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465B Focus Troubleshooting

 

Hello I was told this is the place to be for asking questions about Tektronix scopes.


I rescued a 465B from ebay that showed no trace. When it arrived I was able to get a trace to show just by adjusting the controls. The only issues I can see are a thick trace and a sensitive intensity knob. Although, Having messed with the scope for a few weeks I think the intensity works as it should. Being my first Tektronix scope I started a post on the EEV blog hoping to find more info on repairing it. The kind folks over on the EEV suggested I join the TekScopes group. I am very impressed with this group and how active it is!



Currently I'm starting to suspect the CRT is faulty on this scope.



I have tested the power supply for voltage and ripple. Found a beautiful triplett 630-na to test the high voltage. The tek manual recommended it and I thought it was a great excuse... I mean... a good idea to use the recommended equipment. ;) The HV tested spot on. Tested focus voltage and it seems ok. I also checked all the other lv connections to the CRT for voltage and noise. I also probed the horizontal and for noise.



How can I eliminate the vertical section as the fault? Is it safe to just disconnect the vertical connections?
Also I did read somewhere in the TekScopes group about shorting the vertical connections with a capacitor but I could not find anymore info on that.



I'm just a self taught hobbyist and I've been taking my time. ^-^


EEV post Tektronix 465B Focus Issue - Page 1





Tektronix 465B Focus Issue - Page 1 Tektronix 465B Focus Issue - Page 1



View on www.eevblog.com
Preview by Yahoo
 

It does look like there is something wrong with the focus. Vertical
blurring could be caused by wide bandwidth noise since it is a 100 MHz
oscilloscope but the horizontal blurring matches it so something else
is going on.

Your measurement of TP4217 does not look correct at all. It needs to
be made with a x10 probe to preserve bandwidth and should look like a
clean square wave.

As far as noise from the vertical amplifier, turning on the 20 MHz
bandwidth limit will lower the noise level. You can also disconnect
the CRT leads from the vertical amplifier but be careful not to damage
the CRT.

On 28 Mar 2016 07:03:31 -0700, you wrote:

Hello I was told this is the place to be for asking questions about Tektronix scopes.

I rescued a 465B from ebay that showed no trace. When it arrived I was able to get a trace to show just by adjusting the controls. The only issues I can see are a thick trace and a sensitive intensity knob. Although, Having messed with the scope for a few weeks I think the intensity works as it should. Being my first Tektronix scope I started a post on the EEV blog hoping to find more info on repairing it. The kind folks over on the EEV suggested I join the TekScopes group. I am very impressed with this group and how active it is!

Currently I'm starting to suspect the CRT is faulty on this scope.

I have tested the power supply for voltage and ripple. Found a beautiful triplett 630-na to test the high voltage. The tek manual recommended it and I thought it was a great excuse... I mean... a good idea to use the recommended equipment. ;) The HV tested spot on. Tested focus voltage and it seems ok. I also checked all the other lv connections to the CRT for voltage and noise. I also probed the horizontal and for noise.

How can I eliminate the vertical section as the fault? Is it safe to just disconnect the vertical connections?
Also I did read somewhere in the TekScopes group about shorting the vertical connections with a capacitor but I could not find anymore info on that.

I'm just a self taught hobbyist and I've been taking my time. ^-^

EEV post Tektronix 465B Focus Issue - Page 1

 

On 28/03/16 19:29, David davidwhess@... [TekScopes] wrote:

You can also disconnect
the CRT leads from the vertical amplifier but be careful not to damage
the CRT.
Nor damage the wires.
 

---In TekScopes@..., <davidwhess@...> wrote :

It does look like there is something wrong with the focus. Vertical
blurring could be caused by wide bandwidth noise since it is a 100 MHz
oscilloscope but the horizontal blurring matches it so something else
is going on.

Your measurement of TP4217 does not look correct at all. It needs to
be made with a x10 probe to preserve bandwidth and should look like a
clean square wave.

As far as noise from the vertical amplifier, turning on the 20 MHz
bandwidth limit will lower the noise level. You can also disconnect
the CRT leads from the vertical amplifier but be careful not to damage
the CRT.


I was able to get a clean square wave on TP4217 with a new set of probes. I edited the post and labeled the clean waveform picture, ops. ^^ The old probe was one of those cheap twist for x1 or x10 and gave intermittent signals.

There's no difference when turning on the 20 MHz limit. I do see something that bothers me on one of the connections to the CRT. If I can capture a picture of it I'll post it.
 

On 28 Mar 2016 12:26:24 -0700, you wrote:

---In TekScopes@..., <davidwhess@...> wrote :

It does look like there is something wrong with the focus. Vertical
blurring could be caused by wide bandwidth noise since it is a 100 MHz
oscilloscope but the horizontal blurring matches it so something else
is going on.

Your measurement of TP4217 does not look correct at all. It needs to
be made with a x10 probe to preserve bandwidth and should look like a
clean square wave.

As far as noise from the vertical amplifier, turning on the 20 MHz
bandwidth limit will lower the noise level. You can also disconnect
the CRT leads from the vertical amplifier but be careful not to damage
the CRT.


I was able to get a clean square wave on TP4217 with a new set of probes. I edited the post and labeled the clean waveform picture, ops. ^^ The old probe was one of those cheap twist for x1 or x10 and gave intermittent signals.
I see that now. TP4217 looks fine.

There's no difference when turning on the 20 MHz limit. I do see something that bothers me on one of the connections to the CRT. If I can capture a picture of it I'll post it.
Four things occur to me which could cause this:

1. The CRT may be old. When you turn the intensity up, does the trace
spread before becoming bright? Check for double peaking where the
brightness dips as the intensity is increased.

2. Ripple on the -2450 volt cathode supply would vary the horizontal
and vertical deflection making the display look fuzzy away from the
center. The one photograph does not show this so I do not think this
is the problem. You could carefully look for ripple on cathode supply
using a high voltage capacitor and precharge resistor to coupling it
to an oscilloscope probe.

3. Could there be an external AC magnetic field? Check the location
of the oscilloscope for this. Is the CRT shield in place?

4. Noise on both the horizontal and vertical CRT outputs could cause
this. Disconnecting the vertical CRT leads will reveal if this is the
case.
 

I haven't been following this discussion, but would recommend considering another possibility (if not already mentioned) that a gain stage may be oscillating at a very high frequency, adding apparent fuzziness to the trace. If this is happening in the vertical, and the BW limit has no effect, then it's likely past the BW limiter, like in the output amplifier. If it's in the horizontal, there may be sweep position or rate effects. Maybe lead dress or component position, or a loose grounding screw? I suppose this could also happen in the Z-axis, but you'd probably see it in the test point signal.

Ed
 

The photograph makes it look like the thickness is equal in the
horizontal and vertical direction.

On 28 Mar 2016 19:35:28 -0700, you wrote:

I haven't been following this discussion, but would recommend considering another possibility (if not already mentioned) that a gain stage may be oscillating at a very high frequency, adding apparent fuzziness to the trace. If this is happening in the vertical, and the BW limit has no effect, then it's likely past the BW limiter, like in the output amplifier. If it's in the horizontal, there may be sweep position or rate effects. Maybe lead dress or component position, or a loose grounding screw? I suppose this could also happen in the Z-axis, but you'd probably see it in the test point signal.

Ed
 

---In TekScopes@..., <davidwhess@...> wrote :

On 28 Mar 2016 12:26:24 -0700, you wrote:

There's no difference when turning on the 20 MHz limit. I do see something that bothers me on one of the connections to the CRT. If I can capture a picture of it I'll post it.

Four things occur to me which could cause this:
>1. The CRT may be old. When you turn the intensity up, does the trace
spread before becoming bright? Check for double peaking where the
brightness dips as the intensity is increased.

>2. Ripple on the -2450 volt cathode supply would vary the horizontal
and vertical deflection making the display look fuzzy away from the
center. The one photograph does not show this so I do not think this
is the problem. You could carefully look for ripple on cathode supply
using a high voltage capacitor and precharge resistor to coupling it
to an oscilloscope probe.

>3. Could there be an external AC magnetic field? Check the location
of the oscilloscope for this. Is the CRT shield in place?

>4. Noise on both the horizontal and vertical CRT outputs could cause
this. Disconnecting the vertical CRT leads will reveal if this is the
case.


The trace brightens up very quickly and no dips that I can see. It starts to widen just a little when turned all the way up ( I de-focused the trace and did that very quickly. Turning the intensity up high makes my skin crawl. ~shivers~ )

I'm not sure yet if I would try that. You say precharge resistor? In parallel with the HV capacitor?

The only other gear on the bench is the leader scope and my Keithley DMM. Also I did power on the scope in a different room when I first got it.

I Disconnected the vertical CRT leads and the trace curved a little but it showed no difference in thickness. The vertical pins on the CRT look a little crusty to my eye. I'm going to double check for noise on the LV connections to the CRT and then check the physical connections.
 

Wow, I'm still not used to yahoo groups. I have yet to figure out how to quote messages. ^^;;


On 28 Mar 2016 12:26:24 -0700, you wrote:

>>There's no difference when turning on the 20 MHz limit. I do see something that bothers me on one of the connections to the CRT. If I can capture a picture of it I'll post it.


>Four things occur to me which could cause this:

>1. The CRT may be old. When you turn the intensity up, does the trace
spread before becoming bright? Check for double peaking where the
brightness dips as the intensity is increased.

>2. Ripple on the -2450 volt cathode supply would vary the horizontal
and vertical deflection making the display look fuzzy away from the
center. The one photograph does not show this so I do not think this
is the problem. You could carefully look for ripple on cathode supply
using a high voltage capacitor and precharge resistor to coupling it
to an oscilloscope probe.

>3. Could there be an external AC magnetic field? Check the location
of the oscilloscope for this. Is the CRT shield in place?

>4. Noise on both the horizontal and vertical CRT outputs could cause
this. Disconnecting the vertical CRT leads will reveal if this is the
case.


The trace brightens up very quickly and no dips that I can see. It starts to widen just a little when turned all the way up ( I de-focused the trace and did that very quickly. Turning the intensity up high makes my skin crawl. ~shivers~ )

I'm not sure yet if I would try that. You say precharge resistor? In parallel with the HV capacitor?

The only other gear on the bench is the leader scope and my Keithley DMM. Also I did power on the scope in a different room when I first got it.

I Disconnected the vertical CRT leads and the trace curved a little but it showed no difference in thickness. The vertical pins on the CRT look a little crusty to my eye. I'm going to double check for noise on the LV connections to the CRT and then check the physical connections.
 

On 31/03/16 01:05, dieselchair@... [TekScopes] wrote:

Wow, I'm still not used to yahoo groups. I have yet to figure out how to quote messages. ^^;;
I reply using use my normal email program (seamonkey/thunderbird, but that's not important), and let it do the quoting. I also delete the "pretty picture" that Yahoo adds at the bottom, which I presume is the source of the "line noise" in some postings.

The other trick is to "send in plain text only" - not "html" nor "html and plain text".
[email protected]
 

I have a collection of scopes, about 1/2 are Tek. This last weekend was very wet and humid. I fired up one of my 'brand X' (100 MHz) scopes to work on a preamp and was surprised to see the focus was very blurred. Could not get it sharp. Turned it off, suspecting problems.
Went back out today and since the weather had turned warm and dry, the scope focus sharpened up nicely....
First time I've seen that. Scope has a plastic dust cover on it.
FWIW

ron
N4UE

-----Original Message-----
From: dieselchair@... [TekScopes] <TekScopes@...>
To: TekScopes <TekScopes@...>
Sent: Wed, Mar 30, 2016 8:06 pm
Subject: [TekScopes] Re: 465B Focus Troubleshooting






Wow, I'm still not used to yahoo groups. I have yet to figure out how to quote messages. ^^;;


On 28 Mar 2016 12:26:24 -0700, you wrote:

>>There's no difference when turning on the 20 MHz limit. I do see something that bothers me on one of the connections to the CRT. If I can capture a picture of it I'll post it.


>Four things occur to me which could cause this:

>1. The CRT may be old. When you turn the intensity up, does the trace
spread before becoming bright? Check for double peaking where the
brightness dips as the intensity is increased.

>2. Ripple on the -2450 volt cathode supply would vary the horizontal
and vertical deflection making the display look fuzzy away from the
center. The one photograph does not show this so I do not think this
is the problem. You could carefully look for ripple on cathode supply
using a high voltage capacitor and precharge resistor to coupling it
to an oscilloscope probe.

>3. Could there be an external AC magnetic field? Check the location
of the oscilloscope for this. Is the CRT shield in place?

>4. Noise on both the horizontal and vertical CRT outputs could cause
this. Disconnecting the vertical CRT leads will reveal if this is the
case.


The trace brightens up very quickly and no dips that I can see. It starts to widen just a little when turned all the way up ( I de-focused the trace and did that very quickly. Turning the intensity up high makes my skin crawl. ~shivers~ )

I'm not sure yet if I would try that. You say precharge resistor? In parallel with the HV capacitor?

The only other gear on the bench is the leader scope and my Keithley DMM. Also I did power on the scope in a different room when I first got it.

I Disconnected the vertical CRT leads and the trace curved a little but it showed no difference in thickness. The vertical pins on the CRT look a little crusty to my eye. I'm going to double check for noise on the LV connections to the CRT and then check the physical connections.

[Non-text portions of this message have been removed]









[Non-text portions of this message have been removed]
 

On 30 Mar 2016 16:33:28 -0700, you wrote:

The trace brightens up very quickly and no dips that I can see. It starts to widen just a little when turned all the way up ( I de-focused the trace and did that very quickly. Turning the intensity up high makes my skin crawl. ~shivers~ )
So it is not a problem with the CRT. When they get old, the trace
becomes both dim and less well focused because of lack of emission
from the cathode.

There is another possibility with the CRT though. If the oscilloscope
has been in storage for an extended time, I think trace gas can
accumulate in the CRT. Operating the oscilloscopes for hours to days
may clear this condition.

This happed with my 7603 which was not used for 10+ years. A couple
of days of leaving it running with a bright but defocused trace made a
marked improvement in brightness and focus.

If you do this, for now I would only leave the 465B running while
supervised in case something happens.

I'm not sure yet if I would try that. You say precharge resistor? In parallel with the HV capacitor?
The high voltage capacitor goes to the high voltage test point and a
high value resistor, like 10M, goes between the open end of the
capacitor and ground. That way when the high voltage starts, the
resistor bleeds charge from the capacitor until it reaches ground and
it becomes safe to apply your oscilloscope probe or voltmeter.

The only other gear on the bench is the leader scope and my Keithley DMM. Also I did power on the scope in a different room when I first got it.

I Disconnected the vertical CRT leads and the trace curved a little but it showed no difference in thickness. The vertical pins on the CRT look a little crusty to my eye. I'm going to double check for noise on the LV connections to the CRT and then check the physical connections.
With the deflection plate leads disconnected, a slight curvature in
the trace is expected.

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