7904A and X-Y Display -- Help !


jsternmd
 

My 7904A seems to work fine for routine use with my various Vertical amps and timebases. However, I can't get it to work in X-Y display. I have a 7A26 in right vertical and a 7B70 in the left (A) Horizontal slot with nothing in the B-horizontal slot. As I understand the instruction manual, the 7B80 should be switched from Timebase to Amplifier and external/DC selected but when I do this the 7904A "A" slot light next to the Intensity control goes out and the "B" light comes on and the display is lost. There is nothing in the B slot. Also the Horizontal Mode button selected for A or B lights up correctly but has no effect on the getting anyt display. If I try to reverse things by putting the 7B70 into the B horizontal slot with the A slot empty, it works normally as timebase with B light on but as soon as I switch it to Amplifer mode, the B light goes out and the A light comes on. I tried to input two simple sine wave signals 90deg out of phase but cannot get anything on display. I also have a 7B80 with X-Y option and it shows the exact same behavior switching from timebase to X-Y.

Any advice approeciated.

Jerry
K1JOS


 

The time bases that have the option for XY are meant to remain in their
horizontal slots and work with vertical amplifiers that are in their
vertical slots. DO not swap the time base and the vertical plugin. Keep the
time base in a horizontal slot and the vertical amplifier plugin in a
vertical slot.

I went through the following setup on my 7934 which you can duplicate for
yourself to see how this is supposed to work. It is identical on all 7000
scopes as far as I recall:
The 7A26 in either one of the vertical slots. The vertical mode switch is
pressed for that slot.
The 7B80 in either one of the horizontal slots. The Horizontal mode switch
is pressed for that slot.
The Display Mode push button on the 7B80 is OUT (X-Y position).
The Y signal is connected to CH 2 of the 7A26.
The X signal is connected to CH 1 of the 7A26.
The input coupling switch for each 7A26 channel can be set to AC or DC as
needed.
The Display mode switch of the 7A26 is set to CH 2.
The Trigger Source switch of the 7A26 is set to CH 1.

If you look carefully at the blue area on the 7B80 it says "X Via Internal
Trigger Path". What that means is the X signal is selected by the 7A26
Trigger Source switch - which is switched to CH 1 in the last step above.

When you have TWO VERTICAL PLUGINS then you can put one in a vertical slot
and one in a horizontal slot and generate XY signals that way.

When you have TWO HORIZONTAL TIME BASES then you can put one in a vertical
slot and one in a horizontal slot and generate a RASTER. A raster is what a
TV set generates so the beam covers the entire screen.

Dennis

-----Original Message-----
From: jsternmd@..., Sent: Saturday, April 13, 2013 9:35 PM

My 7904A seems to work fine for routine use with my various Vertical amps
and timebases. However, I can't get it to work in X-Y display. I have a
7A26 in right vertical and a 7B70 in the left (A) Horizontal slot with
nothing in the B-horizontal slot. As I understand the instruction manual,
the 7B80 should be switched from Timebase to Amplifier and external/DC
selected but when I do this the 7904A "A" slot light next to the Intensity
control goes out and the "B" light comes on and the display is lost. There
is nothing in the B slot. Also the Horizontal Mode button selected for A or
B lights up correctly but has no effect on the getting anyt display. If I
try to reverse things by putting the 7B70 into the B horizontal slot with
the A slot empty, it works normally as timebase with B light on but as soon
as I switch it to Amplifer mode, the B light goes out and the A light comes
on. I tried to input two simple sine wave signals 90deg out of phase but
cannot get anything on display. I also have a 7B80 with X-Y option and it
shows the exact same behavior switching from timebase to X-Y.

Any advice approeciated.

Jerry
K1JOS


Albert <aodiversen@...>
 

Try with S4488 (corner logic board A13) in the other position?
Albert

--- In TekScopes@..., "jsternmd@..." <jsternmd@...> wrote:

My 7904A seems to work fine for routine use with my various Vertical amps and timebases. However, I can't get it to work in X-Y display. I have a 7A26 in right vertical and a 7B70 in the left (A) Horizontal slot with nothing in the B-horizontal slot. As I understand the instruction manual, the 7B80 should be switched from Timebase to Amplifier and external/DC selected but when I do this the 7904A "A" slot light next to the Intensity control goes out and the "B" light comes on and the display is lost. There is nothing in the B slot. Also the Horizontal Mode button selected for A or B lights up correctly but has no effect on the getting anyt display. If I try to reverse things by putting the 7B70 into the B horizontal slot with the A slot empty, it works normally as timebase with B light on but as soon as I switch it to Amplifer mode, the B light goes out and the A light comes on. I tried to input two simple sine wave signals 90deg out of phase but cannot get anything on display. I also have a 7B80 with X-Y option and it shows the exact same behavior switching from timebase to X-Y.

Any advice approeciated.

Jerry
K1JOS


 

Jerry,

I did this on a 7934 and a 7854 (I don't have a 7904A). They both worked
identically and your 7904A should work the same as these two did.

First: I am very suspicious of this sentence: "As soon as I push the X-Y
button on the scope the screen goes blank except for the 7A26 mV readout".
You must have the vertical plugin set to a Volts/Div setting or else the
waveform will be moving way off the screen to the left and right.

Second, Do this test: set both channels of the amplifier to ground and press
the BeamFinder. It should locate the beam somewhere and you should be able
to bring it back on screen with the two position controls of the 7A26.

Third, just to be safe lower the frequency of the signal to 100KHz. Most of
the 7000 mainframes have very limited horizontal amplification capability
and I am not sure what the 7904 is capable of. It will be one of the
specifications listed in the very front of the service manual however.

Fourth: When the 7B80 is in Time Base mode (Display Mode push button is IN)
the On-Screen Readout shows the Time Base speed but when I pop the Display
Mode push button on the 7B80 to the OUT position there is no time base
anymore so that portion of the readout goes blank. But the Vertical
Amplifier on-Screen Readout remains in either setting of the Display Mode
push button.

Fifth: make sure the A Trigger Source and B Trigger Source push buttons on
either side of the Horizontal Mode switches are set to VERT MODE just to be
safe. This is necessary since the 7B80 gets its horizontal signal from the
trigger path and the Trigger Source pushbuttons determine that path as well
as the 7A26 Trigger Source switch.

FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does.

Dennis

-----Original Message-----
From: Albert, Sent: Sunday, April 14, 2013 9:41 AM

Try with S4488 (corner logic board A13) in the other position?
Albert
---

-----Original Message-----
From: jsternmd@..., Sent: Sunday, April 14, 2013 5:56 PM

Hi Dennis

Thanks for the excellent advice. I set it up exactly to each detail as you
said and then I inputted a simple 1mhz 2Vpp sine signal split into Ch1 and
Ch 2 on the 7A26. Before I pushed in the Display Mode Blue area X-Y button,
i had a nice single trace of the CH2 signal on the scope ( as you said to
only have the 7A26 display on CH2 and the Trigger on CH1). As soon as I
push the X-Y button on the scope the screen goes blank except for the 7A26
mV readout. May I ask to you to check one thing - I have the 7A26 in the
vertical Right slot and the 7B80 in the horizontal A slot (thats the left of
the two horizontal slots). Before I push in the X-Y button the 7904A
mainframe has the "A" light lit up - the light up near the intensity button.
As soon as I press in X-Y the A light goes out and the B light comes on even
though the B slot is empty.

best
Jerry


Albert <aodiversen@...>
 

Hi Jerry and Dennis,

I now verified this with a 7B53A in the A horizontal slot and in Ampl position of the Time/div knob. The switch S4488 should be slided inwards for normal XY-operation. In the slided outwards position the Z-axis is controlled by the time base amplifier in the B slot (Meaning that the trace is visible while the B sweep is running and blanked during retrace.) The B Intensity bulb gets lit then.
Like in the 7104, the switch is located at the right side of the 'scope, at the upper corner of the A13 board. Best viewed from rear side of the 'scope and switchable with your finger.

Albert

---
FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does.

Dennis

Try with S4488 (corner logic board A13) in the other position?
Albert


jsternmd
 

Hi Dennis,

I made notes below next to each of your check steps. I am pretty certain
it's the XY Delay board problem. I had some success see my notes to you
below.

Jerry

-----Original Message-----
From: Dennis Tillman [mailto:dennis@...]
Sent: Monday, April 15, 2013 02:16
To: jsternmd@...; TekScopes@...
Subject: RE: 7904A and X-Y Display -- Help !

Jerry,

I did this on a 7934 and a 7854 (I don't have a 7904A). They both worked
identically and your 7904A should work the same as these two did.

First: I am very suspicious of this sentence: "As soon as I push the X-Y
button on the scope the screen goes blank except for the 7A26 mV readout".
You must have the vertical plugin set to a Volts/Div setting or else the
waveform will be moving way off the screen to the left and right.
[[[YES, CHECKED AND VERIFED]]]


Second, Do this test: set both channels of the amplifier to ground and press
the BeamFinder. It should locate the beam somewhere and you should be able
to bring it back on screen with the two position controls of the 7A26.
[[[ YES, WHEN IN TIMEBASE DISPLAY MODE; BUT NO, NOTHING IN XY DISPLAY MODE
EXCEPT THE VERTICAL AMP Volts/Div IS DISPLAYED IN THE READOUT]]]

Third, just to be safe lower the frequency of the signal to 100KHz. Most of
the 7000 mainframes have very limited horizontal amplification capability
and I am not sure what the 7904 is capable of. It will be one of the
specifications listed in the very front of the service manual however.
[[[YES, CHECKED USING A 1KHZ SINE SIGNAL AND NO CHANGE]]]

Fourth: When the 7B80 is in Time Base mode (Display Mode push button is IN)
the On-Screen Readout shows the Time Base speed but when I pop the Display
Mode push button on the 7B80 to the OUT position there is no time base
anymore so that portion of the readout goes blank. But the Vertical
Amplifier on-Screen Readout remains in either setting of the Display Mode
push button. [[[YES, VERTICAL AMPLIFER READOUT REMAINS ON AND TIMEBASE
READOUT DISAPPEARS BUT NOTHING ELSE SEEN ON SCREEN ]]]

Fifth: make sure the A Trigger Source and B Trigger Source push buttons on
either side of the Horizontal Mode switches are set to VERT MODE just to be
safe. This is necessary since the 7B80 gets its horizontal signal from the
trigger path and the Trigger Source pushbuttons determine that path as well
as the 7A26 Trigger Source switch. [[[YES, CHECKED AND VERIFED]]]

FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does. [[[THIS IS WHERE MY SCOPE GOES WRONG... WITH TIMEBASE IN SLOT A AND
DISPLAY MODE CHANGED TO XY - THE 'A' LIGHT GOES OUT AND THE 'B' LIGHT COMES
ON - EVEN THOUGH THERE IS NOTHING IN B SLOT. I WONDERED WHAT WOULD HAPPEN
IF I HAD TWO 7B80'S IN THE A AND B SLOTS. I HAVE OTHER 7B80 BUT IT DOESN'T
HAVE THE XY SO I PUT THE XY ONE IN SLOT 'A' AND THE OTHER IN SLOT 'B'. I
TOOK A 1KZ SINE AND SPLIT IT TO VERT AMP CH1 AND CH2 AND SET BOTH TO THE
SAME VOLTS/DIV. I SET THE TRIGGER TO CH1 AND DISPLAY TO CH2. WHEN I PUSHED
THE 7B80 DISPLAY MODE TO XY - THE SCOPE SWITCHED TO THE B CHANNEL BUT I GOT
THE ANTOICPATED LISSAJOUS PATTERN !!!! I WANT TO CONFIRM AND GET TWO
SIGNALS OUT OF PHASE TO DOUBLE CHECK. THEN I WILL CHECK HOW THE SCOPE
BEHAVES WHEN I CHANGE THE TWO SWITCHES ALBERT MENTIONED ON THE XY DELAY
BOARD. I STILL BELIEVE THEY DON'T WORK AS DESCRIBED IN THE MANUAL IN THE
NORMAL TIMEBASE SETUP. I THINK IT'S A BAD BOARD AND ONE OF THE RELAYS IS
NOT SWITCHING CORRECTLY BUT I MAY HAVE A WORK-AROUND. ANY THOUGHTS?

Dennis

-----Original Message-----
From: Albert, Sent: Sunday, April 14, 2013 9:41 AM

Try with S4488 (corner logic board A13) in the other position?
Albert
---

-----Original Message-----
From: jsternmd@..., Sent: Sunday, April 14, 2013 5:56 PM

Hi Dennis

Thanks for the excellent advice. I set it up exactly to each detail as you
said and then I inputted a simple 1mhz 2Vpp sine signal split into Ch1 and
Ch 2 on the 7A26. Before I pushed in the Display Mode Blue area X-Y button,
i had a nice single trace of the CH2 signal on the scope ( as you said to
only have the 7A26 display on CH2 and the Trigger on CH1). As soon as I
push the X-Y button on the scope the screen goes blank except for the 7A26
mV readout. May I ask to you to check one thing - I have the 7A26 in the
vertical Right slot and the 7B80 in the horizontal A slot (thats the left of
the two horizontal slots). Before I push in the X-Y button the 7904A
mainframe has the "A" light lit up - the light up near the intensity button.
As soon as I press in X-Y the A light goes out and the B light comes on even
though the B slot is empty.

best
Jerry


 

Good idea. I would have suggested it but I didn't know you hand another
timebase. If you switch the CH2 Polarity to Invert it should switch the
angle of the pattern 90 degrees. Alternatively you can stick a few
microfarad capacitor in one of the two signals going to the vertical plugin
to generate a phase delay. It will lower the amplitude and you will probably
have to increase the frequency a few orders of magnitude but you will
eventually get an oval at some combination of frequency and capacitance
since the capacitance is in series with the 1Mohm of the vertical amplifier.

I don't know what relays Tek used in the 7904A but often they are in
sockets. If they are in sockets and they are the same try swapping them. If
they are in sockets and they are not the same try removing them and applying
a voltage to each coil and listen for the click. The coil voltage and coil
contacts should be marked on the relay. If they are reed relays try holding
a magnet next to each one and you will hear a very, very faint click as the
contacts close or open.

If the relay's only have a Tek part number on them someone on the user group
will be able to tell you what the pin outs and coil voltages are.

Dennis

-----Original Message-----
From: Jerry [mailto:jsternmd@...]
Sent: Monday, April 15, 2013 5:04 PM

Hi Dennis,
I made notes below next to each of your check steps. I am pretty certain
it's the XY Delay board problem. I had some success see my notes to you
below.
Jerry

<snip>
I WONDERED WHAT WOULD HAPPEN IF I HAD TWO 7B80'S IN THE A AND B SLOTS. I
HAVE OTHER 7B80 BUT IT DOESN'T HAVE THE XY SO I PUT THE XY ONE IN SLOT 'A'
AND THE OTHER IN SLOT 'B'. I TOOK A 1KZ SINE AND SPLIT IT TO VERT AMP CH1
AND CH2 AND SET BOTH TO THE SAME VOLTS/DIV. I SET THE TRIGGER TO CH1 AND
DISPLAY TO CH2. WHEN I PUSHED THE 7B80 DISPLAY MODE TO XY - THE SCOPE
SWITCHED TO THE B CHANNEL BUT I GOT
THE ANTOICPATED LISSAJOUS PATTERN !!!! I WANT TO CONFIRM AND GET TWO
SIGNALS OUT OF PHASE TO DOUBLE CHECK. THEN I WILL CHECK HOW THE SCOPE
BEHAVES WHEN I CHANGE THE TWO SWITCHES ALBERT MENTIONED ON THE XY DELAY
BOARD. I STILL BELIEVE THEY DON'T WORK AS DESCRIBED IN THE MANUAL IN THE
NORMAL TIMEBASE SETUP. I THINK IT'S A BAD BOARD AND ONE OF THE RELAYS IS
NOT SWITCHING CORRECTLY BUT I MAY HAVE A WORK-AROUND. ANY THOUGHTS?


Albert <aodiversen@...>
 

Hi Jerry,

I didn't mention two switches at a XY Delay board (if that's what you mean). I mentioned the so-called "X-Y Z-axis Selector" switch, located at the Logic board. Your 7904A does exactly what it should do when this switch is in the outward position called "X-Y Time Base Controlled Z-axis".
The purpose of that facility is that you can blank portions of the XY-curve that would be far too bright or would display as a one very bright spot.

Albert


[Dennis:]
---
FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does.
[Jerry:]
---
[[[THIS IS WHERE MY SCOPE GOES WRONG... WITH TIMEBASE IN SLOT A AND
DISPLAY MODE CHANGED TO XY - THE 'A' LIGHT GOES OUT AND THE 'B' LIGHT COMES
ON - EVEN THOUGH THERE IS NOTHING IN B SLOT. I WONDERED WHAT WOULD HAPPEN
IF I HAD TWO 7B80'S IN THE A AND B SLOTS. I HAVE OTHER 7B80 BUT IT DOESN'T
HAVE THE XY SO I PUT THE XY ONE IN SLOT 'A' AND THE OTHER IN SLOT 'B'. I
TOOK A 1KZ SINE AND SPLIT IT TO VERT AMP CH1 AND CH2 AND SET BOTH TO THE
SAME VOLTS/DIV. I SET THE TRIGGER TO CH1 AND DISPLAY TO CH2. WHEN I PUSHED
THE 7B80 DISPLAY MODE TO XY - THE SCOPE SWITCHED TO THE B CHANNEL BUT I GOT
THE ANTOICPATED LISSAJOUS PATTERN !!!! I WANT TO CONFIRM AND GET TWO
SIGNALS OUT OF PHASE TO DOUBLE CHECK. THEN I WILL CHECK HOW THE SCOPE
BEHAVES WHEN I CHANGE THE TWO SWITCHES ALBERT MENTIONED ON THE XY DELAY
BOARD. I STILL BELIEVE THEY DON'T WORK AS DESCRIBED IN THE MANUAL IN THE
NORMAL TIMEBASE SETUP. I THINK IT'S A BAD BOARD AND ONE OF THE RELAYS IS
NOT SWITCHING CORRECTLY BUT I MAY HAVE A WORK-AROUND. ANY THOUGHTS?


jsternmd
 

Thanks Albert, I will check alter today and report back.

Jerry

--- In TekScopes@..., "Albert" <aodiversen@...> wrote:

Hi Jerry,

I didn't mention two switches at a XY Delay board (if that's what you mean). I mentioned the so-called "X-Y Z-axis Selector" switch, located at the Logic board. Your 7904A does exactly what it should do when this switch is in the outward position called "X-Y Time Base Controlled Z-axis".
The purpose of that facility is that you can blank portions of the XY-curve that would be far too bright or would display as a one very bright spot.

Albert


[Dennis:]
---
FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does.
[Jerry:]
---
[[[THIS IS WHERE MY SCOPE GOES WRONG... WITH TIMEBASE IN SLOT A AND
DISPLAY MODE CHANGED TO XY - THE 'A' LIGHT GOES OUT AND THE 'B' LIGHT COMES
ON - EVEN THOUGH THERE IS NOTHING IN B SLOT. I WONDERED WHAT WOULD HAPPEN
IF I HAD TWO 7B80'S IN THE A AND B SLOTS. I HAVE OTHER 7B80 BUT IT DOESN'T
HAVE THE XY SO I PUT THE XY ONE IN SLOT 'A' AND THE OTHER IN SLOT 'B'. I
TOOK A 1KZ SINE AND SPLIT IT TO VERT AMP CH1 AND CH2 AND SET BOTH TO THE
SAME VOLTS/DIV. I SET THE TRIGGER TO CH1 AND DISPLAY TO CH2. WHEN I PUSHED
THE 7B80 DISPLAY MODE TO XY - THE SCOPE SWITCHED TO THE B CHANNEL BUT I GOT
THE ANTOICPATED LISSAJOUS PATTERN !!!! I WANT TO CONFIRM AND GET TWO
SIGNALS OUT OF PHASE TO DOUBLE CHECK. THEN I WILL CHECK HOW THE SCOPE
BEHAVES WHEN I CHANGE THE TWO SWITCHES ALBERT MENTIONED ON THE XY DELAY
BOARD. I STILL BELIEVE THEY DON'T WORK AS DESCRIBED IN THE MANUAL IN THE
NORMAL TIMEBASE SETUP. I THINK IT'S A BAD BOARD AND ONE OF THE RELAYS IS
NOT SWITCHING CORRECTLY BUT I MAY HAVE A WORK-AROUND. ANY THOUGHTS?


jsternmd
 

Thanks Albert for first discovering I was using the 7904 manual instead of the 7904A. Also for telling me about the the Z-axis switch S4488 function that is not documented in the 7904A service manual! Changing this switch setting brought the scope back to 'expected' XY operation. I found it interesting that my scope was actually working exactly as the manual said (with S4488 in default setting) where with 2 amps, one in vertical slot and another in A horizontal slot, that you indeed still needed a timebase in the B slot. I found that by pure chance. Now with the the Z-axis switch S4488 in opposite setting the scope works in X-Y with only 2 vertical amps and no timebase. I wish the manual would explain what the benefits or downsides are for each switch settings. At least now its documented here for others to search and find.

best
Jerry

--- In TekScopes@..., "Albert" <aodiversen@...> wrote:

Hi Jerry,

I didn't mention two switches at a XY Delay board (if that's what you mean). I mentioned the so-called "X-Y Z-axis Selector" switch, located at the Logic board. Your 7904A does exactly what it should do when this switch is in the outward position called "X-Y Time Base Controlled Z-axis".
The purpose of that facility is that you can blank portions of the XY-curve that would be far too bright or would display as a one very bright spot.

Albert


[Dennis:]
---
FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does.
[Jerry:]
---
[[[THIS IS WHERE MY SCOPE GOES WRONG... WITH TIMEBASE IN SLOT A AND
DISPLAY MODE CHANGED TO XY - THE 'A' LIGHT GOES OUT AND THE 'B' LIGHT COMES
ON - EVEN THOUGH THERE IS NOTHING IN B SLOT. I WONDERED WHAT WOULD HAPPEN
IF I HAD TWO 7B80'S IN THE A AND B SLOTS. I HAVE OTHER 7B80 BUT IT DOESN'T
HAVE THE XY SO I PUT THE XY ONE IN SLOT 'A' AND THE OTHER IN SLOT 'B'. I
TOOK A 1KZ SINE AND SPLIT IT TO VERT AMP CH1 AND CH2 AND SET BOTH TO THE
SAME VOLTS/DIV. I SET THE TRIGGER TO CH1 AND DISPLAY TO CH2. WHEN I PUSHED
THE 7B80 DISPLAY MODE TO XY - THE SCOPE SWITCHED TO THE B CHANNEL BUT I GOT
THE ANTOICPATED LISSAJOUS PATTERN !!!! I WANT TO CONFIRM AND GET TWO
SIGNALS OUT OF PHASE TO DOUBLE CHECK. THEN I WILL CHECK HOW THE SCOPE
BEHAVES WHEN I CHANGE THE TWO SWITCHES ALBERT MENTIONED ON THE XY DELAY
BOARD. I STILL BELIEVE THEY DON'T WORK AS DESCRIBED IN THE MANUAL IN THE
NORMAL TIMEBASE SETUP. I THINK IT'S A BAD BOARD AND ONE OF THE RELAYS IS
NOT SWITCHING CORRECTLY BUT I MAY HAVE A WORK-AROUND. ANY THOUGHTS?