how to check oscilloscope?

[Sam Kaan]

Hi, I just got an old Tektronix 7904 under my ownership now. However
it doesn't seems to work but I am not sure. Could someone please help
me figure
out if it is indeed kaputz/DOA or if its just that I am a newbie and
don't know how to use it properly. Last time I touched a scope was
back in school a few years ago.

What I did was turned toggle the power on, I hear click click and
lights and such comes on.

BUT!! BUT!! I don't see that little bright dot anywhere, even when I
cranked up
the intensity knob.

Unfortunately I don't have a manual and not sure how to approach it.
There
are 4 plug-ins and they have lights coming on all over but not sure if
the
unit as a whole works.

If the bright spots is not there, then the scope is likely bad right?

 

[Jim Yanik]

[email protected] (Sam Kaan) wrote in

Hi, I just got an old Tektronix 7904 under my ownership now. However
it doesn't seems to work but I am not sure. Could someone please help
me figure
out if it is indeed kaputz/DOA or if its just that I am a newbie and
don't know how to use it properly. Last time I touched a scope was
back in school a few years ago.

What I did was turned toggle the power on, I hear click click and
lights and such comes on.

The scope has to have at least two plug-ins to work. examples are 7A26 for
the vertical,and 7B80 for the horizontal.If it still clicks,you have a
power supply or HV problem.If this is the case,it needs repair.(clicking
indicated the PS is TRYING to start,but some problem(failed part) is
preventing it.It has a series-resonant switching power supply.

BUT!! BUT!! I don't see that little bright dot anywhere, even when I
cranked up
the intensity knob.

Unfortunately I don't have a manual and not sure how to approach it.
There
are 4 plug-ins and they have lights coming on all over but not sure if
the
unit as a whole works.

If the bright spots is not there, then the scope is likely bad right?

Well,the power supply needs work,then you can find out if the rest of the
scope works.The PS is a pretty complex item,so I don't recommend you
attempt repairs yourself.
TEKTRONIX no longer services these scopes,considers them obsolete.
Other cal/repair companies may be able to fix it.For how much,I don't know.

 

[Sam Kaan]

[email protected] (Sam Kaan) wrote in


The scope has to have at least two plug-ins to work. examples are 7A26 for
the vertical,and 7B80 for the horizontal.If it still clicks,you have a
power supply or HV problem.If this is the case,it needs repair.(clicking
indicated the PS is TRYING to start,but some problem(failed part) is
preventing it.It has a series-resonant switching power supply.

Actually I forgot to say. The clicking stops after exactly 3 clicks upon a
cold start.
After that there are no more clicks.

It currently has these pluggins:
7A13 Differential Comparator Plug-in
Dual Trace Amplifier Plug-in
7B52 Dual Time Plug-in
Delaying Time Base Plug-in

Do you think there are any ways to at least get that dot to show up?
Perhaps if I remove two of the 4 plug-ins, that should at least simplify the
testing a bit
Which one to remove I don't know.

 

[Dr. Anton Squeegee]

Hi, I just got an old Tektronix 7904 under my ownership now. However
it doesn't seems to work but I am not sure. Could someone please help
me figure out if it is indeed kaputz/DOA or if its just that I am a newbie and
don't know how to use it properly. Last time I touched a scope was
back in school a few years ago.

<snippety>

The 7904, if you'll pardon a somewhat biased observation (from one
who owns a pair of them), one of the finest analog O-scopes that
Tektronix ever turned out. The base mainframe has a bandwidth of 500MHz.

What I did was turned toggle the power on, I hear click click and
lights and such comes on.

BUT!! BUT!! I don't see that little bright dot anywhere, even when I
cranked up the intensity knob.

Do you have plug-ins installed, and are they in the proper
compartments?

In order to use a 7904 (or any 7000 series 'scope for that
matter), you need to install at least one timebase plug-in (7Bxx series)
and one vertical amp plug-in (7Axx series).

For basic use up to 250MHz, I would recommend the following plug-
in population. For the vertical side, one or two 7A26 plug-ins.
Alternatively, if you want to have a ready combination that will serve
up to the full bandwidth of the mainframe, put a 7A26 in one of the
vertical compartments and a 7A29 in the other.

For the horizontal side, you should use at least one 7B92A
timebase. Ideally, for full measurement flexibility, including delta
time, you should place a 7B92A in the far right horizontal compartment
and a 7B85 in the one immediately to its left.

That's about all I can tell you for now, lacking further details.
Manuals and plug-ins frequently appear on Ebay, and 7A26's are getting
very inexpensive (less than $50).

Happy hunting. That's a fine instrument -- well worth the effort
to get it going.


--
Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
kyrrin a/t bluefeathertech d-o=t c&o&m
Motorola Radio Programming & Service Available -
http://www.bluefeathertech.com/rf.html
"Quando Omni Flunkus Moritati" (Red Green)

 

[Sam Kaan]

Well, I currently have the two
7A13 Differential Comparator Plug-in
7B52 Dual Time Plug-in

Plus two that is missing the knob with the label, but the person who I got
it from says its
Delaying Time Base Plug-in
Dual Trace Amplifier Plug-in

But she knows nothing else of it.

Here, I just took a low rez picture of the thing with my digicam:
http://www.geocities.com/corvette_2050/tek/my7904b.jpg


Anyway, if I pull out the two unknowns, and just leave the 7A13 an 7B52,
would I be
able to get something to work? Right now I just want to do the basics with
it, measuring
some simple signal on an embedded computer bus.

I am hoping that the thing is still salvagable without spending a lot of
cash to service it.

Thanks.

 

[mike]

[email protected] (Sam Kaan) wrote in




The scope has to have at least two plug-ins to work. examples are 7A26 for
the vertical,and 7B80 for the horizontal.If it still clicks,you have a
power supply or HV problem.If this is the case,it needs repair.(clicking
indicated the PS is TRYING to start,but some problem(failed part) is
preventing it.It has a series-resonant switching power supply.



Well,the power supply needs work,then you can find out if the rest of the
scope works.The PS is a pretty complex item,so I don't recommend you
attempt repairs yourself.
TEKTRONIX no longer services these scopes,considers them obsolete.
Other cal/repair companies may be able to fix it.For how much,I don't know.

Jim's the expert and can probably be more precise.
You should be able to see the on-screen readout as long as you have
the plugin selector switch set to a hole that has a plugin.
If you can't get that to work you probably need repair.

My 7704 has three intensity controls and a passel of switches that
must be in the right position to get the green worm. Not to mention
all the position controls that have to be set.
I'd start with one vertical and one sweep plug-in. Make sure ALL the
selector switches from the plugin channel to the vertical hole are all
set for the correct input. Ditto for the horizontal. Put it in
autotrigger mode. Center the position pots.

There should be some connectors on the back where you can see the signal
out and sweep gates etc.
In a very quiet room, turn it on and listen for the high voltage to come
up. Hard to describe, but it's that tiny "whoosh-crackle" sound you get
when your TV turns on.

Put some effort into figuring it out before you write it off.
I bought a DC503A. Borrowed a manual and was half an hour into
troubleshooting the gate generator before I realized I had it in display
HOLD mode.
And I used to manage the group that designed it...probably shouldn't
have admitted that ;-)
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Rich Grise]

look very carefully at every button on the front of the thing,
and see if there's one labeled "beam finder." If _that_
doesn't put a dot on the screen, then you have a broken
'scope. If it does, then note where on the screen it is -
usually toward the lower left or upper left corner. If you
have a stubby trace, then the sweep is working, and you need
to adjust the vertical position to bring it onto the screen.
There might also be a way to get the sweep to free-run, to
turn the vertical up and down until you see the trace.

For more than that, I'd have to be there.

Good Luck!
Rich

 

[JeffM]

Tektronix 7904

has these pluggins:
7A13 Differential Comparator Plug-in
Dual Trace Amplifier Plug-in
7B52 Dual Time Plug-in
Delaying Time Base Plug-in

any ways to at least get that dot to show up?
Sam Kaan

Use the simplest configuration:
comparator and amp for X and for Y (no timebase).
If you can't get a dot on the screen using the Position controls,
it's likely time to hand it over to a pro.

 

[Jim Yanik]

Actually I forgot to say. The clicking stops after exactly 3 clicks
upon a cold start.
After that there are no more clicks.

If the scope has it's controls lit up and clicking has stopped,it's
possible that the CRT grid bias may need adjustment,or that the CRT DC
restorer diodes have failed,or a problem in the Z-axis drive circuits,or
lastly,the CRT itself has died.

It currently has these pluggins:
7A13 Differential Comparator Plug-in

Dual Trace Amplifier Plug-in

7A11,7A18,7A24 or 7A26? Or is the knob missing?

7B52 Dual Time Plug-in
Delaying Time Base Plug-in

7B71,7B85,7B15,7B92/A? or 7B53/A(not really 100% compatible,but still
usable.

Do you think there are any ways to at least get that dot to show up?
Perhaps if I remove two of the 4 plug-ins, that should at least
simplify the testing a bit
Which one to remove I don't know.

Pull one vertical PI and one Horizontal PI. I'd test each 7B PI as they
develop the Z-axis drive for the mainframe.Also,the delaying TB can inhibit
the other horiz PI.(for delayed sweeps or intensified displays)
Actually,I'd test with the dual trace amplifier PI and the 7B85,leave the
other two out.(also try with the room lights off to see if there's a faint
trace or a glow from the CRT beam.)

Don't plug or unplug them with the scope energized,they're not made for
that.

As someone else mentioned,there's a beamfind button on one of the intensity
controls,that should show your trace if it's off-screen,or a dot if the
sweep circuits are bad.

 

[Jim Yanik]

<snippety>

The 7904, if you'll pardon a somewhat biased observation (from
one
who owns a pair of them), one of the finest analog O-scopes that
Tektronix ever turned out. The base mainframe has a bandwidth of
500MHz.


Do you have plug-ins installed, and are they in the proper
compartments?

In order to use a 7904 (or any 7000 series 'scope for that
matter), you need to install at least one timebase plug-in (7Bxx
series) and one vertical amp plug-in (7Axx series).

For basic use up to 250MHz, I would recommend the following plug-
in population. For the vertical side, one or two 7A26 plug-ins.
Alternatively, if you want to have a ready combination that will serve
up to the full bandwidth of the mainframe, put a 7A26 in one of the
vertical compartments and a 7A29 in the other.

I generally don't recommend the 7A29 or 7A24 as they are 50 ohm input
PIs,and most people prefer the 1meg inputs,and don't miss the loss of BW.
Hams may want a 50ohm input PI,though.

His 7A13 is very nice,but the input FETS are easy to blow,and the
differential inputs are hard to fix and cal properly.

 

[Jim Yanik]

Well, I currently have the two
7A13 Differential Comparator Plug-in
7B52 Dual Time Plug-in

Plus two that is missing the knob with the label, but the person who I
got it from says its
Delaying Time Base Plug-in
Dual Trace Amplifier Plug-in

But she knows nothing else of it.

Here, I just took a low rez picture of the thing with my digicam:
http://www.geocities.com/corvette_2050/tek/my7904b.jpg

Ok,the other vertical PI (with all those tiny buttons)is a 7A11,much
older,lower bandwidth,75Mhz BW,IIRC.If it works;OK,but if not,I'd not spend
any time on it.Buy a 7A26 or a 7A18 instead.

The unknown horiz PI looks like a 7B53 PI,inteded for use in the 7600
series mainframes,but useable in the 7904,except for the highest sweep
speeds,the leading edge of the sweep wiln not be displayed.

At least you have the later,good version of the 7A13,with the electronic DC
offset readout,the older ones had a mechanical readout,and were prone to
trouble.

 

[Paul Burridge]

Actually I forgot to say. The clicking stops after exactly 3 clicks upon a
cold start.
After that there are no more clicks.

It currently has these pluggins:
7A13 Differential Comparator Plug-in
Dual Trace Amplifier Plug-in
7B52 Dual Time Plug-in
Delaying Time Base Plug-in

Do you think there are any ways to at least get that dot to show up?

You probably need to fool around with more knobs. There's a confusing
array of knobs on most scopes, most of which don't tell you what they
really do. I've had this problem on countless occasions, and every
time there was no fault with the scope; I just hadn't twiddled the
right combination of knobs to their right settings. Keep twiddling and
something will probably show up sooner or later.
HTH.

 

[Paul Burridge]

As someone else mentioned,there's a beamfind button on one of the intensity
controls,that should show your trace if it's off-screen,or a dot if the
sweep circuits are bad.

When I press beam find I get an elongated, stationary vertical dot of
intense brightness. But there's nothing wrong with the scope (that
correct setting up wouldn't cure).

 

[Sam Kaan]

More pictures, perhaps someone can have a look and can think of something.

http://www.geocities.com/corvette_2050/tek/closeup1.JPG
http://www.geocities.com/corvette_2050/tek/closeup10.JPG
http://www.geocities.com/corvette_2050/tek/closeup2.JPG
http://www.geocities.com/corvette_2050/tek/closeup3.JPG
http://www.geocities.com/corvette_2050/tek/closeup4.JPG
http://www.geocities.com/corvette_2050/tek/closeup5.JPG
http://www.geocities.com/corvette_2050/tek/closeup6.JPG
http://www.geocities.com/corvette_2050/tek/closeup7.JPG
http://www.geocities.com/corvette_2050/tek/closeup9.JPG

 

[Sam Kaan]

If the scope has it's controls lit up and clicking has stopped,it's
possible that the CRT grid bias may need adjustment,or that the CRT DC
restorer diodes have failed,or a problem in the Z-axis drive circuits,or
lastly,the CRT itself has died.



7A11,7A18,7A24 or 7A26? Or is the knob missing?

7B71,7B85,7B15,7B92/A? or 7B53/A(not really 100% compatible,but still
usable.

Pull one vertical PI and one Horizontal PI. I'd test each 7B PI as they
develop the Z-axis drive for the mainframe.Also,the delaying TB can inhibit
the other horiz PI.(for delayed sweeps or intensified displays)
Actually,I'd test with the dual trace amplifier PI and the 7B85,leave the
other two out.(also try with the room lights off to see if there's a faint
trace or a glow from the CRT beam.)

I think I can see some kind of beam. One of the thing I did is that
after the click has stopped on a cold startup. I would repeatedly toggle
the
power toggle on and off. And each time, I can see something get drawn
on the screen. A 2 traces similar to a lightning bolt, and two short lines.
But they all disappears withing milliseconds.

By the way, repeated toggling doesn't not make that 3 clicking sounds.
The 3 clicking sounds comes only at the first power up after a long off
state
ie if I leave it off for 10 minutes, turn it on and hear 3 click sounds.
Repeated
off/on doesn't make the sound.

 

[mike]

More pictures, perhaps someone can have a look and can think of something.

http://www.geocities.com/corvette_2050/tek/closeup4.JPG
http://www.geocities.com/corvette_2050/tek/closeup5.JPG

looks like you haven't pushed any of the horizontal or vertical
mode buttons???
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Sam Kaan]

Pull one vertical PI and one Horizontal PI. I'd test each 7B PI as they
develop the Z-axis drive for the mainframe.Also,the delaying TB can inhibit
the other horiz PI.(for delayed sweeps or intensified displays)
Actually,I'd test with the dual trace amplifier PI and the 7B85,leave the
other two out.(also try with the room lights off to see if there's a faint
trace or a glow from the CRT beam.)

I pulled out the two plugs-ins as you said. Now there are only two PI
and it did seem to make a difference. I am getting a dot or perhaps two
on the screen. But the diameter is very large. It looks like a half-moon
and with halo around it when I press on the beam find (on the intensity
knob).
As can be seen in this picture here. At least we can conclude that not
everything is broken.

http://www.geocities.com/corvette_2050/tek/gotsomething1.JPG

There are two rows of large pushbuttons as you can see here just right above
the plug-in compartments. It makes a difference depending on which one are
pressed.
If some are off, the dot disappears from the screen.

The knobs I am talking about has on them these labels.
LEFT, ALT, ADD, CHOP, RIGHT [for the vertical mode group]
A, ALT, CHOP, B [ for the horizontal mode group]

However I think they [these buttons] are out of order because they like to
pop off when
you press on them and falls on the floor. I have to pick it up and push it
back in. It has
been through many hands and I suspect none of the buttons are representative
of
what they should be.

The two plugins says:
DELAYING TIME BASE (on the left)
DUAL TIME BASE (on the right)
By the way, the plug-ins on the right, even though it is missing the button
to label
which model it is. I am able to see on the inside compartment and on the
left side cover
of this Dual time base, the cover says "TYPE 7A12 LEFT SIDE COVER". However
this can be missleading because it appears to pop off the unit very easily,
as happens
when I was handling it.

I am beginning to have a bit of hope for this scope after all, but just a
tiny bit because
for a complex system like this, who knows what else is wrong with it.

 

[Paul Burridge]

looks like you haven't pushed any of the horizontal or vertical
mode buttons???

Yeah, and there are far too many buttons, anyway. BTW, WTF is "BIG
OUT" for in closeup5.jpg?

 

[Sam Kaan]

Yeah, and there are far too many buttons, anyway. BTW, WTF is "BIG
OUT" for in closeup5.jpg?

actually you mean "SIG OUT"?

 

[Rich Grise]

When I went to the website, I got:
-------------------------------------------------------
We're sorry, but this page is currently unavailable for viewing.
If this site belongs to you, please read this help page for more
information and assistance.

etc.

So, _where_ exactly is this crescent on the screen? Have you tried
turning the position controls while holding the button down? You
should be able to move it to both sides, and top to bottom (almost)
- center it, and let the "beam find" button go.

If you can't make it move, then there's something wrong somewhere
in that deflection circuit (up-down = vertical, r-l = horizontal),
and it's time for a tech.

Good Luck!
Rich

Pull one vertical PI and one Horizontal PI. I'd test each 7B PI as they
develop the Z-axis drive for the mainframe.Also,the delaying TB can inhibit
the other horiz PI.(for delayed sweeps or intensified displays)
Actually,I'd test with the dual trace amplifier PI and the 7B85,leave the
other two out.(also try with the room lights off to see if there's a faint
trace or a glow from the CRT beam.)

I pulled out the two plugs-ins as you said. Now there are only two PI
and it did seem to make a difference. I am getting a dot or perhaps two
on the screen. But the diameter is very large. It looks like a half-moon
and with halo around it when I press on the beam find (on the intensity
knob).
As can be seen in this picture here. At least we can conclude that not
everything is broken.

http://www.geocities.com/corvette_2050/tek/gotsomething1.JPG

There are two rows of large pushbuttons as you can see here just right above
the plug-in compartments. It makes a difference depending on which one are
pressed.
If some are off, the dot disappears from the screen.

The knobs I am talking about has on them these labels.
LEFT, ALT, ADD, CHOP, RIGHT [for the vertical mode group]
A, ALT, CHOP, B [ for the horizontal mode group]

However I think they [these buttons] are out of order because they like to
pop off when
you press on them and falls on the floor. I have to pick it up and push it
back in. It has
been through many hands and I suspect none of the buttons are representative
of
what they should be.

The two plugins says:
DELAYING TIME BASE (on the left)
DUAL TIME BASE (on the right)
By the way, the plug-ins on the right, even though it is missing the button
to label
which model it is. I am able to see on the inside compartment and on the
left side cover
of this Dual time base, the cover says "TYPE 7A12 LEFT SIDE COVER". However
this can be missleading because it appears to pop off the unit very easily,
as happens
when I was handling it.

I am beginning to have a bit of hope for this scope after all, but just a
tiny bit because
for a complex system like this, who knows what else is wrong with it.

As someone else mentioned,there's a beamfind button on one of the intensity
controls,that should show your trace if it's off-screen,or a dot if the
sweep circuits are bad.