a year or more ago. John Sykes, who is a member of this forum and lived not far from me, gave it a look-over and got it working fine. I also have two non-working 466s and a 475A as well as the 468 I am trying to get calibrated. I have original Tek manuals for all of these scopes, plus the 465 and 465B manuals (I don’t own these scopes, so it must be Acquisition Syndrome on my part). It is only the 465B and 468 that have this procedure in the manuals. It has to be said that the Vertical Output Amplifiers of these two look similar, but the others being decidedly different. The 475A Vertical Output Amplifier uses some hybrids which are no doubt proprietary. My electronic knowledge is limited and self-taught, so I find difficulty working out some things. I, too cannot see how the procedure is operating – what does “Vert Alt Sync” have to do with anything here?

David Hess has said that the 468 is basically a 465B with the added DSO hardware. So, while it is a complex and strange beast, there should be probably more people who have a 465B and might have come across this.

Colin.

From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 15 February 2017 11:20
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] LF Compensation of a 468 scope





Hello Colin,
You may get disappointed that my answer is more like a validation of your
problem than an answer properly said.
I own a 464 which Is slightly off in the low freq. compensation as well.
No matter I do, even setting all those trimpots to their minimum, the
square wave tops and bottoms remain still lightly inclined (the tops begin
higher and then fall a little and the bottoms the opposite).
I never noticed any instruction to disconnect some peltola internal cable
on my service manual, but I may have skipped something accidentally.
I fail to understand what in the world this signal of alternate trace sync
has anything to do with the subject of adjusting the lo-freq compensation.
Now you triggered my curiosity and I will look into the 468's SM to see if
I can see any explanation to your issue and perhaps I'll find some
explanation to my problem as well.
I was holding this problem (of mine) for a later time when I'd have solved
other minor issues that are also pending, but now it's itching!
Brgrds,
Fabio


Fabio Trevisan
 

Hello Colin,
I studied a little bit the LF frequency compensation instructions for the
468 and I think I figured out what was their intention.
This scope uses the "Trig-View" internal "pseudo" 3rd channel to display
the Stored waveform.
Unlike the CH1 and CH2 analog pre-amplifiers which have their own Frequency
compensation networks, the "pseudo" 3rd channel input doesn't have any.
Therefore, in order for the display of the Stored waveform (which is low
frequency) to be faithful, it has to rely on the Low Frequency compensation
network of the OUTPUT Amplifier ALONE.
Since there's no way to feed a signal directly into the Vertical OUTPUT
Amplifier to adjust its LF compensation in an independent way, they came
out with this "trick":

By feeding a square wave to Vert Alt Sync Pulse input (on diagram 3,
coordinates 9A) they use the 'scope's own ALT switching circuit as a square
wave source to the Vertical OUTPUT section to be calibrated for LF
Compensation.

The instructions doesn't state very clearly what we should do with the CH1
or CH2 inputs themselves, but if we assume that they want us to leave them
both disconnected (or ideally, set at GND), then the only voltage coming
from each channel is the Channel Position level of each one.

Now, the "key" phrase from the setup instructions, on page 4-40 is:
"Set the generator output frequency to 1KHz AND ADJUST CH1 AND CH2 POSTION
CONTROLS FOR A 6-DIVISION DISPLAY.

By feeding the Vert Alt Sync signal with a waveform that is TWICE the
frequency of the Horizontal scan (which there's no way to happen if J686
would be wired to its regular signal), they created a way to make the scope
alternate from CH1 to CH2 in the middle of the Horizontal sweep and allow
us to see the waveform of its own switching from CH1 to CH2 and vv..

The waveform we see is therefore the transition from CH1 position's level
to CH2 position's level which are, each one, at a static level.

On table 4-14 (at the same page), it's easy to overlook its name, but it
tells "Vertical OUTPUT Low-Frequency compensation, and R335, R329 and R325
are there exclusively for the OUTPUT Amplifier 1Khz and 10KHz LF
compenasation.

Further down, by feeding square waves to CH1 and CH2 respectively, we have
R165 and R172 exclusive to CH1 preamplifier, and R371 and R373 exlcusive to
CH2.

100KHz adjust exists only on the Output amplifier and its adjustment is a
compromise between the "pseudo" 3rd channel for the Stored waveform, and
CH1 and CH2.

My scope (464, as well as other fully analog scopes) don't need that
adjusting of the OUTPUT Amplifier ALONE because the vertical signals are
ALWAYS coming through the pre-amplifiers.
It doesn't matter much if the pre-amplifiers are under compensated and the
output amplifier is over (or vv), as long as the overall result is correct.

Mistery solved.

FOR YOU, on the practical side, this step is the easiest one to achieve,
without requiring any special generator, because any TTL 1KHz oscillator
can do. The waveform doesn't need to be particularly flat or with no
overshoots because the "actual" waveform seen will be generated internally
by the CH1 / CH2 switcher.
You only need to assure that the signal arrives noiseless on U509A,

Rgrds,

Fabio




2017-02-15 10:17 GMT-02:00 'Colin Herbert' colingherbert@blueyonder.co.uk
[TekScopes] <TekScopes@yahoogroups.com>:



Hi Fabio,

I too have a 464, which was giving me problems a year or more ago. John
Sykes, who is a member of this forum and lived not far from me, gave it a
look-over and got it working fine. I also have two non-working 466s and a
475A as well as the 468 I am trying to get calibrated. I have original Tek
manuals for all of these scopes, plus the 465 and 465B manuals (I don’t own
these scopes, so it must be Acquisition Syndrome on my part). It is only
the 465B and 468 that have this procedure in the manuals. It has to be said
that the Vertical Output Amplifiers of these two look similar, but the
others being decidedly different. The 475A Vertical Output Amplifier uses
some hybrids which are no doubt proprietary. My electronic knowledge is
limited and self-taught, so I find difficulty working out some things. I,
too cannot see how the procedure is operating – what does “Vert Alt Sync”
have to do with anything here?

David Hess has said that the 468 is basically a 465B with the added DSO
hardware. So, while it is a complex and strange beast, there should be
probably more people who have a 465B and might have come across this.

Colin.

From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 15 February 2017 11:20
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] LF Compensation of a 468 scope

Hello Colin,
You may get disappointed that my answer is more like a validation of your
problem than an answer properly said.
I own a 464 which Is slightly off in the low freq. compensation as well.
No matter I do, even setting all those trimpots to their minimum, the
square wave tops and bottoms remain still lightly inclined (the tops begin
higher and then fall a little and the bottoms the opposite).
I never noticed any instruction to disconnect some peltola internal cable
on my service manual, but I may have skipped something accidentally.
I fail to understand what in the world this signal of alternate trace sync
has anything to do with the subject of adjusting the lo-freq compensation.
Now you triggered my curiosity and I will look into the 468's SM to see if
I can see any explanation to your issue and perhaps I'll find some
explanation to my problem as well.
I was holding this problem (of mine) for a later time when I'd have solved
other minor issues that are also pending, but now it's itching!
Brgrds,
Fabio

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



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


 

I am sorry for being late replying to your post. I kept reading it
and thinking about it but then I get distracted.

The 464 and 466 are based off of the 465 and the 468 is based on the
465B. The 464/465/466 and the 465B/468 are really distinct series. So
the 465B is the one to compare to the 468.

I was able to figure out from the schematic what they are doing with
the Vert Alt Sync Pulse. The idea here is that driving the Vert Alt
Sync Pulse allows the vertical channel switch to generate a clean
square wave to drive the vertical amplifier with the peaks set by the
position controls.

As to why? Compensating the vertical amplifier separately from the
vertical preamplifiers should produce better results. Maybe doing the
calibration in two stages was faster or produced fewer units which
needed rework.

Notice though that the transient response is still compensated through
the vertical inputs because the edge produced by the channel switch is
not fast enough to calibrate this for the vertical amplifier.

The various mainframe oscilloscopes like the 7000 series also use
separate calibrations for the vertical signal path but that is
understandable given their modular design. The compact oscilloscopes
which replaced the 465B series did not use two stages of vertical
calibration though.

On Tue, 14 Feb 2017 18:24:10 -0000, you wrote:

Hi,

Does no-one have any observations on this? Perhaps I am being too impatient.

I have ordered two of these BNC-to-Peltola adapters from Qservice, so I have
to wait a while for the delivery before I can do much more. I remain puzzled
that this scope and the 465B and no others in the 46x series (or the 475A)
use this method.

Colin.

From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 11 February 2017 20:45
To: TekScopes@yahoogroups.com
Subject: RE: [TekScopes] Gain calibration of a 468 scope

Continuing..

I have now got to the point of trying to adjust the Low-Frequency
Compensation. I believe that I have a little too much overshoot. The manual
states that on the Vertical Output Board there are five pre-sets which
affect the compensation at 1, 10 and 100KHz. That is all very well, but
there is also a requirement set the 486 vertical mode to CH1 & CH2 alternate
and disconnect P686 from J686 on the interface board. This J686 is the "Vert
Alt Sync". Now, either a 1KHz square-wave from a low-frequency generator (an
SG502?), or the fast-rise negative output from the PG506 is connected to
J686 with a BNC-to-Peltola adaptor. I don't understand why this is
considered to be necessary or what it is actually doing.

Neither the 464 nor the 465 scopes use this protocol, but the 465B does. Is
it really necessary, or could one simply leave P686/J686 connected and just
adjust the pre-set on the Vertical Output Board?

Any help and information appreciated. I am learning a lot fiddling with this
beast.

Colin.


Fabio Trevisan
 

David,
I keep asking myself when I'm going to have your writing synthesis
abilities.
It took me a couple of hours to write my last post (and we were probably
writing at the same time) and I couldn't manage to make it more concise
without compromising what I thought was essential...
And after I read your post it felt like a breeze to read! Short and easy to
grasp!
Simply amazing!
Back to the topic, I wonder if you would agree with my assertion that
splitting the LF comp, in the case of the 468, may come handy to assure a
faithful display of the stored waveform? especially considering that the
path through which the stored waveform is displayed doesn't have any sort
of adjustable compensation (besides the ones at the very output amplifier).
I know that this assumption doesn't hold true, or at least not necessary
for the 465b, since it doesn't have a stored waveform to display and that
any LF comp that could be slightly off on the vertical output amplifier
could still be harmlessly compensated at the pre-amps (or vice versa) but
maybe, since the circuits are alike, and since the 468 needed the
procedure, it was just sensible to make the procedure for the 465b the same
way.

Thanks for setting the bar high!

Brgrds,
Fabio.
P.S. my 464 is kind of on hold for a while, reason I haven't been posting
any feedback lately.
Been dealing with reorganization of my shack, to comply with some spousal
requirements (it was a mess!)

On Feb 16, 2017 10:12 PM, "David davidwhess@gmail.com [TekScopes]" <
TekScopes@yahoogroups.com> wrote:



I am sorry for being late replying to your post. I kept reading it
and thinking about it but then I get distracted.

The 464 and 466 are based off of the 465 and the 468 is based on the
465B. The 464/465/466 and the 465B/468 are really distinct series. So
the 465B is the one to compare to the 468.

I was able to figure out from the schematic what they are doing with
the Vert Alt Sync Pulse. The idea here is that driving the Vert Alt
Sync Pulse allows the vertical channel switch to generate a clean
square wave to drive the vertical amplifier with the peaks set by the
position controls.

As to why? Compensating the vertical amplifier separately from the
vertical preamplifiers should produce better results. Maybe doing the
calibration in two stages was faster or produced fewer units which
needed rework.

Notice though that the transient response is still compensated through
the vertical inputs because the edge produced by the channel switch is
not fast enough to calibrate this for the vertical amplifier.

The various mainframe oscilloscopes like the 7000 series also use
separate calibrations for the vertical signal path but that is
understandable given their modular design. The compact oscilloscopes
which replaced the 465B series did not use two stages of vertical
calibration though.

On Tue, 14 Feb 2017 18:24:10 -0000, you wrote:

Hi,

Does no-one have any observations on this? Perhaps I am being too
impatient.

I have ordered two of these BNC-to-Peltola adapters from Qservice, so I
have
to wait a while for the delivery before I can do much more. I remain
puzzled
that this scope and the 465B and no others in the 46x series (or the 475A)
use this method.

Colin.

From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 11 February 2017 20:45
To: TekScopes@yahoogroups.com
Subject: RE: [TekScopes] Gain calibration of a 468 scope

Continuing..

I have now got to the point of trying to adjust the Low-Frequency
Compensation. I believe that I have a little too much overshoot. The
manual
states that on the Vertical Output Board there are five pre-sets which
affect the compensation at 1, 10 and 100KHz. That is all very well, but
there is also a requirement set the 486 vertical mode to CH1 & CH2
alternate
and disconnect P686 from J686 on the interface board. This J686 is the
"Vert
Alt Sync". Now, either a 1KHz square-wave from a low-frequency generator
(an
SG502?), or the fast-rise negative output from the PG506 is connected to
J686 with a BNC-to-Peltola adaptor. I don't understand why this is
considered to be necessary or what it is actually doing.

Neither the 464 nor the 465 scopes use this protocol, but the 465B does.
Is
it really necessary, or could one simply leave P686/J686 connected and
just
adjust the pre-set on the Vertical Output Board?

Any help and information appreciated. I am learning a lot fiddling with
this
beast.

Colin.

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


 

On Fri, 17 Feb 2017 01:12:15 -0200, you wrote:

David,
I keep asking myself when I'm going to have your writing synthesis
abilities.
It took me a couple of hours to write my last post (and we were probably
writing at the same time) and I couldn't manage to make it more concise
without compromising what I thought was essential...
And after I read your post it felt like a breeze to read! Short and easy to
grasp!
Simply amazing!
Sometimes my paragraphs and sentences get out of hand and I have to
break everything up into clauses and rearrange them. As far as
writing quality, practice helps a lot.

Back to the topic, I wonder if you would agree with my assertion that
splitting the LF comp, in the case of the 468, may come handy to assure a
faithful display of the stored waveform? especially considering that the
path through which the stored waveform is displayed doesn't have any sort
of adjustable compensation (besides the ones at the very output amplifier).
I know that this assumption doesn't hold true, or at least not necessary
for the 465b, since it doesn't have a stored waveform to display and that
any LF comp that could be slightly off on the vertical output amplifier
could still be harmlessly compensated at the pre-amps (or vice versa) but
maybe, since the circuits are alike, and since the 468 needed the
procedure, it was just sensible to make the procedure for the 465b the same
way.
That is an interesting idea but you identified one of the reasons it
is not why Tektronix did it; the 465B came before the 468 and the 465B
has no stored display.

I suspect it was simply to make the calibration easier by having fewer
interactions compared to the 465. Later designs like the 22xx series
had fewer adjustments because better transistor amplifiers were used.


Fabio Trevisan
 

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

On Feb 17, 2017 5:13 AM, "David davidwhess@gmail.com [TekScopes]" <
TekScopes@yahoogroups.com> wrote:



On Fri, 17 Feb 2017 01:12:15 -0200, you wrote:

David,
I keep asking myself when I'm going to have your writing synthesis
abilities.
It took me a couple of hours to write my last post (and we were probably
writing at the same time) and I couldn't manage to make it more concise
without compromising what I thought was essential...
And after I read your post it felt like a breeze to read! Short and easy
to
grasp!
Simply amazing!
Sometimes my paragraphs and sentences get out of hand and I have to
break everything up into clauses and rearrange them. As far as
writing quality, practice helps a lot.

Back to the topic, I wonder if you would agree with my assertion that
splitting the LF comp, in the case of the 468, may come handy to assure a
faithful display of the stored waveform? especially considering that the
path through which the stored waveform is displayed doesn't have any sort
of adjustable compensation (besides the ones at the very output
amplifier).
I know that this assumption doesn't hold true, or at least not necessary
for the 465b, since it doesn't have a stored waveform to display and that
any LF comp that could be slightly off on the vertical output amplifier
could still be harmlessly compensated at the pre-amps (or vice versa) but
maybe, since the circuits are alike, and since the 468 needed the
procedure, it was just sensible to make the procedure for the 465b the
same
way.
That is an interesting idea but you identified one of the reasons it
is not why Tektronix did it; the 465B came before the 468 and the 465B
has no stored display.

I suspect it was simply to make the calibration easier by having fewer
interactions compared to the 465. Later designs like the 22xx series
had fewer adjustments because better transistor amplifiers were used.


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


Chuck Harris <cfharris@...>
 

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@gmail.com [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


 

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


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@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 17 February 2017 14:08
To: TekScopes@yahoogroups.com
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@gmail.com [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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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.


 

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


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


Chuck Harris <cfharris@...>
 

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 davidwhess@gmail.com [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 <davidwhess@gmail.com>
------------------------------------


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

Yahoo Groups Links




 

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.


 

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.


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 davidwhess@gmail.com [TekScopes] <
TekScopes@yahoogroups.com>:



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]


Chuck Harris <cfharris@...>
 

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 davidwhess@gmail.com [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.


Colin Herbert
 

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.



There is a caveat here in the use of the PG506, which I eventually worked
out. I couldn't get a square-wave on the 468 using this method with the
PG506 fast-rise negative output. I wondered if the PG506 was actually
getting the channel switch to work. I tried using a TTL output from another
square-wave generator and this did work! This generator was giving a fixed
5V peak-to-peak square-wave, whereas the fast-rise outputs of the PG506 are
only about 2V maximum; obviously not enough to activate the switching.
Unfortunately, the manual, though stating that any alternative signal-source
must be "TTL compatible", doesn't say that the PG506 fast-rise outputs
aren't.

Onward and upward.

Colin.