tektronix D755 CRT problem?

[IZ8DWF]

Hello all,

I hope this newsgroup is still alive, I see too much spam lately

I'm repairing an old telequipment/tektronix D755 scope for a friend.
So far it started with no power supply voltages, turned out to be
some cold solder joints in the power supply PCB.
Then it was badly out of focus, tracked down to open high value
resistors on the EHT PCB. After mounting back the EHT pcb it developed
a short on the +105V rail (which wasn't there before). It turned out
to be two shorted transistors on the Z amplifier PCB.
One transistor is a BC207 and the other is reported on the manual as
FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm
going to substitute it with an high voltage NPN (it sits right
between the 105V supply and ground).
What seems strange to me is these two transistors dying after
repairing the EHT board. However the EHT is now fully functional and I
have a clean focused track, however with full brightness until I
substitute the two transistors in the Z board.
I also set the -2500V cathode bias correctly in the mean time.
Now I have a full scan track on the CRT, time base seems to work
correctly and also track vertical position works, but I see a very
strange effect on the track:
with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods; going
further up the circular arc changes direction (track goes down instead
of up vertically) and behaves in a specular manner.
Now I don't know if there's something that needs tuning in the X or Y
drivers,
unfortunately this scope requires specialized calibration plugins to
go through all calibration steps, so I won't be able to follow the
exact calibration procedure.
However it seems to me that this kind of fault could be due to a bad
CRT, I'm not sure the Y voltage on the plaques should be variable
during a bare horizontal sweep, but also I'm not a great expert of
scopes.
Any hint is welcome.

Best regards
Francesco IZ8DWF

 

[N_Cook]

IZ8DWF <francesco.messineo@gmail.com> wrote in message
news:09a6b09e-4b5b-4715-a795-34c5e0863fe9@w10g2000yqf.googlegroups.com...

Hello all,

I hope this newsgroup is still alive, I see too much spam lately

I'm repairing an old telequipment/tektronix D755 scope for a friend.
So far it started with no power supply voltages, turned out to be
some cold solder joints in the power supply PCB.
Then it was badly out of focus, tracked down to open high value
resistors on the EHT PCB. After mounting back the EHT pcb it developed
a short on the +105V rail (which wasn't there before). It turned out
to be two shorted transistors on the Z amplifier PCB.
One transistor is a BC207 and the other is reported on the manual as
FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm
going to substitute it with an high voltage NPN (it sits right
between the 105V supply and ground).
What seems strange to me is these two transistors dying after
repairing the EHT board. However the EHT is now fully functional and I
have a clean focused track, however with full brightness until I
substitute the two transistors in the Z board.
I also set the -2500V cathode bias correctly in the mean time.
Now I have a full scan track on the CRT, time base seems to work
correctly and also track vertical position works, but I see a very
strange effect on the track:
with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods; going
further up the circular arc changes direction (track goes down instead
of up vertically) and behaves in a specular manner.
Now I don't know if there's something that needs tuning in the X or Y
drivers,
unfortunately this scope requires specialized calibration plugins to
go through all calibration steps, so I won't be able to follow the
exact calibration procedure.
However it seems to me that this kind of fault could be due to a bad
CRT, I'm not sure the Y voltage on the plaques should be variable
during a bare horizontal sweep, but also I'm not a great expert of
scopes.
Any hint is welcome.

Best regards
Francesco IZ8DWF

Telequipment was taken over by Tektronix about that time,
maybe this repair tip is useful
Telequipment D63 scope.
3 faults
1 High voltage discharge crackling noise. Even in the dark it was not
possible
to see where the discharge was so probably in the step-up transformer or the
quintupler. The display was too bright and
post deflection accelerator voltage was too high so much reduced trace size
in X and Y sense.The CRT 2cnd electrode should have been
-1400V but was -2200V so EHT probably 19KV rather than 12KV. The EHT
oscillator
transistor had been replaced by a BD131. Changing the biasing pot to the
preliminary
drive transistor made no effect. The primary of the oscillator o/p
transformer
was 56V pk-pk not the correct 38V. Although the DC voltage varied from 60V
to
100V altering this pot. One of the DC divider chain of 3x 5.1M was o/c so
no -ve
feedback component to the oscillator.
2 Traces in X sense squashed up to left of screen. One of the SPS5286 final
X plate drivers was o/c replaced with BF338.
3 The third trace (Right hand PI) was static bright line with no Y
displacement.
A trace to 330 ohm,1.5W dropper from 105V to one of the Y2 driver
transistors was loose
and touching another trace. With this 330 removed and no 105V then no 3rd
trace, only
appearing on beam finding activation.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

 

[Michael A. Terrell]

IZ8DWF wrote:

Hello all,

I hope this newsgroup is still alive, I see too much spam lately

That's because you're posting through Google, where the spam comes
from. Get a real NNTP news serve and you will see very little.


--
You can't have a sense of humor, if you have no sense!

 

[frank]

Michael A. Terrell <mike.terrell@earthlink.net> wrote:

That's because you're posting through Google, where the spam comes
from. Get a real NNTP news serve and you will see very little.

ok, I did it and you're right (fortunately the old "tin" newsreader
still exists).
However is a bit a p.i.t.a. to remember (or find out) what news server
to use whenever I change ISP (I travel lot).

Best regards and sorry for the OT.

Frank IZ8DWF

 

[frank]

N_Cook <diverse@tcp.co.uk> wrote:

Telequipment was taken over by Tektronix about that time,
maybe this repair tip is useful

Well, yes, it at least gives me an idea on the SPS5286 substitution.
I'm still very curious to understand if the fault I'm observing can be
caused by an electronic problem or if it's definitely a bad CRT, that
changes very much the time I'm going to spend fixing this scope.
Thanks and best regards.

Frank IZ8DWF

 

[frank]

nesesu <neil_sutcliffe@telus.net> wrote:

with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods; going
further up the circular arc changes direction (track goes down instead
of up vertically) and behaves in a specular manner.
Now I don't know if there's something that needs tuning in the X or  Y
drivers,
unfortunately this scope requires specialized calibration plugins to
go through all calibration steps, so I won't be able to follow the
exact calibration procedure.
However it seems to me that this kind of fault could be due to a bad
CRT, I'm not sure the Y voltage on the plaques should be variable
during a bare horizontal sweep, but also I'm not a great expert of
scopes.
Any hint is welcome.

On the 'no signal' trace distortion, check the position of the trace
vertical position pot for centre of rotation and see if the trace is
in approximately the centre of the screen [vertically]. It is possible
that the DC balance of the vertical amplifier is way off centre due to
misadjustment or a component failure in the vertical amplifier or
failure of one of the V+ or V- supply lines into the amp. I have seen
what I think you are describing if the trace is shifted off screen
due to a large DC component on the measured signal and trying to bring
it on screen with the vertical position control.

the vertical position control works as expected. The track is only
distorted in the vicinity of the right side and when it is positioned in
the upper half of the screen.

have very poor
resistors. I eventually changed out all resistors in the power supply
and several in the trigger circuits. The high value ones [10meg] in
the feedback divider mostly went open circuit causing the EHT to
skyrocket, and the metal film ones in the trigger circuits became very
temperature sensitive and would drift like crazy. Otherwise a nice
60MHz 'scope. Still going strong.

Yes, this had 4 open resistors in the EHT board, one 10M, two 4M7 and
one 3M9. So this scope probably suffered for a long time from too high
EHT, I don't know wether this can damage the CRT and result in the very
localized track distortion I'm observing.
After replacing the EHT resistors and reconnecting everything it
developed a short in the 105V supply caused by two transistors in the Z
grid controller going short circuit. This fact still puzzles me, but I
measured the voltages on the two dead transistor's sockets and they look
very normal.
I checked already all resistors in the power supply board, in the EHT
board and in the Z driver board.
I'm in travel until the end of the week, so I won't be able to replace
the two blown transistors before next saturday and see what happens.
Thanks for all suggestions.

Frank IZ8DWF

 

[nesesu]

On Sep 7, 7:23 am, IZ8DWF <francesco.messi...@gmail.com> wrote:

Hello all,

I hope this newsgroup is still alive, I see too much spam lately

I'm repairing an old telequipment/tektronix D755 scope for a friend.
So far it started with no  power supply voltages, turned out to be
some cold solder joints in the power supply PCB.
Then it was badly out of focus, tracked down to open high value
resistors on the EHT PCB. After mounting back the EHT pcb it developed
a short on the +105V rail (which wasn't there before). It turned out
to be two shorted transistors on the Z amplifier PCB.
One transistor is a BC207 and the other is reported on the manual as
FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm
going to substitute it with an high voltage NPN (it sits right
between  the 105V supply and ground).
What seems strange to me is these two transistors dying after
repairing the EHT board. However the EHT is now fully functional and I
have a clean  focused track, however with full brightness until I
substitute the two transistors in the Z board.
I also set the -2500V cathode bias correctly in the mean time.
Now I have a full scan track on the CRT, time base seems to work
correctly and also track vertical position works, but I see a very
strange effect on the track:
with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods; going
further up the circular arc changes direction (track goes down instead
of up vertically) and behaves in a specular manner.
Now I don't know if there's something that needs tuning in the X or  Y
drivers,
unfortunately this scope requires specialized calibration plugins to
go through all calibration steps, so I won't be able to follow the
exact calibration procedure.
However it seems to me that this kind of fault could be due to a bad
CRT, I'm not sure the Y voltage on the plaques should be variable
during a bare horizontal sweep, but also I'm not a great expert of
scopes.
Any hint is welcome.

Best regards
Francesco IZ8DWF

On the 'no signal' trace distortion, check the position of the trace
vertical position pot for centre of rotation and see if the trace is
in approximately the centre of the screen [vertically]. It is possible
that the DC balance of the vertical amplifier is way off centre due to
misadjustment or a component failure in the vertical amplifier or
failure of one of the V+ or V- supply lines into the amp. I have seen
what I think you are describing if the trace is shifted off screen
due to a large DC component on the measured signal and trying to bring
it on screen with the vertical position control.
The Telequipment 'scopes of that period [I have a D83] have very poor
resistors. I eventually changed out all resistors in the power supply
and several in the trigger circuits. The high value ones [10meg] in
the feedback divider mostly went open circuit causing the EHT to
skyrocket, and the metal film ones in the trigger circuits became very
temperature sensitive and would drift like crazy. Otherwise a nice
60MHz 'scope. Still going strong.

Neil S.

 

[Michael A. Terrell]

frank wrote:

Michael A. Terrell <mike.terrell@earthlink.net> wrote:

That's because you're posting through Google, where the spam comes
from. Get a real NNTP news serve and you will see very little.

ok, I did it and you're right (fortunately the old "tin" newsreader
still exists).
However is a bit a p.i.t.a. to remember (or find out) what news server
to use whenever I change ISP (I travel lot).

Best regards and sorry for the OT.

No problem. The more people we can get off of Google Groups the
better. Then you can use Nfilter to drop anything posted from Google,
and not see enough spam to worry about.


--
You can't have a sense of humor, if you have no sense!

 

[Jorgen Lund-Nielsen]

IZ8DWF wrote:

Hello all,

-snip-

Now I have a full scan track on the CRT, time base seems to work
correctly and also track vertical position works, but I see a very
strange effect on the track:
with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods;

However it seems to me that this kind of fault could be due to a bad
CRT,

The effect is independent of the sweep rate?
I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.
If it depends on sweep speed there is a very little chance for ripple
on the supply to show some weird screens but that's normaly not position
sensitive.

Jorgen
dj0ud

 

[Jim Yanik]

Jorgen Lund-Nielsen <jorgen.lund-nielsen@xyz123desy.de> wrote in
news:h8aj2v$fvr$1@it-news01.desy.de:

IZ8DWF wrote:

Hello all,


-snip-

Now I have a full scan track on the CRT, time base seems to work
correctly and also track vertical position works, but I see a very
strange effect on the track:
with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods;


However it seems to me that this kind of fault could be due to a bad
CRT,

The effect is independent of the sweep rate?
I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.
If it depends on sweep speed there is a very little chance for ripple
on the supply to show some weird screens but that's normaly not position
sensitive.

Jorgen
dj0ud

could be a geometry problem with the CRT;
input a square wave signal,one cycle per division,turn up intensity to see
the (fast)rising edges,and check for distortion.
If your tube is bad,this will show it.
If so,the internal mesh lens(final gun element) is probably boogered.
you may even see or hear tiny bits of glass rattling inside,from a cracked
E-gun rod.Those usually show up on the phosphor as dark or bright
spots,though.

One very rare possibility;look for tiny magnets(beam centering magnets)
that have come loose and migrated elsewhere on the tube or MU-
shield.they're about 2mm in diameter and length.

CRTs typically have some trace bowing,should be less than a minor
division (0.2div) at top and bottom of graticule.

This assumes all power supplies are at proper V and within ripple specs.

--
Jim Yanik
jyanik
at
kua.net

 

[frank]

Jorgen Lund-Nielsen <jorgen.lund-nielsen@xyz123desy.de> wrote:

However it seems to me that this kind of fault could be due to a bad
CRT,

The effect is independent of the sweep rate?

yes, totally independent of sweep rate. Track bending is always at the
same position and of the same entity.

I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.

That's also my guess, but as I said, I'm not at
all an expert of scopes and CRT.
Maybe not a deflection plate as for more than half screen the track
looks absolutely perfect? Could it be a problem caused by too high
EHT for a long time?

Thanks
Frank IZ8DWF

 

[frank]

Jim Yanik <jyanik@abuse.gov> wrote:

[snip]

However it seems to me that this kind of fault could be due to a bad
CRT,

The effect is independent of the sweep rate?
I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.
If it depends on sweep speed there is a very little chance for ripple
on the supply to show some weird screens but that's normaly not position
sensitive.

Jorgen
dj0ud


could be a geometry problem with the CRT;
input a square wave signal,one cycle per division,turn up intensity to see
the (fast)rising edges,and check for distortion.

I'm going to do this test on saturday when back home.

If your tube is bad,this will show it.
If so,the internal mesh lens(final gun element) is probably boogered.
you may even see or hear tiny bits of glass rattling inside,from a cracked
E-gun rod.Those usually show up on the phosphor as dark or bright
spots,though.

There's a totally dark spot in the lower half of the CRT, not big,
probably it's the size of the electron beam when focused in a single
dot. Infacts I thought it's due to a burning caused for just leaving a
bright dot for a too long time on the CRT (just guessing of course).
I haven't found any other spot with noticeable brightness difference.

One very rare possibility;look for tiny magnets(beam centering magnets)
that have come loose and migrated elsewhere on the tube or MU-
shield.they're about 2mm in diameter and length.

I had thought about magnets too, but couldn't find any around.
I'm not sure about what a MU-shield is, but the whole neck of the tube
is inside a metallic tube that goes from the socket to the point where
the tube starts to enlarge. Of course it has openings for the X and Y
plates connections.

CRTs typically have some trace bowing,should be less than a minor
division (0.2div) at top and bottom of graticule.

This assumes all power supplies are at proper V and within ripple specs.

power supplies are at proper V, I didn't check for ripples (and I will
do anyway) but I'd assume that deflection artifacts due to supply ripple
wouldn't appear only on a spot less than a quarter of the whole CRT
area.

Thanks
Frank IZ8DWF

 

[Jim Yanik]

frank <frank@ottone.venere.it> wrote in
news:4aa90d39$0$697$5fc30a8@news.tiscali.it:

Jorgen Lund-Nielsen <jorgen.lund-nielsen@xyz123desy.de> wrote:


However it seems to me that this kind of fault could be due to a bad
CRT,

The effect is independent of the sweep rate?

yes, totally independent of sweep rate. Track bending is always at the
same position and of the same entity.

I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.

That's also my guess, but as I said, I'm not at
all an expert of scopes and CRT.
Maybe not a deflection plate as for more than half screen the track
looks absolutely perfect?

Could it be a problem caused by too high
EHT for a long time?

Not trace geometry problems.
too much beam current can burn the phosphor and create a dark spot or line.
and raising the EHT voltage shrinks the display size and then your gain
adjusts don't have enough range.

Deflection plates are pretty hard to bend.It's more likely that the glass
rods that hold the plates in position are broken or cracked,or a mesh
expansion lens has had it's screen distorted or torn.I don't know if your
CRT has the mesh lens,though.(they result in a less focused or more fuzzy
beam spot.)


--
Jim Yanik
jyanik
at
kua.net

 

[frank]

Jim Yanik <jyanik@abuse.gov> wrote:

The effect is independent of the sweep rate?

yes, totally independent of sweep rate. Track bending is always at the
same position and of the same entity.

I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.


Deflection plates are pretty hard to bend.It's more likely that the glass
rods that hold the plates in position are broken or cracked,or a mesh
expansion lens has had it's screen distorted or torn.I don't know if your
CRT has the mesh lens,though.(they result in a less focused or more fuzzy
beam spot.)

I've replaced the two dead transistors in the Z driver board, now the
intensity control works. I replaced the unknown FRB749/SPS5286 with a
BF393 (probably overkill, but I hadn't anything else with the same
E-B-C configuration).
The focus control is however dependent on what's displayed on the CRT, I
have to tweak it when passing from a straight line to a 4 or more
divisions sine wave.
I have made two screenshots of the CRT distortion, one 20 MHz sinewave
and one 50 MHz sinewave both occupying most of the CRT area. Any way to
make them public easy?
If someone is curious and wants to have a look at them, send me an email
to iz8dwf at amsat dot org.

Best regards

Frank IZ8DWF

 

[frank]

Jim Yanik <jyanik@abuse.gov> wrote:

could be a geometry problem with the CRT;
input a square wave signal,one cycle per division,turn up intensity to see
the (fast)rising edges,and check for distortion.

the vertical edges are straigth on the "good" part of the CRT area, they
just aren't focused, look a bit broader than the horizontal lines.
Distortion is visible on the "bad" part of the CRT, but that's with any
kind of signal.

Regards.
Frank IZ8DWF

 

[Bob Larter]

frank wrote:

Jim Yanik <jyanik@abuse.gov> wrote:
The effect is independent of the sweep rate?
yes, totally independent of sweep rate. Track bending is always at the
same position and of the same entity.

I would expect a hard mechanical hit has loosen or bended a deflection
plate,
that often shows such effects and they are then sweeptime independent.
Deflection plates are pretty hard to bend.It's more likely that the glass
rods that hold the plates in position are broken or cracked,or a mesh
expansion lens has had it's screen distorted or torn.I don't know if your
CRT has the mesh lens,though.(they result in a less focused or more fuzzy
beam spot.)



I've replaced the two dead transistors in the Z driver board, now the
intensity control works. I replaced the unknown FRB749/SPS5286 with a
BF393 (probably overkill, but I hadn't anything else with the same
E-B-C configuration).
The focus control is however dependent on what's displayed on the CRT, I
have to tweak it when passing from a straight line to a 4 or more
divisions sine wave.

Sounds like your EHT is dodgy. I'm sorry I can't be any more help than that.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

 

[frank]

Hi all,
I had time to investigate further the problem on this scope and I just
found out that the metallic shield around the CRT's neck is slightly
magnetized. I suppose this is a mu-metal tube to shield external
magnetic fields. I just don't know if a slight magnetizatization is
normal. The shield attracts a small blade in every place I tried, so
seems like a total magnetization.
Comments and hints are welcome.

Best regards
Frank IZ8DWF

 

[Jim Yanik]

frank <frank@ottone.venere.it> wrote in
news:4aca1238$0$694$5fc30a8@news.tiscali.it:

Hi all,
I had time to investigate further the problem on this scope and I just
found out that the metallic shield around the CRT's neck is slightly
magnetized. I suppose this is a mu-metal tube to shield external
magnetic fields. I just don't know if a slight magnetizatization is
normal. The shield attracts a small blade in every place I tried, so
seems like a total magnetization.
Comments and hints are welcome.

Best regards
Frank IZ8DWF

AFAIK,the CRT shield should not be magnetic.
Are you sure your blade is not magnetized?

try a TV degaussing coil to remove the magnetism from the shield.

--
Jim Yanik
jyanik
at
localnet
dot com

 

[frank]

Jim Yanik <jyanik@abuse.gov> wrote:

frank <frank@ottone.venere.it> wrote in
news:4aca1238$0$694$5fc30a8@news.tiscali.it:



AFAIK,the CRT shield should not be magnetic.
Are you sure your blade is not magnetized?

you're right, I picked a magnetized blade
I checked with another one and this time I couldn't find any
magnetization on the shield.
So problem "solved".

try a TV degaussing coil to remove the magnetism from the shield.

it has none, as far as I can tell at least. I read that a mu-metal
shield shouldn't magnetize at all.

Thanks for hint and best regards

Frank

 

[Sjouke Burry]

frank wrote:

Hi all,
I had time to investigate further the problem on this scope and I just
found out that the metallic shield around the CRT's neck is slightly
magnetized. I suppose this is a mu-metal tube to shield external
magnetic fields. I just don't know if a slight magnetizatization is
normal. The shield attracts a small blade in every place I tried, so
seems like a total magnetization.
Comments and hints are welcome.

Best regards
Frank IZ8DWF
Probably the blade is magnetized, it is made of hardened steel,

and can easily be magnitized.
The Mu metal attracts any magnet quite well, because it is so
soft(magnetically).