7854: Excessive stored trace brightness.

#77999

Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David

#78001

I would not count on the potentiometer being good.  With age the wiper
in that type tends to become open and intermittent which can be
repaired by cleaning it.  I need to clean those potentiometers on my
7854.

I would start by doing the power supply and z-axis calibration
outlined in the service manual.

The readout and storage intensity controls are pretty simple
(schematic 17, 13, 34, to 13) but I wonder if the problem is caused by
the stored intensity intensify function being stuck on.  I am not sure
how that is handled.

On Mon, 9 Jul 2012 10:33:40 -0400, David Holland
<david.w.holland@...> wrote:

Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David

#78002

Are the parts that are supposed to be blanked getting blanked?
-ls-

David Holland <david.w.holland@...> wrote:

Show quoted text

Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David


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#78006

Yup.    Blanking looks good to me...

I'll double check the pot, and make certain its smooth.   It looks
like I have some reading to do in the manual....(this is certainly
going to be umm.. interesting)

David

Show quoted text

On Mon, Jul 9, 2012 at 11:32 AM,  <larrys@...> wrote:
Are the parts that are supposed to be blanked getting blanked?
-ls-

David Holland <david.w.holland@...> wrote:
Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David


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

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#78008

Do you have a dual time base, a 7B87, or a 7B80/7B85 (or equivalent)
which you could use to check if the A intensified by B display is
correct?

I wonder if your readout troubles are related.  The stored trace
intensify command could be controlled through the readout or from the
same place the readout is controlled from.  Verifying it should be as
simple as disabling that function by pulling a transistor but I did a
cursory search of the service manual and did not find out it.

You might try fiddling with the contrast adjustment on the front of
the oscilloscope to see if that affects anything.  I know mine has
trouble with the intensified display when using some dual time bases.

On Mon, 9 Jul 2012 12:36:13 -0400, David Holland
<david.w.holland@...> wrote:

Yup.    Blanking looks good to me...

I'll double check the pot, and make certain its smooth.   It looks
like I have some reading to do in the manual....(this is certainly
going to be umm.. interesting)

David


On Mon, Jul 9, 2012 at 11:32 AM,  <larrys@...> wrote:
Are the parts that are supposed to be blanked getting blanked?
-ls-

David Holland <david.w.holland@...> wrote:
Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David


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

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#78017

Yeah, I've a 7B85 down in the basement.  I'll plug things in and
fiddle with them (probably tomorrow) and see what shows up. (Or
doesn't.)

Before running to far down the rabbit hole, I should probably describe
the readout symptoms.

AFACT, the readout doesn't display units.    I get a number in the
correct locations, that will change depending on the settings of the
plugins, but there's no units associated with them.

Perhaps its related, perhaps not.

Anyways, thank you for the pointers.

David

Show quoted text

On Mon, Jul 9, 2012 at 1:08 PM, David <davidwhess@...> wrote:
Do you have a dual time base, a 7B87, or a 7B80/7B85 (or equivalent)
which you could use to check if the A intensified by B display is
correct?

I wonder if your readout troubles are related.  The stored trace
intensify command could be controlled through the readout or from the
same place the readout is controlled from.  Verifying it should be as
simple as disabling that function by pulling a transistor but I did a
cursory search of the service manual and did not find out it.

You might try fiddling with the contrast adjustment on the front of
the oscilloscope to see if that affects anything.  I know mine has
trouble with the intensified display when using some dual time bases.

On Mon, 9 Jul 2012 12:36:13 -0400, David Holland
<david.w.holland@...> wrote:

Yup.    Blanking looks good to me...

I'll double check the pot, and make certain its smooth.   It looks
like I have some reading to do in the manual....(this is certainly
going to be umm.. interesting)

David


On Mon, Jul 9, 2012 at 11:32 AM,  <larrys@...> wrote:
Are the parts that are supposed to be blanked getting blanked?
-ls-

David Holland <david.w.holland@...> wrote:
Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David


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#78019

The readout problem is straightforward although it may not be easy to
find the specific fault and access may be limited.  I am sure it is
not related to the intensity problem unless Tektronix did something
really weird.

On Mon, 9 Jul 2012 20:30:20 -0400, David Holland
<david.w.holland@...> wrote:

Yeah, I've a 7B85 down in the basement.  I'll plug things in and
fiddle with them (probably tomorrow) and see what shows up. (Or
doesn't.)

Before running to far down the rabbit hole, I should probably describe
the readout symptoms.

AFACT, the readout doesn't display units.    I get a number in the
correct locations, that will change depending on the settings of the
plugins, but there's no units associated with them.

Perhaps its related, perhaps not.

Anyways, thank you for the pointers.

David


On Mon, Jul 9, 2012 at 1:08 PM, David <davidwhess@...> wrote:
Do you have a dual time base, a 7B87, or a 7B80/7B85 (or equivalent)
which you could use to check if the A intensified by B display is
correct?

I wonder if your readout troubles are related.  The stored trace
intensify command could be controlled through the readout or from the
same place the readout is controlled from.  Verifying it should be as
simple as disabling that function by pulling a transistor but I did a
cursory search of the service manual and did not find out it.

You might try fiddling with the contrast adjustment on the front of
the oscilloscope to see if that affects anything.  I know mine has
trouble with the intensified display when using some dual time bases.

On Mon, 9 Jul 2012 12:36:13 -0400, David Holland
<david.w.holland@...> wrote:

Yup.    Blanking looks good to me...

I'll double check the pot, and make certain its smooth.   It looks
like I have some reading to do in the manual....(this is certainly
going to be umm.. interesting)

David


On Mon, Jul 9, 2012 at 11:32 AM,  <larrys@...> wrote:
Are the parts that are supposed to be blanked getting blanked?
-ls-

David Holland <david.w.holland@...> wrote:
Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David


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

Yahoo! Groups Links



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

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

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Rob

#78020

I have had to bugger around in the readout circuits in my exploits a bit. I
am no expert but a couple of things I have learned (I wish on my own but I
plagiarized some). In addition, I cannot address the brightness issue and
you may be right they may be related. If they are however, I bet it is a
noise/power supply issue. I suspect a bad cap on the readout board perhaps.

Anyway, if it is readout only:
Given that it's units only missing in the words. 
Does the Identify display correctly or is it missing letters (it should
display if the vertical is in a horizontal as well)? 
If you remove Q-2225 (at least that is the number in a 7904 double check for
a 7854) do all 40 zeros show? (should be a row of 40 across the top and
bottom).
Depending on the answers to those questions you will know if you have a
character generation problem or not. 
If it is a character problem you can move U1251-U1255 around to isolate the
bad chip. 
If it isn't you are in a data collection place (the identify should likely
have had letters in all places but misspelled perhaps). 
This is U1130,1170 which you can switch to see if it changes channels....
Probably vertical... 
If Identify misspelled really bad or symptoms don't change swapping above.
Check U1166 or U1186

I hope this helps
Rob
P.S. All the numbers above are for a 7904. I assume they are the same in a
7854.

Show quoted text

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of David Holland
Sent: Monday, July 09, 2012 7:30 PM
To: TekScopes@...
Subject: Re: [TekScopes] 7854: Excessive stored trace brightness.

Yeah, I've a 7B85 down in the basement.  I'll plug things in and fiddle with
them (probably tomorrow) and see what shows up. (Or
doesn't.)

Before running to far down the rabbit hole, I should probably describe the
readout symptoms.

AFACT, the readout doesn't display units.    I get a number in the
correct locations, that will change depending on the settings of the
plugins, but there's no units associated with them.

Perhaps its related, perhaps not.

Anyways, thank you for the pointers.

David

On Mon, Jul 9, 2012 at 1:08 PM, David <davidwhess@...> wrote:
Do you have a dual time base, a 7B87, or a 7B80/7B85 (or equivalent) 
which you could use to check if the A intensified by B display is 
correct?

I wonder if your readout troubles are related.  The stored trace 
intensify command could be controlled through the readout or from the 
same place the readout is controlled from.  Verifying it should be as 
simple as disabling that function by pulling a transistor but I did a 
cursory search of the service manual and did not find out it.

You might try fiddling with the contrast adjustment on the front of 
the oscilloscope to see if that affects anything.  I know mine has 
trouble with the intensified display when using some dual time bases.

On Mon, 9 Jul 2012 12:36:13 -0400, David Holland 
<david.w.holland@...> wrote:

Yup.    Blanking looks good to me...

I'll double check the pot, and make certain its smooth.   It looks
like I have some reading to do in the manual....(this is certainly 
going to be umm.. interesting)

David

On Mon, Jul 9, 2012 at 11:32 AM,  <larrys@...> wrote:
Are the parts that are supposed to be blanked getting blanked?
-ls-

David Holland <david.w.holland@...> wrote:
Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the 
service manual would be most relevant to read...

thanks in advance...

David

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

Yahoo! Groups Links

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

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

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

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#78029

The readout in the 7854 is radically different - it is part of one of the logic boards.

Dave

Show quoted text

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.


P.S. All the numbers above are for a 7904. I assume they are the same in a 7854.

Albert

#78035

Sure, though readout acquire, as viewed from the plugins, is still the old method. Missing units in readout could be caused by a fault during the corresponding time slot TS-9. I would first of all check for the presence of the -15V pulses at the main interface connectors, contacts B39. All 4 plugin positions have the B39 contacts in common so all should show the TS-9 pulse.
Albert

Show quoted text

--- In TekScopes@..., "David C. Partridge" <david.partridge@...> wrote:

The readout in the 7854 is radically different - it is part of one of the logic boards.

Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.

P.S. All the numbers above are for a 7904. I assume they are the same in a 7854.

#78041

The 7854 readout is part of the display board.  The inputs use the
same Tektronix custom ICs (analog multiplexor and analog to digital
converter) as the other 7000 series readouts but the output is
generated by the display board raster line generator as it is
processed by the microprocessor leading to the interesting display
antics during acquisition when interrupts are being continuously
processed.

Since the problem only affects specific character positions including
at least most if not all of row 4 on the character map, I suspect that
the readout problem occurs from the row decoder (U1220 155-0014-01) to
the binary encoder (U1230 74LS147).  Possibly one bit in the data bus
driver (U830 81LS97) is bad.

Just reseating the chips could be enough to fix it if those icky TI
sockets were used, U1220 could be swapped with U1520 or replaced with
a decoder from another 7000 series readout board, or U1230 or U830 may
need replacing.  A replacement 74LS147 should be easy to find for
U1230.  U830 could be swapped with U930 and if bad, replaced with a
81LS97 or apparently the 74LS467 equivalent.

I do not see anything that would link this problem to the intensity
issue.

On Tue, 10 Jul 2012 08:54:11 +0100, "David C. Partridge"
<david.partridge@...> wrote:

The readout in the 7854 is radically different - it is part of one of the logic boards.

Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.


P.S. All the numbers above are for a 7904. I assume they are the same in a 7854.

#78042

The readout acquire uses the same ICs although it is encoded to binary
and processed by the microprocessor before being displayed.

I left out checking for the time slot pulse in my post.  If it was
missing, then there could be short/open or U2320 (155-0017-00) which
is another custom Tektronix readout could be bad.

On Tue, 10 Jul 2012 13:13:23 -0000, "Albert" <aodiversen@...>
wrote:

Sure, though readout acquire, as viewed from the plugins, is still the old method. Missing units in readout could be caused by a fault during the corresponding time slot TS-9. I would first of all check for the presence of the -15V pulses at the main interface connectors, contacts B39. All 4 plugin positions have the B39 contacts in common so all should show the TS-9 pulse.
Albert

--- In TekScopes@..., "David C. Partridge" <david.partridge@...> wrote:

The readout in the 7854 is radically different - it is part of one of the logic boards.

Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.


P.S. All the numbers above are for a 7904. I assume they are the same in a 7854.

Albert

#78060

Hi David,

When readout intensity is ok then there is not much room left for faults in stored mode waveform intensity. The path for the "intensity setting current" starts at the intensity pot A5R65, signal WFMINT, diagram <11>. Then WFMINT is used in A21 <13> (for other purposes) and also passed to A29 <34>, A29Q1830, a CA3046 transistor array. The output ROZAXIS is common for readout intensity and stored waveform intensity. Switching between these is done by ~CHRZON and ~WFMZON.

The simplest fault would be that the ground terminal of R65 is disconnected internally. Then you still have smooth operation but you can't reduce intensity far enough. Try to measure resistance from slider to ground, perhaps via A29R1832|R1831 (that would need removal of board A29?) if you can't reach the pot.

I'm afraid the board locations are such that you need extender boards for measurements on the working 'scope. Q1830 and Q1930 are of the same type but swapping these 5-fold arrays might produce other problems.
Albert

Show quoted text

--- In TekScopes@..., David Holland <david.w.holland@...> wrote:

Hello all,

I'm kind of getting around to poking at my 7854 scope.

The stored trace brightness is excessive, and the adjustment won't go
down any further.      I'm reasonably certain the pot is good, since
it'll smoothy vary over what range it does have.   (From "too bright",
to "cr*p, I'd better turn that down before the CRT gets burnt").
Normal display, and readout's are of a more reasonable intensity, and
have goodly range.     (Readout isn't working correctly, but that's
not the problem I want to concentrate first on.)

Before I crack the covers, any suggestions of which parts of the
service manual would be most relevant to read...

thanks in advance...

David

Rob

#78074

Thanks guys on the impromptu training on the 7854. I had thought to be
helpful but I should have checked for similarities first before posting. At
least it is good to hear that if/when I finally get one of those my
experience in the other 7000's won't be a total waste. the readout is
basically the same on all of the other scopes I have. Anyway, my hope is I
did not confuse the original poster in my exuberance to finally have a
chance of helping for a change *smile*.

Show quoted text

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of David
Sent: Tuesday, July 10, 2012 10:14 AM
To: TekScopes@...
Subject: Re: [TekScopes] Re: 7854: Excessive stored trace brightness.

The readout acquire uses the same ICs although it is encoded to binary and
processed by the microprocessor before being displayed.

I left out checking for the time slot pulse in my post.  If it was missing,
then there could be short/open or U2320 (155-0017-00) which is another
custom Tektronix readout could be bad.

On Tue, 10 Jul 2012 13:13:23 -0000, "Albert" <aodiversen@...>
wrote:

Sure, though readout acquire, as viewed from the plugins, is still the oldmethod. Missing units in readout could be caused by a fault during the
corresponding time slot TS-9. I would first of all check for the presence of
the -15V pulses at the main interface connectors, contacts B39. All 4 plugin
positions have the B39 contacts in common so all should show the TS-9 pulse.
Albert

--- In TekScopes@..., "David C. Partridge"<david.partridge@...> wrote:

The readout in the 7854 is radically different - it is part of one of thelogic boards. 

Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On 
Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.

P.S. All the numbers above are for a 7904. I assume they are the samein a 7854.

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

Yahoo! Groups Links

#78126

I appreciate all the help, even the confusing help. :-)

I admit I'm kind of slow getting back to things.  (Unfortunately "real
life" keeps getting in the way of my hobbies.)

I did check out the A/B intensity display, and all was as expected there.

At this point, (as far as I can tell), the plan of attack should be:

1) Check PSU & Z-Axis calibration.
2) Check the signal path's as mentioned by Albert and see what I find.

I do have a set of extender boards I purchased from David Partridge a
while ago, for just this sort of thing, so I'm good there as well.

David

Show quoted text

On Tue, Jul 10, 2012 at 8:43 PM, Rob <rgwood@...> wrote:
Thanks guys on the impromptu training on the 7854. I had thought to be
helpful but I should have checked for similarities first before posting. At
least it is good to hear that if/when I finally get one of those my
experience in the other 7000's won't be a total waste. the readout is
basically the same on all of the other scopes I have. Anyway, my hope is I
did not confuse the original poster in my exuberance to finally have a
chance of helping for a change *smile*.

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of David
Sent: Tuesday, July 10, 2012 10:14 AM
To: TekScopes@...
Subject: Re: [TekScopes] Re: 7854: Excessive stored trace brightness.

The readout acquire uses the same ICs although it is encoded to binary and
processed by the microprocessor before being displayed.

I left out checking for the time slot pulse in my post.  If it was missing,
then there could be short/open or U2320 (155-0017-00) which is another
custom Tektronix readout could be bad.

On Tue, 10 Jul 2012 13:13:23 -0000, "Albert" <aodiversen@...>
wrote:

Sure, though readout acquire, as viewed from the plugins, is still the oldmethod. Missing units in readout could be caused by a fault during the
corresponding time slot TS-9. I would first of all check for the presence of
the -15V pulses at the main interface connectors, contacts B39. All 4 plugin
positions have the B39 contacts in common so all should show the TS-9 pulse.
Albert

--- In TekScopes@..., "David C. Partridge"<david.partridge@...> wrote:

The readout in the 7854 is radically different - it is part of one of thelogic boards.

Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On
Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.


P.S. All the numbers above are for a 7904. I assume they are the samein a 7854.

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7854: Excessive stored trace brightness.