Topics

A 2465 teaser...

 

We will probably never know for sure.  But it certainly isn't the way I or my associates would have designed it.  The function is basically an Op Amp inverter with an input mux.  And why would Ch5 ever be added to anything?  I'm not being critical and I don't claim to know, just saying that as an engineer I would have used something more simple.

On ‎Sunday‎, ‎November‎ ‎12‎, ‎2017‎ ‎10‎:‎42‎:‎03‎ ‎AM‎ ‎CST, Sigurður Ásgeirsson siggi@... [TekScopes] <TekScopes@...> wrote:

 
On Sun, 12 Nov 2017 at 11:05 Chuck Harris cfharris@... [TekScopes] <
TekScopes@...> wrote:



I don't believe the complexity in feeding CH5 is a mistake, but rather
an issue of partitioning... I believe the channel switch hybrid works
exactly the way it was supposed to work.
You can see the data sheet for the channel switch die here <
http://w140.com/tek_made_sm.pdf>, it's "named" 203-0211-90 and appears on
page 6-29 (or 271). It seems pretty clear that this is behaving as
intended, and that the #5 input is designed for a much lower bandwidth
signal than #1/2 and #3/4.
I'm guessing the original intent would have been for the readout signal to
be in a quiescent state when not displayed, or maybe it was simply less
expensive to do the low-bandwidth switching externally to the hybrid.






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

 

Chuck,
The only thing I see wrong with that description is I have to read it more carefully.  I am embarrassed to admit that I only skimmed that description in the past and wrote my description without even looking at the manual (except for the schematic).  So I agree, lets use the manual.  Duhh
I, too, am an engineer, retired after 35+ years.  Worked during my college years as a radio/TV technician and repairman.  So nearly 40 years around oscilloscopes before assembling my home lab with three Tek scopes and a parts mule.  Ive been retired 18 years now.  I hate shotgunning for repair.  It leaves me wondering if I fixed the problem or covered it up.  Worse, like you said, I never learn how it was designed to work.  That's why the amplifier/switch arrangement for Ch5 input to U400 stands out to me.  It just doesn't fit with my expectation for quality design work.   Your explanation of why its that way (someone wrote the spec wrong) is as good as any. 
Anyway, glad you got it fixed.
Mac

On ‎Sunday‎, ‎November‎ ‎12‎, ‎2017‎ ‎10‎:‎05‎:‎39‎ ‎AM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
Hi Mac,

Your description looks to be ok. But what is wrong with the description
in the manual? It is available for our use, and seems pretty clear to me.

My shame is I should have read it earlier as it contained lots of clues
that would have gotten me off of the bad power boondoggle.

2 or 3 years ago, I did read the description, and concluded that U475 or
U485 was likely bad. I replaced U475, but hesitated to change U485 when
replacing U475 had no effect. I don't like replacing parts shotgun style,
much preferring learning over efficiency. Shotgunning when done rationally
is very fast, but you never really learn what the failure was, just the
area where the failure occurred.

I think the difference in the way I approach repairs is because I am an
electrical engineer, not a technician. The failures a technician fixes
are circuits that once worked. Replace the broken part, and operation
will be restored. The failures an engineer fixes may never have worked
correctly. You have to replace the broken idea to fix an engineering
failure.

I don't believe the complexity in feeding CH5 is a mistake, but rather
an issue of partitioning... I believe the channel switch hybrid works
exactly the way it was supposed to work.

Partitioning was the job of the 2465 system engineer.

What I think they did is reason that the display needed to be updated
often, without fail, or it would flicker, or wink out. You can tolerate
a little of that in a trace, but in the display it would look ridiculously
broken. The display has to be stable. Display has to be priority 1.

So, what they did is make a 4 channel MUX for CH1-4, and give it individual
selection lines that mostly mirrored the state of the channel selection
buttons on the front panel.

CH1 and CH2 were special in that tradition required there be a function
where the MUX output is CH1 + CH2... the "ADD" function. That means there
is an adder gate before the CH1/CH2 MUX. I think the designer simply made
that adder gate a 3 input adder that added CH1+CH2+CH5, and presented
the sum to the channel switch MUX.

For this to work, the way you deselect an input is to change it to a
zero, or ground (Ch1 + CH2 + 0= no display...). The VS1 - VS4 selection
lines automatically do that for CH1 - CH4 input, but CH5 has no such
selection line, so the user of the Channel Switch is required to do it
himself.

Earlier I said it was a partitioning problem. I believe the mistake
that happened was the Channel Switch hybrid designer, and the Display
hybrid designer each thought the other was responsible for grounding
out the display input when data was not being displayed. The possibility
is also pretty good that neither designed their part with the 2465, or
the other designer, in mind. They designed to spec.

The simple fix would have been to add VS5 to deselect the display channel.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:
Chuck, I am sure you understand that "complex circuit" on Ch5 input to U400, the Channel Switch. I think I do but I will document it for others and I hope you will correct any errors.
U485A and U485B form a differential amplifier whose operation depends on current to their common emitters through R483. Diode CR484 is fitted to stop the amplifier by "stealing" the emitter current when U975 pin 1 is high. U485C and U485D form a similar amplifier that is disabled when the Pin 3 of U975 is high. The outputs from these two differential amplifers is joined by the common collectors of U485B and U485D. U975A acts to select one or the other (not both) of these two amplifiers. Amplifier One (U485A and U485B) carries the readout signal as presented on pin 2 of U485A. Amplifier Two (U485C and U485D) "amplifies" the ground that is present on pin 6 of U485C. Thus, the signal at pin 6 of U475C is either Readout Vertical or ground, depending on the logic state of U975A. U475C is a simple emitter follower to provide the current that drives the input to Ch5+ on U400 and other circuitry. That other circuitry includes another differential amplifier (unswitched this time) comprised of U475A and U475B that inverts the Ch5+ signal for input to U475D and on to Ch5-. U475D is another emitter follower to drive current into Ch5- on U400.
Bottom line, a somewhat complex yet straight forward "mux" to present either Readout Vertical or Ground to the inputs of U400 Ch5.
I was surprised by the number of components Tek used to accomplish such a simple task and aks the question: "why doesn't the Channel Switch ignore Ch5 inputs when Ch1 or Ch2 is selected?" I concluded that late in the development of the 2465 they discovered U400 "leaks" Ch5 into Ch1 and Ch2. The dies for U400 were proved and mass production had likely begun. The only solution was to block the "leak" by adding a switch in the pathway for Ch5 input so it is "grounded" when Ch1 or Ch2 is running. No time for special IC's and fidelity was important. So Tek built a kludge to mitigate the problem. It was cheap, and it worked, and it prevented a complex redesign of U400, so it stayed for the 2465A and 2465B.
Mac


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

Siggi
 

On Sun, 12 Nov 2017 at 11:05 Chuck Harris cfharris@... [TekScopes] <
TekScopes@...> wrote:



I don't believe the complexity in feeding CH5 is a mistake, but rather
an issue of partitioning... I believe the channel switch hybrid works
exactly the way it was supposed to work.
You can see the data sheet for the channel switch die here <
http://w140.com/tek_made_sm.pdf>, it's "named" 203-0211-90 and appears on
page 6-29 (or 271). It seems pretty clear that this is behaving as
intended, and that the #5 input is designed for a much lower bandwidth
signal than #1/2 and #3/4.
I'm guessing the original intent would have been for the readout signal to
be in a quiescent state when not displayed, or maybe it was simply less
expensive to do the low-bandwidth switching externally to the hybrid.

Chuck Harris
 

Hi Mac,

Your description looks to be ok. But what is wrong with the description
in the manual? It is available for our use, and seems pretty clear to me.

My shame is I should have read it earlier as it contained lots of clues
that would have gotten me off of the bad power boondoggle.

2 or 3 years ago, I did read the description, and concluded that U475 or
U485 was likely bad. I replaced U475, but hesitated to change U485 when
replacing U475 had no effect. I don't like replacing parts shotgun style,
much preferring learning over efficiency. Shotgunning when done rationally
is very fast, but you never really learn what the failure was, just the
area where the failure occurred.

I think the difference in the way I approach repairs is because I am an
electrical engineer, not a technician. The failures a technician fixes
are circuits that once worked. Replace the broken part, and operation
will be restored. The failures an engineer fixes may never have worked
correctly. You have to replace the broken idea to fix an engineering
failure.

I don't believe the complexity in feeding CH5 is a mistake, but rather
an issue of partitioning... I believe the channel switch hybrid works
exactly the way it was supposed to work.

Partitioning was the job of the 2465 system engineer.

What I think they did is reason that the display needed to be updated
often, without fail, or it would flicker, or wink out. You can tolerate
a little of that in a trace, but in the display it would look ridiculously
broken. The display has to be stable. Display has to be priority 1.

So, what they did is make a 4 channel MUX for CH1-4, and give it individual
selection lines that mostly mirrored the state of the channel selection
buttons on the front panel.

CH1 and CH2 were special in that tradition required there be a function
where the MUX output is CH1 + CH2... the "ADD" function. That means there
is an adder gate before the CH1/CH2 MUX. I think the designer simply made
that adder gate a 3 input adder that added CH1+CH2+CH5, and presented
the sum to the channel switch MUX.

For this to work, the way you deselect an input is to change it to a
zero, or ground (Ch1 + CH2 + 0= no display...). The VS1 - VS4 selection
lines automatically do that for CH1 - CH4 input, but CH5 has no such
selection line, so the user of the Channel Switch is required to do it
himself.

Earlier I said it was a partitioning problem. I believe the mistake
that happened was the Channel Switch hybrid designer, and the Display
hybrid designer each thought the other was responsible for grounding
out the display input when data was not being displayed. The possibility
is also pretty good that neither designed their part with the 2465, or
the other designer, in mind. They designed to spec.

The simple fix would have been to add VS5 to deselect the display channel.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:

Chuck, I am sure you understand that "complex circuit" on Ch5 input to U400, the Channel Switch. I think I do but I will document it for others and I hope you will correct any errors.
U485A and U485B form a differential amplifier whose operation depends on current to their common emitters through R483. Diode CR484 is fitted to stop the amplifier by "stealing" the emitter current when U975 pin 1 is high. U485C and U485D form a similar amplifier that is disabled when the Pin 3 of U975 is high. The outputs from these two differential amplifers is joined by the common collectors of U485B and U485D. U975A acts to select one or the other (not both) of these two amplifiers. Amplifier One (U485A and U485B) carries the readout signal as presented on pin 2 of U485A. Amplifier Two (U485C and U485D) "amplifies" the ground that is present on pin 6 of U485C. Thus, the signal at pin 6 of U475C is either Readout Vertical or ground, depending on the logic state of U975A. U475C is a simple emitter follower to provide the current that drives the input to Ch5+ on U400 and other circuitry. That other circuitry includes another differential amplifier (unswitched this time) comprised of U475A and U475B that inverts the Ch5+ signal for input to U475D and on to Ch5-. U475D is another emitter follower to drive current into Ch5- on U400.
Bottom line, a somewhat complex yet straight forward "mux" to present either Readout Vertical or Ground to the inputs of U400 Ch5.
I was surprised by the number of components Tek used to accomplish such a simple task and aks the question: "why doesn't the Channel Switch ignore Ch5 inputs when Ch1 or Ch2 is selected?" I concluded that late in the development of the 2465 they discovered U400 "leaks" Ch5 into Ch1 and Ch2. The dies for U400 were proved and mass production had likely begun. The only solution was to block the "leak" by adding a switch in the pathway for Ch5 input so it is "grounded" when Ch1 or Ch2 is running. No time for special IC's and fidelity was important. So Tek built a kludge to mitigate the problem. It was cheap, and it worked, and it prevented a complex redesign of U400, so it stayed for the 2465A and 2465B.
Mac

 

Chuck, I am sure you understand that "complex circuit" on Ch5 input to U400, the Channel Switch.  I think I do but I will document it for others and I hope you will correct any errors. 
U485A and U485B form a differential amplifier whose operation depends on current to their common emitters through R483.  Diode CR484 is fitted to stop the amplifier by "stealing" the emitter current when U975 pin 1 is high.  U485C and U485D form a similar amplifier that is disabled when the Pin 3 of U975 is high.  The outputs from these two differential amplifers is joined by the common collectors of U485B and U485D.  U975A acts to select one or the other (not both) of these two amplifiers.  Amplifier One (U485A and U485B) carries the readout signal as presented on pin 2 of U485A.  Amplifier Two (U485C and U485D) "amplifies" the ground that is present on pin 6 of U485C.  Thus, the signal at pin 6 of U475C is either Readout Vertical or ground, depending on the logic state of U975A.  U475C is a simple emitter follower to provide the current that drives the input to Ch5+ on U400 and other circuitry.  That other circuitry includes another differential amplifier (unswitched this time) comprised of U475A and U475B that inverts the Ch5+ signal for input to U475D and on to Ch5-.  U475D is another emitter follower to drive current into Ch5- on U400.  
Bottom line, a somewhat complex yet straight forward "mux" to present either Readout Vertical or Ground to the inputs of U400 Ch5. 
I was surprised by the number of components Tek used to accomplish such a simple task and aks the question: "why doesn't the Channel Switch ignore Ch5 inputs when Ch1 or Ch2 is selected?"  I concluded that late in the development of the 2465 they discovered U400 "leaks" Ch5 into Ch1 and Ch2.  The dies for U400 were proved and mass production had likely begun.  The only solution was to block the "leak" by adding a switch in the pathway for Ch5 input so it is "grounded" when Ch1 or Ch2 is running.  No time for special IC's and fidelity was important.  So Tek built a kludge to mitigate the problem.  It was cheap, and it worked, and it prevented a complex redesign of U400, so it stayed for the 2465A and 2465B.  
Mac

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎07‎:‎25‎:‎58‎ ‎PM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:

 
Glad you got it fixed.  Glad I could help.  Only the returning the many times you have helped me.

Mac

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎04‎:‎53‎:‎00‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
According to tektronix, U485A, U485B, U485C, and U475, form an
analog switch that routes either the readout at the base of U485A,
or the ground at the base of U485C to the output at the emitter of
U475C. The selection voltages are developed by voltage dividers
on the complementary outputs of U975C, and U975A.

Which pretty much fits your description exactly...

Also interesting is readout can only happen when the CH3 and CH4
inputs are deselected (eg. VS3 and VS4 are high). So, readout only
steals the beam from CH1 and CH2, or when no channels are being
displayed.... never from CH3 or CH4.

Why didn't I notice that phrase earlier?

I checked all of the signals once again, and they match what the manual
describes, however, the signal examples are only for when the readout
is turned off. Hmmm?

The nand gate in U975A, that creates signals 51 and 50 are indeed inverses
of each other, and look clean... and they match the waveform examples.

Hmmm? Hmmm?

Ok, why am I in AC mode, and why didn't I notice that the DC signals
at 52 and 53 were straddling ground?

Ah! U485 can do that if the "C" section is broken... Let's replace
it. Now where are my CA3046's? I only bought them a couple of years
ago... Seems like only yesterday....

Found them, replaced U485, and.......

The problem is gone!

Seems to me like I am always tagging a 2N3904 on the side of an open
circuited CA3046.

One of these days I should start recording which section of the
CA3046 pops.

Thanks machineguy for forcing me to think about that ugly mess
tagged on the side of the Channel Switch hybrid.

And thanks to everyone else for their hints, and being a sounding
board.

-Chuck Harris



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




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

 

Glad you got it fixed.  Glad I could help.  Only the returning the many times you have helped me.


Mac

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎04‎:‎53‎:‎00‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
According to tektronix, U485A, U485B, U485C, and U475, form an
analog switch that routes either the readout at the base of U485A,
or the ground at the base of U485C to the output at the emitter of
U475C. The selection voltages are developed by voltage dividers
on the complementary outputs of U975C, and U975A.

Which pretty much fits your description exactly...

Also interesting is readout can only happen when the CH3 and CH4
inputs are deselected (eg. VS3 and VS4 are high). So, readout only
steals the beam from CH1 and CH2, or when no channels are being
displayed.... never from CH3 or CH4.

Why didn't I notice that phrase earlier?

I checked all of the signals once again, and they match what the manual
describes, however, the signal examples are only for when the readout
is turned off. Hmmm?

The nand gate in U975A, that creates signals 51 and 50 are indeed inverses
of each other, and look clean... and they match the waveform examples.

Hmmm? Hmmm?

Ok, why am I in AC mode, and why didn't I notice that the DC signals
at 52 and 53 were straddling ground?

Ah! U485 can do that if the "C" section is broken... Let's replace
it. Now where are my CA3046's? I only bought them a couple of years
ago... Seems like only yesterday....

Found them, replaced U485, and.......

The problem is gone!

Seems to me like I am always tagging a 2N3904 on the side of an open
circuited CA3046.

One of these days I should start recording which section of the
CA3046 pops.

Thanks machineguy for forcing me to think about that ugly mess
tagged on the side of the Channel Switch hybrid.

And thanks to everyone else for their hints, and being a sounding
board.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:
I have a 2465A that once did something I would describe exactly as you have for your 2465. I puzzled out the circuit involving U475, U485 and U975 enough to understand what appears to be its purpose. That purpose is to block readout "noise" from the channel switch when either Ch1 or Ch2 are running. I checked the Ch1 and Ch2 inputs to the channel switch and they were clean, the outputs (waveforms 54 and 55) were not. I tried a different channel switch and got the same results. It seemed to be associated with the readout (turning off readout would stop the noise the "running part" of the trace). So I studied the complex circuit at the input to Ch5 and concluded its purpose was to block the readout when Ch1 or Ch2 are running. In my case, one of the pins of U975 was stuck. I changed U975 and everything got well for me. U975 isn't the only part that is critical to the operation. Its just the easiest to diagnose. A simple test of my theory would be to firmly ground pin 2 of U485. This will prevent all readout vertical from reaching the channel switch. Then if the noise on the trace goes away, the complex circuit is failing. Cant say how without more tests.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎01‎:‎12‎:‎28‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:


Another possibility.

The complexity of the circuitry going to CH5 has made me lift an
eyebrow a couple of times now. I even bought a stash of new CA3046's,
and replaced U475, to no effect, at the time.

I did check 48, 49, 50, 51, 52, 53, 54 and 55 waveforms, and they all
nominally matched what the book shows, when the SUT was set the way the
book said to. But notice I said nominally, the hook was obviously
there on 54 and 55.. 52 and 53 in the manual are just plain wrong as
far as amplitude goes... 150mv, not 5v p-p. I think the rest were ok,
though I may have missed some other clue.

-Chuck Harris


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

Chuck Harris
 

According to tektronix, U485A, U485B, U485C, and U475, form an
analog switch that routes either the readout at the base of U485A,
or the ground at the base of U485C to the output at the emitter of
U475C. The selection voltages are developed by voltage dividers
on the complementary outputs of U975C, and U975A.

Which pretty much fits your description exactly...

Also interesting is readout can only happen when the CH3 and CH4
inputs are deselected (eg. VS3 and VS4 are high). So, readout only
steals the beam from CH1 and CH2, or when no channels are being
displayed.... never from CH3 or CH4.

Why didn't I notice that phrase earlier?

I checked all of the signals once again, and they match what the manual
describes, however, the signal examples are only for when the readout
is turned off. Hmmm?

The nand gate in U975A, that creates signals 51 and 50 are indeed inverses
of each other, and look clean... and they match the waveform examples.

Hmmm? Hmmm?

Ok, why am I in AC mode, and why didn't I notice that the DC signals
at 52 and 53 were straddling ground?

Ah! U485 can do that if the "C" section is broken... Let's replace
it. Now where are my CA3046's? I only bought them a couple of years
ago... Seems like only yesterday....

Found them, replaced U485, and.......

The problem is gone!

Seems to me like I am always tagging a 2N3904 on the side of an open
circuited CA3046.

One of these days I should start recording which section of the
CA3046 pops.

Thanks machineguy for forcing me to think about that ugly mess
tagged on the side of the Channel Switch hybrid.

And thanks to everyone else for their hints, and being a sounding
board.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:

I have a 2465A that once did something I would describe exactly as you have for your 2465. I puzzled out the circuit involving U475, U485 and U975 enough to understand what appears to be its purpose. That purpose is to block readout "noise" from the channel switch when either Ch1 or Ch2 are running. I checked the Ch1 and Ch2 inputs to the channel switch and they were clean, the outputs (waveforms 54 and 55) were not. I tried a different channel switch and got the same results. It seemed to be associated with the readout (turning off readout would stop the noise the "running part" of the trace). So I studied the complex circuit at the input to Ch5 and concluded its purpose was to block the readout when Ch1 or Ch2 are running. In my case, one of the pins of U975 was stuck. I changed U975 and everything got well for me. U975 isn't the only part that is critical to the operation. Its just the easiest to diagnose. A simple test of my theory would be to firmly ground pin 2 of U485. This will prevent all readout vertical from reaching the channel switch. Then if the noise on the trace goes away, the complex circuit is failing. Cant say how without more tests.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎01‎:‎12‎:‎28‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:


Another possibility.

The complexity of the circuitry going to CH5 has made me lift an
eyebrow a couple of times now. I even bought a stash of new CA3046's,
and replaced U475, to no effect, at the time.

I did check 48, 49, 50, 51, 52, 53, 54 and 55 waveforms, and they all
nominally matched what the book shows, when the SUT was set the way the
book said to. But notice I said nominally, the hook was obviously
there on 54 and 55.. 52 and 53 in the manual are just plain wrong as
far as amplitude goes... 150mv, not 5v p-p. I think the rest were ok,
though I may have missed some other clue.

-Chuck Harris

 

I have a 2465A that once did something I would describe exactly as you have for your 2465.  I puzzled out the circuit involving U475, U485 and U975 enough to understand what appears to be its purpose.  That purpose is to block readout "noise" from the channel switch when either Ch1 or Ch2 are running.  I checked the Ch1 and Ch2 inputs to the channel switch and they were clean, the outputs (waveforms 54 and 55) were not.  I tried a different channel switch and got the same results.  It seemed to be associated with the readout (turning off readout would stop the noise the "running part" of the trace).  So I studied the complex circuit at the input to Ch5 and concluded its purpose was to block the readout when Ch1 or Ch2 are running.  In my case, one of the pins of U975 was stuck.  I changed U975 and everything got well for me.  U975 isn't the only part that is critical to the operation.  Its just the easiest to diagnose.  A simple test of my theory would be to firmly ground pin 2 of U485.  This will prevent all readout vertical from reaching the channel switch.  Then if the noise on the trace goes away, the complex circuit is failing.  Cant say how without more tests.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎01‎:‎12‎:‎28‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
Another possibility.

The complexity of the circuitry going to CH5 has made me lift an
eyebrow a couple of times now. I even bought a stash of new CA3046's,
and replaced U475, to no effect, at the time.

I did check 48, 49, 50, 51, 52, 53, 54 and 55 waveforms, and they all
nominally matched what the book shows, when the SUT was set the way the
book said to. But notice I said nominally, the hook was obviously
there on 54 and 55.. 52 and 53 in the manual are just plain wrong as
far as amplitude goes... 150mv, not 5v p-p. I think the rest were ok,
though I may have missed some other clue.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:
Here is what I think, and I'm pretty confident. The channel switch (U400) design "leaks" Ch 5 into Ch1 and Ch2. Tek decided (for some reason) to fix this by adding logic external to the channel switch. That external logic is implemented in U485, U475 and U975. These parts function so that any time horizontal A or horizontal B (A sweep or B sweep) are operating the readout into U400 is clamped to zero at the emitter outputs of U475C and U475D. In your case, they are not functioning, most likely a fault in U975. So, the readout vertical reaches U400 and leaks into Ch1 and Ch2, just like Tek always knew it would.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎11‎:‎47‎:‎44‎ ‎AM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:


Chuck, I think its from the readout. Most likely the readout DO circuitry. Probably in U485, U475, or U975. There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975. If this switch and logic fails, readout is coupled to the vertical channels of the channel switch. I have never tested to see if it affects only Ch1 and Ch2. But I have seen it. Check waveforms 50 and 51 on U975.
Mac

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:


A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:

(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:
I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan

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



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

Chuck Harris
 

Another possibility.

The complexity of the circuitry going to CH5 has made me lift an
eyebrow a couple of times now. I even bought a stash of new CA3046's,
and replaced U475, to no effect, at the time.

I did check 48, 49, 50, 51, 52, 53, 54 and 55 waveforms, and they all
nominally matched what the book shows, when the SUT was set the way the
book said to. But notice I said nominally, the hook was obviously
there on 54 and 55.. 52 and 53 in the manual are just plain wrong as
far as amplitude goes... 150mv, not 5v p-p. I think the rest were ok,
though I may have missed some other clue.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:

Here is what I think, and I'm pretty confident. The channel switch (U400) design "leaks" Ch 5 into Ch1 and Ch2. Tek decided (for some reason) to fix this by adding logic external to the channel switch. That external logic is implemented in U485, U475 and U975. These parts function so that any time horizontal A or horizontal B (A sweep or B sweep) are operating the readout into U400 is clamped to zero at the emitter outputs of U475C and U475D. In your case, they are not functioning, most likely a fault in U975. So, the readout vertical reaches U400 and leaks into Ch1 and Ch2, just like Tek always knew it would.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎11‎:‎47‎:‎44‎ ‎AM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:


Chuck, I think its from the readout. Most likely the readout DO circuitry. Probably in U485, U475, or U975. There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975. If this switch and logic fails, readout is coupled to the vertical channels of the channel switch. I have never tested to see if it affects only Ch1 and Ch2. But I have seen it. Check waveforms 50 and 51 on U975.
Mac

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:


A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:

(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:
I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan





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Posted by: machine guy <@Mac>
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Chuck Harris
 

I'm coming to think it is the power circuitry around the
CH1 and CH2 preamps.

I soldered a wire onto one of the pads where the delay line,
attaches to the PCB, and carefully adjusted the hold off,
on my bench scope, so that I got a clean picture of that
output... which is the direct output of the Channel Switch.

I connected the CH1 Gate signal from the back panel to my
bench scope's CH2, and used it for the trigger. The CH2
display of the gate gives me a clear picture of the beginning
and end of each channel's time slot.

I can easily see each of the channels as it outputs from
the Channel Switch in succession. Hook on CH1 and CH2 and
everything. OBTW, that Hook is the clue, as it is *always*
there on CH1 and CH2.

If you look at the power distribution circuitry around the
preamps, you will see that CH3/CH4 use only +5V and -5V, and
share common RF isolation filter networks to clean up their
sources of power.

CH1/CH2 share a common RF isolation filter networks on +5V,
and common RF isolation filter networks on -5V and -8V.

Ok, why is the hook the clue?

Simple, the hook is the vertical preamplifier's way of saying
that it has squishy power, and doesn't like the channel switch
turning its load on and off when CH1 or CH2 is selected.

When the channel switch connects to CH1 or CH2, it is pulling
the quiescent output level of the preamp towards the load
potential of the channel switch... Oh so slowly. Forming the Hook.

It could also be a bad output amplifier on both CH1 and CH2, but
what are the odds of that happening simultaneously?

I think it is more likely that one of the power supply isolation
RF filters that CH1 and CH2 share is squishy, and I will find
one supply has a nice healthy 5us pulse on it. A mirror image
of the hook that shows on the leading edge (left) of the CH1 and
CH2 traces.

I will check that soon and report back.

-Chuck Harris

machine guy @Mac [TekScopes] wrote:

Chuck, I think its from the readout. Most likely the readout DO circuitry. Probably in U485, U475, or U975. There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975. If this switch and logic fails, readout is coupled to the vertical channels of the channel switch. I have never tested to see if it affects only Ch1 and Ch2. But I have seen it. Check waveforms 50 and 51 on U975.
Mac

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:


A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:

(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

 

Here is what I think, and I'm pretty confident.  The channel switch (U400) design "leaks" Ch 5 into Ch1 and Ch2.  Tek decided (for some reason) to fix this by adding logic external to the channel switch.  That external logic is implemented in U485, U475 and U975.  These parts function so that any time horizontal A or horizontal B (A sweep or B sweep) are operating the readout into U400 is clamped to zero at the emitter outputs of U475C and U475D.  In your case, they are not functioning, most likely a fault in U975.  So, the readout vertical reaches U400 and leaks into Ch1 and Ch2, just like Tek always knew it would.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎11‎:‎47‎:‎44‎ ‎AM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:

 
Chuck, I think its from the readout.  Most likely the readout DO circuitry.  Probably in U485, U475, or U975.  There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975.  If this switch and logic fails, readout is coupled to the vertical channels of the channel switch.  I have never tested to see if it affects only Ch1 and Ch2.  But I have seen it.  Check waveforms 50 and 51 on U975.  
Mac

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:

(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:
I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan

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



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


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Yahoo Groups Links








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

 

Chuck, I think its from the readout.  Most likely the readout DO circuitry.  Probably in U485, U475, or U975.  There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975.  If this switch and logic fails, readout is coupled to the vertical channels of the channel switch.  I have never tested to see if it affects only Ch1 and Ch2.  But I have seen it.  Check waveforms 50 and 51 on U975.  
Mac

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:

(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:
I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan





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Posted by: hahi@...
------------------------------------


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

Yahoo Groups Links





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

Tom Gardner
 

Which pushes the problem area towards the VS1/2/3/4 control signals,
and conceivably the blanking waveform iff it is different for the
different channels.

Puzzling :)

On 10/11/17 16:13, Chuck Harris cfharris@... [TekScopes] wrote:

The glitch is the same regardless of the vertical or horizontal
positions. The attenuator and input AC/DC/50/GND switch makes no
difference.

The only thing that makes a difference is CH1/CH2 vs CH3/CH4. You
can even have all 4 traces on the screen, and CH1/CH2 have the
glitches, but CH3 and CH4 are clean and perfect.

When I look at the input to the delay line, I can see CH1 with its
hook, and CH4 with its perfectly straight trace. If display is on,
both show so much garbage that you can't tell what you are seeing.

The input to the delay line is the output from the Channel Switch
hybrid.

Because CH2 output on the back panel is clean, I think I can assume
the preamps are out of the problem area. Since the problem shows
clearly on the output of the Channel Switch, and the Channel Switch
is a known good hybrid, the problem has to be in the care and feeding
of the Channel Switch....Somewhere.

Unfortunately, there are no test points between the output of the
CH1/CH2 preamps and the Channel Switch inputs... they are all on the
underside of the board, and running the A1 board out of the chassis
is impractical.

-Chuck Harris

Tom Gardner tggzzz@... [TekScopes] wrote:
It would be interesting to know if the "glitch" amplitude depends
on the relative positions of the CH1/2 trace on the screen, with the
inputs grounded and with different DC inputs.

I would identify just about all the analogue switches (e.g. 4053s)
I could find, and see if any of those are being switched too slowly
and/or had too high a resistance and/or whatever was driving their
signals had become too high resistance. Maybe there's a mechanical
failure somewhere in that chain.

I would use another scope to find where the glitch /doesn't/ exist
- trigger on any control signal vaguely associated with the glitch,
potentially even the readout vert/horiz signals diagram 48 and 49
- work back from the Y output deflection voltage, through U600, U400
- then to U100/U200
- then control signals VS1/2/3/4 from U650, checking particularly
if there is any difference between VS1/2 and VS3/4

Chuck Harris
 

The 2465 is not a digital scope. It is analog, but has
a digital microprocessor controlling all of the circuitry.

On the 2465, hybrids handle much of the circuitry. In the
case of the readout, the channel selecting switch is a
hybrid that has 5 identical input channels. 1-4 are CH1
through CH4, CH5 is uniquely for the display input.

The CH5 display is always selected, but CH1 through CH4 can
be individually selected by VS1-VS4.

The Channel Switch hybrid is just a multiplexor. The blanking
signals don't go to the Channel Switch hybrid at all.

So, if it is blanking timing, it should affect all channels
equally, but it doesn't. I can see the ugly stuff in the
CH1 and CH2 slots, directly at the output of the Channel Switch.

The truth is out there...

-Chuck Harris

Harvey White madyn@... [TekScopes] wrote:

On Fri, 10 Nov 2017 01:37:50 -0500, you wrote:

<snip>
An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.

I am pretty certain it has something to do with the display
readout logic.
I don't quite have that scope, but they have to do much the same in
the 7000 series for the readouts.

I think what you're seeing in the ___ |___ _____ are the switching
transients caused by inadequate blanking caused by the readout
generator.

The generator chops the beam, runs the readout, then puts the beam
back where it was. For that transition from trace to readout, the
beam is obviously blanked. If that blanking is messed up, then I
suspect you get the results you see. Won't be particularly sensitive
to sweep speed because it's a digital scope. I'd be looking at
inadequate leading edge sharpness on the chopping blanking pulses
myself.

Harvey

Chuck Harris
 

The only thing in common between the symptoms
I described, and the symptoms you described is
the fact that the scopes were 2465's.

-Chuck Harris

davidazzz8@... [TekScopes] wrote:

So not in case he was with me
I had a Shortened diode cr3011


It made Channel 1
The volt / dev



For improper identification of measured volts

On some of the situations in the switch.


The second problem I had in the front panel
The flexible cable
From borad A6 to borad A5

One of the cables in J652
Probably torn without continuity.

If I'm not mistaken do not remember for sure
It was pin 14 on J652.



This caused the Channel 3 display not to work
channel One And Channel Two
the Trace Were not stable

and Not kept in place
coupling ch1 and ch2 gnd Worked properly

Chuck Harris
 

The glitch is the same regardless of the vertical or horizontal
positions. The attenuator and input AC/DC/50/GND switch makes no
difference.

The only thing that makes a difference is CH1/CH2 vs CH3/CH4. You
can even have all 4 traces on the screen, and CH1/CH2 have the
glitches, but CH3 and CH4 are clean and perfect.

When I look at the input to the delay line, I can see CH1 with its
hook, and CH4 with its perfectly straight trace. If display is on,
both show so much garbage that you can't tell what you are seeing.

The input to the delay line is the output from the Channel Switch
hybrid.

Because CH2 output on the back panel is clean, I think I can assume
the preamps are out of the problem area. Since the problem shows
clearly on the output of the Channel Switch, and the Channel Switch
is a known good hybrid, the problem has to be in the care and feeding
of the Channel Switch....Somewhere.

Unfortunately, there are no test points between the output of the
CH1/CH2 preamps and the Channel Switch inputs... they are all on the
underside of the board, and running the A1 board out of the chassis
is impractical.

-Chuck Harris





Tom Gardner tggzzz@... [TekScopes] wrote:

It would be interesting to know if the "glitch" amplitude depends
on the relative positions of the CH1/2 trace on the screen, with the
inputs grounded and with different DC inputs.

I would identify just about all the analogue switches (e.g. 4053s)
I could find, and see if any of those are being switched too slowly
and/or had too high a resistance and/or whatever was driving their
signals had become too high resistance. Maybe there's a mechanical
failure somewhere in that chain.

I would use another scope to find where the glitch /doesn't/ exist
- trigger on any control signal vaguely associated with the glitch,
potentially even the readout vert/horiz signals diagram 48 and 49
- work back from the Y output deflection voltage, through U600, U400
- then to U100/U200
- then control signals VS1/2/3/4 from U650, checking particularly
if there is any difference between VS1/2 and VS3/4

Harvey White
 

On Fri, 10 Nov 2017 01:37:50 -0500, you wrote:

<snip>
An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.

I am pretty certain it has something to do with the display
readout logic.
I don't quite have that scope, but they have to do much the same in
the 7000 series for the readouts.

I think what you're seeing in the ___ |___ _____ are the switching
transients caused by inadequate blanking caused by the readout
generator.

The generator chops the beam, runs the readout, then puts the beam
back where it was. For that transition from trace to readout, the
beam is obviously blanked. If that blanking is messed up, then I
suspect you get the results you see. Won't be particularly sensitive
to sweep speed because it's a digital scope. I'd be looking at
inadequate leading edge sharpness on the chopping blanking pulses
myself.

Harvey



-Chuck Harris

edbreya@... [TekScopes] wrote:
You may want to check all readily accessible hardware mounting and grounding, particularly around the delay line. If the hooks are constant, then they must be leaking into the circuit past the front-end, and somehow localized so some parts are not affected. The one thing that is commonest to all sections and operations is the grounding. Check that all screws are tightened up snug (and that none are missing), and jiggle things around to see if the symptoms can be aggravated.

To see if you've got some kind of ground loops at the front, jump the apparently unaffected CH3 and CH4 inputs to the CH1 and CH2 - it's easy to just use BNC cables for this.

Ed

Tom Gardner
 

It would be interesting to know if the "glitch" amplitude depends
on the relative positions of the CH1/2 trace on the screen, with the
inputs grounded and with different DC inputs.

I would identify just about all the analogue switches (e.g. 4053s)
I could find, and see if any of those are being switched too slowly
and/or had too high a resistance and/or whatever was driving their
signals had become too high resistance. Maybe there's a mechanical
failure somewhere in that chain.

I would use another scope to find where the glitch /doesn't/ exist
- trigger on any control signal vaguely associated with the glitch,
potentially even the readout vert/horiz signals diagram 48 and 49
- work back from the Y output deflection voltage, through U600, U400
- then to U100/U200
- then control signals VS1/2/3/4 from U650, checking particularly
if there is any difference between VS1/2 and VS3/4

On 10/11/17 14:19, Chuck Harris cfharris@... [TekScopes] wrote:

Hi Tom,

It looks just like what you would see if you had a fast
marker pulse, and you sped up the timebase. At slow
timebase settings, it is just a spike, at faster settings,
it is a fast rise, with a much, much slower decay.

The "hooks" are about 5us in duration. The risetime is
very fast, but the decay takes most of the 5us.

I believe that I am looking at two things. Probably issues
with the channel switch:

1) a glitch that is leaking through whenever the sweep starts
drawing CH1/CH2 on the screen.
2) a stream of display glitches that leak through whenever the
channel switch switches between CH1/CH2 and the CH5, the
display.

I have zero doubt that the random looking grassy pulses are
the transitions when the display takes over the beam from CH1/CH2.
They disappear when the display is turned off.

Everything is complicated by the display timing logic. The 2465
tries to avoid the "holes" in the traces that happen when the
display has to happen, so it has 4 or 5 different modes that
range from alternating display and trace, to random display
writing, to writing display full time. The display timing logic
makes these decisions based on how long it has been since it has
refreshed the display.

-Chuck Harris

Tom Gardner tggzzz@... [TekScopes] wrote:
On 10/11/17 06:37, Chuck Harris cfharris@... [TekScopes] wrote:

An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.
By that do you mean that when you change the timebase
the decay curve is the same time or the same number
of divisions?

I am pretty certain it has something to do with the display
readout logic.
That was the feeling I got from reading your description.

To see whether it is correlated with the display readout
timing, I would try either of two tests:
1) apply an external signal and adjust its frequency to be
the same as (a harmonic or subharmonic) the display
readout frequency so that any "twinkling" in the traces
is more-or-less stationary. (Start with a timebase of
~200us/div). Then see if the hooks are stationary w.r.t.
the intensity variations. If they are, then the display
readout is involved.
2) without an external signal and a ~2ms/div timebase,
change the holdoff until the twinkling is stationary,
and look for the hooks as above.

If display readout is involved, I would find out what is
happening when the display is switching from trace
to readout. I would observe the Y waveform and blanking
signals after the display sequencer IC and near the
display blanking IC. Trigger on either the blanking
waveform or the Y-waveform.

I would be looking for either the blanking signal to
be mistimed or the Y-waveform to be changing too slowly.
I would suspect the latter, so then it is a case of moving
"away from the output" until you find where it isn't
changing too slowly.

Be careful with probe tips around there; I know they
can short signals and destroy ICs :(

Chuck Harris
 

I should have said: "pulses are "related" to the transitions
when the display takes over the screen... The actual transitions
of the beam are invisible... as they should be.

-Chuck Harris

Chuck Harris cfharris@... [TekScopes] wrote:

Hi Tom,

It looks just like what you would see if you had a fast
marker pulse, and you sped up the timebase. At slow
timebase settings, it is just a spike, at faster settings,
it is a fast rise, with a much, much slower decay.

The "hooks" are about 5us in duration. The risetime is
very fast, but the decay takes most of the 5us.

I believe that I am looking at two things. Probably issues
with the channel switch:

1) a glitch that is leaking through whenever the sweep starts
drawing CH1/CH2 on the screen.
2) a stream of display glitches that leak through whenever the
channel switch switches between CH1/CH2 and the CH5, the
display.

I have zero doubt that the random looking grassy pulses are
the transitions when the display takes over the beam from CH1/CH2.
They disappear when the display is turned off.

Everything is complicated by the display timing logic. The 2465
tries to avoid the "holes" in the traces that happen when the
display has to happen, so it has 4 or 5 different modes that
range from alternating display and trace, to random display
writing, to writing display full time. The display timing logic
makes these decisions based on how long it has been since it has
refreshed the display.

-Chuck Harris

Chuck Harris
 

That sort of failure would affect all channels equally.
This is only a problem with CH1 and CH2.

The panel works fully.

-Chuck Harris

davidazzz8@... [TekScopes] wrote:

Have you checked the A6 board
Of the front panel
From experience I when i fix my 2465
Failure in one of the diodes can cause such a problem
You can also do a pot test



Exercise 1



And check that all buttons work correctly.



also check the continuity
from J652 on A6 to

( a sec dev /b sec dev)

col 0
col 1
col 2
col 3
col 4
( volts dev ch1 ch2 )

row 0
row 1
row 2
row 3
row 4















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Posted by: davidazzz8@...
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