475 Time Base Setting Problem


John Clark
 

So, up until last night my 475 had been working reasonably well, with exception of the jitter/wiggle problem I mentioned in my other topic. I plan on looking at LV ripple today.

However, it just presented another problem. I was watching a video by W2AEW on how to use the B delayed sweep and the delay time position knob to determine the rise time of a square wave. I had my function generator plugged in and was attempting to duplicate what he was doing. I was able to figure it out and it all seemed to work. That is, until this new problem. Now, when I have the knobs locked for A, and set the time to either .5s, 5ms, 50us, or .5us the trace gets very slow, i.e. it turns to just a dot moving very slowly across the display. .5s moves slower than 5ms, and 5ms moves slower than 50us, so there's still some kind of adjustment happening there but something went haywire. If I pull to unlock the center Delayed Sweep knob out to where the switch inside activates then the sweep goes to, what appears to be, its normal speed.

It just started doing this after I turned away from it for a few minutes last night. When I looked at it again there was no trace and just this slow moving dot on only the settings listed above.

Any ideas on this one? It looks like I'll have to tear into again it after all...


Thanks for everyone's help so far!


John


 

John,
Did you try exercising all positions and A/B selection of the time base knobs several times? This looks like a bad (dirty) contact on the div/sec drum. I haven't looked into the schematics for common connections for the speed you mention but cleaning will have to be non-selectively anyway. It *could* be a bad transistor or IC contact on the time base board. Make a habit of carefully wiggling all IC's and transistors that you can touch, preferably with power off. Most are in sockets, though that may not be apparent without looking closely.

Don't let this discourage you: There is a limited number of issues that these 'scopes develop in over 70% of cases. It's not a matter of "a new surprise every day"- that is, until you've enjoyed them all...
Basically, it's dirty contacts, including pots (both often treated satisfactorilly by exercising them), (dipped) tantalum caps, the large PSU alu can caps and indicator and graticule lights. Oh yes, a transistor every now and then...

At this stage, make sure you get a service manual if you haven't got it already (PDF from the Internet)
and *a second 'scope*! It may be partially broken because you'll repair it using your 475!

Raymond


John Clark
 

I'm not discouraged. A bit irritated, maybe! I suspected a dirty contact, as well, but all the 5's in the non working times should be a clue and so I thought I'd post to see if someone had a better idea. I did try turning the knob back and forth quite a bit to try and "clean it up" that way. No go.

I'm fortunate that I have a nice original hard copy of the service manual. ‎It has been invaluable!

John

Sent from my BlackBerry 10 smartphone.
From: hewpatek@gmail.com [TekScopes]
Sent: Wednesday, November 4, 2015 08:43
To: TekScopes@yahoogroups.com
Reply To: TekScopes@yahoogroups.com
Subject: [TekScopes] Re: 475 Time Base Setting Problem


John,
Did you try exercising all positions and A/B selection of the time base knobs several times? This looks like a bad (dirty) contact on the div/sec drum. I haven't looked into the schematics for common connections for the speed you mention but cleaning will have to be non-selectively anyway. It *could* be a bad transistor or IC contact on the time base board. Make a habit of carefully wiggling all IC's and transistors that you can touch, preferably with power off. Most are in sockets, though that may not be apparent without looking closely.

Don't let this discourage you: There is a limited number of issues that these 'scopes develop in over 70% of cases. It's not a matter of "a new surprise every day"- that is, until you've enjoyed them all...
Basically, it's dirty contacts, including pots (both often treated satisfactorilly by exercising them), (dipped) tantalum caps, the large PSU alu can caps and indicator and graticule lights. Oh yes, a transistor every now and then...

At this stage, make sure you get a service manual if you haven't got it already (PDF from the Internet)
and *a second 'scope*! It may be partially broken because you'll repair it using your 475!

Raymond

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



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


 

Nothing beats a paper manual. I like to use pdf's to have dozens of manuals with me on my iPad all the time and to search through manuals, next I open the paper manual if I have it.

The "5-" definitely is an important clue. Unfortunately, I'm a bit busy at the moment to dive into the manual.
Drawing 9 should help you further. You will see that drum switch 9 of the non-delaying sweep controls the speeds you mention. As said before, you won't be working on an individual contact to clean once you're in there - which is a bit of a project, as is getting out safely...
Make sure you understand the time/div. knob assembly and the drum mechanism before you take things apart and *take lots of pictures*. Carefully collect all small parts that may fall out.
The time/div. knob assy. is a bit complex but once you understand it, it's not difficult to reassemble and adjust. Make sure you have a set of high quality small hex keys for the knobs' screws.
Instructions for cleaning the switch contacts are elsewhere on this forum. Just make sure to *never* use any abrasive materials. In short, it's a matter of wiping the contacts with a piece of IPA-wetted, preferably chalkless paper (I use my business cards). You may carefully press down on the contact spring notch for cleaning, pinching the paper between contact and PCB.. Make sure you don't bend or even damage the contacts and be aware that they're soldered onto the PCB so never use a soldering iron in the area.

If you're like me, you still have the attenuator you replaced. You may study and clean that as a training exercise.

Raymond


John Clark
 

Thanks Raymond! I was already open to Drawing 9 and already had a pencil mark on switch 9. However, the schematic shows the switch in XY mode so I wasn't sure if that meant switch 9 was my issue or not. The dots only indicate a closed switch when set to XY mode. How would I know where they're all supposed to be when I set it to, for example, 5ms?

I guess I should clarify, I didn't replace the entire attenuator assembly on channel 1. I only replaced one of the plug in modules under the shield cover. I did, however, clean as many of the switches as I could get to easily when I was in there, following some similar instructions I found online for doing so. That's how I found I needed to replace the plug-in module.

John

To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Wed, 4 Nov 2015 06:21:41 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























Nothing beats a paper manual. I like to use pdf's to have dozens of manuals with me on my iPad all the time and to search through manuals, next I open the paper manual if I have it.





The "5-" definitely is an important clue. Unfortunately, I'm a bit busy at the moment to dive into the manual.


Drawing 9 should help you further. You will see that drum switch 9 of the non-delaying sweep controls the speeds you mention. As said before, you won't be working on an individual contact to clean once you're in there - which is a bit of a project, as is getting out safely...


Make sure you understand the time/div. knob assembly and the drum mechanism before you take things apart and *take lots of pictures*. Carefully collect all small parts that may fall out.


The time/div. knob assy. is a bit complex but once you understand it, it's not difficult to reassemble and adjust. Make sure you have a set of high quality small hex keys for the knobs' screws.


Instructions for cleaning the switch contacts are elsewhere on this forum. Just make sure to *never* use any abrasive materials. In short, it's a matter of wiping the contacts with a piece of IPA-wetted, preferably chalkless paper (I use my business cards). You may carefully press down on the contact spring notch for cleaning, pinching the paper between contact and PCB.. Make sure you don't bend or even damage the contacts and be aware that they're soldered onto the PCB so never use a soldering iron in the area.





If you're like me, you still have the attenuator you replaced. You may study and clean that as a training exercise.





Raymond























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


 

John,
Not sure if I understand you. The dot at X-Y means that the contact is also closed when X-Y is selected. The dots means that the corresponding switch (horizontal axis) is closed in corresponding positions shown in the column at left.

The misunderstanding re. "attenuator" happens more often. I tell myself to use the term "attenuator block" for the 6-pin plugin part that you mean and "attenuator assembly" for the PCB with blocks and switches.
I try and avoid using "attenuator module" in these 'scopes for obvious reasons...

BTW, these blocks can be repaired, although it's a bit tricky. You need low-temp solder, a narrow tip and be prepared for oxidised pins to solder to.


Raymond


 

FYI: It's not certain that switch contact 9 is the culprit. It's "just" a common factor.So are R1083 and R1084.

Raymond


John Clark
 

"Attenuator block"...got it!

Re the X-Y question...Nevermind. I answered it myself. I was understanding the drawing wrong. I was reading the "switch shown in X-Y position" to mean the entire block of switches in the top diagram when it's just referring to the rest of the schematic's switch symbols.

John
To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Wed, 4 Nov 2015 07:02:09 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























John,


Not sure if I understand you. The dot at X-Y means that the contact is also closed when X-Y is selected. The dots means that the corresponding switch (horizontal axis) is closed in corresponding positions shown in the column at left.





The misunderstanding re. "attenuator" happens more often. I tell myself to use the term "attenuator block" for the 6-pin plugin part that you mean and "attenuator assembly" for the PCB with blocks and switches.


I try and avoid using "attenuator module" in these 'scopes for obvious reasons...





BTW, these blocks can be repaired, although it's a bit tricky. You need low-temp solder, a narrow tip and be prepared for oxidised pins to solder to.








Raymond



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



















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


John Clark
 

Yes, I do understand that.

Pulling the knob out of the locked position closes S1099 which switches it to delayed sweep and then the trace returns to normal.

John

To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Wed, 4 Nov 2015 07:09:41 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























FYI: It's not certain that switch contact 9 is the culprit. It's "just" a common factor.So are R1083 and R1084.





Raymond



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


 

If I pull to unlock the center Delayed Sweep knob out to where the switch inside activates
then the sweep goes to, what appears to be, its normal speed.
Obviously, this is important as well. Unfortunately, I have no time to dive in now.

Raymond


John Clark
 

So, with the opamp replacements coming for my noise/wiggle issue, I'm going to move back to my time base problem since my trace is now pretty solid. Thanks so much to everyone who helped me with that!

Now for the time base problem. I'll try and provide as much detail as I can. Sorry for the long read. There might even be enough clues in here for me to find it myself, eventually, but I'm not as versed at reading these diagrams as many of you.

I was watching a YT video on how to use the B delayed sweep and the delay time position knob
to determine the rise time of a square wave. I had my function
generator plugged in and was attempting to duplicate what the guy on the video was doing.
I was able to understand it and it all seemed to work on my 475 similar to what he was describing on his 485. I was able to select the wave rise and focus in on it using the B delayed and the delay time position knob. The rise time I calculated even matched what my function generator should be doing, so I was happy! (yes, simple things make a newbie happy.)

Anyway, I looked away for a few minutes--back to the video--and when I turned back to the scope, the trace was gone. I messed with it for a minute and found that when the knobs are locked for A, and the sweep set to either .5s, 5ms, 50us, or .5us the trace gets very slow. It
turns to just a dot moving very slowly across the display. .5s moves
slower (pretty much a standstill) than 5ms, and 5ms moves slower than 50us, so there's still some
kind of adjustment happening there but something is haywire. Also, on these 4 speeds the dot moving across the screen gets very bright. This appears to be normal at slower speeds. If I
pull straight out to unlock the center Delayed Sweep knob just to where S1094 closes then the sweep goes to, what appears to be, its normal
speed.


Also, I notice that if I pull the knob out for delayed sweep, then select another sweep speed with the delayed sweep knob, and then select B delayed sweep I see the same type of slow sweep on the 5ms, 50us and .5us time settings for delayed sweep. The other delayed sweep speeds appear to be normal. Would this not indicate something that is common to both the A and B delayed sweep in those time settings?

When I press "Mix" on those time settings I get a short sweep about 5 graticules wide that looks similar to beam finder.

When I press "A Inten" the sweeps on A act normally. While in "A Inten" if I pull the delay sweep knob out and move it, it focuses the intensity of the sweep on a smaller and smaller section (normal, I believe) until I get to one of the failed sweep speeds where the sweep then goes back to the normal looking sweep selected with A, instead of a smaller, more intensified section of the sweep.




I have turned the A knob a lot, in both locked and unlocked positions, trying to clean it up and get it to work, even intermittently, but it seems to be hard failed. Upon looking at the diagrams yesterday we came to the conclusion earlier that switch #9 on the rear cam of S1050 could be the culprit as it closes for each of those settings plus X-Y. Since it has been working for months and started this problem on it's own, it could be just a dirty switch. However, if it were just a little dirty wouldn't it probably work intermittently? I'd suspect a broken switch except that it started when it was just sitting there running, not moving the knobs, though it could have broken while moving it and then finally just lost contact. There is also R1083 and R1084. However, since the trigger board has to come out in order to check those components I want to do as much troubleshooting as possible prior to disassembly since I won't be able to check voltages. The fact that I see the problem on both A and B delayed seems to me it might be something else besides the switch 9 or those associated resistors. What other components are common to both non-delayed and delayed, but only needed for those particular sweep speeds?

Switch 9 also makes contact in X-Y mode. Is there a simple way to check that X-Y mode is working right? When in X-Y mode I get a dot on Ch1 and Ch2 (normal.) Channel 1 dot moves at a 45 degree angle when I adjust the vertical position knob. Channel 2 moves only vertically.

Thanks,
John



To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Wed, 4 Nov 2015 08:21:43 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























> If I pull to unlock the center Delayed Sweep knob out to where the switch inside activates


then the sweep goes to, what appears to be, its normal speed.


Obviously, this is important as well. Unfortunately, I have no time to dive in now.





Raymond























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


John Clark
 

And because a picture is worth 1000 words (and nobody likes reading a big long 1000 word question like below) here is a YT video of the issue. The intensity isn't really as bright as my phone camera thinks it is. Also, at the end the trace disappears. I get a little frustrated and end the video. Turns out the intensity was down...there wasn't another problem. It's harder to make good YT videos than people think!

John

https://youtu.be/UzYZWYwtS-o



From: johnclark05@outlook.com
To: tekscopes@yahoogroups.com
Subject: RE: [TekScopes] Re: 475 Time Base Setting Problem
Date: Thu, 5 Nov 2015 10:21:49 -0500




So, with the opamp replacements coming for my noise/wiggle issue, I'm going to move back to my time base problem since my trace is now pretty solid. Thanks so much to everyone who helped me with that!

Now for the time base problem. I'll try and provide as much detail as I can. Sorry for the long read. There might even be enough clues in here for me to find it myself, eventually, but I'm not as versed at reading these diagrams as many of you.

I was watching a YT video on how to use the B delayed sweep and the delay time position knob
to determine the rise time of a square wave. I had my function
generator plugged in and was attempting to duplicate what the guy on the video was doing.
I was able to understand it and it all seemed to work on my 475 similar to what he was describing on his 485. I was able to select the wave rise and focus in on it using the B delayed and the delay time position knob. The rise time I calculated even matched what my function generator should be doing, so I was happy! (yes, simple things make a newbie happy.)

Anyway, I looked away for a few minutes--back to the video--and when I turned back to the scope, the trace was gone. I messed with it for a minute and found that when the knobs are locked for A, and the sweep set to either .5s, 5ms, 50us, or .5us the trace gets very slow. It
turns to just a dot moving very slowly across the display. .5s moves
slower (pretty much a standstill) than 5ms, and 5ms moves slower than 50us, so there's still some
kind of adjustment happening there but something is haywire. Also, on these 4 speeds the dot moving across the screen gets very bright. This appears to be normal at slower speeds. If I
pull straight out to unlock the center Delayed Sweep knob just to where S1094 closes then the sweep goes to, what appears to be, its normal
speed.


Also, I notice that if I pull the knob out for delayed sweep, then select another sweep speed with the delayed sweep knob, and then select B delayed sweep I see the same type of slow sweep on the 5ms, 50us and .5us time settings for delayed sweep. The other delayed sweep speeds appear to be normal. Would this not indicate something that is common to both the A and B delayed sweep in those time settings?

When I press "Mix" on those time settings I get a short sweep about 5 graticules wide that looks similar to beam finder.

When I press "A Inten" the sweeps on A act normally. While in "A Inten" if I pull the delay sweep knob out and move it, it focuses the intensity of the sweep on a smaller and smaller section (normal, I believe) until I get to one of the failed sweep speeds where the sweep then goes back to the normal looking sweep selected with A, instead of a smaller, more intensified section of the sweep.




I have turned the A knob a lot, in both locked and unlocked positions, trying to clean it up and get it to work, even intermittently, but it seems to be hard failed. Upon looking at the diagrams yesterday we came to the conclusion earlier that switch #9 on the rear cam of S1050 could be the culprit as it closes for each of those settings plus X-Y. Since it has been working for months and started this problem on it's own, it could be just a dirty switch. However, if it were just a little dirty wouldn't it probably work intermittently? I'd suspect a broken switch except that it started when it was just sitting there running, not moving the knobs, though it could have broken while moving it and then finally just lost contact. There is also R1083 and R1084. However, since the trigger board has to come out in order to check those components I want to do as much troubleshooting as possible prior to disassembly since I won't be able to check voltages. The fact that I see the problem on both A and B delayed seems to me it might be something else besides the switch 9 or those associated resistors. What other components are common to both non-delayed and delayed, but only needed for those particular sweep speeds?

Switch 9 also makes contact in X-Y mode. Is there a simple way to check that X-Y mode is working right? When in X-Y mode I get a dot on Ch1 and Ch2 (normal.) Channel 1 dot moves at a 45 degree angle when I adjust the vertical position knob. Channel 2 moves only vertically.

Thanks,
John



To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Wed, 4 Nov 2015 08:21:43 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























> If I pull to unlock the center Delayed Sweep knob out to where the switch inside activates


then the sweep goes to, what appears to be, its normal speed.


Obviously, this is important as well. Unfortunately, I have no time to dive in now.





Raymond























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


 

Switch 9 also makes contact in X-Y mode. Is there a simple way to check that X-Y mode is
working right? When in X-Y mode I get a dot on Ch1 and Ch2 (normal.) Channel 1 dot moves
at a 45 degree angle when I adjust the vertical position knob. Channel 2 moves only vertically.
I's simple to check X - Y mode:

1. Select X - Y with timebase knob (fully CCW)
2. Depress CH2

Ch1 will be X, CH2 Y.

Raymond


John Clark
 

Oh, duh! It says X-Y right on the Ch2 switch (facepalm.) That appears to be working fine. Ch1 position adjustment moves it horizontal (X) and Ch2 position adjustment moves it vertical (Y). It's OK to smack me if I get too dense!

John

To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Thu, 5 Nov 2015 09:38:53 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























> Switch 9 also makes contact in X-Y mode. Is there a simple way to check that X-Y mode is


working right? When in X-Y mode I get a dot on Ch1 and Ch2 (normal.) Channel 1 dot moves

at a 45 degree angle when I adjust the vertical position knob. Channel 2 moves only vertically.




I's simple to check X - Y mode:





1. Select X - Y with timebase knob (fully CCW)


2. Depress CH2





Ch1 will be X, CH2 Y.





Raymond



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


pdxareaid
 

i quickly scanned the posts and watched the video so i don't have an in depth knowledge of your problem but: a couple years ago i had an seemingly similar experience with a 465M that may help. i had a few sweep modes suddenly behave oddly while the others were ok. having previously cleaned the switches, i concluded the problem was in the sweep rc matrix selected by those switches. carefully noting which sweep modes were failed, using the sweep rc schematic, i was able to deduce a common component for those failed modes (a resistor in the matrix). i went to the board, lifted a resistor lead thinking i'd test it and it fell apart. i replaced it with a near value and all was well. using what i had on hand and since those resistors are low tolerance, i had to make up one and included a trim pot to get it exactly right so my calibration was not thrown off relative to the other sweep modes.
hope this helps.


John Clark
 

I appreciate the feedback. It's quite an endeavor to get to that board to lift resistors. The trigger board has to come out which requires desoldering, etc. Switch 9 in the locked portion of the switch and and R1083 and R1084 affects all those sweep settings but it also affects X-Y which appears to work as normal. I just wanted to get advice first before going in. If I go to the trouble to pull that board I should probably recap the power supply while I'm in there so it's a fairly big job with inherent risk of other damage.

Thanks,
John


To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Thu, 5 Nov 2015 10:12:30 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























i quickly scanned the posts and watched the video so i don't have an in depth knowledge of your problem but: a couple years ago i had an seemingly similar experience with a 465M that may help. i had a few sweep modes suddenly behave oddly while the others were ok. having previously cleaned the switches, i concluded the problem was in the sweep rc matrix selected by those switches. carefully noting which sweep modes were failed, using the sweep rc schematic, i was able to deduce a common component for those failed modes (a resistor in the matrix). i went to the board, lifted a resistor lead thinking i'd test it and it fell apart. i replaced it with a near value and all was well. using what i had on hand and since those resistors are low tolerance, i had to make up one and included a trim pot to get it exactly right so my calibration was not thrown off relative to the other sweep modes.


hope this helps.



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



















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


pdxareaid
 

that is one of the reasons i (blaspheme alert) prefer the 465M to the 465B which i have and is similar in layout to the 475. if something goes wrong on the 465M (really a beefed up 455). pop the cover, yank the module and everything is exposed. the only irritant is most of the transistors are soldered in making them hard to test when something is seriously wrong.
good luck.


John Clark
 

So, here's what I've done today . If someone smarter than me has a diagram for these two boards and has time to give their opinion I'd be extremely grateful. I set up the scope for DC testing as per the Sweep Generator board (8) and the Timing and Horiz Display Switching board (9) instructions on the diagrams. It has you set the Time/div knob to .1mS. This brings +50V directly down to R1089 and R1082 and out to PS-5 and PS-7 pins on the timing board which go to JS-5 and JS-7 on the sweep board. With that .1mS setting I should have -.6VDC on J5-5 pin on the Sweep Generator (8) board which is connected to R1089 on the Timing/Horiz Switching (9) board. I have -.559VDC on that. Tolerances are 20% according to the book.

Then on the J5-7 pin of the Sweep Gen (8) board I should have +.6VDC. This connects to P5-7 and R1082 on the Timing/Horiz (9) board and Q980 and Q974 on the Sweep Gen (8) board. However, here I only have .182VDC. Voltages appear to be correct on the other legs of Q980 and Q974.

Am I safe to suspect a problem with R1082? I don't see anything else in there unless C1083 is bringing the voltage down. I suppose it's also possible that Switch 14 and Switch 8 in the non-delay cam switch could be dirty and reducing the voltage going to PS-7to the low .182VDC. When I select 50ms (one of the problem speeds) that voltage drops to about 3mV. That's the only setting where I see the voltage on PS-7 drop to near zero. With that dropping to nearly zero I have to suspect Switch 9 on the non-delay sweep cam, and/or R1083 and/or R1086.

This is my layman's analysis of the schematic but not sure if I'm missing something.

Thanks,
John

To: tekscopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Thu, 5 Nov 2015 12:12:53 -0500
Subject: RE: [TekScopes] Re: 475 Time Base Setting Problem

And because a picture is worth 1000 words (and nobody likes reading a big long 1000 word question like below) here is a YT video of the issue. The intensity isn't really as bright as my phone camera thinks it is. Also, at the end the trace disappears. I get a little frustrated and end the video. Turns out the intensity was down...there wasn't another problem. It's harder to make good YT videos than people think!

John

https://youtu.be/UzYZWYwtS-o



From: johnclark05@outlook.com
To: tekscopes@yahoogroups.com
Subject: RE: [TekScopes] Re: 475 Time Base Setting Problem
Date: Thu, 5 Nov 2015 10:21:49 -0500




So, with the opamp replacements coming for my noise/wiggle issue, I'm going to move back to my time base problem since my trace is now pretty solid. Thanks so much to everyone who helped me with that!

Now for the time base problem. I'll try and provide as much detail as I can. Sorry for the long read. There might even be enough clues in here for me to find it myself, eventually, but I'm not as versed at reading these diagrams as many of you.

I was watching a YT video on how to use the B delayed sweep and the delay time position knob
to determine the rise time of a square wave. I had my function
generator plugged in and was attempting to duplicate what the guy on the video was doing.
I was able to understand it and it all seemed to work on my 475 similar to what he was describing on his 485. I was able to select the wave rise and focus in on it using the B delayed and the delay time position knob. The rise time I calculated even matched what my function generator should be doing, so I was happy! (yes, simple things make a newbie happy.)

Anyway, I looked away for a few minutes--back to the video--and when I turned back to the scope, the trace was gone. I messed with it for a minute and found that when the knobs are locked for A, and the sweep set to either .5s, 5ms, 50us, or .5us the trace gets very slow. It
turns to just a dot moving very slowly across the display. .5s moves
slower (pretty much a standstill) than 5ms, and 5ms moves slower than 50us, so there's still some
kind of adjustment happening there but something is haywire. Also, on these 4 speeds the dot moving across the screen gets very bright. This appears to be normal at slower speeds. If I
pull straight out to unlock the center Delayed Sweep knob just to where S1094 closes then the sweep goes to, what appears to be, its normal
speed.


Also, I notice that if I pull the knob out for delayed sweep, then select another sweep speed with the delayed sweep knob, and then select B delayed sweep I see the same type of slow sweep on the 5ms, 50us and .5us time settings for delayed sweep. The other delayed sweep speeds appear to be normal. Would this not indicate something that is common to both the A and B delayed sweep in those time settings?

When I press "Mix" on those time settings I get a short sweep about 5 graticules wide that looks similar to beam finder.

When I press "A Inten" the sweeps on A act normally. While in "A Inten" if I pull the delay sweep knob out and move it, it focuses the intensity of the sweep on a smaller and smaller section (normal, I believe) until I get to one of the failed sweep speeds where the sweep then goes back to the normal looking sweep selected with A, instead of a smaller, more intensified section of the sweep.




I have turned the A knob a lot, in both locked and unlocked positions, trying to clean it up and get it to work, even intermittently, but it seems to be hard failed. Upon looking at the diagrams yesterday we came to the conclusion earlier that switch #9 on the rear cam of S1050 could be the culprit as it closes for each of those settings plus X-Y. Since it has been working for months and started this problem on it's own, it could be just a dirty switch. However, if it were just a little dirty wouldn't it probably work intermittently? I'd suspect a broken switch except that it started when it was just sitting there running, not moving the knobs, though it could have broken while moving it and then finally just lost contact. There is also R1083 and R1084. However, since the trigger board has to come out in order to check those components I want to do as much troubleshooting as possible prior to disassembly since I won't be able to check voltages. The fact that I see the problem on both A and B delayed seems to me it might be something else besides the switch 9 or those associated resistors. What other components are common to both non-delayed and delayed, but only needed for those particular sweep speeds?

Switch 9 also makes contact in X-Y mode. Is there a simple way to check that X-Y mode is working right? When in X-Y mode I get a dot on Ch1 and Ch2 (normal.) Channel 1 dot moves at a 45 degree angle when I adjust the vertical position knob. Channel 2 moves only vertically.

Thanks,
John



To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Wed, 4 Nov 2015 08:21:43 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























> If I pull to unlock the center Delayed Sweep knob out to where the switch inside activates


then the sweep goes to, what appears to be, its normal speed.


Obviously, this is important as well. Unfortunately, I have no time to dive in now.





Raymond



























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Am I safe to suspect a problem with R1082?
Only if all timings that are dependent on R1082 are off. Do you have a way to check this? Like 60 Hz mains frequency, the 'scope's calibrator signal and signals from your signal generator if it's accurate enough.

I must admit I haven't studied this but I've been wondering how R1083 + R1084, at 1.809 MOhm, if not being switched into the circuit because of either failing or a bad switch position, can have such an influence on the Miller integrating time as you're seeing. I guess the answer is; It hasn't.
There's probably another contact (or more) in the circuit that isn't functioning correctly because it seems to me that at the problem positions, an RC network isn't established at all (integrator input floating). At the faster speeds, the resistance for the integrator most likely is small compared to R1083 + R1084. I'd expect a 60k or so be open. Sorry for the messy description; I hope it's still of use.

Mapping the R's and C's involved and their ratios, without looking at the exact switch positions, can probably help you further because they should give you the decades of 1-2-5 ratios.

Raymond


John Clark
 

That's just it. R1082 is common to time base settings that are working OK. That's why I kind of shy away from it being a problem but something is definitely dropping the voltage across that circuit. It seems to me that switch 14, if it was dirty, might add some resistance and lower the voltage without affecting the other time base settings that use switches 10-13. It would mean an issue with switch 14 and switch 9 (or a problem in R1083 and/or R1084. But, then, that would mean there would be problems with other time base settings. I realize I'm probably going to have to tear into it but I want to have good evidence that I'm headed to the right place before I go in that far.

John

To: TekScopes@yahoogroups.com
From: TekScopes@yahoogroups.com
Date: Thu, 5 Nov 2015 15:30:16 -0800
Subject: [TekScopes] Re: 475 Time Base Setting Problem


























> Am I safe to suspect a problem with R1082?



Only if all timings that are dependent on R1082 are off. Do you have a way to check this? Like 60 Hz mains frequency, the 'scope's calibrator signal and signals from your signal generator if it's accurate enough.





I must admit I haven't studied this but I've been wondering how R1083 + R1084, at 1.809 MOhm, if not being switched into the circuit because of either failing or a bad switch position, can have such an influence on the Miller integrating time as you're seeing. I guess the answer is; It hasn't.


There's probably another contact (or more) in the circuit that isn't functioning correctly because it seems to me that at the problem positions, an RC network isn't established at all (integrator input floating). At the faster speeds, the resistance for the integrator most likely is small compared to R1083 + R1084. I'd expect a 60k or so be open. Sorry for the messy description; I hope it's still of use.





Mapping the R's and C's involved and their ratios, without looking at the exact switch positions, can probably help you further because they should give you the decades of 1-2-5 ratios.





Raymond



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