Topics

Focus on 475 scope


agaguk_99@...
 
Edited

Hi everyone, i have 2. scope 475. I have recaped both and now full working. But the focus on the second one isn't sharp like the first one. I checked high voltage, 2.72kv for the first and 2.6k for the second. Focus and asting are working but i cant adjust the line more than that. Someone know what could be the problem ?

Thank you :)

Look picture here
https://1drv.ms/f/s!AnKLPDy3pII_gkwm-YPX7azSL7Y2


Velik Kazakov
 

Try to check for bad resistors in the focus line. When repair a scope with bad focus, always fond bad resistors.


agaguk_99@...
 

Thank you for your answer. So far my resistors looks good. Strange thing is, i dont have the same values of resistor on both scope. I thing i have a 475a board model vs 475 if i trust the schematic. the 475a board have the nicer display.
Also I dont know if the old capacitors .0012 4000v could be leaky ? Those one are on the focus circuit too.
Also as per manual. We must have -2450 +-49v On my side on both scope i have -2720v and -2600v. Look a bit high to me. But manual dont tell anywhere how to adjust this.


Chuck Harris
 

Perhaps you are unaware that the 475's CRT has a
mesh dome lens that magnifies both the beam diameter,
and the deflection angle?

This lens was made necessary by the relative low
voltage capability of the high bandwidth transistorized
deflection amplifiers this portable scope required.

Because there is no such thing as a free lunch,
the lens also magnifies the high bandwidth vertical
amplifier's noise, and as such makes the beam larger,
and appear fuzzy.... as if it were out of focus.

This characteristic of the 475 is why I don't have one
for my test bench.

-Chuck Harris

agaguk_99@hotmail.com wrote:

Thank you for your answer. So far my resistors looks good. Strange thing is, i dont have the same values of resistor on both scope. I thing i have a 475a board model vs 475 if i trust the schematic. the 475a board have the nicer display.
Also I dont know if the old capacitors .0012 4000v could be leaky ? Those one are on the focus circuit too.
Also as per manual. We must have -2450 +-49v On my side on both scope i have -2720v and -2600v. Look a bit high to me. But manual dont tell anywhere how to adjust this.


Andre de guerin
 

Yes, check the caps!
From: "agaguk_99@hotmail.com" <agaguk_99@hotmail.com>
To: TekScopes@groups.io
Sent: Thursday, 25 January 2018, 17:14
Subject: Re: [TekScopes] Focus on 475 scope

Thank you for your answer. So far my resistors looks good. Strange thing is, i dont have the same values of resistor on both scope. I thing i have a 475a board model vs 475 if i trust the schematic. the 475a board have the nicer display.
Also I dont know if the old capacitors .0012 4000v could be leaky ? Those one are on the focus circuit too.
Also as per manual. We must have -2450 +-49v    On my side on both scope i have -2720v and -2600v. Look a bit high to me. But manual dont tell anywhere how to adjust this.






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Ed Breya
 

The mesh itself also makes the beam fuzzier - even it is perfect on the way in. Like looking through a screen door. Ed


agaguk_99@...
 
Edited

Is this mesh dome inside ? Maybe i could swap the Crt's and see if problem follow the crt. And also swap the caps... i'll let you know


Chuck Harris
 

The mesh dome is generic, one is as good as the next.
It does not wear out.

The 475 does not have a nice sharp beam. It is inherently
about 1.5 minor divisions wide.

It is at its absolute best when the bandwidth limiter is
turned on to its lowest bandwidth setting.

If the glass screen, or the polycarbonate blast shield,
is dirty, the focus can appear even less sharp than
normal.

The 475A is a little better, as I recall.

-Chuck Harris

agaguk_99@hotmail.com wrote:

Is this mesh dome inside ? Maybe i could swap the Crt's and see if problem follow the crt.


Chuck Harris
 

I'm curious, why are you fixating on the caps?

I have never seen one of the HV caps go bad in a
465 or 475 scope... Triplers, yes, but caps no.

The best, easiest way to see if everything is ok
in the focus circuitry is to notice if the focus
pot passes through a single point of best focus.
By that I mean if the pot is fully CCW, the beam
is fuzzy, and as you turn it CW, it gets less and
less fuzzy, until it reaches a point where it
starts to get more and more fuzzy again.

If that happens, you are as good as you are going
to get with the focus circuitry.

The same should be the case with the astigmatism
control.

If either control doesn't reach a point of best
focus before it runs out of travel, then you have
a problem with resistor tolerance in the focus
divider chain, or the astigmatism divider chain.

Or, possibly an HV supply is out of wack.

If the intensity control "double peaks", then your
problem is a gassy CRT, which will also result in
poor focus/astigmatism.

-Chuck Harris

agaguk_99@hotmail.com wrote:

[Edited Message Follows]

Is this mesh dome inside ? Maybe i could swap the Crt's and see if problem follow the crt. And also swap the caps... i'll let you know




Tom Gardner
 

On 26/01/18 02:09, agaguk_99@hotmail.com wrote:
Is this mesh dome inside ?
Yes. See a disassembled 465 tube at
https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/


agaguk_99@...
 

On Thu, Jan 25, 2018 at 09:38 pm, Chuck Harris wrote:
I'm curious, why are you fixating on the caps?
I know that ceramic capacitor are aging very well. But those 4000v cap wasn't looks like caramic to me so i asked the question. And someone tell me yes check the caps. So i wanted to check those cap :)

The best, easiest way to see if everything is ok
in the focus circuitry is to notice if the focus
pot passes through a single point of best focus.
By that I mean if the pot is fully CCW, the beam
is fuzzy, and as you turn it CW, it gets less and
less fuzzy, until it reaches a point where it
starts to get more and more fuzzy again.
If that happens, you are as good as you are going
to get with the focus circuitry.
Ok,i thing my scope is full working so.


The same should be the case with the astigmatism
control.
yes sameting, working...



If the intensity control "double peaks", then your
problem is a gassy CRT, which will also result in
poor focus/astigmatism.

-Chuck Harris
Thank you chuck, i didn't know that the crt was a problem on that scope. Probably my scope is just working very fine and problem is a bad crt. Thank you all for your answer.


Chuck Harris
 

If your scope intensity double peaks, it could mean
that it is gassy, or it could mean that it has low
emission. Running the CRT will help get the gas
absorbed by the getter, so it may improve.

-Chuck Harris

agaguk_99@hotmail.com wrote:

On Thu, Jan 25, 2018 at 09:38 pm, Chuck Harris wrote:
I'm curious, why are you fixating on the caps?
I know that ceramic capacitor are aging very well. But those 4000v cap wasn't looks like caramic to me so i asked the question. And someone tell me yes check the caps. So i wanted to check those cap :)

The best, easiest way to see if everything is ok
in the focus circuitry is to notice if the focus
pot passes through a single point of best focus.
By that I mean if the pot is fully CCW, the beam
is fuzzy, and as you turn it CW, it gets less and
less fuzzy, until it reaches a point where it
starts to get more and more fuzzy again.
If that happens, you are as good as you are going
to get with the focus circuitry.
Ok,i thing my scope is full working so.


The same should be the case with the astigmatism
control.
yes sameting, working...



If the intensity control "double peaks", then your
problem is a gassy CRT, which will also result in
poor focus/astigmatism.

-Chuck Harris
Thank you chuck, i didn't know that the crt was a problem on that scope. Probably my scope is just working very fine and problem is a bad crt. Thank you all for your answer.


John
 

I've had (bad) noise on the traces of 453 series scopes when the EHT section cover was missing. So it may be worth checking the following on hte "bad" scope:

1. EHT screening all in place and all screws tight.
2. Ditto all earthing screws on pcb(s), and coax lead screens checked/ wiggled.
3. Wiggle transistors in their sockets to clean contacts.

John


 

And the expansion mesh also creates a ghost from secondary emission
which follows and then proceeds the beam spot across the CRT. The
ghosting has the effect of reducing display contrast.

On Thu, 25 Jan 2018 11:22:06 -0800, you wrote:

The mesh itself also makes the beam fuzzier - even it is perfect on the way in. Like looking through a screen door. Ed


 

My 7603 which spent considerable time in storage had its brightness
and sharpness improve after operating for days. if you do this, then
leave the beam intensity low but not off and for safety, only leave
the oscilloscope operating under supervision in case something goes
wrong.

On Fri, 26 Jan 2018 09:21:10 -0500, you wrote:

If your scope intensity double peaks, it could mean
that it is gassy, or it could mean that it has low
emission. Running the CRT will help get the gas
absorbed by the getter, so it may improve.

-Chuck Harris


 

It may be just a difference between the 475 and 475A oscilloscopes.
Someone else may know.

The power supply rejection of the vertical amplifier chain is not
infinite so excessive power supply noise might end up being displayed.
This could be caused by worn out or bad decoupling capacitors.

As a test to determine if the problem is with the circuits or the CRT,
I would short the vertical deflection pins together to see how this
affects the trace thickness. DO NOT short the vertical CRT pins to
ground or any other points as this will likely destroy the vertical
CRT amplifier; only short them together.

I had considerable trouble with Microsoft's sharing service. If I
thought someone could make something worse than Google Drive or Yahoo
Groups, then I was wrong.

On Wed, 24 Jan 2018 06:49:08 -0800, you wrote:

Hi everyone, i have 2. scope 475. I had recap both and now full working. But the line on the second one isn't sharper than first one. I check high voltage. 2.72kv for the first and 2.6k for the second one. So fucus and asting are working but i cant adjust the line more than that. Someone know what will be the problem ?

Thank you :)

Look picture here
https://1drv.ms/f/s!AnKLPDy3pII_gkwm-YPX7azSL7Y2


Chuck Harris
 

Usually, a pretty safe way is to force it to be out of focus
by turning the focus control fully one way, or the other.

-Chuck Harris

David Hess wrote:

My 7603 which spent considerable time in storage had its brightness
and sharpness improve after operating for days. if you do this, then
leave the beam intensity low but not off and for safety, only leave
the oscilloscope operating under supervision in case something goes
wrong.

On Fri, 26 Jan 2018 09:21:10 -0500, you wrote:

If your scope intensity double peaks, it could mean
that it is gassy, or it could mean that it has low
emission. Running the CRT will help get the gas
absorbed by the getter, so it may improve.

-Chuck Harris



donald collie
 

Looked at the photos - yes, that`s about as sharp as mine goes.
Cheers!...............................................................................................................................Don
C.

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On Mon, Jan 29, 2018 at 5:24 PM, Chuck Harris <cfharris@erols.com> wrote:

Usually, a pretty safe way is to force it to be out of focus
by turning the focus control fully one way, or the other.

-Chuck Harris

David Hess wrote:
My 7603 which spent considerable time in storage had its brightness
and sharpness improve after operating for days. if you do this, then
leave the beam intensity low but not off and for safety, only leave
the oscilloscope operating under supervision in case something goes
wrong.

On Fri, 26 Jan 2018 09:21:10 -0500, you wrote:

If your scope intensity double peaks, it could mean
that it is gassy, or it could mean that it has low
emission. Running the CRT will help get the gas
absorbed by the getter, so it may improve.

-Chuck Harris





 

One issue I have seen on the 465 and 475 is the accumulation of black soot on many HV parts. I ran into exactly your problem more than once, and found that carefully cleaning all the soot with Q-tips and water or alcohol, and removing every bit of the (conductive) soot fixed my "insoluble" focus problem. it provides a sneak path for HV leakage, and disturbs the electron gun bias. the older the scope is, the more likely this problem becomes, you have to remove the aluminum HV protective cover to see it!

at the end of the day, keep in mind, this mesh style tube does not ever look really sharp in terms of focus.

this is really worth a try if nothing else seem to be wrong.
all the best,
walter
sphere research corp.


 

Some CRTs with expansion meshes are much better than others though.

My 547 has the sharpest CRT I have seen which makes sense because it
lacks a scan expansion mesh but my 7904 and 7854 are not far behind
and my 100 MHz 22xx models are better than my 2445B which is the
fuzziest non-storage oscilloscope I have now.

Grouped together in order of sharpness.

547 - Lacks scan expansion mesh so this is not surprising.
7904/7854

2230/2232/2246/2247A
7603 - Note: Has larger than standard CRT.

465
2445B - At least mine but other 2465 series seem to be similar.

On Mon, 29 Jan 2018 07:18:43 -0800, you wrote:

...

at the end of the day, keep in mind, this mesh style tube does not ever look really sharp in terms of focus.

this is really worth a try if nothing else seem to be wrong.
all the best,
walter
sphere research corp.