Date   

Re: tektronix 7S14 batteries and time base question

Miguel Work
 


Re: Comments from Peter Keller on the 500 Series Scopes

 

My very first Tek was a 524AD, but it was BIG and heavy. I got it while I was in high school, through working weekends at a surplus store. For me, I found the 535/545 (and 524AD) form factor a bit too big, and eventually preferred the far inferior 10Mhz 561B/564B 500 frames with rectangular CRTs. they are much smaller and lighter, and most of my work easily fit in the 10MHz bandwidth. AND, they look so great with that rectangular bezel!

I still have the 561B and 564B and some 2/3 plug-ins just because I like how they look, even though I have all kinds of much newer Tek scope gear, even TM500 SC504's. Some stuff is just too good to give up.

all the best & merry christmas to everyone!
-walter (walter2 -at- sphere.bc.ca)
sphere research corp.


Former Tek workers project maybe

Bill Higdon
 

From some of what has been posted recently about Tek employees home projects, I think this is another one
https://portland.craigslist.org/wsc/ele/d/banks-vintage-arr-15-military-receiver/7246613294.html

Bill


Re: Looking for pictures of 067-0865-00 calibration fixture for A/P6303 current clamp

 

Jared,

I have a Tek current cal fixture on the stuff season page, see:
https://www.sphere.bc.ca/test/stuffday.html#tekfans

that one is Tektronix 015-0601-50 1-Turn 50 Ohm Current Loop.
Not complex, as Dennis explained, but if you want a Tek one there it is.
Keep in mind, the current through these fixtures is limited by the power dissipation
of the 50 ohm resistor. For higher currents, consider a technique that Fluke uses to
do its clamp on ammeters, use MULTIPLE TURNS on the loop. I have one with 50
turns I use to get up to high currents. It is quite effective, but at low frequencies.

all the best to everybody for Christmas & New Years!
Walter (walter2 -at- sphere.bc.ca)
Sphere Research Corp.


Re: Comments from Peter Keller on the 500 Series Scopes

John Williams
 

I am glad to see that from all the responses the 500 series is still alive and well. I would like to say that these hobbies of ours, be it test gear, film cameras, cars or whatever has been a great defence against the virus. I have found that there is always something to work on, or something else to buy on eBay. Or some reading like Peter’s book or Tekwiki. I see all these people lined up in airports today and I think “what the hell.” I need to replace one of the selenium stacks in one of my 551 power supplies so maybe I will work on that today. Or maybe I will watch another episode of Selling Sunset. Haha. Happy Christmas Eve and to all a good night.


Re: tektronix 7S14 batteries and time base question

Tom Lee
 

I second Dennis's recommendation to go with Ed Breya's brilliant LED conversion method. Don't bother wasting time in search of replacement cells. Many before you have looked far and wide. Suitable substitutes simply do not exist any longer.

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/24/2020 12:24, Dennis Tillman W7pF wrote:
Hi Rick,
How wonderful those Mercury cells were!!!
Congratulations on snagging a 5S14!!! You are going to have a ball with it. The 7S14 and 5S14 plugins are fabulous!!! Those two plugins are truly amazing and completely unrecognized for the brilliant engineering that went into them. In an instant they can turn a simple 5000 series scope capable of only a few MHz into a 1GHz Dual Trace, Dual Time Base scope capable of rock solid triggering with XT as well as XY capability.

I did an extensive search for replacement batteries about a year ago thinking it might be possible to find ones that would last long enough to be worth adding battery holders so they could be easily replaced as needed. I was very disappointed by what I found. I will be the first to admit I may have missed some possibilities so maybe you can find something I missed.
What I found:
1) Either manufacturers no longer bother to make long life cells because there is no money in manufacturing something that may never need to be replaced, or none of the current battery technologies have leakage currents low enough to prevent them from self-discharging within a few years.
2) The relatively short lifetime of any current replacement is a major problem especially when you consider how annoying it will be to remove all the screws to get to the batteries every few years.
Other factors to consider with using cells:
3) Finding cells with the right voltage that have a stable (flat) output voltage over the cells lifetime.
4) Finding cells that will fit the tight space available.
5) Figuring out how to insulate the cell holder so it doesn't touch any nearby parts of the circuit but allows for easy cell replacement every few years.

I decided that replacing the mercury cells was impractical and the only solution was LEDs. I was looking for a small enough combination of LEDs to fit in the available space (I was going to try surface mount parts next) when other responsibilities got in the way. It is something I need to get back to.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Rick
Sent: Thursday, December 24, 2020 10:10 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] tektronix 7S14 batteries and time base question

Love this thread. I am super excited for the delivery of a 5S14N this coming Monday. I assume the batteries will be too low. I'm torn between replacing the batteries and adding holders or go the LED route.

Rick







Re: tektronix 7S14 batteries and time base question

 

On Thu, Dec 24, 2020 at 09:41 PM, Raymond Domp Frank wrote:


You may remember that I used ordinary leds, sets of two each mounted head to
head, in the available space without problems.
See

https://groups.io/g/TekScopes/photo/13004/16?p=Name,,7s14+led,50,1,0,0

Others have used several diodes in each "optocoupler". I used one transmitting and one receiving clear narrow-beam led and aligned them by watching the DC voltage across the receiver generated on my 'scope. I fixed them head-to-head with a tiny drop of cyanoacrylate glue.

Raymond


Re: tektronix 7S14 batteries and time base question

 

On Thu, Dec 24, 2020 at 09:24 PM, Dennis Tillman W7pF wrote:


I decided that replacing the mercury cells was impractical and the only
solution was LEDs. I was looking for a small enough combination of LEDs to fit
in the available space (I was going to try surface mount parts next) when
other responsibilities got in the way. It is something I need to get back to.
Hi Dennis,
You may remember that I used ordinary leds, sets of two each mounted head to head, in the available space without problems. Still works fine. At your request, I sent you a hand drawing a few years ago.

Raymond


Re: Comments from Peter Keller on the 500 Series Scopes

Jokken Feldhaar
 

Hi all,

I own a small herd of 5xx scopes, starting with a 567, more than 30
years ago. To me, one of the most fascinating reads on this planet is
the patent bunch on the 54x and 58x series, the name John Kobbe stands
out to me. Reading his description of the vertical chain amplifier and
the termination was like reading a thriller by Grisham, likewise. I
admire the knowledge and superb engineering that has gone into these
scopes. If I have a question about these scopes, this group is my
resource for maintaining these fine instruments!

Cheers, Jochen DH6FAZ

Am 23.12.2020 um 20:47 schrieb Dennis Tillman W7pF:

Since Sunday when I picked up 106 of Peter Keller's books we have spoken by
phone. During our conversation Peter mentioned a few things I thought I
should pass along since I thought he may have reached the wrong conclusion
from his brief time as a member of TekScopes.

Peter was disappointed that there didn't seem to be much interest in the 500
series of oscilloscopes. I thought it might be nice if Peter heard from how
many TekScopes members have 500 series scopes and what you like most about
your favorite model.

* Peter and I both agree that the pinnacle of the 500 series was the 547. I
was in love with the first one I got to use in 1967 shortly after it was
introduced. That was a fabulous scope!
* We both agree the 500 series was known worldwide for the incredibly tiny
CRT spot size.
* Peter felt the 500 series was the last scope anyone could fix since it
didn't use many specialized parts. I disagree about this because I have
spent years troubleshooting and repairing 7000 series plugins and
mainframes. Tektronix' fabulous documentation makes it just as easy to fix
these 7000 scopes as it does to fix the 500 series but that it just my
opinion.

From the volume of posts we receive it may appear that there is more
interest in the some of the portables but that is because they are so often
broken, replacement parts are impossible to find, they use a lot of surface
mount and custom parts, and Tek stopped providing Circuit Descriptions,
Parts Lists, and schematics to assist in calibrating and repairing them.

Dennis Tillman W7pF




Re: tektronix 7S14 batteries and time base question

 

Hi Rick,
How wonderful those Mercury cells were!!!
Congratulations on snagging a 5S14!!! You are going to have a ball with it. The 7S14 and 5S14 plugins are fabulous!!! Those two plugins are truly amazing and completely unrecognized for the brilliant engineering that went into them. In an instant they can turn a simple 5000 series scope capable of only a few MHz into a 1GHz Dual Trace, Dual Time Base scope capable of rock solid triggering with XT as well as XY capability.

I did an extensive search for replacement batteries about a year ago thinking it might be possible to find ones that would last long enough to be worth adding battery holders so they could be easily replaced as needed. I was very disappointed by what I found. I will be the first to admit I may have missed some possibilities so maybe you can find something I missed.
What I found:
1) Either manufacturers no longer bother to make long life cells because there is no money in manufacturing something that may never need to be replaced, or none of the current battery technologies have leakage currents low enough to prevent them from self-discharging within a few years.
2) The relatively short lifetime of any current replacement is a major problem especially when you consider how annoying it will be to remove all the screws to get to the batteries every few years.
Other factors to consider with using cells:
3) Finding cells with the right voltage that have a stable (flat) output voltage over the cells lifetime.
4) Finding cells that will fit the tight space available.
5) Figuring out how to insulate the cell holder so it doesn't touch any nearby parts of the circuit but allows for easy cell replacement every few years.

I decided that replacing the mercury cells was impractical and the only solution was LEDs. I was looking for a small enough combination of LEDs to fit in the available space (I was going to try surface mount parts next) when other responsibilities got in the way. It is something I need to get back to.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Rick
Sent: Thursday, December 24, 2020 10:10 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] tektronix 7S14 batteries and time base question

Love this thread. I am super excited for the delivery of a 5S14N this coming Monday. I assume the batteries will be too low. I'm torn between replacing the batteries and adding holders or go the LED route.

Rick







--
Dennis Tillman W7pF
TekScopes Moderator


Re: Looking for pictures of 067-0865-00 calibration fixture for A/P6303 current clamp

 

Hi Jared,
The calibrator for this current probe (and all of the most common Tek current probes) is simple to make. The three things you need to make this are a piece of stiff wire about 10" (25cm) long, a BNC connector, and a 49.9ohm 1% 1/4W or 1/2W resistor. It should take no more than an hour to assemble it.

It is driven by the 4.0Vp-p 1KHz square wave calibrator output of any Tek scope. Most Tek calibrator outputs have a BNC but some of the 7000 scopes have small pin jacks to save space. So that is your signal source.

To convert the calibrator's square wave voltage into a current you must use a 50ohm resistor (49.9 ohm 1% 1/4W or 1/2W is the closest value so that is what Tek uses). The current loop itself is nothing more than a stiff (bus-bar) wire bent into a square that is long enough on two of the 4 sides to go through the current probe jaws. I would guess a 2" by 2" square current loop will work just fine. Leave some extra length at each end to connect the bus bar to the resistor and the ground side of the BNC.

One end of the square bus-bar is connected (soldered) to the ground side of the BNC. The other end of the bus-bar goes to the 49.9 ohm resistor. The other side of the 49.9 ohm resistor is connected (soldered) to the center pin of the BNC which will be plugged in to the 4V calibrator output of your scope.

In the specifications section of each scope it indicates the current output of the scope's calibrator when a 50ohm current loop like I just described is connected to it. The current for a 7000 scope is specified as a 40mA 1KHz. square wave when this current loop is connected to the calibrator's 4V output.

The rest of the mechanical parts in the drawing/photo for the A/P6303 calibrator are not important and do not affect the calibration current for your probe.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jared Cabot via groups.io
Sent: Thursday, December 24, 2020 5:32 AM
To: TekScopes@groups.io
Subject: [TekScopes] Looking for pictures of 067-0865-00 calibration fixture for A/P6303 current clamp

Hi all,

I recently picked up an A6303 (P6303) current clamp and I'd like to construct a calibration fixture to enable me to check and adjust it etc.
It looks just like an aluminium shroud with a hex standoff as a current path through the current clamp jaws, the shroud is connected to the shield on the BNC, the hex stand off connected to the center pin of the BNC via a 50ohm resistor on one side and shorted to the shroud on the other (To form a 50ohm terminated shorting loop).
I found a data sheet for it on the Tek Wiki website, and a picture of the side of it via Google, but I'm hoping someone who might have one can take a few shots from different angles with a ruler for size comparison.
Here is the one photo I have found online so far:
https://i.imgur.com/8zoJDiw.jpg

I'm pretty sure I know how it is constructed, but if I can get a few photos, I'll be able to see for sure.

Thanks!
Thanks!







--
Dennis Tillman W7pF
TekScopes Moderator


Re: Tek 465 Negative 8 volt rail issues

Dave Peterson
 

Thank you everyone for the pointers. I will follow those. I think I need to invest in an ESR meter. If this is going to be a problem I'm going to regularly encounter, and I think it's likely if I continue to buy and fix 465s (and I will), then it's worth the expenditure.

For now, it's time to go bake Christmas cookies.

Happy Holidays!
Dave

On Thursday, December 24, 2020, 11:36:12 AM PST, Ed Pavlovic <kc9mmm@comcast.net> wrote:

Dave,
I have current production caps that I just bought from Mouser.  I specifically looked for smaller diameter caps so I wouldn’t have clearance issues between the caps, but a few I just couldn’t get as small as I wanted but they should work.

I did buy the adapters on eBay to get the same footprint as the original ones, I may use a small washer on the pins to make soldering then to the board easier.  Not really looking forward to unsoldering the old cans.

I can post a list of what I bought, but I want to make sure everything fits before I do.

Ed


Re: DM44 on a Tek 475B - what's the externally accessible adjustment?

Tom Lee
 

"Too many scopes"?

No such thing.

Happy Holidays,
One of the other Toms

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/24/2020 11:39, Tom Gardner wrote:

I didn't get an answer, and no longer have a 475A. I do, of course, have too many scopes.





Re: DM44 on a Tek 475B - what's the externally accessible adjustment?

Tom Gardner
 

On 24/12/20 19:31, Paul (AE4PN) wrote:
Hey, Tom, did you ever get an answer? I have a 475A with DM44 scope like that. It has what is known as "Mod PG" which was a mod developed for a big customer named IBM! Such scopes have some changes, most visible being the small toggle switch just to the right of the Delta TIME knob on the DM44 unit. And yes, in the DM44 there is the small circuit board add-on you have found. One of the purposes of the PG mod is to switch-enable the DM44's Delta TIME control to either track the 475A (or other scope) Delay Time Position (DTP) control or to operate independently of the DTP. In a non-PG DM44, the default mode is "Tracking" but one can change to "Independent" mode by changing the plug-in location of a 3-wire cable on the DM44's main board as well as position a jumper plug to an alternate position. I have a bit more info I can share. Also, what have you discovered? Anyone out there have a circuit diagram for "Mod PG"? Has anyone been successful in troubleshooting a DM44 with Mod PG?
I didn't get an answer, and no longer have a 475A. I do, of course, have too many scopes.


Re: Tek 465 Negative 8 volt rail issues

Ed Pavlovic
 

Dave,
I have current production caps that I just bought from Mouser. I specifically looked for smaller diameter caps so I wouldn’t have clearance issues between the caps, but a few I just couldn’t get as small as I wanted but they should work.

I did buy the adapters on eBay to get the same footprint as the original ones, I may use a small washer on the pins to make soldering then to the board easier. Not really looking forward to unsoldering the old cans.

I can post a list of what I bought, but I want to make sure everything fits before I do.

Ed


Re: Tek 465 Negative 8 volt rail issues

Paul Amaranth
 

If you search on ebay for "Tektronix capacitor adapter" you'll get a few hits.

Someone posted gerbers on eevblog if you want to send them out yourself. I
could probably find the link to that. Actually, here's one:

https://www.eevblog.com/forum/repair/making-dave_k_s-capacitor-adapters-at-osh-park/msg1218875/#msg1218875

I made up a pattern in a drawing program and etched some using
pnp-blue. That worked fine, but I just buy them now. A number of
people have independantly come up with this solution, a sure sign
it's a good idea :-)

You could likely use the gerbers if you had access to a pcb milling setup.

"Wholesale replacement of caps" in this instance only refers to the
metal can filter caps. Other than the odd shorted tant I've never had
to replace any other cap and, in general, if it ain't broke don't make
it so :-)

It's possible the other cans could be good for years. To accurately
check them though you'd need to pull them, so ...

An ESR meter might give you some good hints without pulling them. The
ESR will go up as the cap dries out so if you're seeing 100 ohms on a
470uf cap, it's a good candidate for replacement.

You could just replace the bad one and you'd be good for a while. When
I've tried that I'm back in there in a year or so anyway. At least
they don't leak all over and destroy the PCB.

Paul

On Thu, Dec 24, 2020 at 07:08:47PM +0000, Dave Peterson via groups.io wrote:
Thanks Paul!

I see I've scored. I'm going to have to pick your brain on this subject. That is, take advantage of the accumulated wealth of knowledge of the community. You guys are great.

I want to keep this conversation on the forum for the time being as the first thing I've been doing so far is searching and review threads with regards to 465 power caps. It's paying big dividends so far. But it is challenging to peruse the vast results. I'll do a search for adapter boards as this is the first I've heard of it, and am kind of liking the idea. The 465 PS Cap Replacement Guide is good, but I'd like to implement it a little neater. That's no dig against the original writer. We're all indebted for the shared experience. So hopefully my experiences and this conversation can contribute to the knowledge base.

I'm a little on the fence about the wholesale replacement of the original caps, but then I'm an ignorant newb. I hear your point about prophylactic replacement. I'll have to dwell on it for my own comfort. Also know that I replaced C1419/C1418 with new Nichicon ULD 60v caps, so I'm not a original preservationist. I guess I want to strike a balance between need and the risks and efforts of wholesale replacement. I suspect the adapter boards are constructed for replacement of all 5 big caps. I do like the assurance that new over-spec'ed caps provide.

As I say, I'll do some research, but if you have particular advice for adapter boards I'd love to hear from you.

Thanks again!
Dave
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@AuroraGrp.Com | Unix/Linux - We don't do windows


Re: DM44 on a Tek 475B - what's the externally accessible adjustment?

Paul (AE4PN)
 

Hey, Tom, did you ever get an answer? I have a 475A with DM44 scope like that. It has what is known as "Mod PG" which was a mod developed for a big customer named IBM! Such scopes have some changes, most visible being the small toggle switch just to the right of the Delta TIME knob on the DM44 unit. And yes, in the DM44 there is the small circuit board add-on you have found. One of the purposes of the PG mod is to switch-enable the DM44's Delta TIME control to either track the 475A (or other scope) Delay Time Position (DTP) control or to operate independently of the DTP. In a non-PG DM44, the default mode is "Tracking" but one can change to "Independent" mode by changing the plug-in location of a 3-wire cable on the DM44's main board as well as position a jumper plug to an alternate position. I have a bit more info I can share. Also, what have you discovered? Anyone out there have a circuit diagram for "Mod PG"? Has anyone been successful in troubleshooting a DM44 with Mod PG?


Re: Tek 465 Negative 8 volt rail issues

Michael W. Lynch
 

Dave,

I have replaced several sets of these filter caps on the 465/68/475 series. Seller has multiple versions for different brands and types of equipment.

I used these:

Capacitor Adapter 15.5mm triangle recapping vintage equipment Tek 465 kit (x5)

eBay item number:
273254508468

Seller information:
cuog (749 )
100% Positive feedback

I believe that these are the ones that Paul is recommending in his earlier reply.

Good Luck!


--
Michael Lynch
Dardanelle, AR


Re: Tek 465 Negative 8 volt rail issues

Dave Peterson
 

Thanks Paul!

I see I've scored. I'm going to have to pick your brain on this subject. That is, take advantage of the accumulated wealth of knowledge of the community. You guys are great.

I want to keep this conversation on the forum for the time being as the first thing I've been doing so far is searching and review threads with regards to 465 power caps. It's paying big dividends so far. But it is challenging to peruse the vast results. I'll do a search for adapter boards as this is the first I've heard of it, and am kind of liking the idea. The 465 PS Cap Replacement Guide is good, but I'd like to implement it a little neater. That's no dig against the original writer. We're all indebted for the shared experience. So hopefully my experiences and this conversation can contribute to the knowledge base.

I'm a little on the fence about the wholesale replacement of the original caps, but then I'm an ignorant newb. I hear your point about prophylactic replacement. I'll have to dwell on it for my own comfort. Also know that I replaced C1419/C1418 with new Nichicon ULD 60v caps, so I'm not a original preservationist. I guess I want to strike a balance between need and the risks and efforts of wholesale replacement. I suspect the adapter boards are constructed for replacement of all 5 big caps. I do like the assurance that new over-spec'ed caps provide.

As I say, I'll do some research, but if you have particular advice for adapter boards I'd love to hear from you.

Thanks again!
Dave

On Thursday, December 24, 2020, 10:41:39 AM PST, Paul Amaranth <paul@auroragrp.com> wrote:

These caps dry out all the time, it's a very common problem.

The orginal cans are no longer available and, if you find some NOS
ones, they will probably dry out shortly if not already.

Get 105C snap caps and use one of the adapter board floating around to
match the footprint to the 4 terminal footprint of the original.  Use
high quality caps by Nichicon, Panasonic, etc.  Avoid cheap noname or
offbrand Chinese caps.

All 3 ground leads need to be jumpered since Tek used the can as a
jumper and the adapter boards take care of that for you. The adapter
boards make for a very professional repair.  I hate seeing caps hanging
off the board hot glued to something.

If I have to replace one, I generally replace all of them.  Then you
won't have to worry about it for 20 years.

By using very long leads on the adapter boards I have replaced the
caps in situ without pulling the main board by threading them down from
the top.

Unsoldering the main caps is fun, not.  It's easy to overheat the board
and have trace separation.  Be careful and use good desoldering tools
and technique for that.  Some chipquick wouldn't hurt.

  Paul

On Thu, Dec 24, 2020 at 05:59:34PM +0000, Dave Peterson via groups.io wrote:
  Ed, how serendipitous.

I too am looking at a power supply cap issue. Haven't nailed it down yet, but similar line frequency "ripple". In my case the "ripple" is more like unregulated/unfiltered full-wave rectifier output on the 120v and/or 55v rectifier output - there's no significant capacitance. Question is, which cap.

What caught my eye here was "I have the caps on hand". Do you mean you have a stock of original 465 PS caps? Or do you mean you have a supply of modern caps?

I've looked at the "465 Power Supply Capacitor Replacement Guide", and am feeling a little more confident about attacking these caps. But I'm still on the fence about what route I would want to go. I'm very much the "ain't broke, don't fix" kind of guy, and I'll say I'm 51% biased towards keeping things as original as possible.

Curious to know what you meant by on hand, and if you have suggestions for sources.

Dave


  On Thursday, December 24, 2020, 09:11:31 AM PST, Ed Pavlovic <kc9mmm@comcast.net> wrote:

  I recently picked up a used Tek 465 scope for a price I liked and could pick
it up without having is shipped, and I'm getting started on checking it
over.  Everything works, although both traces even when grounded look like
there's an uncompensated probe attached.  But I figured I would look at the
low voltage power rails before I try to track down that issue.  Early 465,
with a B010XXX serial number, needs some cleaning and TLC.



All the test points for the rails except the -8 volt are within spec which
was low, so I hooked up a probe to the test point.  I'm getting a 1.75 volt
ripple at a frequency of 120 Hz, which makes me suspect the filter cap is
bad.  I have the caps on hand to replace all of those power supply caps on
all the rails, but before I go and do that, I just figured I would ask if I
should look at anything else on this power rail before I change the filter
cap so I don't kill it right away.  There's also a 33uf tantalum cap on that
rail to ground that I will replace as well as the service manual calls it
out as a 10 volt rated cap, which isn't much to spare on 8 volts.

I uploaded a photo to a folder I created of a shot of the ripple on the -8
volt rail here:



https://groups.io/g/TekScopes/photo/258534/0?p=Created,,,20,2,0,0



If the link doesn't work the name of the folder is "Tek 465 Negative 8 volt
rail" and there's only one picture in there.



Ed Pavlovic

KC9MMM













!DSPAM:5fe4d72d33731209017941!
--
Paul Amaranth, GCIH            | Manchester MI, USA
Aurora Group of Michigan, LLC  |  Security, Systems & Software
paul@AuroraGrp.Com              |  Unix/Linux - We don't do windows


Mechanism of CRT Double Peaking (UPDATED)

 

Peter Keller just provided me with a revision to the 4th paragraph of my original explanation for why the brightness of a well-used CRT first brightens then dims before finally brightening again as the intensity control is increased (known as "Double Peaking"). Here is the updated explanation.
-----------
Under ordinary circumstances a cloud of electrons boil away from the surface of the cathode as a result of the heater heating it to incandescence in a vacuum. This is sometimes called the Edison Effect because he noticed it first in light bulb filaments. It is more correctly called Thermionic Emission. This cloud forms around the cathode and is known as the space charge region.

The cathode is intentionally coated with a material that lowers the work function. The work function determines how much energy will be required for an electron to leave the surface of the cathode and join the cloud of electrons surrounding the cathode. A low work function means more emission.

In ordinary circumstances the space charge region is the source of the electron beam, not the cathode. The cathode is "filling" the space charge region with electrons that will be attracted to a positively charged surface and ultimately focused along the way into a tight beam.

As the cathode ages the chemical coating used to lower the work function wears out. The space charge cloud can no longer provide enough electrons to form a bright beam. After depletion of the space cloud, the current is being drawn from the damaged center area of the cathode coating which decreases available current. Further increasing the intensity control (lowering the negative grid bias) begins to draw current from the undamaged periphery of the cathode and increases beam current again but with a resulting larger spot size.
-----------
Dennis Tillman W7pF

-----Original Message-----
Sent: Saturday, February 23, 2019 9:28 AM
Subject: RE: [TekScopes] Mechanism of CRT Double Peaking

Hi Fabio,
Your explanation is plausible except for one more bit of vacuum tube physics that has to be taken into account: Under ordinary circumstances a cloud of electrons boil away from the surface of the cathode as a result of the heater heating it to incandescence in a vacuum. This is sometimes called the Edison Effect because he noticed it first in light bulb filaments. It is more correctly called Thermionic Emission. This cloud forms around the cathode and is known as the space charge region.
The cathode is intentionally coated with a material that lowers the work function. The work function determines how much energy will be required for an electron to leave the surface of the cathode and join the cloud of electrons surrounding the cathode. A low work function means more emission.
In ordinary circumstances the space charge region is the source of the electron beam, not the cathode. The cathode is "filling" the space charge region with electrons that will be attracted to a positively charged surface and ultimately focused along the way into a tight beam.
As the cathode ages the chemical coating used to lower the work function wears out. The space charge cloud can no longer provide enough electrons to form a bright beam. The user turns the brightness up to compensate. This increases the positive voltage attracting the electrons. Eventually the user increases this positive voltage to the point where the space charge region is completely depleted and electrons are being stripped directly off the cathode increasing the brightness temporarily. This is not good.
There is a way to extend the life of the cathode in a CRT as most people who worked in a TV repair store are familiar with it. I recall it as "rejuvenation". It is a simple process. If you increase the voltage going to the heater this will heat the cathode to a very high temperature which will burn off the weakly emitting surface of the cathode exposing fresh chemical coating from underneath. There is the risk of burning out the filament that must be considered when you do this. There is another variation of this which boosted the filament voltage on a permanent basis. This allowed customers to temporarily get more life out of a weak filament. Since filament life decreases rapidly with increasing voltage this won't postpone the need for a new CRT for very long. As you mentioned Ed Breya has some experience with this process so maybe he can explain it in more detail.

For more on the subject of Space Charge you can go to:
https://en.wikipedia.org/wiki/Space_charge
https://en.wikipedia.org/wiki/Thermionic_emission
https://en.wikipedia.org/wiki/Grid-leak_detector
https://en.wikipedia.org/wiki/Vacuum_tube

Dennis Tillman W7PF

-----Original Message-----
From: Fabio Trevisan
Sent: Tuesday, February 19, 2019 1:12 PM

Hello,
My first Tek, I used to own until a few months ago, had a weak CRT
since day one (when I got it from a local, Brazilian auction site).
One of the first thing that I noticed, besides the low intensity, was
that it had this double-peaking thing (which I was unfamiliar with).
It was when I joined this group and the messages should still be there.
Back then David Hess (where's him? Haven't seen anything from him for
a
while) pointed me to Ed Breya (both are renowned members), as Ed had
some experience in rejuvenating CRTs.
At the same time, I learnt about the extensive information available
on the tekwiki site (google for TekWiki... you'll find it).
At TekWiki, there in the "Manuals, Catalogs and Other Publications",
there's a section called "Concepts Series".
Between the "Concepts Series" publications, the very first one is the
"Oscilloscope Cathode-Ray Tubes, 2nd ed.
LInk here for your convenience:
http://w140.com/tekwiki/images/6/62/062-0852-
01.pdf
Between pages 10 and 14 there's a detailed explanation of how the
electric field lines from the grid interact with that of the cathode,
to create an electrical equivalent to a photo camera's "aperture"
ring... which is actually what broadens or pinches how much of the Cathode surface is "opened"
for emissions.
So,, follows what I conclude from the understanding:
When set to a lower intensity, the cathode effectively emits only from
the center, and as we crank up the intensity, it opens up more and
more area of the cathode to emit the beam. Therefore, the center of
the cathode is the most used portion and is the one that wears out first.
As the CRT ages, the central spot of the cathode wears out and you can
only obtain some intensity as you open the "aperture"more and more,
exposing the next section (an emitting ring now, not anymore a circle).
That concentric wearing "pattern" ends up imprinted on the cathode
and, in my opinion, is what causes the double peaking...
On a worn out cathode, when you start cranking up the brightness,
initially the aperture is drawing electrons from the "worn out" area
which, despite it's worn out, it responds to the intensity
control...(more C.W. more brightness, only that at a lesser degree"...
As you keep opening up the "aperture", you expose more and more of the
outer rings (which are also more worn out than the center, so the
intensity decreases)...
Up until a point that you open the aperture so much that you expose
the outermost ring of the cathode which is still not worn out...
(thus, enters the second "peak").
At this point, although you managed to get more electrons and more
beam intensity, the size of the spot is already too big and the
Focusing anode can no longer focus the beam correctly... (or the Focus
Tracking potentiometer can no longer track correctly the increase of
the intensity control) and ultimately, the Focus and Astigmatism is ruined.

Part of my conclusion can be flawed, but overall, I think this is the
mechanism.

Rgrds,

Fabio



--
Dennis Tillman W7pF
TekScopes Moderator

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