Topics

New Member Dead 7704


 

On 07 Jan 2017 13:54:27 -0800, you wrote:

...

Also no preferred method of discharging the CRT, other then "Oh yeah, you just grab the anode plug and touch the chassis with it! No problem!"
That *is* the preferred method in this case. The connector which is
in line with the anode lead pulls straight apart leaving the terminal
on the CRT plug exposed which can then be shorted to the chassis.
Twisting the connector halves will help in pulling it apart.

This is actually a good test to see if the high voltage multiplier is
working. Pull it apart while the oscilloscope is on, carefully, and
watch the screen for a change as the anode discharges. The connector
does not have to be pulled all the way apart.

Also couldn't see any blinking neon light thing in the power unit that's supposed to discharge some large caps.
They discharge pretty quickly. The neon flasher will only operate
when the unit is turned on as a warning that AC line power is applied.


 

I wouldn't touch those. They are made of Kovar and are very soft / delicate.

Dennis Tillman W7PF

-----Original Message-----
Sent: Saturday, January 07, 2017 1:54 PM
Subject: RE: [TekScopes] Re: New Member Dead 7704

<snip>
I think my CRT problem must be oxidized deflection plate pins.
Too chicken to mess with them yet.
------------------------------------
Posted by: volvoboy242@...
------------------------------------


volvoboy242@...
 

Ok, my main issue is that the gun is intermittently shooting up and off the sreen.
Other then an intermittent open, I don't know what else could make it behave so randomly.
Is there anything between the vertical amp and CRT that can be disconnected, cleaned and re connected?
Those pins need to be dealt with in the process of removing and installing the CRT, right?
Those pins have to be fooled with to get the CRT out don't they?


Chuck Harris
 

How about an intermittent short?

volvoboy242@... [TekScopes] wrote:

Ok, my main issue is that the gun is intermittently shooting up and off the sreen.
Other then an intermittent open, I don't know what else could make it behave so randomly.
Is there anything between the vertical amp and CRT that can be disconnected, cleaned and re connected?
Those pins need to be dealt with in the process of removing and installing the CRT, right?
Those pins have to be fooled with to get the CRT out don't they?


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


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

Yahoo Groups Links




volvoboy242@...
 

I guess maybe but, in my experience, shorts leave something burned and stinky. That should tell you something of my "experience".


Also, there isn't anything in either the generic troubleshooting or the 7704 manual that specifically deals with the problem of both readout and trace being shot off screen.
Pressing or locking the beam finder brings them both into view.
But once released, the readout and trace are gone.
I haven't found anything that details CRT diag either.
I guess either it works or it doesn't.


Mine is very bright and the Z for both readout and trace I have set to about 1/4 to 1/3 intensity.


And then, all of a sudden, It'll shoot everything on screen like nothing's wrong.


Chuck Harris
 

There are lots of high impedance differential paths in this
scope that will take shorts without releasing the holy smoke.

Your efforts at trying to blame the CRT for open circuits are
not useful. The CRT is such high impedance that most of its
pins, that aren't directly part of the gun assembly, can store
charge for minutes at a time when left open circuit. I include
the deflection plates, and the geometry, focus, and control
grid of the gun. I had one CRT where the weld connecting to
the control grid broke inside of the envelope, and the scope
started with a dim trace, and over the course of a couple of
minutes it slowly got brighter and brighter until it wasn't
safe to leave it turned on. Otherwise, everything else seemed
to work. The CRT isn't a light bulb, or a motor, where
everything stops the instant an open circuit happens.

Losing a single deflection plate to open circuit usually shows
up as losing 1/2 of the deflection amplitude, and some of the
deflection range, one side of center, or the other.

The A number 1 problem with the 7704 is the tantalum capacitor.
They can short permanently, for an instant now and then, only
when above some voltage, but not below, or not at all.

I would bet that your entire problem lies in the population of
tantalum capacitors that filter the power on the vertical and/or
the horizontal board. I have never, and I do mean never, found
a 7704, or 7904 that didn't lose at least one capacitor on the
+15V or -15V, +5V, or +50V lines on the horizontal, or vertical
deflection amplifier boards. There is even a 1uF tantalum right
on the beam finder input to the horizontal, and the vertical
deflection amplifier board. If one is shorted, the beam will be
out of position, without the benefit of turning the intensity up.

Tektronix specified the maximum voltage of these capacitors way
too close to the normal operating voltage of the circuits they
filtered... typically using 16V or 20V caps on 15V circuits, and
6.3V caps on 5V circuits. NASA determined that for reliable
operation the rated voltage of dry slug tantalum caps should be
2.5 times the maximum voltage under operation.

Leave the CRT alone. If it has shown the display even once,
it is good enough.

Another area where the 7704, and its same aged brethren fail
is the blue colored harmonica connectors. Something was wrong
with the pigment used that it caused the plastic to crumble after
a few years passed. There is one such connector on the Horizontal
deflection amplifier, as I recall.

-Chuck Harris

volvoboy242@... [TekScopes] wrote:

I guess maybe but, in my experience, shorts leave something burned and stinky.
That should tell you something of my "experience".


Also, there isn't anything in either the generic troubleshooting or the 7704
manual that specifically deals with the problem of both readout and trace being
shot off screen. Pressing or locking the beam finder brings them both into view.
But once released, the readout and trace are gone. I haven't found anything that
details CRT diag either. I guess either it works or it doesn't.


Mine is very bright and the Z for both readout and trace I have set to about 1/4
to 1/3 intensity.


And then, all of a sudden, It'll shoot everything on screen like nothing's wrong.






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


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

Yahoo Groups Links


volvoboy242@...
 

I think the CRT is OK....Just sayin'.




https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/lightbox/749043417?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#zax/749043417 https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/lightbox/749043417?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#zax/749043417

Why it doesn't do this all the time is what I'm trying to figure out.
It worked long enough today for me to get a few waveforms and to find a problem with channel 2 in my 7B53A.


volvoboy242@...
 

I reseated the pins on the vertical amp deflection plate outputs.
No change.
I hacked all low voltage test points on the vertical board.
All good.
It magically started working so I pulled a couple waveforms off my phone.
Here's one
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/lightbox/45856422?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#


 

Random or intermittent problems are a troubleshooting nightmare because they can come and go when you seem to be getting closer to the source of the problem. One very useful technique when tracking down random / intermittent problems is a can of freeze spray with a nozzle so you can concentrate the spray in a tight area. Freezing a part will cause any temperature sensitive areas to start acting up (if they have been quiet) or stop an intermittent part from acting up. This technique can be done quickly which helps if you are troubleshooting a problem that comes and goes too fast to catch it by the usual means.

There are many different brands of Freeze Spray. They all work fine. Few people realize that a can of freeze spray is the same as a can of compressed air. The only difference is where the pickup tube is located inside the can. If you turn a can of freeze spray upside down it comes out as canned air. Also, canned air can be used as freeze spray in a pinch by turning the can upside down.

Dennis Tillman W7PF

-----Original Message-----
Sent: Sunday, January 08, 2017 6:42 AM
Subject: RE: [TekScopes] Re: New Member Dead 7704

Ok, my main issue is that the gun is intermittently shooting up and off the sreen.
Other then an intermittent open, I don't know what else could make it behave so randomly.
Is there anything between the vertical amp and CRT that can be disconnected, cleaned and re connected?
Those pins need to be dealt with in the process of removing and installing the CRT, right?
Those pins have to be fooled with to get the CRT out don't they?
------------------------------------
Posted by: volvoboy242@...
------------------------------------


 

Think what it means when both trace and readout disappear off screen! It
means you have narrowed down the problem to something both of those have in
common, namely your problem is in the Vertical Deflection system or the
Z-Axis blanking.
So concentrate your efforts there. Read the Theory of Operation and look at
the schematics for areas where the readout circuitry is fed into the
Vertical Deflection system. Since both move off screen at the same time the
problem must lie after that point.

Think what it means that the beam finder brings the trace back on screen!
The vertical deflection system or Z-axis amplifiers are not completely dead.
They do work at least partially. This helps to narrow down the problem even
further. If the problem is the vertical deflection amplifier then there may
be some DC level that is pushing the beam too far in one direction to
display it until the beam finder truncates the output of the vertical
amplifier to force it onscreen. Take a look at the beam finder section in
the Theory of Operation to see how it works and then hook onto the scope at
the point that the beam finder has forced the beam back onscreen. Check the
voltages there and compare what happens when the beam is off-screen to
on-screen. DO the deflection amplifier voltages match what is on the
schematic?

The deflection amplifier is differential which means each vertical
deflection plate will have the opposite voltage on it so the net deflection
is balanced and a net zero. When the beam is off screen the voltage will
almost certainly be unbalanced and there will be a noticeable difference
between both sides of the deflection amplifier. What do you measure when the
beam is onscreen versus offscreen. If the voltages look right then the
problem may not be in this section. Instead it might be in the Z-Axis
amplifier.

That narrows down your search area considerably. Now you are onto something.
Examine the Vertical Deflection board and the Z-Axis board for any signs of
debris, loose parts, suspicious solder joints, etc. Keep in mind using the
freeze spray to temperature stress the parts and get them to act up.

You are making progress in locating the problem and you are discovering how
to troubleshoot electronic circuits as you do this. It won't happen
overnight but after years of locating the source of problems in electronic
circuits you will have learned a very useful skill. Persistence pays off
with intermittent problems.

Dennis Tillman W7PF

-----Original Message-----
Sent: Sunday, January 08, 2017 12:14 PM
Subject: Re: [TekScopes] Re: New Member Dead 7704

I guess maybe but, in my experience, shorts leave something burned and
stinky. That should tell you something of my "experience".

Also, there isn't anything in either the generic troubleshooting or the
7704 manual that specifically deals with the problem of both readout and
trace being shot off screen.
Pressing or locking the beam finder brings them both into view.
But once released, the readout and trace are gone.
I haven't found anything that details CRT diag either.
I guess either it works or it doesn't.

Mine is very bright and the Z for both readout and trace I have set to
about 1/4 to 1/3 intensity.

And then, all of a sudden, It'll shoot everything on screen like nothing's
wrong.
------------------------------------
Posted by: volvoboy242@...
------------------------------------


M P <volvoboy242@...>
 

I just wanted to thank everyone for the helpful replies so far.I am very grateful for all the advice and insight.
I'm trying to figure out what systems work and which don't rather then blindly checking random components one by one.So far as per the manual, I've checked the power unit, the rectifier and the low voltage regulator board.I checked the voltage at the labeled test pins of the vertical output board also, not following any specific test procedure.Although not balls on, all voltages so far were very close to what they should be.I haven't scoped anything yet. My little Velleman is good for <5mHz I think.....when it works.
I haven't checked the actual vertical outputs as I haven't found that procedure in the manual.I did re seat the spring loaded locking pin things and, after watching the very fragile pins on the CRT, decided to never tough those again.I have no idea what the vertical deflection plate voltages are supposed to be and I don't want to zap my DVOM or myself with 6kV or something.I've been bitten by ~10kV diaging automotive ignition secondaries so I know it can jump at you from a distance.
I would be absolutely shocked if there wasn't a few bad caps on several boards throughout this scope, seeing as it's older then me and I've recapped radios that were completely dead after 35 years.I'd like to know exactly which ones are acting up before firing the cap cannon. I think there are a lot more tantalums in this scope then electrolytics in the last radio I did.
If you look through the album I've been uploading to, you'll see some of the waveforms I've been able to pull off my phone.My tone generator made a pretty nice sine and triangle wave but a very dirty square wave and sawtooth.There is a pic  of one of those waveforms chopped with the square wave from the 7704 (which is clean).
Other then the readout (for which there is a big section in the maintenance chapter) most of the scope seems to function...sporadically.
Once I find out what's involved in checking the vertical outs, I'll update again.If anyone has experience with that, please let me know!


ThanksMikeKD2MBG

On Monday, January 9, 2017 12:55 PM, "'Dennis Tillman' @Dennis_Tillman_W7pF [TekScopes]" <TekScopes@...> wrote:


  Think what it means when both trace and readout disappear off screen! It
means you have narrowed down the problem to something both of those have in
common, namely your problem is in the Vertical Deflection system or the
Z-Axis blanking.
So concentrate your efforts there. Read the Theory of Operation and look at
the schematics for areas where the readout circuitry is fed into the
Vertical Deflection system. Since both move off screen at the same time the
problem must lie after that point.

Think what it means that the beam finder brings the trace back on screen!
The vertical deflection system or Z-axis amplifiers are not completely dead.
They do work at least partially. This helps to narrow down the problem even
further. If the problem is the vertical deflection amplifier then there may
be some DC level that is pushing the beam too far in one direction to
display it until the beam finder truncates the output of the vertical
amplifier to force it onscreen. Take a look at the beam finder section in
the Theory of Operation to see how it works and then hook onto the scope at
the point that the beam finder has forced the beam back onscreen. Check the
voltages there and compare what happens when the beam is off-screen to
on-screen. DO the deflection amplifier voltages match what is on the
schematic?

The deflection amplifier is differential which means each vertical
deflection plate will have the opposite voltage on it so the net deflection
is balanced and a net zero. When the beam is off screen the voltage will
almost certainly be unbalanced and there will be a noticeable difference
between both sides of the deflection amplifier. What do you measure when the
beam is onscreen versus offscreen. If the voltages look right then the
problem may not be in this section. Instead it might be in the Z-Axis
amplifier.

That narrows down your search area considerably. Now you are onto something.
Examine the Vertical Deflection board and the Z-Axis board for any signs of
debris, loose parts, suspicious solder joints, etc. Keep in mind using the
freeze spray to temperature stress the parts and get them to act up.

You are making progress in locating the problem and you are discovering how
to troubleshoot electronic circuits as you do this. It won't happen
overnight but after years of locating the source of problems in electronic
circuits you will have learned a very useful skill. Persistence pays off
with intermittent problems.

Dennis Tillman W7PF



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


Jerry Massengale <jmassen418@...>
 

Hi,

I doubt that you have CRT issues. It sounds more like control problems.
Getting the focus and intensity pot and the beam finder switch cleaned is a
good start. Use a good contact cleaner is important. I do not know what
that stuff is that you have mentioned but am suspious.

jerry

On Sun, Jan 8, 2017 at 8:42 AM, volvoboy242@... [TekScopes] <
TekScopes@...> wrote:



Ok, my main issue is that the gun is intermittently shooting up and off
the sreen.
Other then an intermittent open, I don't know what else could make it
behave so randomly.
Is there anything between the vertical amp and CRT that can be
disconnected, cleaned and re connected?
Those pins need to be dealt with in the process of removing and installing
the CRT, right?
Those pins have to be fooled with to get the CRT out don't they?


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


M P <volvoboy242@...>
 

CRC brand QD Electronic Cleaner; Isohexane, diflouroethane, hexane and ethanol.It's pretty much my go-to since age 19 fixing F-14 Tomcats.
The video showing vigorous twisting of various knobs and random punching of switches was me trying to find scratchy pots and bad contacts. 
Once the scope lights off (at random), I can get a look long enough to see some dirty pots and switches.I thought there might have been some corrosion on some of the through pins and plugs, but they all look very shiny and clean. Also, wiggling and finger thumping have no effect on the symptoms.I'm sure a good teardown and deoxidizing would help a lot, but I no longer think that it would solve the vertical output issues I've been observing.It seems more likely a component failure. But which cap....and which board? And yes. I'm sure there are several.
This pic shows the scope working "normally" yesterday, except for the garbled readout.https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/45856422?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/45856422

 ThanksMikeKD2MBG

On Monday, January 9, 2017 5:55 PM, "Jerry Massengale jmassen418@... [TekScopes]" <TekScopes@...> wrote:


  Hi,

I doubt that you have CRT issues. It sounds more like control problems.
Getting the focus and intensity pot and the beam finder switch cleaned is a
good start. Use a good contact cleaner is important. I do not know what
that stuff is that you have mentioned but am suspious.

jerry

]

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Jerry Massengale <jmassen418@...>
 

hi,

good job. maybe a little focus/astig but looking good

jerry

On Mon, Jan 9, 2017 at 6:11 PM, M P volvoboy242@... [TekScopes] <
TekScopes@...> wrote:



CRC brand QD Electronic Cleaner; Isohexane, diflouroethane, hexane and
ethanol.It's pretty much my go-to since age 19 fixing F-14 Tomcats.
The video showing vigorous twisting of various knobs and random punching
of switches was me trying to find scratchy pots and bad contacts.
Once the scope lights off (at random), I can get a look long enough to see
some dirty pots and switches.I thought there might have been some corrosion
on some of the through pins and plugs, but they all look very shiny and
clean. Also, wiggling and finger thumping have no effect on the
symptoms.I'm sure a good teardown and deoxidizing would help a lot, but I
no longer think that it would solve the vertical output issues I've been
observing.It seems more likely a component failure. But which cap....and
which board? And yes. I'm sure there are several.
This pic shows the scope working "normally" yesterday, except for the
garbled readout.https://groups.yahoo.com/neo/groups/TekScopes/
photos/photostream/lightbox/45856422?orderBy=mtime&
sortOrder=desc&photoFilter=ALL#zax/45856422

ThanksMikeKD2MBG

On Monday, January 9, 2017 5:55 PM, "Jerry Massengale jmassen418@...
[TekScopes]" <TekScopes@...> wrote:


Hi,

I doubt that you have CRT issues. It sounds more like control problems.
Getting the focus and intensity pot and the beam finder switch cleaned is a
good start. Use a good contact cleaner is important. I do not know what
that stuff is that you have mentioned but am suspious.

jerry

]

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volvoboy242@...
 

Since the scope seems to intermittently light off after a long warm-up, this is probably the way I'll do it but only after isolating the problem to a specific system or even board.



"Random or intermittent problems are a troubleshooting nightmare because they can come and go when you seem to be getting closer to the source of the problem. One very useful technique when tracking down random / intermittent problems is a can of freeze spray with a nozzle so you can concentrate the spray in a tight area. Freezing a part will cause any temperature sensitive areas to start acting up (if they have been quiet) or stop an intermittent part from acting up. This technique can be done quickly which helps if you are troubleshooting a problem that comes and goes too fast to catch it by the usual means."


 

Checking the power supplies was important for you to do to confirm they are working. There is nothing wrong with the CRT and you should stop wasting time and energy on it. The problem is almost certainly in the Vertical deflection amplifier. This board does not use high voltages which you should have found out by now by reading the theory of operation and by looking at the voltages in the vertical amplifier section of the schematics. Don't keep thinking you have to be taking readings at the CRT. You do not need to measure voltages at the vertical deflection plates. There is nothing that will tell you that will be useful. There may not be an actual procedure for checking the vertical output per se. That is not useful here. Instead read the theory of operation on how the vertical amplifier chain works.

The vertical amplifier is a differential amplifier consisting of several stages of amplification. Because it is differential one side goes to one deflection plate and an almost identical opposite side goes to the other plate of the CRT. It is almost certain your malfunction is caused by an imbalance on one side causing the beam to go off screen. This makes it easy to troubleshoot because you check each side of the chain of transistors to see where the voltages suddenly go wrong. That will be the area to look at in more detail. Trace the DC voltages through each transistor stage for each side of the differential amplifier. Write them in pencil in the schematic.

You would probably do more harm than good by replacing all the caps hoping that will fix something. It is far better to at least isolate the problem down to a specific part of a circuit then making an educated guess as to what are the most likely ones that can explain the voltages you are measuring.

Remember Freeze spray is your friend when you have intermittent problems.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Monday, January 09, 2017 1:59 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: New Member Dead 7704

I just wanted to thank everyone for the helpful replies so far.I am very grateful for all the advice and insight.
I'm trying to figure out what systems work and which don't rather then blindly checking random components one by one.So far as per the manual, I've checked the power unit, the rectifier and the low voltage regulator board.I checked the voltage at the labeled test pins of the vertical output board also, not following any specific test procedure.Although not balls on, all voltages so far were very close to what they should be.I haven't scoped anything yet. My little Velleman is good for <5mHz I think.....when it works.
I haven't checked the actual vertical outputs as I haven't found that procedure in the manual.I did re seat the spring loaded locking pin things and, after watching the very fragile pins on the CRT, decided to never tough those again.I have no idea what the vertical deflection plate voltages are supposed to be and I don't want to zap my DVOM or myself with 6kV or something.I've been bitten by ~10kV diaging automotive ignition secondaries so I know it can jump at you from a distance.
I would be absolutely shocked if there wasn't a few bad caps on several boards throughout this scope, seeing as it's older then me and I've recapped radios that were completely dead after 35 years.I'd like to know exactly which ones are acting up before firing the cap cannon. I think there are a lot more tantalums in this scope then electrolytics in the last radio I did.
If you look through the album I've been uploading to, you'll see some of the waveforms I've been able to pull off my phone.My tone generator made a pretty nice sine and triangle wave but a very dirty square wave and sawtooth.There is a pic of one of those waveforms chopped with the square wave from the 7704 (which is clean).
Other then the readout (for which there is a big section in the maintenance chapter) most of the scope seems to function...sporadically.
Once I find out what's involved in checking the vertical outs, I'll update again.If anyone has experience with that, please let me know!


ThanksMikeKD2MBG



On Monday, January 9, 2017 12:55 PM, "'Dennis Tillman' @Dennis_Tillman_W7pF [TekScopes]" <TekScopes@...> wrote:


Think what it means when both trace and readout disappear off screen! It means you have narrowed down the problem to something both of those have in common, namely your problem is in the Vertical Deflection system or the Z-Axis blanking.
So concentrate your efforts there. Read the Theory of Operation and look at the schematics for areas where the readout circuitry is fed into the Vertical Deflection system. Since both move off screen at the same time the problem must lie after that point.

Think what it means that the beam finder brings the trace back on screen!
The vertical deflection system or Z-axis amplifiers are not completely dead.
They do work at least partially. This helps to narrow down the problem even further. If the problem is the vertical deflection amplifier then there may be some DC level that is pushing the beam too far in one direction to display it until the beam finder truncates the output of the vertical amplifier to force it onscreen. Take a look at the beam finder section in the Theory of Operation to see how it works and then hook onto the scope at the point that the beam finder has forced the beam back onscreen. Check the voltages there and compare what happens when the beam is off-screen to on-screen. DO the deflection amplifier voltages match what is on the schematic?

The deflection amplifier is differential which means each vertical deflection plate will have the opposite voltage on it so the net deflection is balanced and a net zero. When the beam is off screen the voltage will almost certainly be unbalanced and there will be a noticeable difference between both sides of the deflection amplifier. What do you measure when the beam is onscreen versus offscreen. If the voltages look right then the problem may not be in this section. Instead it might be in the Z-Axis amplifier.

That narrows down your search area considerably. Now you are onto something.
Examine the Vertical Deflection board and the Z-Axis board for any signs of debris, loose parts, suspicious solder joints, etc. Keep in mind using the freeze spray to temperature stress the parts and get them to act up.

You are making progress in locating the problem and you are discovering how to troubleshoot electronic circuits as you do this. It won't happen overnight but after years of locating the source of problems in electronic circuits you will have learned a very useful skill. Persistence pays off with intermittent problems.

Dennis Tillman W7PF



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------------------------------------
Posted by: M P <volvoboy242@...>
------------------------------------


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

Yahoo Groups Links


Brent Watson <brentleew2003@...>
 

Like others have said.
 I'd change all the power supply caps and the problem will probably go away. It's always worked for me.



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


Chuck Harris
 

Truly a bad idea! Especially since this isn't likely
to be a power supply problem.

[I would look closely at the two metal film power resistors
on the output stage of the vertical amp. They, and
others of their make, are commonly intermittent.]

First RTFM, and then check the power supply voltages for
correct amplitude and ripple. If the amplitude and ripple
are ok, the caps, even if not perfect, are good enough for
this use.

You are likely to destroy the scope by trying to remove
and replace the capacitors. Tektronix had a habit of using
huge ground planes without providing thermal reliefs. It
takes a very high wattage temperature controlled soldering
iron for their safe removal, 70-100W and 650F temperature.
Let the temperature go above 650F, and the board will
delaminate.

Many power supply caps are fit into too small holes in the
circuit boards. If the solder isn't totally melted, the
plated through holes will pull out with the lead, ruining
the circuit board. If you don't happen to notice, you will
have introduced yet another problem to diagnose and fix...
Only this time, you will waste hours and hours of your time
because you already replaced the capacitor, and *know* that
it can't possibly be bad.

The schematics aren't detailed enough to show which
connections are made by the plated through holes.

Put your head to work, and diagnose the problem. There is
ample instruction on how to do this in the very fine manual.

If you don't think you are up to figuring out the problem, do
yourself a big favor and buy a ready to go scope, perhaps
a Rigol? It will cost you about 3 dinners at a nice
restaurant with a friend... Two if you like to pop a cork.

The next guy to take a stab at fixing your scope will
thank you!

-Chuck Harris

Brent Watson brentleew2003@... [TekScopes] wrote:

Like others have said. I'd change all the power supply caps and the problem will
probably go away. It's always worked for me.







------------------------------------ Posted by: Brent Watson
<brentleew2003@...> ------------------------------------


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

Yahoo Groups Links


Brent Watson <brentleew2003@...>
 

I'm sorry for that poor contribution. I don't know the 7704 and should not have commented. I read Dennis's post after I sent mine, and tried to delete it, but too late.
Brent


From: "Chuck Harris cfharris@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Tuesday, January 10, 2017 6:49 AM
Subject: Re: [TekScopes] Re: New Member Dead 7704

  Truly a bad idea! Especially since this isn't likely
to be a power supply problem.

[I would look closely at the two metal film power resistors
on the output stage of the vertical amp. They, and
others of their make, are commonly intermittent.]

First RTFM, and then check the power supply voltages for
correct amplitude and ripple. If the amplitude and ripple
are ok, the caps, even if not perfect, are good enough for
this use.

You are likely to destroy the scope by trying to remove
and replace the capacitors. Tektronix had a habit of using
huge ground planes without providing thermal reliefs. It
takes a very high wattage temperature controlled soldering
iron for their safe removal, 70-100W and 650F temperature.
Let the temperature go above 650F, and the board will
delaminate.

Many power supply caps are fit into too small holes in the
circuit boards. If the solder isn't totally melted, the
plated through holes will pull out with the lead, ruining
the circuit board. If you don't happen to notice, you will
have introduced yet another problem to diagnose and fix...
Only this time, you will waste hours and hours of your time
because you already replaced the capacitor, and *know* that
it can't possibly be bad.

The schematics aren't detailed enough to show which
connections are made by the plated through holes.

Put your head to work, and diagnose the problem. There is
ample instruction on how to do this in the very fine manual.

If you don't think you are up to figuring out the problem, do
yourself a big favor and buy a ready to go scope, perhaps
a Rigol? It will cost you about 3 dinners at a nice
restaurant with a friend... Two if you like to pop a cork.

The next guy to take a stab at fixing your scope will
thank you!

-Chuck Harris

Brent Watson brentleew2003@... [TekScopes] wrote:
Like others have said. I'd change all the power supply caps and the problem will
probably go away. It's always worked for me.



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



------------------------------------ Posted by: Brent Watson
<brentleew2003@...> ------------------------------------


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

Yahoo Groups Links

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


Brent Watson <brentleew2003@...>
 

I apologize  for that poor contribution. I don't know the 7704 and should not have commented. I read Dennis's post after I sent mine, and tried to delete it, but too late.
Please disregard.Brent

From: "Chuck Harris cfharris@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Tuesday, January 10, 2017 6:49 AM
Subject: Re: [TekScopes] Re: New Member Dead 7704

  Truly a bad idea! Especially since this isn't likely
to be a power supply problem.

[I would look closely at the two metal film power resistors
on the output stage of the vertical amp. They, and
others of their make, are commonly intermittent.]

First RTFM, and then check the power supply voltages for
correct amplitude and ripple. If the amplitude and ripple
are ok, the caps, even if not perfect, are good enough for
this use.

You are likely to destroy the scope by trying to remove
and replace the capacitors. Tektronix had a habit of using
huge ground planes without providing thermal reliefs. It
takes a very high wattage temperature controlled soldering
iron for their safe removal, 70-100W and 650F temperature.
Let the temperature go above 650F, and the board will
delaminate.

Many power supply caps are fit into too small holes in the
circuit boards. If the solder isn't totally melted, the
plated through holes will pull out with the lead, ruining
the circuit board. If you don't happen to notice, you will
have introduced yet another problem to diagnose and fix...
Only this time, you will waste hours and hours of your time
because you already replaced the capacitor, and *know* that
it can't possibly be bad.

The schematics aren't detailed enough to show which
connections are made by the plated through holes.

Put your head to work, and diagnose the problem. There is
ample instruction on how to do this in the very fine manual.

If you don't think you are up to figuring out the problem, do
yourself a big favor and buy a ready to go scope, perhaps
a Rigol? It will cost you about 3 dinners at a nice
restaurant with a friend... Two if you like to pop a cork.

The next guy to take a stab at fixing your scope will
thank you!

-Chuck Harris

Brent Watson brentleew2003@... [TekScopes] wrote:
Like others have said. I'd change all the power supply caps and the problem will
probably go away. It's always worked for me.



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



------------------------------------ Posted by: Brent Watson
<brentleew2003@...> ------------------------------------


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

Yahoo Groups Links

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