2467B focus/astigmatism anomaly...

#159167

On Tue, Aug 20, 2019 at 05:14 PM, Chuck Harris wrote:

I tested the cap's leakage at 1KV, and it was 0.3uA.

1uA is 1V on 1M. And here we have impedances around 10M.

It is concerning, especially since the cap has 1.4kV
across it.
Good news!

Raymond

Chuck Harris

#159153

I tested the cap's leakage at 1KV, and it was 0.3uA.

1uA is 1V on 1M.  And here we have impedances around 10M.

It is concerning, especially since the cap has 1.4kV
across it.

-Chuck Harris

satbeginner wrote:

Show quoted text

Maybe the cap IS leaking, and heated some of the resistors on the right hand side of the schematic up?

After disconnecting the cap, the resistors cooled back down and found their normal DC working area, hence the focus stayed ok?

Still interested,

Leo

#159098

Maybe the cap IS leaking, and heated some of the resistors on the right hand side of the schematic up?

After disconnecting the cap, the resistors cooled back down and found their normal DC working area, hence the focus stayed ok?

Still interested,

Leo

Chuck Harris

#159097

An interesting change has occurred from yesterday...

Yesterday, I removed C4403, turned the scope on, adjusted
the focus and astigmatism, played a little bit, and concluded
the dynamic focus circuitry did very little, if anything...

I then turned the scope's beam off, and walked away for a
few hours.

When I returned, and turned the scope's beam back on, I found
the focus was bad once again.  So, after tweaking the astigmatism
to sharpen the beam up, I turned the scope off for the night.

This morning, when I turned the scope back on, and turned its
intensity back up, it was still sharp... when totally cold!

Sharp when hot, and still sharp when cold.... Exciting!

I turned the beam back off, and walked away for an hour, and
when I came back and turned the beam back on, it was still
sharp.  Then I turned the beam back off, and walked away for
another hour, and turned the beam back on, and it was still
sharp.

I am going to let it percolate for 2 or 3 hours and try again,
and I will report back.

Perhaps C4403 is involved?

I'm having a little trouble explaining last night's focus
shift shortly after I removed the capacitor.

-Chuck Harris

Raymond Domp Frank wrote:

Show quoted text

Hi Chuck,

On Mon, Aug 19, 2019 at 01:58 AM, Chuck Harris wrote:


The scope has been running all afternoon with the
capacitor removed, and frankly I can't even tell it
is gone... A lot of circuitry for such little affect.

However, the anomaly continues exactly as before.
Thanks for checking that, so it's off my mind.

I agree re. Q4432, Q4440, and all of
those high resistance resistors in the focus chain.

Interesting and intriguing!

Raymond

#159090

Hi Chuck,

On Mon, Aug 19, 2019 at 01:58 AM, Chuck Harris wrote:


The scope has been running all afternoon with the
capacitor removed, and frankly I can't even tell it
is gone... A lot of circuitry for such little affect.

However, the anomaly continues exactly as before.
Thanks for checking that, so it's off my mind.

I agree re. Q4432, Q4440, and all of
those high resistance resistors in the focus chain.

Interesting and intriguing!

Raymond

Chuck Harris

#159087

Hi Raymond,

The scope has been running all afternoon with the
capacitor removed, and frankly I can't even tell it
is gone...  A lot of circuitry for such little affect.

However, the anomaly continues exactly as before.

I think it is time to pick on Q4432, Q4440, and all of
those high resistance resistors in the focus chain.

-Chuck Harris

Raymond Domp Frank wrote:

Show quoted text

Hi Chuck,

On Sun, Aug 18, 2019 at 07:52 PM, Chuck Harris wrote:


C4403's intended purpose is to make moving changes to the
focus as the trace moves about on the screen.
Yes

Lifting one leg should stabilize the focus issues, if C4403,
or the dynamic focus circuitry, is part of the problem.
I'm curious, probably easy to do.

Raymond

#159076

Hi Chuck,

On Sun, Aug 18, 2019 at 07:52 PM, Chuck Harris wrote:


C4403's intended purpose is to make moving changes to the
focus as the trace moves about on the screen.
Yes

Lifting one leg should stabilize the focus issues, if C4403,
or the dynamic focus circuitry, is part of the problem.
I'm curious, probably easy to do.

Raymond

Chuck Harris

#159072

Hi Raymond,

I agree that C4403 needs to be investigated.  It will be
a week, or so, before I have any parts in hand to try.

C4403's intended purpose is to make moving changes to the
focus as the trace moves about on the screen.

Lifting one leg should stabilize the focus issues, if C4403,
or the dynamic focus circuitry, is part of the problem.

Something killed Q4402, be it old age, physical abuse, or
a random zap from C4403.  Ordinarily, I would think that the
backwards diodes CR4410 and CR4411 should have protected against
all but the largest insults...  Maybe they are compromised
too?

-Chuck Harris

Raymond Domp Frank wrote:

Show quoted text

On Sat, Aug 17, 2019 at 03:14 PM, Chuck Harris wrote:


My thought is the only way a new astigmatism voltage for optimal focus can be
required is if the beam current has changed between the gun's anode #1 and the
astigmatism lens. If you can believe the stacking of the electrodes shown in
the schematic, the focus lens comes before the astigmatism, and after the
anode #1.

And, it is the toughest to meter, as it is 1.4KV roughly, and about 15M
resistance.

I didn't give Q4432 much thought... perhaps I should, as it would shift the
focus
electrode voltage quite nicely... It is an MPSA94, which is about as you
surmised,
VCBO and VCEO = 400V
The beam current definitely changes each time between turning intensity down and up again, as the case describes. Although I can understand a jittery/jumping Q4432 role, is there a reason *not* to suspect C4403 leakage? You *did* find a broken Q4402 earlier, didn't you? And where/how does the intensity down/then up scenario cause Q4432 to make its jump, since normally there's (virtually) no (change in) current in the focus electrode circuit? Still sitting on my earlier post...

Raymond

#159054

AAAAAAAAAAAAAAccccccck!

#159053

On Sat, Aug 17, 2019 at 03:14 PM, Chuck Harris wrote:


My thought is the only way a new astigmatism voltage for optimal focus can be
required is if the beam current has changed between the gun's anode #1 and the
astigmatism lens. If you can believe the stacking of the electrodes shown in
the schematic, the focus lens comes before the astigmatism, and after the
anode #1.

And, it is the toughest to meter, as it is 1.4KV roughly, and about 15M
resistance.

I didn't give Q4432 much thought... perhaps I should, as it would shift the
focus
electrode voltage quite nicely... It is an MPSA94, which is about as you
surmised,
VCBO and VCEO = 400V
The beam current definitely changes each time between turning intensity down and up again, as the case describes. Although I can understand a jittery/jumping Q4432 role, is there a reason *not* to suspect C4403 leakage? You *did* find a broken Q4402 earlier, didn't you? And where/how does the intensity down/then up scenario cause Q4432 to make its jump, since normally there's (virtually) no (change in) current in the focus electrode circuit? Still sitting on my earlier post...

Raymond

#159035

Hi Chuck

I agree with you except that I am old enough to remember things like projection TV's where the EHT was not low current. I have also worked on high voltage electrostatic clean room filters which were not at all friendly.

After some sixty years of experience I just don't like holding probes when measuring high voltages. Even the involuntary reflex from a low current shock can cause serious injury.

I have seen Radio Hams using these 40KV probes to check the Anode voltages on PA Tubes (Valves) with about 3KV present which is directly mains derived and extremely lethal. 

Best wishes, keep up your enjoyable postings.

George.     On Saturday, August 17, 2019, 2:24:53 PM GMT+1, Chuck Harris <cfharris@...> wrote:
 
 Hi George,

There are rules, and there are rules.? Behavior when working
on a 40KV power line is very different than behavior when
working on a 40KV CRT anode supply.? One can source enough
current to vaporize the probe, and the other can merely taunt
the probe with its cross words...

These probes are meant for low current applications, and to be
held by hand.? There are sever warnings about not using the probe
on high current or without the grounding clip attached to a
ground... for obvious reasons.

-Chuck Harris

george edmonds via Groups.Io wrote:

Show quoted text

? Hi Chuck

I am quite amused by the 40KV probe as rule one is that you do not hand hold such an item when applied to power.

Best of luck

George
? ? ? On Friday, August 16, 2019, 11:02:22 PM GMT+1, Chuck Harris <cfharris@...> wrote:
? 
? Hi George,

Chuck Harris

#159034

Hi George,

There are rules, and there are rules.  Behavior when working
on a 40KV power line is very different than behavior when
working on a 40KV CRT anode supply.  One can source enough
current to vaporize the probe, and the other can merely taunt
the probe with its cross words...

These probes are meant for low current applications, and to be
held by hand.  There are sever warnings about not using the probe
on high current or without the grounding clip attached to a
ground... for obvious reasons.

-Chuck Harris

george edmonds via Groups.Io wrote:

Show quoted text

 Hi Chuck

I am quite amused by the 40KV probe as rule one is that you do not hand hold such an item when applied to power.

Best of luck

George
     On Friday, August 16, 2019, 11:02:22 PM GMT+1, Chuck Harris <cfharris@...> wrote:
 
 Hi George,

Chuck Harris

#159033

Hi Ed,

Ed Breya via Groups.Io wrote:
Well, now yuzz made me look. After looking through the whole thread, I got curious about the actual situation. I got the 2465B/2467B manual and found the CRT section schematics. I was surprised at how different these are between the 65B and 67B.

Anyway, as I understand it, the issue is that the trace drifts out of focus slightly over time, and apparently it can't be re-focused (with the focus pot) the same without also adjusting the astigmatism pot. Is this basically the deal?No, only the astigmatism pot has to be moved... resulting in a
brand new value on the astigmatism grid for optimum focus.

It looks like only the focus voltage is fairly high, and in a fairly high resistance environment, so the measurement desired is mainly about getting a good reading on this medium-HV signal, without upsetting the circuit too much. Right?Basically, the astigmatism control supply is a low impedance voltage source that
goes from 0 to 87V.  It drives an electrostatic lens that focuses on one axis, and
the focus grid drives a second electrostatic lens that focuses on the other axis.

Together the astigmatism and focus bring the beam to a circular shape.

My thought is the only way a new astigmatism voltage for optimal focus can be
required is if the beam current has changed between the gun's anode #1 and the
astigmatism lens.  If you can believe the stacking of the electrodes shown in
the schematic, the focus lens comes before the astigmatism, and after the anode #1.

And, it is the toughest to meter, as it is 1.4KV roughly, and about 15M resistance.

I didn't give Q4432 much thought... perhaps I should, as it would shift the focus
electrode voltage quite nicely... It is an MPSA94, which is about as you surmised,
VCBO and VCEO = 400V

-Chuck Harris



First, I'd expect the focus voltage to drift a little with ambient temperature and due to self-heating differences at different focus settings. The Q4432 circuit is just a shunt-feedback amplifier, with the base near zero volts, but subject to Vbe change with temperature. CR4433 may give a bit of temperature compensation, but not much. I assume Q4432 being PNP, is probably a small-signal type like TO-92 or SMT size, good for Vcbo around 350-400V. The collector operating voltage should be around 0-300V, and the current should be relatively constant around 85 uA. So, the bottom of the focus string should be able to move by about 300V, attenuated by the string to a much lower amount at the electrode. The dissipation in Q4432 is small, but can change quite a bit, from nearly zero to about 25 mW.

So, my first guess would be that the focus drift is normal - it is a simple analog circuit, after all - and the reason the astigmatism control needs to help, is that the focus voltage gets to be slightly out of range for full control. If you adjust R4430 "focus range" pot more toward the bottom side, it may expand it enough to regain the proper coverage. Whenever you change the front panel focus control, it changes the dissipation in Q4432, so it may take some time to settle at a new operating point, and will still be subject to drift. If Q4432 is readily accessible, heating or cooling it may show what's going on.

Another possibility is that Q4432 is on the verge of breakdown, which could have big effects, including apparent bi-stable behavior. Be sure it's the right specified type, and is not compromised. If all the normal operating conditions are right, it shouldn't be subjected to more than 400V or so, with any settings. Even if it did break down, the current is very limited, so shouldn't be able to damage it.

Good luck.

Ed

#159021

True enough that higher voltage can actually be safer than LV.  And from what I understand, larger currents of around 1 A can actually stop the heart, and it will start beating on its own again.  Smaller currents, above 70 mA IIRC, will cause the heart to fibrillate, a condition from which it will not recover on its own.  Hence the defibrillators that pump an amp or so through the heart to get it to stop and then start up again.  Not that I've actually tried this!  I did get a zap from 120 VDC a couple years ago at work, perhaps through my wrist strap.  We're not supposed to wear the ESD wrist straps when working with >50 V or >25 A circuits, but the RF power amplifiers are quite ESD sensitive, so we are supposed to wear them.  Nobody has figured out exactly what we really should be doing yet.

I understand that ESD strikes from static electricity in dry areas can exceed 30 kV.  I worked at one place in the mid 1990's where we had carpeted floors, and in the winter in Southern California when the Santa Ana winds blow very dry air off the desert, we would generate some pretty serious sparks.  The path to the nearest restroom was a few hundred feet long, and we would get bad zaps when grabbing the door handle, so I took to pulling out a key and letting it get the zap.
Ancient history now.

Jim

Show quoted text

------ Original Message ------
From: "Chuck Harris" <cfharris@...>
To: [email protected]
Sent: 8/16/2019 9:04:01 PM
Subject: Re: [TekScopes] 2467B focus/astigmatism anomaly...

Hi Jim,

The 15KV is pretty harmless, as it has very little capacitance,
and as a result, few Joules of energy, and very low current.

It won't stop a healthy heart, just annoy the owner.

Getting across the anode supply feels like you got a good zap from
a carpet in the winter... and is about as harmful.

The low voltage power supply's inverter is loaded with voltages that
can kill you.

-Chuck Harris

Jim Ford wrote:
 All the more reason why I don't fiddle with high voltage.  If a CRT in one of my instruments dies, I'll look for a replacement instrument or buy an LCD kit.  The one with the dead CRT will then become a parts mule.I don't want to become a human french fry!Jim Ford

#159019

Try tapping on the tube.? It may have a disconnected element internally.

Regards

Show quoted text

On 8/16/2019 11:55 PM, Chuck Harris wrote:
Hi Raymond,

The required astigmatism voltage wanders all over the
place, never to return to a previous position.

So, let's say it starts at 23.7V.  After running for an hour
without a beam on, I turn the beam on, and it will be fuzzy.

I can restore the very best sharp focus by adjusting just the
astigmatism voltage.... to a new value.

Now, I let the scope run for another hour beam off... and when
I turn the beam back up, it will again be fuzzy, and once again
I can return it to best sharp focus by adjusting just the
astigmatism voltage...

The astigmatism voltage is rock stable, and will always be exactly
where I set it.  But the voltage required for focus wanders up and
down.

I never have to touch the focus adjustment.

-Chuck Harris

Raymond Domp Frank wrote:
Chuck,
I assume that after fiddling with the astig setting, you always end up at the setting before turning down intensity?
Did  you try not touching the astig setting after turning intensity back up? Will focus eventually return even when not touching astig?
I'm thinking about leakage somewhere or lack of DC coupling (almost the same here).

With intensity off, beam current goes to almost zero.

I have seen setups where the astig electrode is closer to the cathode than the focus electrode. That does not seem to be so in this CRT but could it be that the dot distortion caused by disturbing the round shape of the beam (by operating the astig control) after upping the intensity causes a slight charge leak to/from the focus hole in the focus electrode, slightly adjusting the potential on the focus electrode?
This raises the question of whether the "dark" situation might actually be the correct situation, which is then *disturbed* after upping intensity or the other way around?
I would expect the currents on the focus and astig electrodes to be virtually zero during normal operation but the beam could approach the edge of the focus hole when it's not round, causing some leakage.
Perhaps replacing C4403 (since it carries a substantial DC voltage during operation, a slight leakage may disturb things) would be a good idea without having to resort to more extreme measures to see the high impedance HV's. As you say, regular HV probes (like a Fluke 40K8, for DC) or a Tek P6015A or the like won't be of much help in many places here.

Raymond

Chuck Harris

#159017

I'm not at all nervous about anode voltages, I have
been zapped numerous times.

The 2465 family's CRT is right tight to a mumetal shield,
so Tektronix glued the anode cap on with silicon RTV.

The anode wire does have a connector, but I would rather
not have to cobble things too much.

-Chuck Harris

Ken Eckert wrote:

Show quoted text

When I worked in a military cal centre we had lots of Tek 545s, HP 180s and
a smattering of 465s.

There was more than a fair share of weak CRTs and HV supplies issues.

I used to slide the HV probe under the rubber cap where the HV clipped onto
the CRT. If you are nervous doing that pre position  it, then turn on the
scope.

And it is sage advise, one hand behind the back or in the pocket.

The rest is experience and respect....

Ken

Chuck Harris

#159015

Hi Jim,

The 15KV is pretty harmless, as it has very little capacitance,
and as a result, few Joules of energy, and very low current.

It won't stop a healthy heart, just annoy the owner.

Getting across the anode supply feels like you got a good zap from
a carpet in the winter... and is about as harmful.

The low voltage power supply's inverter is loaded with voltages that
can kill you.

-Chuck Harris

Jim Ford wrote:

Show quoted text

All the more reason why I don't fiddle with high voltage.  If a CRT in one of my instruments dies, I'll look for a replacement instrument or buy an LCD kit.  The one with the dead CRT will then become a parts mule.I don't want to become a human french fry!Jim Ford

Chuck Harris

#159013

Hi Raymond,

The required astigmatism voltage wanders all over the
place, never to return to a previous position.

So, let's say it starts at 23.7V.  After running for an hour
without a beam on, I turn the beam on, and it will be fuzzy.

I can restore the very best sharp focus by adjusting just the
astigmatism voltage.... to a new value.

Now, I let the scope run for another hour beam off... and when
I turn the beam back up, it will again be fuzzy, and once again
I can return it to best sharp focus by adjusting just the
astigmatism voltage...

The astigmatism voltage is rock stable, and will always be exactly
where I set it.  But the voltage required for focus wanders up and
down.

I never have to touch the focus adjustment.

-Chuck Harris

Raymond Domp Frank wrote:

Show quoted text

Chuck,
I assume that after fiddling with the astig setting, you always end up at the setting before turning down intensity?
Did  you try not touching the astig setting after turning intensity back up? Will focus eventually return even when not touching astig?
I'm thinking about leakage somewhere or lack of DC coupling (almost the same here).

With intensity off, beam current goes to almost zero.

I have seen setups where the astig electrode is closer to the cathode than the focus electrode. That does not seem to be so in this CRT but could it be that the dot distortion caused by disturbing the round shape of the beam (by operating the astig control) after upping the intensity causes a slight charge leak to/from the focus hole in the focus electrode, slightly adjusting the potential on the focus electrode?
This raises the question of whether the "dark" situation might actually be the correct situation, which is then *disturbed* after upping intensity or the other way around?
I would expect the currents on the focus and astig electrodes to be virtually zero during normal operation but the beam could approach the edge of the focus hole when it's not round, causing some leakage.
Perhaps replacing C4403 (since it carries a substantial DC voltage during operation, a slight leakage may disturb things) would be a good idea without having to resort to more extreme measures to see the high impedance HV's. As you say, regular HV probes (like a Fluke 40K8, for DC) or a Tek P6015A or the like won't be of much help in many places here.

Raymond

#159011

Well, now yuzz made me look. After looking through the whole thread, I got curious about the actual situation. I got the 2465B/2467B manual and found the CRT section schematics. I was surprised at how different these are between the 65B and 67B.

Anyway, as I understand it, the issue is that the trace drifts out of focus slightly over time, and apparently it can't be re-focused (with the focus pot) the same without also adjusting the astigmatism pot. Is this basically the deal?

It looks like only the focus voltage is fairly high, and in a fairly high resistance environment, so the measurement desired is mainly about getting a good reading on this medium-HV signal, without upsetting the circuit too much. Right?

First, I'd expect the focus voltage to drift a little with ambient temperature and due to self-heating differences at different focus settings. The Q4432 circuit is just a shunt-feedback amplifier, with the base near zero volts, but subject to Vbe change with temperature. CR4433 may give a bit of temperature compensation, but not much. I assume Q4432 being PNP, is probably a small-signal type like TO-92 or SMT size, good for Vcbo around 350-400V. The collector operating voltage should be around 0-300V, and the current should be relatively constant around 85 uA. So, the bottom of the focus string should be able to move by about 300V, attenuated by the string to a much lower amount at the electrode. The dissipation in Q4432 is small, but can change quite a bit, from nearly zero to about 25 mW.

So, my first guess would be that the focus drift is normal - it is a simple analog circuit, after all - and the reason the astigmatism control needs to help, is that the focus voltage gets to be slightly out of range for full control. If you adjust R4430 "focus range" pot more toward the bottom side, it may expand it enough to regain the proper coverage. Whenever you change the front panel focus control, it changes the dissipation in Q4432, so it may take some time to settle at a new operating point, and will still be subject to drift. If Q4432 is readily accessible, heating or cooling it may show what's going on.

Another possibility is that Q4432 is on the verge of breakdown, which could have big effects, including apparent bi-stable behavior. Be sure it's the right specified type, and is not compromised. If all the normal operating conditions are right, it shouldn't be subjected to more than 400V or so, with any settings. Even if it did break down, the current is very limited, so shouldn't be able to damage it.

Good luck.

Ed

#159007

When I worked in a military cal centre we had lots of Tek 545s, HP 180s and
a smattering of 465s.

There was more than a fair share of weak CRTs and HV supplies issues.

I used to slide the HV probe under the rubber cap where the HV clipped onto
the CRT. If you are nervous doing that pre position  it, then turn on the
scope.

And it is sage advise, one hand behind the back or in the pocket.

The rest is experience and respect....

Ken

As a side note, all the 545s used metallized ceramic standoffs for point to
point wiring. Every scope came with a little spool of silver bearing solder
to solder on those ceramic pieces..



On Friday, August 16, 2019, Ed Breya via Groups.Io <edbreya=
[email protected]> wrote:

John said:
"I've got a 40kV Glassman 2kW I'm in the middle of converting from
positive to negative."

Going negative, eh? Sounds like something for a fusor project. I have two
Spellman negative 60 kV, 3 kW units, planned for fusor work long ago. Alas,
I'll probably never get around to it - it's still decades down the priority
list.

Anyway, back to the discussion, I gather that the desired measurements are
in the 3 kV region or less, and are DC. Everyone working on old scopes
should have at least a medium (3-5 kV) HVDC probe if they want to poke
around in the CRT cathode circuits. It seems that there should be plenty of
cheap new or used ones available. I've collected a few TV service 30-40 kV
type ones over the years for very cheap - they're not needed much any more
for their original purpose, but I guess there are also plenty of people
fooling around with HV for other fun purposes.

The old TV ones made for anode ultor measurements can be used down to
medium ranges with good resolution if you're using a DVM. The top-ends of
the ones I've seen are typically 600-1000 megs, and the bottom-ends hooked
to much lower resistances to scale it for the intended readout device,
sometimes even a small built-in meter movement. Regardless of what the
probe was supposed to go with, you can always modify the bottom circuitry
to work with whatever you want. What counts is the big top resistor being
intact, and a high enough value to not overload the DUT. The rest is very
adaptable, especially if the whole thing can be disassembled. So, you don't
have to find an "exact" right probe that matches your meter, since you can
tweak it around as necessary. This opens up a lot more options for finding
something usable.

For simple analysis, treat the big resistor as a current source from the
applied HV. The bottom end will always be nearly zero, comparatively, so
it's a constant current at a certain applied HV. That's what's available to
produce your desired low voltage output signal - it's just a matter of
scaling things properly to convenient values with the bottom end
termination circuitry.

BTW, that current is also the amount that will be available to flow into
you or your equipment if you fail to properly ground the bottom end of the
dividing system. It should be small, like 50 uA or less worst-case, so not
a serious shock hazard. But, it's backed up by the applied HV, so it can
jump just the same, and it can be quite a surprise. That's also why the big
top resistor has to be good - properly rated for the voltage, properly
packaged, and clean. If it were to break down, it won't isolate you
anymore, so instead of a little tingle or snap, you could get a very
noticeable shock, depending on the HV source impedance and the breakdown
characteristics. Always be sure to properly connect that ground lead before
use, and you won't need to experience any of these reminders.

You can roll your own too, suitable for low power, 3 kV applications,
without getting too fancy, packaging-wise, if you have a decent collection
of HV, high-value resistors, and can make things out of plastic tubing.

Ed

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2467B focus/astigmatism anomaly...