Topics

Tek 465 intensity problem


Tom Gardner
 

I've picked up a discarded Tek465 which is functional except the screen lights up the room and the intensity control has no effect - a traditional problem, I believe. The focus control has significant effect but, fortunately, cannot completely focus the intense beam. Initial experiments with the Z axis circuits make me believe it is functioning correctly, and that maybe the tube is faulty. If the tube is faulty, I doubt I'll bother to get a replacement, since I have a perfectly functional HP1740A :) Your experience and wisdom would be appreciated.

I'm not familiar with CRTs, but my understanding is that the beam intensity is controlled by the grid-cathode voltage. For generic Tek scopes, if the grid is more than 65V more negative than the cathode, no electrons should reach the screen. My measurements, detailed below, indicate that the grid is at least 120V more negative, and hence the tube should be cutoff. Is that correct?

Unrelated repair work: replaced one blown decoupling capacitor, beneficially cleaned "ViewTrig" and "Beam Find" switch contacts with IPA. The PSU lines and capacitors all appear OK, to my pleasant surprise.

HV diodes replaced: CR1482, CR1483, CR1487, CR1488, possibly unnecessarily. HV capacitors replaced: none, simply because they are not easily available, the HV voltages appear sane, and there would be no point if the tube was faulty.

Scope: Tek 465, s/n < 250000
Documents: service manual for 250000-up (free file "tek-465-late.pdf"), and the generic "Tektronix Troubleshooting Scopes", and this group, of course.
HV measurement technique: handheld 3.5digit DMM with a homemade 100Mohm 1000:1 potential divider (3*33Mohm+1*100kohm resistors).

Looking at the low voltage Z-axis output, TP 1486, shows reasonable waveforms
0Vpp to 80Vpp depending on position of the intensity control periods and voltages commensurate with sweep speed and timebase options (e.g. A inten) Thus that seems to be working OK

So with the timebase chosen so that TP1486 has a 50% duty cycle, what are the voltages at the CRT's tubeholder?

HV measurements with the CRT connected:
pin 2, cathode, -2458V and that does not change when intensity and focus change => working correctly pin 3, grid, -2580V with TP1486 0Vpp => Vkg = -120V pin 3, grid, -2553V TP1486 50Vpp=> Vkg = -95V, and a 25V change (i.e. half of 50Vpp, as I would expect) pin 4, focus, -1060V to -1670V depending on focus control position, i.e. not -1700V Overall that appears - to me - to be delivering reasonable voltages to the CRT's grid and that therefore the HV capacitors are not faulty.

HV measurements with the CRT disconnected:
pin 2, cathode, -2498V i.e. -50V relative to connected voltage pin 3, grid, TP1486 0Vpp -2585V i.e. ~same as when connected and not relative to Vk, and Vkg reduced to -85V pin 3, grid, TP1486 50Vpp -2560V i.e. ~same as when connected and same 25V change pin 4, focus, -1600 to -1900 depending on focus control position
So, do those measurements indicate a problem inside the tube with the grid?
If so then are there any possible quick fixes?
Are there any more tests I could usefully make?
If there are then I'll try them and, if necessary, order replacement HV capacitors.

Thanks in advance.


Tom Gardner
 

**This is the same as the root message, but with the "formatting" reinserted so that it might be readable, sigh **

I've picked up a discarded Tek465 which is functional except the screen lights up the room and the intensity control has no effect - a traditional problem, I believe. The focus control has significant effect but, fortunately, cannot completely focus the intense beam. Initial experiments with the Z axis circuits make me believe it is functioning correctly, and that maybe the tube is faulty. If the tube is faulty, I doubt I'll bother to get a replacement, since I have a perfectly functional HP1740A :) Your experience and wisdom would be appreciated.

I'm not familiar with CRTs, but my understanding is that the beam intensity is controlled by the grid-cathode voltage. For generic Tek scopes, if the grid is more than 65V more negative than the cathode, no electrons should reach the screen. My measurements, detailed below, indicate that the grid is at least 120V more negative, and hence the tube should be cutoff. Is that correct?

Unrelated repair work: replaced one blown decoupling capacitor, beneficially cleaned "ViewTrig" and "Beam Find" switch contacts with IPA. The PSU lines and capacitors all appear OK, to my pleasant surprise.

HV diodes replaced: CR1482, CR1483, CR1487, CR1488, possibly unnecessarily. HV capacitors replaced: none, simply because they are not easily available, the HV voltages appear sane, and there would be no point if the tube was faulty.

Scope: Tek 465, s/n < 250000
Documents: service manual for 250000-up (free file "tek-465-late.pdf"), and the generic "Tektronix Troubleshooting Scopes", and this group, of course.
HV measurement technique: handheld 3.5digit DMM with a homemade 100Mohm 1000:1 potential divider (3*33Mohm+1*100kohm resistors).

Looking at the low voltage Z-axis output, TP 1486, shows reasonable waveforms
- 0Vpp to 80Vpp depending on position of the intensity control
- periods and voltages commensurate with sweep speed and timebase options (e.g. A inten)
Thus that seems to be working OK

So with the timebase chosen so that TP1486 has a 50% duty cycle, what are the voltages at the CRT's tubeholder?

HV measurements with the CRT connected:
- pin 2, cathode, -2458V and that does not change when intensity and focus change => working correctly
- pin 3, grid, -2580V with TP1486 0Vpp => Vkg = -120V
- pin 3, grid, -2553V TP1486 50Vpp=> Vkg = -95V, and a 25V change (i.e. half of 50Vpp, as I would expect)
- pin 4, focus, -1060V to -1670V depending on focus control position, i.e. not -1700V
Overall that appears - to me - to be delivering reasonable voltages to the CRT's grid and that therefore the HV capacitors are not faulty.

HV measurements with the CRT disconnected:
- pin 2, cathode, -2498V i.e. -50V relative to connected voltage
- pin 3, grid, TP1486 0Vpp -2585V i.e. ~same as when connected and not relative to Vk, and Vkg reduced to -85V
- pin 3, grid, TP1486 50Vpp -2560V i.e. ~same as when connected and same 25V change
- pin 4, focus, -1600 to -1900 depending on focus control position

So, do those measurements indicate a problem inside the tube with the grid?
If so then are there any possible quick fixes?
Are there any more tests I could usefully make?
If there are then I'll try them and, if necessary, order replacement HV capacitors.

Thanks in advance.


EB4APL
 

If you are pretty sure that the Z axis amplifier is working and unless
the tube has its internal grid connection broken, the most common cause
is the DC restorer.
Troubleshooting it in an analytical way is not an easy task, so I would
use the shotgun approach: replace all diodes and HV capacitors in the
circuit, one must remove them to test for leakage (the common cause and
not easy to test) so instead to solder them again it is better to
replace them with new parts, they are cheap.
I have seen the problem on several scopes before, not only Tek but
others also, and BTW, most of them uses the same circuit, probably
borrowed from Tek. I also have read in this lists more DC restorer
problems recently.

After writing this I checked the schematics and now I see that you
already have replaced all the DC restorer diodes so the suspicious parts
are the capacitors C1487 and C1488. The diodes should be of the low
leakage type and new 6 kV capacitors should exhibit low leakage also.
In the 3 scopes that I found recently with just the same symptoms the
problem was the diodes, which all tested ok with a DVM test. What
diodes you used for replacing CR1482-CR1488?
The strange thing for me is that the grid voltage appears to be changing
as it should be so maybe the tube is bad.
The waveforms that you see on TP1486 are the unblanking signals so they
changes with the sweep. You could make the scope to stop sweeping using
the trigger controls or the single sweep setting. Be sure that before
trying this the beam is fully defocused and better move it away the
screen, you don't want to burn the phosphor if already it is not.

Good luck,
Ignacio EB4APL


El 12/02/2016 a las 20:18, tggzzz@... [TekScopes] wrote:

I've picked up a discarded Tek465 which is functional except the
screen lights up the room and the intensity control has no effect - a
traditional problem, I believe. The focus control has significant
effect but, fortunately, cannot completely focus the intense beam.
Initial experiments with the Z axis circuits make me believe it is
functioning correctly, and that maybe the tube is faulty. If the tube
is faulty, I doubt I'll bother to get a replacement, since I have a
perfectly functional HP1740A :) Your experience and wisdom would be
appreciated.

I'm not familiar with CRTs, but my understanding is that the beam
intensity is controlled by the grid-cathode voltage. For generic Tek
scopes, if the grid is more than 65V more negative than the cathode,
no electrons should reach the screen. My measurements, detailed below,
indicate that the grid is at least 120V more negative, and hence the
tube should be cutoff. Is that correct?

Unrelated repair work: replaced one blown decoupling capacitor,
beneficially cleaned "ViewTrig" and "Beam Find" switch contacts with
IPA. The PSU lines and capacitors all appear OK, to my pleasant surprise.

HV diodes replaced: CR1482, CR1483, CR1487, CR1488, possibly
unnecessarily. HV capacitors replaced: none, simply because they are
not easily available, the HV voltages appear sane, and there would be
no point if the tube was faulty.

Scope: Tek 465, s/n < 250000
Documents: service manual for 250000-up (free file
"tek-465-late.pdf"), and the generic "Tektronix Troubleshooting
Scopes", and this group, of course.
HV measurement technique: handheld 3.5digit DMM with a homemade
100Mohm 1000:1 potential divider (3*33Mohm+1*100kohm resistors).

Looking at the low voltage Z-axis output, TP 1486, shows reasonable
waveforms
0Vpp to 80Vpp depending on position of the intensity control periods
and voltages commensurate with sweep speed and timebase options (e.g.
A inten) Thus that seems to be working OK

So with the timebase chosen so that TP1486 has a 50% duty cycle, what
are the voltages at the CRT's tubeholder?

HV measurements with the CRT connected:
pin 2, cathode, -2458V and that does not change when intensity and
focus change => working correctly pin 3, grid, -2580V with TP1486 0Vpp
=> Vkg = -120V pin 3, grid, -2553V TP1486 50Vpp=> Vkg = -95V, and a
25V change (i.e. half of 50Vpp, as I would expect) pin 4, focus,
-1060V to -1670V depending on focus control position, i.e. not -1700V
Overall that appears - to me - to be delivering reasonable voltages to
the CRT's grid and that therefore the HV capacitors are not faulty.

HV measurements with the CRT disconnected:
pin 2, cathode, -2498V i.e. -50V relative to connected voltage pin 3,
grid, TP1486 0Vpp -2585V i.e. ~same as when connected and not relative
to Vk, and Vkg reduced to -85V pin 3, grid, TP1486 50Vpp -2560V i.e.
~same as when connected and same 25V change pin 4, focus, -1600 to
-1900 depending on focus control position
So, do those measurements indicate a problem inside the tube with the
grid?
If so then are there any possible quick fixes?
Are there any more tests I could usefully make?
If there are then I'll try them and, if necessary, order replacement
HV capacitors.

Thanks in advance.




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Tom Gardner
 

Thanks for your thoughts.

My understanding is that the grid is physically between the cathode and everything else, and is responsible for controlling the beam current. (Or not in this case!) Other electrodes should not be able to "override" the grid blanking the beam.

I've retaken the CRT voltage measurements in a slightly different way, and, to the best of my understanding the cathode and grid voltages are sufficient. If that is the case then it looks like the tube is faulty, presumably with the grid being disconnected.

The cathode is adjusted to: Vk = -2450V, as per the manual, without problem.
The grid is a bit more negative: Vkg = -42Vmean to -147Vmean depending on the position of the CRT bias trimpot and the front-panel intensity control. (i.e. Vg is around -2550V+-50V). The change in Vmean corresponds to the TP1486 waveform's Vpp and duty cycle. Hence I believe the DC restorer is working correctly.
If "most" Tek CRTs blank at around -65V, then those voltages should definitely be blanking the beam, and none of the other electrodes could prevent blanking.

The focus is -1060V to -1670V depending on the focus front-panel control. The manual indicates -1700V. If the CRT is disconnected, the focus is -1600V to -1900V. With neither understanding nor evidence I presuming that is a secondary problem that might be caused by the excessive beam current.
I haven't measured the anode, but many electrons are hitting the screen so I doubt that's a problem.
The other voltages are all reasonable, but I'm guessing they are also unimportant at this stage.

I've preferred to keep the beam sweeping and off the phosphor to spread the load across the tube. The easiest way of removing any blanking signal on TP1486 is simply to turn the intensity knob fully CCW. As expected, the grid voltage is then at its most negative.

CR1482/7/8 replaced with BAV21 (250V 250mA, 100nA, 50ns), CR1483 replaced with a STTH1R06 (600V 1A, 1uA, 45ns).
CR1421 and associated capacitors not replaced since the cathode voltage (Vk) seems good.
DC restorer capacitors not replaced since the grid-cathode voltage (Vkg) looks large enough to cutoff the beam. But my understanding of CRTs is limited and I'd like a sanity check!

So, unless I've misunderstood the necessary grid voltages to ensure beam cutoff, I'll presume the tube is faulty :(

On 14/02/16 00:29, EB4APL eb4apl@... [TekScopes] wrote:
If you are pretty sure that the Z axis amplifier is working and unless
the tube has its internal grid connection broken, the most common cause
is the DC restorer.
Troubleshooting it in an analytical way is not an easy task, so I would
use the shotgun approach: replace all diodes and HV capacitors in the
circuit, one must remove them to test for leakage (the common cause and
not easy to test) so instead to solder them again it is better to
replace them with new parts, they are cheap.
I have seen the problem on several scopes before, not only Tek but
others also, and BTW, most of them uses the same circuit, probably
borrowed from Tek. I also have read in this lists more DC restorer
problems recently.

After writing this I checked the schematics and now I see that you
already have replaced all the DC restorer diodes so the suspicious parts
are the capacitors C1487 and C1488. The diodes should be of the low
leakage type and new 6 kV capacitors should exhibit low leakage also.
In the 3 scopes that I found recently with just the same symptoms the
problem was the diodes, which all tested ok with a DVM test. What
diodes you used for replacing CR1482-CR1488?
The strange thing for me is that the grid voltage appears to be changing
as it should be so maybe the tube is bad.
The waveforms that you see on TP1486 are the unblanking signals so they
changes with the sweep. You could make the scope to stop sweeping using
the trigger controls or the single sweep setting. Be sure that before
trying this the beam is fully defocused and better move it away the
screen, you don't want to burn the phosphor if already it is not.

Good luck,
Ignacio EB4APL


El 12/02/2016 a las 20:18, tggzzz@... [TekScopes] wrote:
I've picked up a discarded Tek465 which is functional except the
screen lights up the room and the intensity control has no effect - a
traditional problem, I believe. The focus control has significant
effect but, fortunately, cannot completely focus the intense beam.
Initial experiments with the Z axis circuits make me believe it is
functioning correctly, and that maybe the tube is faulty. If the tube
is faulty, I doubt I'll bother to get a replacement, since I have a
perfectly functional HP1740A :) Your experience and wisdom would be
appreciated.

I'm not familiar with CRTs, but my understanding is that the beam
intensity is controlled by the grid-cathode voltage. For generic Tek
scopes, if the grid is more than 65V more negative than the cathode,
no electrons should reach the screen. My measurements, detailed below,
indicate that the grid is at least 120V more negative, and hence the
tube should be cutoff. Is that correct?

Unrelated repair work: replaced one blown decoupling capacitor,
beneficially cleaned "ViewTrig" and "Beam Find" switch contacts with
IPA. The PSU lines and capacitors all appear OK, to my pleasant surprise.

HV diodes replaced: CR1482, CR1483, CR1487, CR1488, possibly
unnecessarily. HV capacitors replaced: none, simply because they are
not easily available, the HV voltages appear sane, and there would be
no point if the tube was faulty.

Scope: Tek 465, s/n < 250000
Documents: service manual for 250000-up (free file
"tek-465-late.pdf"), and the generic "Tektronix Troubleshooting
Scopes", and this group, of course.
HV measurement technique: handheld 3.5digit DMM with a homemade
100Mohm 1000:1 potential divider (3*33Mohm+1*100kohm resistors).

Looking at the low voltage Z-axis output, TP 1486, shows reasonable
waveforms
0Vpp to 80Vpp depending on position of the intensity control periods
and voltages commensurate with sweep speed and timebase options (e.g.
A inten) Thus that seems to be working OK

So with the timebase chosen so that TP1486 has a 50% duty cycle, what
are the voltages at the CRT's tubeholder?

HV measurements with the CRT connected:
pin 2, cathode, -2458V and that does not change when intensity and
focus change => working correctly pin 3, grid, -2580V with TP1486 0Vpp
=> Vkg = -120V pin 3, grid, -2553V TP1486 50Vpp=> Vkg = -95V, and a
25V change (i.e. half of 50Vpp, as I would expect) pin 4, focus,
-1060V to -1670V depending on focus control position, i.e. not -1700V
Overall that appears - to me - to be delivering reasonable voltages to
the CRT's grid and that therefore the HV capacitors are not faulty.

HV measurements with the CRT disconnected:
pin 2, cathode, -2498V i.e. -50V relative to connected voltage pin 3,
grid, TP1486 0Vpp -2585V i.e. ~same as when connected and not relative
to Vk, and Vkg reduced to -85V pin 3, grid, TP1486 50Vpp -2560V i.e.
~same as when connected and same 25V change pin 4, focus, -1600 to
-1900 depending on focus control position
So, do those measurements indicate a problem inside the tube with the
grid?
If so then are there any possible quick fixes?
Are there any more tests I could usefully make?
If there are then I'll try them and, if necessary, order replacement
HV capacitors.

Thanks in advance.






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EB4APL
 

Hi,
I'm afraid that you are right, the grid is not controlling the beam.
Is it possible that the socket grid pin is not contacting the tube pin,
or is there any corrosion there?
I don't remember if this tube has a glass neck, but if yes maybe you
could verify visually if the grid internal contact is loose.
And you are right, the grid is the first electrode after the cathode, in
fact it is not a grid but a capped cylinder with a hole on the cap.
Anyway sometimes it is difficult to ascertain which electrode is which
but you could see if there are anything loose there.

Good luck,
Ignacio


El 14/02/2016 a las 2:34, Tom Gardner tggzzz@... [TekScopes] escribió:

Thanks for your thoughts.

My understanding is that the grid is physically between the cathode and
everything else, and is responsible for controlling the beam current.
(Or not in
this case!) Other electrodes should not be able to "override" the grid
blanking
the beam.

I've retaken the CRT voltage measurements in a slightly different way,
and, to
the best of my understanding the cathode and grid voltages are
sufficient. If
that is the case then it looks like the tube is faulty, presumably
with the grid
being disconnected.

The cathode is adjusted to: Vk = -2450V, as per the manual, without
problem.
The grid is a bit more negative: Vkg = -42Vmean to -147Vmean depending
on the
position of the CRT bias trimpot and the front-panel intensity
control. (i.e. Vg
is around -2550V+-50V). The change in Vmean corresponds to the TP1486
waveform's
Vpp and duty cycle. Hence I believe the DC restorer is working correctly.
If "most" Tek CRTs blank at around -65V, then those voltages should
definitely
be blanking the beam, and none of the other electrodes could prevent
blanking.

The focus is -1060V to -1670V depending on the focus front-panel
control. The
manual indicates -1700V. If the CRT is disconnected, the focus is
-1600V to
-1900V. With neither understanding nor evidence I presuming that is a
secondary
problem that might be caused by the excessive beam current.
I haven't measured the anode, but many electrons are hitting the
screen so I
doubt that's a problem.
The other voltages are all reasonable, but I'm guessing they are also
unimportant at this stage.

I've preferred to keep the beam sweeping and off the phosphor to
spread the load
across the tube. The easiest way of removing any blanking signal on
TP1486 is
simply to turn the intensity knob fully CCW. As expected, the grid
voltage is
then at its most negative.

CR1482/7/8 replaced with BAV21 (250V 250mA, 100nA, 50ns), CR1483
replaced with a
STTH1R06 (600V 1A, 1uA, 45ns).
CR1421 and associated capacitors not replaced since the cathode
voltage (Vk)
seems good.
DC restorer capacitors not replaced since the grid-cathode voltage
(Vkg) looks
large enough to cutoff the beam. But my understanding of CRTs is
limited and I'd
like a sanity check!

So, unless I've misunderstood the necessary grid voltages to ensure
beam cutoff,
I'll presume the tube is faulty :(

On 14/02/16 00:29, EB4APL eb4apl@... [TekScopes] wrote:
If you are pretty sure that the Z axis amplifier is working and unless
the tube has its internal grid connection broken, the most common cause
is the DC restorer.
Troubleshooting it in an analytical way is not an easy task, so I would
use the shotgun approach: replace all diodes and HV capacitors in the
circuit, one must remove them to test for leakage (the common cause and
not easy to test) so instead to solder them again it is better to
replace them with new parts, they are cheap.
I have seen the problem on several scopes before, not only Tek but
others also, and BTW, most of them uses the same circuit, probably
borrowed from Tek. I also have read in this lists more DC restorer
problems recently.

After writing this I checked the schematics and now I see that you
already have replaced all the DC restorer diodes so the suspicious parts
are the capacitors C1487 and C1488. The diodes should be of the low
leakage type and new 6 kV capacitors should exhibit low leakage also.
In the 3 scopes that I found recently with just the same symptoms the
problem was the diodes, which all tested ok with a DVM test. What
diodes you used for replacing CR1482-CR1488?
The strange thing for me is that the grid voltage appears to be changing
as it should be so maybe the tube is bad.
The waveforms that you see on TP1486 are the unblanking signals so they
changes with the sweep. You could make the scope to stop sweeping using
the trigger controls or the single sweep setting. Be sure that before
trying this the beam is fully defocused and better move it away the
screen, you don't want to burn the phosphor if already it is not.

Good luck,
Ignacio EB4APL


El 12/02/2016 a las 20:18, tggzzz@... [TekScopes] wrote:
I've picked up a discarded Tek465 which is functional except the
screen lights up the room and the intensity control has no effect - a
traditional problem, I believe. The focus control has significant
effect but, fortunately, cannot completely focus the intense beam.
Initial experiments with the Z axis circuits make me believe it is
functioning correctly, and that maybe the tube is faulty. If the tube
is faulty, I doubt I'll bother to get a replacement, since I have a
perfectly functional HP1740A :) Your experience and wisdom would be
appreciated.

I'm not familiar with CRTs, but my understanding is that the beam
intensity is controlled by the grid-cathode voltage. For generic Tek
scopes, if the grid is more than 65V more negative than the cathode,
no electrons should reach the screen. My measurements, detailed below,
indicate that the grid is at least 120V more negative, and hence the
tube should be cutoff. Is that correct?

Unrelated repair work: replaced one blown decoupling capacitor,
beneficially cleaned "ViewTrig" and "Beam Find" switch contacts with
IPA. The PSU lines and capacitors all appear OK, to my pleasant
surprise.

HV diodes replaced: CR1482, CR1483, CR1487, CR1488, possibly
unnecessarily. HV capacitors replaced: none, simply because they are
not easily available, the HV voltages appear sane, and there would be
no point if the tube was faulty.

Scope: Tek 465, s/n < 250000
Documents: service manual for 250000-up (free file
"tek-465-late.pdf"), and the generic "Tektronix Troubleshooting
Scopes", and this group, of course.
HV measurement technique: handheld 3.5digit DMM with a homemade
100Mohm 1000:1 potential divider (3*33Mohm+1*100kohm resistors).

Looking at the low voltage Z-axis output, TP 1486, shows reasonable
waveforms
0Vpp to 80Vpp depending on position of the intensity control periods
and voltages commensurate with sweep speed and timebase options (e.g.
A inten) Thus that seems to be working OK

So with the timebase chosen so that TP1486 has a 50% duty cycle, what
are the voltages at the CRT's tubeholder?

HV measurements with the CRT connected:
pin 2, cathode, -2458V and that does not change when intensity and
focus change => working correctly pin 3, grid, -2580V with TP1486 0Vpp
=> Vkg = -120V pin 3, grid, -2553V TP1486 50Vpp=> Vkg = -95V, and a
25V change (i.e. half of 50Vpp, as I would expect) pin 4, focus,
-1060V to -1670V depending on focus control position, i.e. not -1700V
Overall that appears - to me - to be delivering reasonable voltages to
the CRT's grid and that therefore the HV capacitors are not faulty.

HV measurements with the CRT disconnected:
pin 2, cathode, -2498V i.e. -50V relative to connected voltage pin 3,
grid, TP1486 0Vpp -2585V i.e. ~same as when connected and not relative
to Vk, and Vkg reduced to -85V pin 3, grid, TP1486 50Vpp -2560V i.e.
~same as when connected and same 25V change pin 4, focus, -1600 to
-1900 depending on focus control position
So, do those measurements indicate a problem inside the tube with the
grid?
If so then are there any possible quick fixes?
Are there any more tests I could usefully make?
If there are then I'll try them and, if necessary, order replacement
HV capacitors.

Thanks in advance.




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


----------------------------------------------------------
Posted by: tggzzz@...
----------------------------------------------------------

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Tom Gardner
 

On 14/02/16 02:27, EB4APL eb4apl@... [TekScopes] wrote:
Hi,
I'm afraid that you are right, the grid is not controlling the beam.
Is it possible that the socket grid pin is not contacting the tube pin,
or is there any corrosion there?
No :(

I've already used IPA and gently pinched the connector contacts, to no effect.

Since the scope is so much more difficult to disassemble than my HP1740A, I can't really be bothered to look at the tube's internals.

At this point I'm not sure whether to break it up for spares or wait until a tube becomes available, or simply to visit the local tip.

Thanks for your help. (And thanks to contributors to the earlier threads that enabled me to localise the fault).

I don't remember if this tube has a glass neck, but if yes maybe you
could verify visually if the grid internal contact is loose.
And you are right, the grid is the first electrode after the cathode, in
fact it is not a grid but a capped cylinder with a hole on the cap.
Anyway sometimes it is difficult to ascertain which electrode is which
but you could see if there are anything loose there.


Albert Otten
 

Hi Tom,
Thinking of Ed's rejuvenation experiments with his 576 CRT, I suggest you might test for connectivity of the grid electrode in a similar way. All you have to do is remove the CRT socket, supply the correct heater voltage to the CRT and check that the "circuit" between grid and kathode pins conducts as a diode. Use some tens of volts of DC supply and a large series resistor to make g positive w.r.t. k, and measure the current. A few uA max is enough to distinguish between connected and disconnected grid terminal.
Albert


Tom Gardner
 

On 14/02/16 11:18, aodiversen@... [TekScopes] wrote:

Hi Tom,
Thinking of Ed's rejuvenation experiments with his 576 CRT, I suggest you
might test for connectivity of the grid electrode in a similar way. All you
have to do is remove the CRT socket, supply the correct heater voltage to the
CRT and check that the "circuit" between grid and kathode pins conducts as a
diode. Use some tens of volts of DC supply and a large series resistor to make
g positive w.r.t. k, and measure the current. A few uA max is enough to
distinguish between connected and disconnected grid terminal.
Albert
Good idea; thanks.

Unfortunately using 50V confirms a disconnected grid. Oh well :(


Malcolm Hunter
 

On 15 February 2016 at 19:00, Tom Gardner tggzzz@... [TekScopes] <
TekScopes@...> wrote:

Good idea; thanks.

Unfortunately using 50V confirms a disconnected grid. Oh well :(
​Reasonably priced tubes do seem to ​pop up from time to time, so all hope
is not lost.

Malcolm


Bert Haskins
 

On 2/15/2016 2:00 PM, Tom Gardner tggzzz@... [TekScopes] wrote:

On 14/02/16 11:18, aodiversen@... [TekScopes] wrote:

Hi Tom,
Thinking of Ed's rejuvenation experiments with his 576 CRT, I
suggest you
might test for connectivity of the grid electrode in a similar way.
All you
have to do is remove the CRT socket, supply the correct heater
voltage to the
CRT and check that the "circuit" between grid and kathode pins
conducts as a
diode. Use some tens of volts of DC supply and a large series
resistor to make
g positive w.r.t. k, and measure the current. A few uA max is enough to
distinguish between connected and disconnected grid terminal.
Albert
Good idea; thanks.

Unfortunately using 50V confirms a disconnected grid. Oh well :(
I have a good 154-0777-00 ( 465M ) CRT.
Is there anyway to find out what the difference might be?





Tad
 

The 0777 CRT will run just fine in a 465...I've done it many times.
You'll have a nice scope when you're finished.

Tad


Tom Gardner
 

On 16/02/16 00:15, fqo63ta@... [TekScopes] wrote:

The 0777 CRT will run just fine in a 465...I've done it many times.
This raises the question of how to determine which tubes can work in which scopes, especially tubes (such at the 0777) that weren't originally used in the (465) scope. My only sources are:
http://www.reprise.com/host/tektronix/reference/crt.asp
and
http://www.sphere.bc.ca/test/tek-crts.html

Are there any others?


Brian Bloom
 

I sent you a link to a 0777 CRT brand new for uber cheap over on EEV. I don't know how to PM anyone on here. I hope you scoop it up in time!!! I'm quite tempted to snatch it up myself, but I know that you actually need it.


Tom Gardner
 

On 16/02/16 00:15, fqo63ta@... [TekScopes] wrote:

The 0777 CRT will run just fine in a 465...I've done it many times.
You'll have a nice scope when you're finished.
I notice the 465M CRT voltages are different to those in the 465 (e.g. grid 2kV
vs 2.45kV).

When you retrofit an 0777 in a 465, which voltages do you use?


Tom Gardner
 

On 16/02/16 23:41, analogaddict013@... [TekScopes] wrote:

I sent you a link to a 0777 CRT brand new for uber cheap over on EEV. I don't
know how to PM anyone on here. I hope you scoop it up in time!!! I'm quite
tempted to snatch it up myself, but I know that you actually need it.
Thanks; I've only just noticed that PM. It is the first time I've seen an
individual make such a cross-domain link :)

It isn't as cheap as it first appears!
That one is £17.44+32.78 postage = £50.22
Another one is £34.94+12.55 = £47.49
I don't know why the P&P is so different. Both go via the "global shipping
programme", so (given the content of the other long thread) I'm not entirely
sure whether there might be extra hidden charges :(

And then there's a 0731 for £69 from Greece. Choices choices...


Tad
 

When looking at replacement possibilities, I check the CRT's physical size. Then I look at the pin out. Next are the operating voltages. A minor difference in voltages doesn't matter...as long as they are close.
I have used 2246 CRT's in 465's, and vice versa...they run just fine. In that case you have to do some diddleing with the HV lead, but it's just long enough.

HTH... Tad


Brian Bloom
 

The one reason why the 0777 stands out is that it is NOS. There has never been an opportunity for the phosphor to get used/burnt up.

I don't have a clue of whether anyone here in the US could save you anything on shipping costs. The shipping for me would be $17.58usd, and it's on the other side of the country. I've got no clue as to the actual cost of shipping from me to you.


Tom Gardner
 

On 18/02/16 00:10, analogaddict013@... [TekScopes] wrote:

The one reason why the 0777 stands out is that it is NOS. There has never been
an opportunity for the phosphor to get used/burnt up.
Yes.

Is there any significant chance of leaks and gas ingress? (You can tell I don't
know much about valves/tubes :) )

I don't have a clue of whether anyone here in the US could save you anything
on shipping costs. The shipping for me would be $17.58usd, and it's on the
other side of the country. I've got no clue as to the actual cost of shipping
from me to you.
I wouldn't want put someone else to any trouble. The money saved, if any, would
be small and outweighed by the inconvenience.

The US internal shipping cost is a useful point of information. From my end I
see the other tube is "Postage: US $17.96 (approx. £12.55)", so it is all a bit
of a mystery to me.


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


Brian Bloom
 

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

On 18/02/16 00:10, analogaddict013@... mailto:analogaddict013@... [TekScopes] wrote:
>>
>> The one reason why the 0777 stands out is that it is NOS. There has never been
>> an opportunity for the phosphor to get used/burnt up.
>>
>Yes.

>Is there any significant chance of leaks and gas ingress? (You can tell I don't
>know much about valves/tubes :) )

I'm also fairly new to all of this myself, but from what I do know, such a tube would have been 'factory checked' and would not have been sold by Tektronix if it had any issues. As for whether its storage/handling between then and now could have degraded it or caused issues, I don't know.

However, I do see a big difference between "NOS" and "Tested" in regard to tubes. ..If only the seller could take it for a quick spin in a proper working scope in order to confirm it's condition. But at that price, I doubt it's worth it to the seller to go thru that much effort in order to sell it. I suppose it couldn't hurt to ask, though.


 

Ask the seller if there is any lose broken glass bits in the neck of the tube. Also, ask about the getter flash coating on the neck. It must be silver and not dull white. The neck pins should also be carefully protected for the shipping process. You can't just wrap the tube in bubble wrap and expect it to survive shipping damage.

Ideally the tube still has the factory shipping package.

Regards

----- Original Message -----
From: analogaddict013@... [TekScopes]
To: TekScopes@...
Sent: Wednesday, February 17, 2016 7:32 PM
Subject: Re: [TekScopes] Re: Tek 465 intensity problem






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

On 18/02/16 00:10, analogaddict013@... mailto:analogaddict013@... [TekScopes] wrote:
>>
>> The one reason why the 0777 stands out is that it is NOS. There has never been
>> an opportunity for the phosphor to get used/burnt up.
>>
>Yes.

>Is there any significant chance of leaks and gas ingress? (You can tell I don't
>know much about valves/tubes :) )

I'm also fairly new to all of this myself, but from what I do know, such a tube would have been 'factory checked' and would not have been sold by Tektronix if it had any issues. As for whether its storage/handling between then and now could have degraded it or caused issues, I don't know.

However, I do see a big difference between "NOS" and "Tested" in regard to tubes. ..If only the seller could take it for a quick spin in a proper working scope in order to confirm it's condition. But at that price, I doubt it's worth it to the seller to go thru that much effort in order to sell it. I suppose it couldn't hurt to ask, though.