Date   
Re: And you think our collecting hobby is crazy and takes up a lot of space

n4buq
 

According to the article, one of them was given to him. If that's his house in the background, though, that may account for a few things.

Thanks,
Barry - N4BUQ

Hi John,

Thanks for this - it does make us all seem a bit more ‘normal’ doesn’t it? I
just wonder how Michel funds buying jets!

Cheers,
Mel

--
you can never have enough oscilloscopes, DMMs, valve testers or soldering
irons . . .

Re: And you think our collecting hobby is crazy and takes up a lot of space

Vintage Test
 

Hi John,

Thanks for this - it does make us all seem a bit more ‘normal’ doesn’t it? I just wonder how Michel funds buying jets!

Cheers,
Mel

--
you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . .

Re: 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.
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

Re: 7844 power supply

Peter Gottlieb
 

Well, I now highly suspect the shorted winding.  I disconnected the HV winding from circuitry and am getting overcurrent on main transformer drive.  Since I also disconnected the LV rectifiers and am still getting the overcurrent, something in there is shorted.

Unless I get lucky and find a parts 7844 or a presumably unobtanium replacement transformer, this scope seems like it's destined for the heap.

Too bad because it has the beautiful blue phosphor and a really strong CRT with low hours on the scope.

Peter

On 11/18/2019 12:50 AM, Ed Breya via Groups.Io wrote:
The last time I fixed a 7844 PS, it was fortunately just two or so rather large (100 or 220 uF 20V) Ta caps shorted, on the pre-regulator supply board, for the +/- 15V inside the PS module - the one inside the cage, just below the outer, upper, linear regulator board. It's a PITA to get to, but doable - if you take enough stuff apart, you can run it while opened up and make measurements. The fix is fairly straightforward, but there's very little space, so you'll have to get creative with alternative cap arrangements. I manged to do all the work from the component side, avoiding having to remove the board, which is even more a PITA.

If it's not those or something else like another Ta cap in the scope circuits, it could be the dreaded HV winding failure on the main transformer. Hope for the lesser grief.

Ed


.

Re: 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

Re: Tektronix 7904, no horizontal trace on input B

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



Re: Tektronix 7904, no horizontal trace on input B

 

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

Re: Tektronix 7904, no horizontal trace on input B

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

And you think our collecting hobby is crazy and takes up a lot of space

John Williams
 

Re: Tek 576 noise and looping

 

Hi Peter,
That is a clever idea to compare high frequency transistors (low Miller Effect capacitance results in small loops) to low frequency transistors (greater capacitance means larger loops).

Even bigger loops will be found in power transistors because their E, B, and C have to be larger to handle the currents. Their loops will be large provided their current gain (Beta) is good. But as a rule of thumb the gain of a power transistor is optimized to peak at high currents since it will be controlling high power. At very low currents a power transistor's current gain may not be very much resulting in a relatively small loop even though it has a large capacitance between base and collector.

The size of the loop is directly proportional to the capacitance between base and collector and to the Beta (current gain) of the transistor. So a transistor with low gain will have a smaller loop than a transistor with very high Beta all things being equal.

Dennis Tillman W7PF

-----Original Message-----
From: peter bunge
Sent: Friday, November 15, 2019 12:27 PM
Subject: Re: [TekScopes] Tek 576 noise and looping

<SNIP>
Thanks for your explanation which I tested by comparing curves with slow and fast transistors. I had some trouble trying to find a couple with matching gain but the high freq (900 MHz) transistor performed perfectly with nice curves at low current. On the other hand the slow transistor had huge loops.




--
Dennis Tillman W7PF
TekScopes Moderator

Re: 465B soft start?

Tom Gardner
 

On 18/11/19 22:58, Raymond Domp Frank wrote:
On Mon, Nov 18, 2019 at 10:01 PM, Tom Gardner wrote:

Switching power supplies operate to keep the output power constant.
Hence if the input voltage is lower, the input current must be higher.

Increased current stresses components such as the rectifiers and
switching transistors.
The 465B has a linear power supply.
Doh! Too much time spent inside a 2465 :(

Apologies.

Re: 465B soft start?

 

On Mon, Nov 18, 2019 at 10:01 PM, Tom Gardner wrote:


Switching power supplies operate to keep the output power constant.
Hence if the input voltage is lower, the input current must be higher.

Increased current stresses components such as the rectifiers and
switching transistors.
The 465B has a linear power supply.
Low-spec'ed (if so) working voltage hasn't affected those caps for the past 35-odd years. Also, those are (were?) high-quality caps so I wouldn't bother. Epoxy-encapsulated (i.e. dipped) tantalum caps are another matter, especially those with little voltage headroom. The 465B is stuffed with those but slow-on won't help those much I'm afraid. Lots about all that to be found in this group.

Raymond
Power-on current surge puts a strain on the bridge rectifiers. That has been an issue with some 465's.

Re: TDS3012B NVRAM

 

On Mon, Nov 18, 2019 at 11:45 PM, Raymond Domp Frank wrote:


You'll find what you're looking for in my message #157465 in thread " Clock
error on TDS3034", including link to pictures of the Dallas Module.
That's if you're prepared to open the module.

Raymond

Re: TDS3012B NVRAM

 

Hi Ferenc,
You'll find what you're looking for in my message #157465 in thread " Clock error on TDS3034", including link to pictures of the Dallas Module.

Raymond

Re: Tektronix 7904, no horizontal trace on input B

 

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

Re: Tektronix 7904, no horizontal trace on input B

 

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

Raymond

Re: Tektronix 7904, no horizontal trace on input B

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


Re: Tektronix 7904, no horizontal trace on input B

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


Re: 465B soft start?

Tom Gardner
 

Switching power supplies operate to keep the output power constant.
Hence if the input voltage is lower, the input current must be higher.

Increased current stresses components such as the rectifiers and
switching transistors.

On 18/11/2019, treasurer@... <treasurer@...> wrote:
Is there much to be gained by powering up my 465B with a variac, lamp dimmer
or similar arrangement? It popped (shorted) its 1200 ufd C4429 at power on
recently. This cap does not have much head room with its 100 volt rating and
70 volts sitting on it. I'm thinking a soft power up may help prolong the
life of those caps. Any thoughts or past experience from the group using
this power-on approach?
Bruce



465B soft start?

treasurer@...
 

Is there much to be gained by powering up my 465B with a variac, lamp dimmer or similar arrangement? It popped (shorted) its 1200 ufd C4429 at power on recently. This cap does not have much head room with its 100 volt rating and 70 volts sitting on it. I'm thinking a soft power up may help prolong the life of those caps. Any thoughts or past experience from the group using this power-on approach?
Bruce