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485 super weak brightness control

 

Hi folks,

I acquired some time ago 485 and finally today I managed to spend some time with it.

Cleaning of the switches and moving all the controls back and forth several times brought back most things so I could see both channels working.

Most challenging was/is the

TIME base,
wouldn't scale correctly but after really many turns it now basically does what it should with exception of the last 3 fastest settings. There the picture is all to the side and slowly drifts in but it's very distorted and quite clearly there's something wrong.

I can also see ghost signals like if there was massive jitter or even better described some bounced signals as if the transmission wasnt terminated and I think I can see the signal partly during blank phase. Things are better at higher frequencies and worse on lower. When I enable second time base it's only very faint picture only visible with B intensity at max. But when I move the display all the way to the right I can see very bright spots on the left side so maybe the horizontal signal doesn't have proper connection somewhere and only deflection I can see is some crosstalk?

power supply

all the low voltages look OK, but the high voltage 2950V I can only measure some 2000 volts. It is my first time using 1000x probe for multimeter so I have checked on my lab psu and 30V was correctly showing as 30mV. So it's likely the voltage coming from the multiplier is too low unless the voltmeter impedance despite being so high could drop it down?
Brightness settings both total and B have basically no effect what so ever. They do something but very fine adjustment only.

Fan isn't spinning, any replacement ideas?

Channel 2

1x LED doesn't work
The 1M/50Ohm illumination doesn't work
The 10x and 100x works ok
There was a tantalum in the ch2 attenuator 22uF/15V I replaced because it had a head blown off but it didn't really change anything.
I tested around U80 and shorted the pins associated with lightbulb for 1M/50 and reset lightbulb and pin associated with 1x diode and it looks like these are faulty because they don't light up and there is correct 5V on the other side.

Overall the picture probably could be adjusted to be rather nice and sharp if the brightness could be turned down and the ghost images resolved.
Appreciate if those who been through 485 before could give me some heads up what to look at first and where to look to solve the issues.

P. S. It's got 7 small knobs missing, anybody happen to have some spare parts donor?
 

I checked the DC recovery diodes in the HV supply as listed in other thread
(all under the plastic cover and they are ok.
Any other suggestions?

On Mon, 15 Mar 2021, 22:30 Ondrej Pavelka, <info@...>
wrote:

Hi folks,

I acquired some time ago 485 and finally today I managed to spend some
time with it.

Cleaning of the switches and moving all the controls back and forth
several times brought back most things so I could see both channels
working.

Most challenging was/is the

TIME base,
wouldn't scale correctly but after really many turns it now basically does
what it should with exception of the last 3 fastest settings. There the
picture is all to the side and slowly drifts in but it's very distorted and
quite clearly there's something wrong.

I can also see ghost signals like if there was massive jitter or even
better described some bounced signals as if the transmission wasnt
terminated and I think I can see the signal partly during blank phase.
Things are better at higher frequencies and worse on lower. When I enable
second time base it's only very faint picture only visible with B intensity
at max. But when I move the display all the way to the right I can see very
bright spots on the left side so maybe the horizontal signal doesn't have
proper connection somewhere and only deflection I can see is some
crosstalk?

power supply

all the low voltages look OK, but the high voltage 2950V I can only
measure some 2000 volts. It is my first time using 1000x probe for
multimeter so I have checked on my lab psu and 30V was correctly showing as
30mV. So it's likely the voltage coming from the multiplier is too low
unless the voltmeter impedance despite being so high could drop it down?
Brightness settings both total and B have basically no effect what so
ever. They do something but very fine adjustment only.

Fan isn't spinning, any replacement ideas?

Channel 2

1x LED doesn't work
The 1M/50Ohm illumination doesn't work
The 10x and 100x works ok
There was a tantalum in the ch2 attenuator 22uF/15V I replaced because it
had a head blown off but it didn't really change anything.
I tested around U80 and shorted the pins associated with lightbulb for
1M/50 and reset lightbulb and pin associated with 1x diode and it looks
like these are faulty because they don't light up and there is correct 5V
on the other side.

Overall the picture probably could be adjusted to be rather nice and sharp
if the brightness could be turned down and the ghost images resolved.
Appreciate if those who been through 485 before could give me some heads
up what to look at first and where to look to solve the issues.

P. S. It's got 7 small knobs missing, anybody happen to have some spare
parts donor?
 


I checked the DC recovery diodes in the HV supply as listed in other thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM diode check doesn't discover problems.

It looks like the behavior of your 485 is different in different settings. As the first issue are you debugging the intensity? Do you have too bright, or too dim brightness, does intensity control make any difference?

HV supply voltage of 2000V is low. What is the impedance of your 1000x probe? Looking at the schematic ¡°CATH REG¡± can only adjust ~ +/-100V so this is outside adjustment range. Should be the first thing to fix since everything else with intensity depends on it.

Feedback loop around U1624 uses +50V at the top of R1642B as the reference. Is +50V correct?
If you look at pins 2 and 3 of U1624 what voltage do you measure (should be essentially the same and close to 0V).

Once you fix the HV supply if intensity is still problematic:
There are multiple posts about DC restorer issues I recommend you search "DC restorer" "Grid Bias" for useful information.

The ones I know well are: Topic #180156: Although different equipment, this topic has some DC restorer debug. Topic #180235??is a 485 with intensity control issues. In particular looking at the circuitry around intensity control pots. If you review both chains from start there is some relevant information.

For time base issues, in the problematic 3 fastest settings how does the "A Sweep" and "A gate" signals look? They are easily available at the back of the scope with a BNC connection.
Ozan
 

Thanks so much for your reply, it makes it much easier to approach.

I cleaned up the HV test point and I get now some 2800V. What makes me
little bit nervous is I don't know if the cause was just the contact
cleaning or if something else have changed. Nevertheless it changed in the
right direction. I think the whole section under the plastic cover needs
cleaning it's covered in black dust.

I will de-solder the diodes in the DC restorer and measure leakage current.

The intensity is too bright

I'll get back to you when I follow up on the other measurements tomorrow.
The HV probe is labeled as HV-40 but can't find any manufacturer markings.
It's definitely from 80s or 70s

On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:


I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

It looks like the behavior of your 485 is different in different settings.
As the first issue are you debugging the intensity? Do you have too bright,
or too dim brightness, does intensity control make any difference?

HV supply voltage of 2000V is low. What is the impedance of your 1000x
probe? Looking at the schematic ¡°CATH REG¡± can only adjust ~ +/-100V so
this is outside adjustment range. Should be the first thing to fix since
everything else with intensity depends on it.

Feedback loop around U1624 uses +50V at the top of R1642B as the
reference. Is +50V correct?
If you look at pins 2 and 3 of U1624 what voltage do you measure (should
be essentially the same and close to 0V).

Once you fix the HV supply if intensity is still problematic:
There are multiple posts about DC restorer issues I recommend you search
"DC restorer" "Grid Bias" for useful information.

The ones I know well are: Topic #180156: Although different equipment,
this topic has some DC restorer debug. Topic #180235 is a 485 with
intensity control issues. In particular looking at the circuitry around
intensity control pots. If you review both chains from start there is some
relevant information.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the back
of the scope with a BNC connection.
Ozan
 

I cleaned up the HV test point and I get now some 2800V. What makes me
little bit nervous is I don't know if the cause was just the contact
cleaning or if something else have changed. Nevertheless it changed in the
right direction. I think the whole section under the plastic cover needs
cleaning it's covered in black dust.
It may be some conductive residue. HV section have some high M-ohm resistors, any conductive residue will change things. 2800V is low (looking at +50V supply and opamp inputs is still a good debug flow for HV) but probably close enough for intensity debug. Too bright intensity is not good for the screen so it is better to take care of it sooner than later.

I will de-solder the diodes in the DC restorer and measure leakage current.
Before you desolder the diodes, can you check if you can control brightness with "GRID BIAS" adjustment (R1660)? Also, as you change intensity knob, can you control the voltage at "Z OUT DC" test point (TP1781), and at what range?

Ozan
 

Hi,

After having repaired several scopes (not only Tek's, but other brands also) with this symptom they invariably had leaky DC restorer diodes. Testing these diodes for leakage must be done at high voltages, because at normal DVM or component tester applied voltages they usually seems to be ok, but this is not true. Since the main work is to reach and remove the diodes, I would replace all of them, period.

This also applies to the focus circuit DC restorer if one is used. The symptom there is lack of control over the focus.? The way the DC restorers circuit operates may cause stresses in these diodes that make them to fail. I don't know the exact explanation.

Regards,

Ignacio EB4APL

El 16/03/2021 a las 23:33, Ondrej Pavelka escribi¨®:
Thanks so much for your reply, it makes it much easier to approach.

I cleaned up the HV test point and I get now some 2800V. What makes me
little bit nervous is I don't know if the cause was just the contact
cleaning or if something else have changed. Nevertheless it changed in the
right direction. I think the whole section under the plastic cover needs
cleaning it's covered in black dust.

I will de-solder the diodes in the DC restorer and measure leakage current.

The intensity is too bright

I'll get back to you when I follow up on the other measurements tomorrow.
The HV probe is labeled as HV-40 but can't find any manufacturer markings.
It's definitely from 80s or 70s

On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:

I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

It looks like the behavior of your 485 is different in different settings.
As the first issue are you debugging the intensity? Do you have too bright,
or too dim brightness, does intensity control make any difference?

HV supply voltage of 2000V is low. What is the impedance of your 1000x
probe? Looking at the schematic ¡°CATH REG¡± can only adjust ~ +/-100V so
this is outside adjustment range. Should be the first thing to fix since
everything else with intensity depends on it.

Feedback loop around U1624 uses +50V at the top of R1642B as the
reference. Is +50V correct?
If you look at pins 2 and 3 of U1624 what voltage do you measure (should
be essentially the same and close to 0V).

Once you fix the HV supply if intensity is still problematic:
There are multiple posts about DC restorer issues I recommend you search
"DC restorer" "Grid Bias" for useful information.

The ones I know well are: Topic #180156: Although different equipment,
this topic has some DC restorer debug. Topic #180235 is a 485 with
intensity control issues. In particular looking at the circuitry around
intensity control pots. If you review both chains from start there is some
relevant information.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the back
of the scope with a BNC connection.
Ozan





--
El software de antivirus Avast ha analizado este correo electr¨®nico en busca de virus.
 

On Wed, 17 Mar 2021, 00:58 Ozan, <ozan_g@...> wrote:


I cleaned up the HV test point and I get now some 2800V. What makes me
little bit nervous is I don't know if the cause was just the contact
cleaning or if something else have changed. Nevertheless it changed in
the
right direction. I think the whole section under the plastic cover needs
cleaning it's covered in black dust.
It may be some conductive residue. HV section have some high M-ohm
resistors, any conductive residue will change things. 2800V is low (looking
at +50V supply and opamp inputs is still a good debug flow for HV) but
probably close enough for intensity debug. Too bright intensity is not good
for the screen so it is better to take care of it sooner than later.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.

50V point measures 49.73V



I will de-solder the diodes in the DC restorer and measure leakage
current.

Before you desolder the diodes, can you check if you can control
brightness with "GRID BIAS" adjustment (R1660)? Also, as you change
intensity knob, can you control the voltage at "Z OUT DC" test point
(TP1781), and at what range?


Grid bias control has no effect what so ever
The Z OUT DC can be controlled from -70 through -170 and finishes around
-50



Ozan
 

On Wed, 17 Mar 2021, 14:32 Ondrej Pavelka, <info@...>
wrote:



On Wed, 17 Mar 2021, 00:58 Ozan, <ozan_g@...> wrote:


I cleaned up the HV test point and I get now some 2800V. What makes me
little bit nervous is I don't know if the cause was just the contact
cleaning or if something else have changed. Nevertheless it changed in
the
right direction. I think the whole section under the plastic cover needs
cleaning it's covered in black dust.
It may be some conductive residue. HV section have some high M-ohm
resistors, any conductive residue will change things. 2800V is low (looking
at +50V supply and opamp inputs is still a good debug flow for HV) but
probably close enough for intensity debug. Too bright intensity is not good
for the screen so it is better to take care of it sooner than later.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.

50V point measures 49.73V


I will de-solder the diodes in the DC restorer and measure leakage
current.

Before you desolder the diodes, can you check if you can control
brightness with "GRID BIAS" adjustment (R1660)? Also, as you change
intensity knob, can you control the voltage at "Z OUT DC" test point
(TP1781), and at what range?


Grid bias control has no effect what so ever
The Z OUT DC can be controlled from -50 CCW through -170 and finishes
around -70 CW



Ozan
 

On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:


I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

It looks like the behavior of your 485 is different in different settings.
As the first issue are you debugging the intensity? Do you have too bright,
or too dim brightness, does intensity control make any difference?

HV supply voltage of 2000V is low. What is the impedance of your 1000x
probe? Looking at the schematic ¡°CATH REG¡± can only adjust ~ +/-100V so
this is outside adjustment range. Should be the first thing to fix since
everything else with intensity depends on it.

Feedback loop around U1624 uses +50V at the top of R1642B as the
reference. Is +50V correct?
If you look at pins 2 and 3 of U1624 what voltage do you measure (should
be essentially the same and close to 0V).
Pin 2 is zero pin 3 is 1.9V




Once you fix the HV supply if intensity is still problematic:
There are multiple posts about DC restorer issues I recommend you search
"DC restorer" "Grid Bias" for useful information.

The ones I know well are: Topic #180156: Although different equipment,
this topic has some DC restorer debug. Topic #180235 is a 485 with
intensity control issues. In particular looking at the circuitry around
intensity control pots. If you review both chains from start there is some
relevant information.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the back
of the scope with a BNC connection.
Both if the signals don't change past the 10nS setting they stay the same
for 5 2 and 1ns

Ozan
 

On Wed, Mar 17, 2021 at 08:45 AM, Ondrej Pavelka wrote:


On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:




I checked the DC recovery diodes in the HV supply as listed in other
thread

(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.
I would second this part of the post. I had a similar failure on a 475 and traced it to a bad restorer diode. The only way that I was able to locate the problem was using a 5 On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:


I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

I would second this part of the comment. I had a 475 with a similar failure and symptoms. A simple DMM diode test showed the diode as ¡°good¡±. When I removed it and tested it on the 576 curve tracer the problem became apparent, as soon as I checked it on the CT, it exhibited a very strange breakdown at higher voltages. So replacing the diode and associated capacitor with a known good diode and capacitor repaired the problem.

--
Michael Lynch
Dardanelle, AR
 

Behavior of your 485 is changing over time so some of these comments may not apply to current state of the scope but here is my feedback:

If you look at pins 2 and 3 of U1624 what voltage do you measure (should
be essentially the same and close to 0V).
Pin 2 is zero pin 3 is 1.9V
This means the regulation loop (U1624) thinks HV voltage is too low and working as hard as it can to increase the voltage but can't increase anymore (railed). When you look at pin 6 do you see it railed (>10V)? In this condition HV supply is not regulated anymore.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.
However, I also saw this post that now voltage is OK. Did pin 3 also changed to ~ 0V?

Does the HV voltage change when you turn intensity knob? Maybe there is some interaction and when you change some settings HV voltage is changing. Or something might be baking and recovering ...

The Z OUT DC can be controlled from -70 through -170 and finishes around -50
This should not be possible. If I am not missing a supply connection, minimum supply of that block is -5V. In normal operation "Z OUT DC" starts from ~ 14V and goes higher for lower intensity. What voltage do you see at the left side of R1792 (collector of Q1772)?

Only possibility I can see is a DC path from the HV line, which would also explain why HV supply is low. Suspect components are C1684, C1664, C1663. You can also try cleaning the HV area more thoroughly in case there is some conductive residue.

In almost all the "beam too bright" cases HV diodes were bad so it is a good idea to replace them all as a good measure. I am not an expert in HV DC restorers but if you look at past discussions UF4004 was recommended by Tom as a replacement. A random diode won't work.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the back
of the scope with a BNC connection.
Both if the signals don't change past the 10nS setting they stay the same for 5 2 and 1ns
I assume slower than 5ns you see a pulse at "A gate" and a triangle wave at "A sweep". In 5/2/1ns are they both flat? Flat at ground or some other voltage? Is the flat voltage same as the lowest point or highest point of proper signals (when sweep slower than 5ns)?

Ozan
 

I thoroughly cleaned the entire HV area with IPA and now the 2950V is
steady while before it would drift slowly down to 2800 and even further

The grid bias changes the voltage at TP1665 but does not have any visible
impact on the trace

On Wed, 17 Mar 2021, 15:54 Mlynch001, <mlynch002@...> wrote:

On Wed, Mar 17, 2021 at 08:45 AM, Ondrej Pavelka wrote:


On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:




I checked the DC recovery diodes in the HV supply as listed in other
thread

(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.
I would second this part of the post. I had a similar failure on a 475
and traced it to a bad restorer diode. The only way that I was able to
locate the problem was using a 5 On Tue, 16 Mar 2021, 22:42 Ozan, <
ozan_g@...> wrote:


I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.

I would second this part of the comment. I had a 475 with a similar
failure and symptoms. A simple DMM diode test showed the diode as ¡°good¡±.
When I removed it and tested it on the 576 curve tracer the problem
became apparent, as soon as I checked it on the CT, it exhibited a very
strange breakdown at higher voltages. So replacing the diode and
associated capacitor with a known good diode and capacitor repaired the
problem.

--
Michael Lynch
Dardanelle, AR
 

The grid bias changes the voltage at TP1665 but does not have any visible
impact on the trace
What voltage range do you see at TP1665 when you change grid bias. No need to re-measure, just ballpark numbers you remember. Especially the sign, positive or negative voltage.
Ozan
 

The HV area is now lot better

I have cleaned thoroughly the entire area with IPA and now I have steady
-2950. This also resolved the problems at the Obama where now I have
basically zeroes at pins 2 and 3 and 0.6V at pin 6, the grid bias seating
is changing the U1665 but has no visible impact on the trace.


The Z out DC is now 16 to 165V when I regulate this but there is no impact
on the trace


Regarding the timing

I can see triangle wave and pulse on respective outputs and they change
shape /frequency as I change the timing basically making them narrower but
this trend stops and they don't change shape in last few settings but
remain exactly the same. Screen is blank or picture drifts in partially
from the side but cannot be centered

On Wed, 17 Mar 2021, 16:46 Ozan, <ozan_g@...> wrote:

Behavior of your 485 is changing over time so some of these comments may
not apply to current state of the scope but here is my feedback:

If you look at pins 2 and 3 of U1624 what voltage do you measure
(should
be essentially the same and close to 0V).
Pin 2 is zero pin 3 is 1.9V
This means the regulation loop (U1624) thinks HV voltage is too low and
working as hard as it can to increase the voltage but can't increase
anymore (railed). When you look at pin 6 do you see it railed (>10V)? In
this condition HV supply is not regulated anymore.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.
However, I also saw this post that now voltage is OK. Did pin 3 also
changed to ~ 0V?

Does the HV voltage change when you turn intensity knob? Maybe there is
some interaction and when you change some settings HV voltage is changing.
Or something might be baking and recovering ...

The Z OUT DC can be controlled from -70 through -170 and finishes around
-50

This should not be possible. If I am not missing a supply connection,
minimum supply of that block is -5V. In normal operation "Z OUT DC" starts
from ~ 14V and goes higher for lower intensity. What voltage do you see at
the left side of R1792 (collector of Q1772)?

Only possibility I can see is a DC path from the HV line, which would also
explain why HV supply is low. Suspect components are C1684, C1664, C1663.
You can also try cleaning the HV area more thoroughly in case there is some
conductive residue.

In almost all the "beam too bright" cases HV diodes were bad so it is a
good idea to replace them all as a good measure. I am not an expert in HV
DC restorers but if you look at past discussions UF4004 was recommended by
Tom as a replacement. A random diode won't work.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the
back
of the scope with a BNC connection.
Both if the signals don't change past the 10nS setting they stay the same
for 5 2 and 1ns

I assume slower than 5ns you see a pulse at "A gate" and a triangle wave
at "A sweep". In 5/2/1ns are they both flat? Flat at ground or some other
voltage? Is the flat voltage same as the lowest point or highest point of
proper signals (when sweep slower than 5ns)?

Ozan
 

+75 to +110

On Wed, 17 Mar 2021, 17:37 Ozan, <ozan_g@...> wrote:


The grid bias changes the voltage at TP1665 but does not have any visible
impact on the trace
What voltage range do you see at TP1665 when you change grid bias. No need
to re-measure, just ballpark numbers you remember. Especially the sign,
positive or negative voltage.
Ozan
 

The UF4004 vs BAV21.

BAV is same speed with 50ns recovery but 250V, UF4004 is 400V and higher
current at the same speed.

I have BAV21 in stock, I'd have to order in the UF4004s

On Wed, 17 Mar 2021, 16:46 Ozan, <ozan_g@...> wrote:

Behavior of your 485 is changing over time so some of these comments may
not apply to current state of the scope but here is my feedback:

If you look at pins 2 and 3 of U1624 what voltage do you measure
(should
be essentially the same and close to 0V).
Pin 2 is zero pin 3 is 1.9V
This means the regulation loop (U1624) thinks HV voltage is too low and
working as hard as it can to increase the voltage but can't increase
anymore (railed). When you look at pin 6 do you see it railed (>10V)? In
this condition HV supply is not regulated anymore.

I'm not sure what's going on today I turned it on and I measure 2963V on
the HV spot.
However, I also saw this post that now voltage is OK. Did pin 3 also
changed to ~ 0V?

Does the HV voltage change when you turn intensity knob? Maybe there is
some interaction and when you change some settings HV voltage is changing.
Or something might be baking and recovering ...

The Z OUT DC can be controlled from -70 through -170 and finishes around
-50

This should not be possible. If I am not missing a supply connection,
minimum supply of that block is -5V. In normal operation "Z OUT DC" starts
from ~ 14V and goes higher for lower intensity. What voltage do you see at
the left side of R1792 (collector of Q1772)?

Only possibility I can see is a DC path from the HV line, which would also
explain why HV supply is low. Suspect components are C1684, C1664, C1663.
You can also try cleaning the HV area more thoroughly in case there is some
conductive residue.

In almost all the "beam too bright" cases HV diodes were bad so it is a
good idea to replace them all as a good measure. I am not an expert in HV
DC restorers but if you look at past discussions UF4004 was recommended by
Tom as a replacement. A random diode won't work.

For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the
back
of the scope with a BNC connection.
Both if the signals don't change past the 10nS setting they stay the same
for 5 2 and 1ns

I assume slower than 5ns you see a pulse at "A gate" and a triangle wave
at "A sweep". In 5/2/1ns are they both flat? Flat at ground or some other
voltage? Is the flat voltage same as the lowest point or highest point of
proper signals (when sweep slower than 5ns)?

Ozan
 

I have cleaned thoroughly the entire area with IPA and now I have steady
-2950. This also resolved the problems at the Obama where now I have
Please no politics in the group :)

basically zeroes at pins 2 and 3 and 0.6V at pin 6, the grid bias seating
is changing the U1665 but has no visible impact on the trace.
It looks like your HV supply is in good shape now.

The Z out DC is now 16 to 165V when I regulate this but there is no impact
on the trace
With these new measurements observations are pointing to faulty diodes.


Regarding the timing

I can see triangle wave and pulse on respective outputs and they change
shape /frequency as I change the timing basically making them narrower but
this trend stops and they don't change shape in last few settings but
remain exactly the same.
Flat at what voltage level? Compared to good waveforms are these waveforms at the max/min of the good waveforms are at a different DC voltage?

Ozan
 

Trust me it's the phones auto correct turning opamp into obama :)

I think I had the timebase measurements AC coupled and there was no change
in level. I haven't checked the DC. I will do that tomorrow when I'm back
at the workshop.

On Wed, 17 Mar 2021, 18:17 Ozan, <ozan_g@...> wrote:


I have cleaned thoroughly the entire area with IPA and now I have steady
-2950. This also resolved the problems at the Obama where now I have
Please no politics in the group :)

basically zeroes at pins 2 and 3 and 0.6V at pin 6, the grid bias seating
is changing the U1665 but has no visible impact on the trace.
It looks like your HV supply is in good shape now.

The Z out DC is now 16 to 165V when I regulate this but there is no
impact
on the trace
With these new measurements observations are pointing to faulty diodes.


Regarding the timing

I can see triangle wave and pulse on respective outputs and they change
shape /frequency as I change the timing basically making them narrower
but
this trend stops and they don't change shape in last few settings but
remain exactly the same.
Flat at what voltage level? Compared to good waveforms are these waveforms
at the max/min of the good waveforms are at a different DC voltage?

Ozan
 

Trust me it's the phones auto correct turning opamp into obama :)
I know I was just kidding. I also had my share of funny auto correct stories.

I think I had the timebase measurements AC coupled and there was no change
in level. I haven't checked the DC. I will do that tomorrow when I'm back
at the workshop.
The goal is to find if sweep trigger is not coming (no gate) or sweep trigger comes but sweep doesn't happen. DC levels are important to understand the state of the sweep circuit.

At the end most likely it will be a dirty switch. You can make a good guess which one by looking sheet <10> and see which switches only change in the last three settings. However, we can also pinpoint which one by electrical debug.

BAV21 looks OK as a replacement, it has better specs in some areas. The swing at the restorer input is ~ 100Vpp max, so the diodes in the grid bias area will not see more than 200V. Close to max spec but could be a good initial debug step. If others see a problem please chime in.

Ozan
 

So..... I changed following diodes for BAV21 and there was no change to the
behaviour.
CR1651
CR1646
CR1664
CR1666
CR1660
CR1656
CR1652

No intensity control and no grid bias control. I was desperate but then I
noticed there's another one hiding covered by a cap CR1663 and that was the
one!! Now I have nice sharp trace and working intensity control!

I need to calibrate it, I can get good image but as soon as I change either
of the intensity controls it gets blurry.


Now with the HV supply sorted I can progress further with the list of
problems

Vertical amp Channel 1 is in top condition all LEDs working lightbulb
working
Vertical amp channel 2 has the 1x LED gone and lightbulb gone. Otherwise
cleaning brought it back to 100% working order.

Horizontal A sweep

Works flawlessly from 5second to about 10uS. From 5uS the image starts ever
so slightly moving to the right but I still get enough to fill the full
screen when I use the horizontal position control because there is a
reserve, at 10nS I pretty much run out of the reserve. I can still center
it to have full 10 divisions but there is no reserve at all.
The 5 2 and 1nS settings are not working at all.



Horizontal B sweep.

I don't get any B sweep from 5s to 20ms then I get good B sweep for 10mS to
0.2uS
0.1uS to 10nS is showing only first 4 divisions from the left
5 2 and 1 ns is working correctly.

The delay time position potenciometer is not in best shape, it gets stuck
quite often and needs to be wiggled about to move it further. The latching
is somehow damaged.


Power supply fan.

Is quite picky, it sometimes runs and sometimes it does not.

On Wed, 17 Mar 2021, 19:17 Ozan, <ozan_g@...> wrote:


Trust me it's the phones auto correct turning opamp into obama :)
I know I was just kidding. I also had my share of funny auto correct
stories.

I think I had the timebase measurements AC coupled and there was no
change
in level. I haven't checked the DC. I will do that tomorrow when I'm back
at the workshop.
The goal is to find if sweep trigger is not coming (no gate) or sweep
trigger comes but sweep doesn't happen. DC levels are important to
understand the state of the sweep circuit.

At the end most likely it will be a dirty switch. You can make a good
guess which one by looking sheet <10> and see which switches only change in
the last three settings. However, we can also pinpoint which one by
electrical debug.

BAV21 looks OK as a replacement, it has better specs in some areas. The
swing at the restorer input is ~ 100Vpp max, so the diodes in the grid bias
area will not see more than 200V. Close to max spec but could be a good
initial debug step. If others see a problem please chime in.

Ozan

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