Topics

Tektronix 7904, no horizontal trace on input B

Hermansson Amplification
 

I just bought a Tektronix 7904 scope and realized that something is wrong with horizontal input B.
Horizontal input A works fine but I got no trace on input B. If I press
“Beam finder” slightly I see dots for each inputs that are activated in the vertical slots and if I press the “Beam finder” harder I see bigger dots. I only see the dots when I press “Beam finder” otherwise the screen is blank when horizontal input B is activated.

Any suggestions where to start? I’ve measured the voltages on the Z axis-PCB but nothing alarming there. Since the horizontal Input A works something might be bad on the “Trigger B”-PCB? Or where should I start?

Here’s a video showing the problem:
https://youtu.be/KuPenOSkyJg

I’ve been exercising the switches and cleaned the input slots for the plugin.

Thank you in advance!

Best regards,
Henric

 

Hi Henric,

The first thing I notice in your video is that when you put the time base in the B horizontal slot the 7904 recognizes it is present because it displays the on-screen readout time/div setting on the bottom right part of the CRT. That is a good sign.

The second thing I see is when you press the beam finder with the time base in the B horizontal slot the display you get shows there is no sweep present. You are just getting the vertical signals on the screen without any horizontal sweep.

There could be two reasons for this:
A) The time base is not getting a trigger signal when it is in the B horizontal slot. One test you can perform would be to feed a trigger signal into the external trigger input of the time base while in the B slot to see if this fixes the problem. If it does then that confirms the trigger signal is not getting to the B slot from the vertical amplifiers. That could be caused by a bad front panel B Trigger Source Switch or by the IC that controls the routing of this trigger signal which would be on the backplane PC board. Also on the rear of the 7904 (or maybe even on the front of it) there are several BNC connectors which bring out critical signals like trigger signals. If there is a B Trigger Signal output make sure it is present on the front or rear panel BNC connectors.

B) The time base is being triggered and generating a sweep but the sweep is not getting from the time base to the horizontal amplifier in the mainframe. Once again there are ICs on the back plane that route this sweep signal. The B Sweep signal comes out to one of the front or rear BNC connectors so you should check for it there. If it isn't there and if it isn't on the CRT the IC that switches it thru from the back plane to the horizontal amplifier may not be getting the right switching signal or it may be bad.

The most important thing you can do for your own benefit and to learn how your new scope works is to read the Theory of Operation section of the service manual. It explains everything! For the moment read the parts about how the trigger signal is routed and how the sweep signal is routed to learn the various places they go in the scope and which ICs they go through to get a better idea of exactly where to look for where it is being blocked.

Dennis Tillman W7PF

-----Original Message-----
From: @HermanssonAmplification
Sent: Monday, November 18, 2019 7:06 AM
Subject: [TekScopes] Tektronix 7904, no horizontal trace on input B

I just bought a Tektronix 7904 scope and realized that something is wrong with horizontal input B.
Horizontal input A works fine but I got no trace on input B. If I press “Beam finder” slightly I see dots for each inputs that are activated in the vertical slots and if I press the “Beam finder” harder I see bigger dots. I only see the dots when I press “Beam finder” otherwise the screen is blank when horizontal input B is activated.

Any suggestions where to start? I’ve measured the voltages on the Z axis-PCB but nothing alarming there. Since the horizontal Input A works something might be bad on the “Trigger B”-PCB? Or where should I start?

Here’s a video showing the problem:
https://youtu.be/KuPenOSkyJg

I’ve been exercising the switches and cleaned the input slots for the plugin.
Thank you in advance!

Best regards,
Henric




--
Dennis Tillman W7PF
TekScopes Moderator

Robert Dixon
 

Hi Henric,
It looks like the trigger light on the timebase responds the same in both slots. Try varying your input amp settings to confirm this. If so, it suggests that the trigger signal is getting to the timebase, and the problem is B) as Dennis describes below.

Good luck. The 7904 is a nice piece of gear.
BTW, YouTube video for diagnosis is a great way to make the symptoms clear.

Robert Dixon

On 19 Nov 2019, at 06:24, Dennis Tillman W7PF <@Dennis_Tillman_W7PF> wrote:

Hi Henric,

The first thing I notice in your video is that when you put the time base in the B horizontal slot the 7904 recognizes it is present because it displays the on-screen readout time/div setting on the bottom right part of the CRT. That is a good sign.

The second thing I see is when you press the beam finder with the time base in the B horizontal slot the display you get shows there is no sweep present. You are just getting the vertical signals on the screen without any horizontal sweep.

There could be two reasons for this:
A) The time base is not getting a trigger signal when it is in the B horizontal slot. One test you can perform would be to feed a trigger signal into the external trigger input of the time base while in the B slot to see if this fixes the problem. If it does then that confirms the trigger signal is not getting to the B slot from the vertical amplifiers. That could be caused by a bad front panel B Trigger Source Switch or by the IC that controls the routing of this trigger signal which would be on the backplane PC board. Also on the rear of the 7904 (or maybe even on the front of it) there are several BNC connectors which bring out critical signals like trigger signals. If there is a B Trigger Signal output make sure it is present on the front or rear panel BNC connectors.

B) The time base is being triggered and generating a sweep but the sweep is not getting from the time base to the horizontal amplifier in the mainframe. Once again there are ICs on the back plane that route this sweep signal. The B Sweep signal comes out to one of the front or rear BNC connectors so you should check for it there. If it isn't there and if it isn't on the CRT the IC that switches it thru from the back plane to the horizontal amplifier may not be getting the right switching signal or it may be bad.

The most important thing you can do for your own benefit and to learn how your new scope works is to read the Theory of Operation section of the service manual. It explains everything! For the moment read the parts about how the trigger signal is routed and how the sweep signal is routed to learn the various places they go in the scope and which ICs they go through to get a better idea of exactly where to look for where it is being blocked.

Dennis Tillman W7PF

-----Original Message-----
From: @HermanssonAmplification
Sent: Monday, November 18, 2019 7:06 AM
Subject: [TekScopes] Tektronix 7904, no horizontal trace on input B

I just bought a Tektronix 7904 scope and realized that something is wrong with horizontal input B.
Horizontal input A works fine but I got no trace on input B. If I press “Beam finder” slightly I see dots for each inputs that are activated in the vertical slots and if I press the “Beam finder” harder I see bigger dots. I only see the dots when I press “Beam finder” otherwise the screen is blank when horizontal input B is activated.

Any suggestions where to start? I’ve measured the voltages on the Z axis-PCB but nothing alarming there. Since the horizontal Input A works something might be bad on the “Trigger B”-PCB? Or where should I start?

Here’s a video showing the problem:
https://youtu.be/KuPenOSkyJg

I’ve been exercising the switches and cleaned the input slots for the plugin.
Thank you in advance!

Best regards,
Henric




--
Dennis Tillman W7PF
TekScopes Moderator


Robert Dixon
 

Hi again Henric,

Another trick that may help narrow the problem, is understanding that 7000 series CROs can be put into X-Y mode by putting a vertical amplifier in a "timebase" slot, and then drive both amps with the same signal. I can't remember if you need a timebase for this, but the manuals will help.

Robert Dixon

On 19 Nov 2019, at 07:42, Robert Dixon via Groups.Io <lymphominator=yahoo.com@groups.io> wrote:

Hi Henric,
It looks like the trigger light on the timebase responds the same in both slots. Try varying your input amp settings to confirm this. If so, it suggests that the trigger signal is getting to the timebase, and the problem is B) as Dennis describes below.

Good luck. The 7904 is a nice piece of gear.
BTW, YouTube video for diagnosis is a great way to make the symptoms clear.

Robert Dixon

On 19 Nov 2019, at 06:24, Dennis Tillman W7PF <@Dennis_Tillman_W7PF> wrote:

Hi Henric,

The first thing I notice in your video is that when you put the time base in the B horizontal slot the 7904 recognizes it is present because it displays the on-screen readout time/div setting on the bottom right part of the CRT. That is a good sign.

The second thing I see is when you press the beam finder with the time base in the B horizontal slot the display you get shows there is no sweep present. You are just getting the vertical signals on the screen without any horizontal sweep.

There could be two reasons for this:
A) The time base is not getting a trigger signal when it is in the B horizontal slot. One test you can perform would be to feed a trigger signal into the external trigger input of the time base while in the B slot to see if this fixes the problem. If it does then that confirms the trigger signal is not getting to the B slot from the vertical amplifiers. That could be caused by a bad front panel B Trigger Source Switch or by the IC that controls the routing of this trigger signal which would be on the backplane PC board. Also on the rear of the 7904 (or maybe even on the front of it) there are several BNC connectors which bring out critical signals like trigger signals. If there is a B Trigger Signal output make sure it is present on the front or rear panel BNC connectors.

B) The time base is being triggered and generating a sweep but the sweep is not getting from the time base to the horizontal amplifier in the mainframe. Once again there are ICs on the back plane that route this sweep signal. The B Sweep signal comes out to one of the front or rear BNC connectors so you should check for it there. If it isn't there and if it isn't on the CRT the IC that switches it thru from the back plane to the horizontal amplifier may not be getting the right switching signal or it may be bad.

The most important thing you can do for your own benefit and to learn how your new scope works is to read the Theory of Operation section of the service manual. It explains everything! For the moment read the parts about how the trigger signal is routed and how the sweep signal is routed to learn the various places they go in the scope and which ICs they go through to get a better idea of exactly where to look for where it is being blocked.

Dennis Tillman W7PF

-----Original Message-----
From: @HermanssonAmplification
Sent: Monday, November 18, 2019 7:06 AM
Subject: [TekScopes] Tektronix 7904, no horizontal trace on input B

I just bought a Tektronix 7904 scope and realized that something is wrong with horizontal input B.
Horizontal input A works fine but I got no trace on input B. If I press “Beam finder” slightly I see dots for each inputs that are activated in the vertical slots and if I press the “Beam finder” harder I see bigger dots. I only see the dots when I press “Beam finder” otherwise the screen is blank when horizontal input B is activated.

Any suggestions where to start? I’ve measured the voltages on the Z axis-PCB but nothing alarming there. Since the horizontal Input A works something might be bad on the “Trigger B”-PCB? Or where should I start?

Here’s a video showing the problem:
https://youtu.be/KuPenOSkyJg

I’ve been exercising the switches and cleaned the input slots for the plugin.
Thank you in advance!

Best regards,
Henric




--
Dennis Tillman W7PF
TekScopes Moderator


 

Silly question: Are you sure the A-slot Intensity isn't completely off? Could be the knob or a pot with problems.

Raymond

 

On Mon, Nov 18, 2019 at 11:34 PM, Raymond Domp Frank wrote:


Silly question: Are you sure the A-slot Intensity isn't completely off?
That should be "B"-slot of course

Raymond

Hermansson Amplification
 

Thank you all for your responses!

I will look into the manual and read about the routing path!

To answer some of your questions:
Yes, the trigger light lights up on the timebase when I apply signal to the vertical input. And you can see the vertical signal on the upper trace when I press beam finder.

I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.

The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it. And I guess I wouldn’t see the dots when I press beam finder either if the potentiometer was the problem?

I believe this scope has been stored for a long time but it seems to be in very good shape for its age. It was inspected by Tektronix in Sweden in 1972 so it’s an early unit.

Best regards,
Henric

 

On Tue, Nov 19, 2019 at 01:48 AM, Hermansson Amplification wrote:


The intensity B knob is very very stiff! I needed a pliers to move it, but it
affects the brightness of the beam finder dots when I turn it.
Hi Henric,
A bit of oil on the bearing from the outside and some turning works wonders on the movement of the B intensity pot.

Just a few observations/thoughts:
- The fact that the dots and bars at the left are visible at all could indicate that grid bias is adjusted significantly higher in your 'scope than in the two that I just checked or that something is wrong in that part of the 'scope. If so, it's no problem and doesn't seem directly related to your main problem. It could also mean that someone tried adjusting a few internal pots to see what would happen, trying to repair a perceived intensity problem, possibly causing other misadjustments.
- As I understand it, you got this 'scope as it is now, so be prepared for surprises.
- In the 7904 Service Manual, only the reduction in the traces is mentioned when pressing (or pulling) Beam Find (BF). In my 'scopes, intensity remains the same, no dots appear and the trace becomes defocused. Both are later models. In many 'scopes BF does increase intensity.
- BF does not affect the size of the readout in your 'scope, so that indicates an early model, as you already mentioned.
- On your 'scope, I see a slight horizontal movement of the readout between A/Alt vs. Chop/B when activating BF. Could be normal but not in my 'scopes.
- After activating BF, It seems as if you have two depths of depression, since I think I can see your finger move and I hear a few clicks and there's a slight change in the image vertically (dots and bars). Could be another difference between old and new models: My BF switches definitely only have one full-in and one full-out position from the "BF Off" position.

And I guess I wouldn’t see the dots when I press beam finder either if the potentiometer was the problem?
Pressing BF seems to change the Z behavior in your 'scope (see above) so you do but the fact that the dots respond to the B intensity setting suggests the pot isn't the problem. Not sure though.

Raymond

Kyle Rhodes
 

Hi Henric,

I don't have much to add, everyone else seems to have that under
control... Just wanted to say, and sorry for the off-topic message, it's
great to see a fellow guitar amp builder here!


Thanks,
Kyle


--
KSR Amps / Rhodes Amplification, LLC
Kyle Rhodes
(513) 480-2677
kyle@...
http://www.ksramps.com
http://kylerhodesdesigns.com


On Mon, Nov 18, 2019 at 9:11 PM Raymond Domp Frank <@Raymond>
wrote:

On Tue, Nov 19, 2019 at 01:48 AM, Hermansson Amplification wrote:


The intensity B knob is very very stiff! I needed a pliers to move it,
but it
affects the brightness of the beam finder dots when I turn it.
Hi Henric,
A bit of oil on the bearing from the outside and some turning works
wonders on the movement of the B intensity pot.

Just a few observations/thoughts:
- The fact that the dots and bars at the left are visible at all could
indicate that grid bias is adjusted significantly higher in your 'scope
than in the two that I just checked or that something is wrong in that part
of the 'scope. If so, it's no problem and doesn't seem directly related to
your main problem. It could also mean that someone tried adjusting a few
internal pots to see what would happen, trying to repair a perceived
intensity problem, possibly causing other misadjustments.
- As I understand it, you got this 'scope as it is now, so be prepared for
surprises.
- In the 7904 Service Manual, only the reduction in the traces is
mentioned when pressing (or pulling) Beam Find (BF). In my 'scopes,
intensity remains the same, no dots appear and the trace becomes defocused.
Both are later models. In many 'scopes BF does increase intensity.
- BF does not affect the size of the readout in your 'scope, so that
indicates an early model, as you already mentioned.
- On your 'scope, I see a slight horizontal movement of the readout
between A/Alt vs. Chop/B when activating BF. Could be normal but not in my
'scopes.
- After activating BF, It seems as if you have two depths of depression,
since I think I can see your finger move and I hear a few clicks and
there's a slight change in the image vertically (dots and bars). Could be
another difference between old and new models: My BF switches definitely
only have one full-in and one full-out position from the "BF Off" position.

And I guess I wouldn’t see the dots when I press beam finder either if
the potentiometer was the problem?
Pressing BF seems to change the Z behavior in your 'scope (see above) so
you do but the fact that the dots respond to the B intensity setting
suggests the pot isn't the problem. Not sure though.

Raymond



 

Hi Henric,

<SNIPS>
...the trigger light lights up on the timebase when I apply signal to the vertical input...
I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.
ANSWER
This tells me that the trigger path is OK and the signal path in the B Horizontal slot is bad. Both of the signal sources you tried (the sweep from the time base, as well as the signal from the vertical amplifier) did not get through to the horizontal amplifier board. The problem is probably a switching IC on the rear interface board so follow the part in the Theory that discusses how the time base sweep signal or a vertical amplifier signal (same difference) goes from the plugin in the B Horizontal Slot to the Horizontal Amplifier.

<SNIP>
The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it.
SOLUTION
This may be the problem causing the signal to be missing from the B Horizontal slot. Be very careful you do not break the shaft of this pot by trying to turn it if it is seized up! Do not turn it anymore until you are able to lubricate it. What has probably happened is some old lubricant has dried out between the shaft and the walls of the bushing it goes through.

This is how to free up the intensity pot: Unplug the scope and rest it on its rear panel so the front of the scope faces the ceiling. Remove the B intensity knob with a hex wrench. Carefully apply a few drops of contact cleaner onto the shaft of the pot. Gravity will cause the contact cleaner to drip down the shaft into the bushing and into the pot. Wait a few minutes and reapply a few more drops down the shaft. It should soon be free enough that you may be able to turn the shaft of the pot with your fingers. If not, apply a few more drops, wait a few more minutes then put the knob back on and tighten it. Then gently try turning the knob. It should be a lot less difficult to turn once the contact cleaner has dissolved the old lubricant. Reapply contact cleaner if necessary then try turning with the knob until it starts to move freely. Eventually the pot will turn freely. At that point you should rotate it back and forth through its full range to be sure it is free everywhere.

Dennis Tillman W7PF

-----Original Message-----
From: Hermansson Amplification
Sent: Monday, November 18, 2019 4:48 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

Thank you all for your responses!

I will look into the manual and read about the routing path!

To answer some of your questions:
Yes, the trigger light lights up on the timebase when I apply signal to the vertical input. And you can see the vertical signal on the upper trace when I press beam finder.

I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.

The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it. And I guess I wouldn’t see the dots when I press beam finder either if the potentiometer was the problem?

I believe this scope has been stored for a long time but it seems to be in very good shape for its age. It was inspected by Tektronix in Sweden in 1972 so it’s an early unit.

Best regards,
Henric



--
Dennis Tillman W7PF
TekScopes Moderator

 

On Tue, Nov 19, 2019 at 04:59 AM, Dennis Tillman W7PF wrote:

Hi Dennis,
A few remarks on your response:

Unplug the scope and rest it on its rear panel so the front of the scope faces the ceiling.
ISTR getting warnings that it is possible for loose particles to fall into the gun area when turning an old CRT screen-up, causing deformation in the image or (partial) blocking of the beam.
Dennis, with your experience, did you ever hear about this?


This tells me that the trigger path is OK and the signal path in the B Horizontal slot is bad.
I don't understand why the lack of a visible B-trace has to be caused by a fault in the signal path. Aah, you're mentioning the B-intensity pot as I did. Let's hope my stupid question wasn't as stupid after all and your advice solves the problem for Henric:

Carefully apply a few drops of contact cleaner onto the shaft of the pot.
In my experience, the volatile components in most contact oils evaporate with time, causing the pot to become stuck again within a few months. That's why I prefer the use of normal oil for freeing shafts. The right normal oils (or even WD-40) keep their original viscosity much longer.

It's also possible that after the shaft turns smoothly again, the wiper contact in the pot is scratch or even absent. That's when I apply contact cleaner, preferably after (slightly) opening the pot. Depending on the pot, that may be unfeasible.

Raymond

 

Hi Raymond,
Regarding your concern about my suggestion to Henric that he rest his scope on its rear panel so the front of the scope faces the ceiling when lubricating a stuck pot: You said "loose particles [can] fall into the gun area when turning an old CRT screen-up".

I am not aware of this happening but I can ask someone who worked on CRTs at Tek.
Dennis Tillman W7PF

-----Original Message-----
From: Raymond Domp Frank
Sent: Monday, November 18, 2019 8:53 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

On Tue, Nov 19, 2019 at 04:59 AM, Dennis Tillman W7PF wrote:

Hi Dennis,
A few remarks on your response:

Unplug the scope and rest it on its rear panel so the front of the scope faces the ceiling.
<SNIP>
ISTR getting warnings that it is possible for loose particles to fall into the gun area when turning an old CRT screen-up, causing deformation in the image or (partial) blocking of the beam.
Dennis, with your experience, did you ever hear about this?
Raymond

-----Original Message-----
From: Dennis Tillman W7PF
Sent: Monday, November 18, 2019 7:59 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

Hi Henric,

<SNIPS>
..the trigger light lights up on the timebase when I apply signal to the vertical input...
I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.
ANSWER
This tells me that the trigger path is OK and the signal path in the B Horizontal slot is bad. Both of the signal sources you tried (the sweep from the time base, as well as the signal from the vertical amplifier) did not get through to the horizontal amplifier board. The problem is probably a switching IC on the rear interface board so follow the part in the Theory that discusses how the time base sweep signal or a vertical amplifier signal (same difference) goes from the plugin in the B Horizontal Slot to the Horizontal Amplifier.

<SNIP>
The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it.
SOLUTION
This may be the problem causing the signal to be missing from the B Horizontal slot. Be very careful you do not break the shaft of this pot by trying to turn it if it is seized up! Do not turn it anymore until you are able to lubricate it. What has probably happened is some old lubricant has dried out between the shaft and the walls of the bushing it goes through.

This is how to free up the intensity pot: Unplug the scope and rest it on its rear panel so the front of the scope faces the ceiling. Remove the B intensity knob with a hex wrench. Carefully apply a few drops of contact cleaner onto the shaft of the pot. Gravity will cause the contact cleaner to drip down the shaft into the bushing and into the pot. Wait a few minutes and reapply a few more drops down the shaft. It should soon be free enough that you may be able to turn the shaft of the pot with your fingers. If not, apply a few more drops, wait a few more minutes then put the knob back on and tighten it. Then gently try turning the knob. It should be a lot less difficult to turn once the contact cleaner has dissolved the old lubricant. Reapply contact cleaner if necessary then try turning with the knob until it starts to move freely. Eventually the pot will turn freely. At that point you should rotate it back and forth through its full range to be sure it is free everywhere.

Dennis Tillman W7PF

-----Original Message-----
From: Hermansson Amplification
Sent: Monday, November 18, 2019 4:48 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

Thank you all for your responses!

I will look into the manual and read about the routing path!

To answer some of your questions:
Yes, the trigger light lights up on the timebase when I apply signal to the vertical input. And you can see the vertical signal on the upper trace when I press beam finder.

I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.

The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it. And I guess I wouldn’t see the dots when I press beam finder either if the potentiometer was the problem?

I believe this scope has been stored for a long time but it seems to be in very good shape for its age. It was inspected by Tektronix in Sweden in 1972 so it’s an early unit.

Best regards,
Henric





--
Dennis Tillman W7PF
TekScopes Moderator

 

On Tue, Nov 19, 2019 at 11:38 PM, Dennis Tillman W7PF wrote:


I am not aware of this happening but I can ask someone who worked on CRTs at
Tek.
Hi Dennis,
I would very much appreciate if at some time convenient to you, you could check. I remember reading this some time ago but it could possibly only apply to very old (by age or by design) CRT's or only with low quality ones.

Raymond

Carl Hallberg
 

Wouldn't we have found this out with all the 475, 476, 485, etc scopes that have the crt face up?
Carl

On Tuesday, November 19, 2019, 6:23:29 PM CST, Raymond Domp Frank <@Raymond> wrote:

On Tue, Nov 19, 2019 at 11:38 PM, Dennis Tillman W7PF wrote:


I am not aware of this happening but I can ask someone who worked on CRTs at
Tek.
Hi Dennis,
I would very much appreciate if at some time convenient to you, you could check. I remember reading this some time ago but it could possibly only apply to very old (by age or by design) CRT's or only with low quality ones.

Raymond

 

Hi Raymond,
I checked with my friend, the director of Tektronix Laboratories in the mid 1980s. He did the vertical deflection of the 7104 CRT which has a 3dB point above 3GHz.

He says everything inside the CRT is either spot welded or fused glass so there is nothing that can possibly become loose. After the CRT is sealed the getter is flashed to capture any microscopic particles that might be present. When the mainframe is shipped there is nothing that would prevent it from being shipped upright and from being knocked around and bounced in the cargo container or truck.

There is nothing that can become loose inside a CRT according to my friend.

I can add my own less professional, but more practical, solution to loose particles if there should be any after following my suggestion to free the intensity pot shaft:
If a particle should ever end up in the path of the beam then momentarily turn the scope down on its front to allow the loose particle to move to the front of the CRT. Then when the scope is put in its normal position that particle will settle to the bottom front part of the CRT where it won't be in the way of the beam.

Dennis Tillman W7PF

-----Original Message-----
From: Dennis Tillman W7PF
Sent: Tuesday, November 19, 2019 2:38 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

Hi Raymond,
Regarding your concern about my suggestion to Henric that he rest his scope on its rear panel so the front of the scope faces the ceiling when lubricating a stuck pot: You said "loose particles [can] fall into the gun area when turning an old CRT screen-up".

I am not aware of this happening but I can ask someone who worked on CRTs at Tek.
Dennis Tillman W7PF

-----Original Message-----
From: Raymond Domp Frank
Sent: Monday, November 18, 2019 8:53 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

On Tue, Nov 19, 2019 at 04:59 AM, Dennis Tillman W7PF wrote:

Hi Dennis,
A few remarks on your response:

Unplug the scope and rest it on its rear panel so the front of the scope faces the ceiling.
<SNIP>
ISTR getting warnings that it is possible for loose particles to fall into the gun area when turning an old CRT screen-up, causing deformation in the image or (partial) blocking of the beam.
Dennis, with your experience, did you ever hear about this?
Raymond

-----Original Message-----
From: Dennis Tillman W7PF
Sent: Monday, November 18, 2019 7:59 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

Hi Henric,

<SNIPS>
.the trigger light lights up on the timebase when I apply signal to the vertical input...
I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.
ANSWER
This tells me that the trigger path is OK and the signal path in the B Horizontal slot is bad. Both of the signal sources you tried (the sweep from the time base, as well as the signal from the vertical amplifier) did not get through to the horizontal amplifier board. The problem is probably a switching IC on the rear interface board so follow the part in the Theory that discusses how the time base sweep signal or a vertical amplifier signal (same difference) goes from the plugin in the B Horizontal Slot to the Horizontal Amplifier.

<SNIP>
The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it.
SOLUTION
This may be the problem causing the signal to be missing from the B Horizontal slot. Be very careful you do not break the shaft of this pot by trying to turn it if it is seized up! Do not turn it anymore until you are able to lubricate it. What has probably happened is some old lubricant has dried out between the shaft and the walls of the bushing it goes through.

This is how to free up the intensity pot: Unplug the scope and rest it on its rear panel so the front of the scope faces the ceiling. Remove the B intensity knob with a hex wrench. Carefully apply a few drops of contact cleaner onto the shaft of the pot. Gravity will cause the contact cleaner to drip down the shaft into the bushing and into the pot. Wait a few minutes and reapply a few more drops down the shaft. It should soon be free enough that you may be able to turn the shaft of the pot with your fingers. If not, apply a few more drops, wait a few more minutes then put the knob back on and tighten it. Then gently try turning the knob. It should be a lot less difficult to turn once the contact cleaner has dissolved the old lubricant. Reapply contact cleaner if necessary then try turning with the knob until it starts to move freely. Eventually the pot will turn freely. At that point you should rotate it back and forth through its full range to be sure it is free everywhere.

Dennis Tillman W7PF

-----Original Message-----
From: Hermansson Amplification
Sent: Monday, November 18, 2019 4:48 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B

Thank you all for your responses!

I will look into the manual and read about the routing path!

To answer some of your questions:
Yes, the trigger light lights up on the timebase when I apply signal to the vertical input. And you can see the vertical signal on the upper trace when I press beam finder.

I’ve tried to put a vertical amplifier into the horizontal input B but I can only see the read out volt/div on the display.

The intensity B knob is very very stiff! I needed a pliers to move it, but it affects the brightness of the beam finder dots when I turn it. And I guess I wouldn’t see the dots when I press beam finder either if the potentiometer was the problem?

I believe this scope has been stored for a long time but it seems to be in very good shape for its age. It was inspected by Tektronix in Sweden in 1972 so it’s an early unit.

Best regards,
Henric
--
Dennis Tillman W7PF
TekScopes Moderator




--
Dennis Tillman W7PF
TekScopes Moderator

 


He says everything inside the CRT is either spot welded or fused glass so
there is nothing that can possibly become loose.
Thanks, Dennis. That is what I assumed. Rather, I was thinking about phosphor particles, possibly.


Wouldn't we have found this out with all the 475, 476, 485, etc scopes that have the crt face up?
That makes sense, Carl. I remember now that I read it in relation to transporting older mainframes, especially. The 465-475-485-etc. 'scopes aren't exactly known for this kind of problem.
Probably a red herring.

Raymond

 

Hi Raymond,
I forgot to mention that my friend said the phosphor is inert. You can burn it by leaving a very bright beam in one spot but it won't come off.

Dennis Tillman W7PF

-----Original Message-----
From: Raymond Domp Frank
Sent: Tuesday, November 19, 2019 5:41 PM
Subject: Re: [TekScopes] Tektronix 7904, no horizontal trace on input B


He says everything inside the CRT is either spot welded or fused glass
so there is nothing that can possibly become loose.
Thanks, Dennis. That is what I assumed. Rather, I was thinking about phosphor particles, possibly.


Wouldn't we have found this out with all the 475, 476, 485, etc scopes that have the crt face up?
That makes sense, Carl. I remember now that I read it in relation to transporting older mainframes, especially. The 465-475-485-etc. 'scopes aren't exactly known for this kind of problem.
Probably a red herring.

Raymond





--
Dennis Tillman W7PF
TekScopes Moderator

 

On Wed, Nov 20, 2019 at 06:07 AM, Dennis Tillman W7PF wrote:


I forgot to mention that my friend said the phosphor is inert. You can burn it
by leaving a very bright beam in one spot but it won't come off.
There goes another fairy tale. Thanks Dennis!

Raymond

Hermansson Amplification
 

Hi Kyle!
Very happy to see you stepped in just to say hello!
Have a nice day and keep up the good work! :)

All the best,
Henric

Hermansson Amplification
 

Big thanks for your replies and for sharing your knowledge, very appreciated!

The B intensity is lubricated and works fine!

I had a look at the theory of operation. I guess I should replace U305 as a start? How about Q275 and Q367? Could these two transistors cause this problem as well?

Here are some pics showing the block diagram and schematics:

https://www.forumbilder.com/images/2019/11/20/1D80462C-944C-4038-89A6-D3350896ED16.jpg

https://www.forumbilder.com/images/2019/11/20/FB51B4EF-20FD-45CA-A8C2-440374E88ACC.jpg

https://www.forumbilder.com/images/2019/11/20/B7C9A350-D3DD-4F35-9145-32130D4D4C10.jpg

https://www.forumbilder.com/images/2019/11/20/C9F67AAE-ED0A-4B25-9F78-58882E59B65A.jpg

Best regards,
Henric