Topics

New Member Dead 7704


volvoboy242@...
 

I brought my old 7704 (non A) down from the attic the other day to find that the CRT is dead.
I see a quick flash upon power down but, other then that, there is no display.
I am guessing that the HV power supply has kicked the bucket.
I've done no diag since I know the voltage is high and my knowledge doesn't extend to testing deadly secondary potential.
I've never had the covers off or had any work done (in 20 years).
Is this a serviceable part or do I need to source a known good power supply?
I'm hoping it's something as simple as a re cap but this might be a bit beyond me.


If any of you guys would Elmer me through this resto project, I'd be much obliged!
No rush.


The only other scope I have is an old Velleman hand held that works occasionally. I have some basic Ham survival tools.
I planned to gear up around this old main frame for radio alignments and things of that nature.
Overkill I know.


Off topic; Also in my attic, there is an old pre war Allen B Dumont "oscillograph" I played with when I was a kid.
I'd like to get that one working again some day. Just for fun.


Thanks
Mike
KD2MBG


Roger Evans
 

If you have the flash on power down then the filament and EHT are probably ok. Either the beam is deflected off screen due to a fault in the X or Y amplifiers (check voltages on the deflection plates with a very slow sweep speed and adjusting the vertical position on the plugin, the plate voltages should move symmetrically in opposite directions) or you have a problem in the z axis circuit which can be more tricky to trace without a second scope.

Regards

Roger


Jerry Massengale <jmassen418@...>
 

Mike,

Where do you see a quick flash? Is it green on the crt? You can at least
check for the -3Kv HV primary. Your unit may have something as simple as a
deflection plate connection loose. What plugins are you using?

jerry

On Thu, Dec 29, 2016 at 3:42 PM, volvoboy242@... [TekScopes] <
TekScopes@...> wrote:



I brought my old 7704 (non A) down from the attic the other day to find
that the CRT is dead.
I see a quick flash upon power down but, other then that, there is no
display.
I am guessing that the HV power supply has kicked the bucket.
I've done no diag since I know the voltage is high and my knowledge
doesn't extend to testing deadly secondary potential.
I've never had the covers off or had any work done (in 20 years).
Is this a serviceable part or do I need to source a known good power
supply?
I'm hoping it's something as simple as a re cap but this might be a bit
beyond me.


If any of you guys would Elmer me through this resto project, I'd be much
obliged!
No rush.


The only other scope I have is an old Velleman hand held that works
occasionally. I have some basic Ham survival tools.
I planned to gear up around this old main frame for radio alignments and
things of that nature.
Overkill I know.


Off topic; Also in my attic, there is an old pre war Allen B Dumont
"oscillograph" I played with when I was a kid.
I'd like to get that one working again some day. Just for fun.


Thanks
Mike
KD2MBG





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


volvoboy242@...
 

The numbers are gone too. Doesn't the screen glow normally? I have a vague recollection of how it used to work.
I thought it had a glow visible with the grid ilum dimmed all the way.


Are there diagnostic pins or connectors to test? Something to probe without damaging anything?


Joseph Tatum
 

The only glow I expected would be the scale illumination. But this is like a trace that fills almost the whole screen.


 

How about a short video to Youtube?

----- Original Message -----
From: tatumj1@... [TekScopes]
To: TekScopes@...
Sent: Thursday, December 29, 2016 7:16 PM
Subject: [TekScopes] Re: New Member Dead 7704



The only glow I expected would be the scale illumination. But this is like a trace that fills almost the whole screen.


Joseph Tatum
 

Rght now I have the control board out of it so cannot turn it back on until I get it re-assembled. I do not have a YouTube account right now.


volvoboy242@...
 

Well the youtube vid fixed it. Good suggestion!
As soon as I started recording on my phone, the screen lit up.
I was able to get focus and I could read my volts/div.
I tried to get a trace but the screen faded very slowly and went out.
Much button pushing and knob wiggling later (plus pulling the beam finder) and I got this:
2016-12-29_22-02-45_281.jpg https://drive.google.com/open?id=0BydAjXF94zykWGR1ellCMi1SbFE

https://drive.google.com/open?id=0BydAjXF94zykWGR1ellCMi1SbFE

2016-12-29_22-02-45_281.jpg https://drive.google.com/open?id=0BydAjXF94zykWGR1ellCMi1SbFE Sign in Main menu



View on drive.google.com https://drive.google.com/open?id=0BydAjXF94zykWGR1ellCMi1SbFE
Preview by Yahoo







Seems like a good cleaning, re seating some chips and maybe a couple caps and I'll be able to calibrate and use it.
Will share videos shortly.


volvoboy242@...
 


volvoboy242@...
 

OK so that row of nine mystery traces are actually numbers. A row of ones in horizontal mode B and nine fours in all other modes.


Chuck Harris
 

You are thinking of HP, this is a Tektronix scope.
The scale illumination is by incandescent bulbs side
lighting the screen.

Nine times out of ten, when a 7000 series scope is dead, the
problem is simply a shorted tantalum capacitor somewhere on
a power supply input filter to one of the circuit boards.

Also, a 7000 series scope without plugins will not show a
screen trace, or dot.

-Chuck Harris

tatumj1@... [TekScopes] wrote:

The only glow I expected would be the scale illumination. But this is like a trace that fills almost the whole screen.





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Posted by: tatumj1@...
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Yahoo Groups Links




volvoboy242@...
 

I have no time base. I think I sold (more likely "loaned") it years ago. Just bought one off ebay for $20 shipped. We'll see. With the 7A18 in either slot, I can get ch1 and ch2 dots and the voltage scale at the top of the screen (after a lot of fiddling).
It's very intermittent and most of the time the only thing on the screen is the garbled dashes in the picture and only with the beam finder pushed or locked.
I pulled the side covers off and everything looks shiny and new.


I can't "make" it work. It just pops to life at random.
Sometimes just a dot...sometimes just the volt/div...sometimes the garbled dashes which are actually numbers (either 1s or 4s) that are usually too far up the x scale to see without the beam finder.
And once, the whole scope seemed to just function normally, like magic.


Pretty tidy inside. Most of the boards are clearly labeled and easy to get at and service.
If I can find what is causing the intermittent operation of the display, diaging the rest of the scope will be possible.


volvoboy242@...
 

My new 7B53A came in the mail today. Had to double check the eBay listing because of the heavy drop check damage.
The picture that would have shown such damage was conspicuously missing, but for $20 shipped........Meh!
I plugged it in with the 7A18 and, for a minute, I had a readout and was able to watch time and volt values change with the knobs. No dots though.
I pushed the beam finder and it all went away.
With the beam finder locked, I have everything and can move the dots X and Y but, without the beam finder, it's all off screen. Up, I think.


So now I'm pretty sure most of this scope works.
Just need to figure out what's loose or cracked.


volvoboy242@...
 


 

Hi,
It looks like the readout ICs are acting up.

There are several possible causes. The most common one is dirty or oxidized
contact between the back of the plugin and the mainframe backplane. This can
be remedied by cleaning the contacts with contact cleaner. Never clean gold
contacts with an eraser.

I would also locate a manual so you can then find out where the readout
board is located. It would be a good idea to reseat all the readout ICs and
any other active devices on that board.

In either case you can put the plugin into any other slot of the scope (the
Tek 7000 scopes are universally flexible in this respect) and the readout
numbers and letters will probably change due to different contact resistance
in the other slots. That will be a clue that contact resistance is the
cause.

Dennis Tillman W7PF

-----Original Message-----
Sent: Tuesday, January 03, 2017 2:22 PM
Subject: [TekScopes] Re: New Member Dead 7704

And now this;
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/light
box/2034395368?orderBy=ordinal&amp;sortOrder=asc&amp;photoFilter=ALL
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/light
box/2034395368?orderBy=ordinal&amp;sortOrder=asc&amp;photoFilter=ALL
------------------------------------
Posted by: volvoboy242@...
------------------------------------


volvoboy242@...
 

I have another plugin coming.
I'll see if that changes the display.
Another symptom; I let it cool down for an hour and powered it up.
After 1 minute, the display flashed 1 second. Readout and traces.
It did it again 1 minute later and then nothing for the next 20 minutes.
Holding the beam finder showed the squished readout and traces.


 

Your oscilloscope is a complex instrument but it is very well documented. Our troubleshooting suggestions will help you to narrow down the source of the problems you are having. You need to start using the troubleshooting recommendations members of the group make and then report back what the results were if we are to help you.

Have you downloaded a manual yet for the scope mainframe and for each plugin you will own? In it you will find all of the mechanical drawings and those will show you the location of the readout boards.

If you don't know where to find a manual just ask us.

In my previous reply I suggested moving the plugin to another slot, horizontal or vertical to narrow down the problem to the scope or the plugin. You can put the timebase in a vertical slot (Left Vert or Right Vert). What you should see is the trace going vertical instead of horizontal but other than that it will work. You can put the plugin in the other horizontal slot (B Horiz) which will almost certainly change the random readout characters you are getting. What happened when you did this?

Dennis Tillman W7PF

-----Original Message-----
Sent: Wednesday, January 04, 2017 5:18 AM
Subject: [TekScopes] Re: New Member Dead 7704

I have another plugin coming.
I'll see if that changes the display.
Another symptom; I let it cool down for an hour and powered it up.
After 1 minute, the display flashed 1 second. Readout and traces.
It did it again 1 minute later and then nothing for the next 20 minutes.
Holding the beam finder showed the squished readout and traces.

------------------------------------
Posted by: volvoboy242@...
------------------------------------


volvoboy242@...
 

I have swapped the plugins around in their respective A/B slots but I haven't tried swapping them from horizontal/vertical slots. I have wiggle tested the plugins to see if bad contacts might be causing problems.
I tried wiggling what I think are the 10 number chips on the display board as one or two of the characters seem to flicker intermittently.
What I did made no difference.
I'll give your suggestion a try.
The display works very intermittently and usually, the only way to see anything on screen is with the beam finder.
I tried to pull a square wave off my handheld while the 7704 was "working" but got this:
https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/462709026?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/462709026 https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/462709026?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/462709026


I downloaded the generic troubleshooting guide and the 7704 operating manual.

I haven't found the manuals for my plugins yet.


The other time plugin I accidentally bought ( ) should arrive today.
I'll be able to switch back and forth for comparison.
I'll post results later.


It would probably help if I knew something about electronics, other then having excellent unsoldering/soldering skills and being able to read a DVOM!
I'm pretty sure that everything has to actually hit the screen of the CRT before I can troubleshoot the rest of the scope.
Again, the boards inside look very clean, not stinky and nothing I can see is burned or exploded.
I haven't touched anything with DVOM or scope as I wouldn't know what I was looking at anyway.
If you guys want me to check something or try swapping something around, let me know.
I'll post back with pics, videos or values....whatever you ask for.
I've got good hands but I'm a monkey with a Tek 7704.


 

Your knowledge of soldering and a DVOM isn't going to help you to understand
an oscilloscope. Get a copy of "The XYZs of Oscilloscopes" and read it
thoroughly before doing anything else.

You can download it here
http://info.tek.com/rs/584-WPH-840/images/XYZs-of-Oscilloscopes_03W_8605_7.p
df

From your YouTube video it appears you are just randomly turning knobs and
buttons. Read the section of the 7704 manual called "First Time Operation"
and set the scope and plugins up accordingly. You will need a BNC to BNC
cable to connect the vertical plugin to the calibrator.

Now each time you start the scope you will be in a known good state that
should result (when all is finally OK with the scope) in a square wave
waveform on the CRT.

Once you have the cable you need and you have set the scope up properly
according to the First Time Operation instructions take a picture or a
YouTube video of what happens on the CRT. Also take a picture of the entire
front of the scope so we can verify it is setup properly.

Never mind about the readout. That is a minor problem for now. Getting a
good display is your big problem. After seeing the pictures I don't think
swapping the 7B53 over to a vertical slot will tell us anything more.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Thursday, January 05, 2017 9:31 AM
To: TekScopes@...
Subject: [TekScopes] Re: New Member Dead 7704

I have swapped the plugins around in their respective A/B slots but I
haven't tried swapping them from horizontal/vertical slots. I have wiggle
tested the plugins to see if bad contacts might be causing problems.
I tried wiggling what I think are the 10 number chips on the display board
as one or two of the characters seem to flicker intermittently.
What I did made no difference.
I'll give your suggestion a try.
The display works very intermittently and usually, the only way to see
anything on screen is with the beam finder.
I tried to pull a square wave off my handheld while the 7704 was "working"
but got this:

https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/46
2709026?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/462709026
https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/46
2709026?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/462709026


I downloaded the generic troubleshooting guide and the 7704 operating
manual.

I haven't found the manuals for my plugins yet.


The other time plugin I accidentally bought ( ) should arrive today.
I'll be able to switch back and forth for comparison.
I'll post results later.


It would probably help if I knew something about electronics, other then
having excellent unsoldering/soldering skills and being able to read a DVOM!
I'm pretty sure that everything has to actually hit the screen of the CRT
before I can troubleshoot the rest of the scope.
Again, the boards inside look very clean, not stinky and nothing I can see
is burned or exploded.
I haven't touched anything with DVOM or scope as I wouldn't know what I was
looking at anyway.
If you guys want me to check something or try swapping something around,
let me know.
I'll post back with pics, videos or values....whatever you ask for.
I've got good hands but I'm a monkey with a Tek 7704.








------------------------------------
Posted by: volvoboy242@...
------------------------------------


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

Yahoo Groups Links


Jerry Massengale <jmassen418@...>
 

The readout is encoded by current through the interface and is sensitive to
poor contact. a good cleaning of the connectors at the rear of each module
is needed. also ensure the modules a latched in.


On Thu, Jan 5, 2017 at 11:31 AM, volvoboy242@... [TekScopes] <
TekScopes@...> wrote:



I have swapped the plugins around in their respective A/B slots but I
haven't tried swapping them from horizontal/vertical slots. I have wiggle
tested the plugins to see if bad contacts might be causing problems.
I tried wiggling what I think are the 10 number chips on the display board
as one or two of the characters seem to flicker intermittently.
What I did made no difference.
I'll give your suggestion a try.
The display works very intermittently and usually, the only way to see
anything on screen is with the beam finder.
I tried to pull a square wave off my handheld while the 7704 was "working"
but got this:
https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/
462709026?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/462709026
https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/
462709026?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/462709026


I downloaded the generic troubleshooting guide and the 7704 operating
manual.

I haven't found the manuals for my plugins yet.


The other time plugin I accidentally bought ( ) should arrive today.
I'll be able to switch back and forth for comparison.
I'll post results later.


It would probably help if I knew something about electronics, other then
having excellent unsoldering/soldering skills and being able to read a
DVOM!
I'm pretty sure that everything has to actually hit the screen of the CRT
before I can troubleshoot the rest of the scope.
Again, the boards inside look very clean, not stinky and nothing I can see
is burned or exploded.
I haven't touched anything with DVOM or scope as I wouldn't know what I
was looking at anyway.
If you guys want me to check something or try swapping something around,
let me know.
I'll post back with pics, videos or values....whatever you ask for.
I've got good hands but I'm a monkey with a Tek 7704.