Topics

2235 power supply noise

Michael Pickwell <mikepickwell@...>
 

Hello folks,


This is my first post so please be gentle!


I have a 2235 which I occasionally use for amateur hobby use repairing audio equipment.


Just recently it has started making a high pitched whining sound which appears to be coming from the power supply section. Although it still *works* I find the noise worrying and I think the trace is also getting more fuzzy too.


My first thought was to check for tired electrolytic capacitors in the power supply. I had a look yesterday and none of them show any external signs of failure (i.e. bulging or leaking electrolyte).


My soldering skills are good and I have a decent DMM a steady hand and a keen eye but I'm no electronics engineer.


Any advice will be gratefully received.


Best regards, Mike

Nenad Filipovic
 

Hi Mike,

In most well designed equipment, electrolytic capacitors do not tend to
bulge and leak that easily like in cheap Chinese stuff. Your 2235 is surely
old enough to exceed the lifespan of capacitors in areas where ripple
currents are high (and have a high slew rate, typical for SMPS). My advice
would be to stop using the scope to prevent any potential damage, and
straightforwardly replace all electrolytic capacitors in the power supply,
with (perhaps) the exception of the mains bulk capacitor (the big one which
filters the rectified mains supply). The cost of full replacement is
usually less than the effort you would spend to find exactly the one
responsible for the symptoms, not to mention future proof repair. The
replacements should all be of the 105°C temperature rating and low ESR.

Filtering capacitors on the mains transformer secondary side are usually
the most important (these usually fail first due to stress), there you
should use some good reputable brand like Nippon Chemi-con, Rubycon,
Panasonic..., you should consult the catalogs to see which series are
recommended for SMPS, there exist some 2-3 letter markings for the series.
Also note the size, low ESR radial capacitors are usually tall and thin,
and sometimes it can be difficult to find a replacement that exactly fits.
You should not have a problem to use a slightly larger capacity rating than
the original.

Some peculiar SMPS designs sometimes employ special rating components, so
there is a slight chance that some of those "small, less significant"
capacitors is somehow special. More experienced members of this group might
help here, but anyways it is always a good practice to examine each part
you remove to check whether its ratings/specs are standard.

I'm no expert for 2xxx series, but you should also check the high voltage
power supply (if it is separate from the main power supply). Stresses in
high voltage low current converters are usually inferior compared to their
counterparts, but still any SMPS is suspect for eating out its own
(electrolytic) capacitors.

You should also be able to find good advice in the archives of this group,
capacitor replacements are very common, I'm sure more experienced members
have already covered this topic many times, much more precisely than me.

Best Regards,
Nenad FIlipovic


On Thu, Oct 5, 2017 at 8:21 AM, Michael Pickwell mikepickwell@...
[TekScopes] <TekScopes@...> wrote:



Hello folks,

This is my first post so please be gentle!

I have a 2235 which I occasionally use for amateur hobby use repairing
audio equipment.

Just recently it has started making a high pitched whining sound which
appears to be coming from the power supply section. Although it still
*works* I find the noise worrying and I think the trace is also getting
more fuzzy too.

My first thought was to check for tired electrolytic capacitors in the
power supply. I had a look yesterday and none of them show any external
signs of failure (i.e. bulging or leaking electrolyte).

My soldering skills are good and I have a decent DMM a steady hand and a
keen eye but I'm no electronics engineer.

Any advice will be gratefully received.

Best regards, Mike

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


Michael Pickwell <mikepickwell@...>
 

Thank you for the very helpful reply Nenad.

If someone familiar with this model could tell me which capacitors are the most likely culprits I'll get those changed first.

Will I need to do any recalibration after changing caps in the PSU?

Best regards, Mike



Sent from my Samsung device

-------- Original message --------
From: "Nenad Filipovic ilmuerte@... [TekScopes]" <TekScopes@...>
Date: 05/10/2017 09:27 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise



Hi Mike,

In most well designed equipment, electrolytic capacitors do not tend to
bulge and leak that easily like in cheap Chinese stuff. Your 2235 is surely
old enough to exceed the lifespan of capacitors in areas where ripple
currents are high (and have a high slew rate, typical for SMPS). My advice
would be to stop using the scope to prevent any potential damage, and
straightforwardly replace all electrolytic capacitors in the power supply,
with (perhaps) the exception of the mains bulk capacitor (the big one which
filters the rectified mains supply). The cost of full replacement is
usually less than the effort you would spend to find exactly the one
responsible for the symptoms, not to mention future proof repair. The
replacements should all be of the 105C temperature rating and low ESR.

Filtering capacitors on the mains transformer secondary side are usually
the most important (these usually fail first due to stress), there you
should use some good reputable brand like Nippon Chemi-con, Rubycon,
Panasonic..., you should consult the catalogs to see which series are
recommended for SMPS, there exist some 2-3 letter markings for the series.
Also note the size, low ESR radial capacitors are usually tall and thin,
and sometimes it can be difficult to find a replacement that exactly fits.
You should not have a problem to use a slightly larger capacity rating than
the original.

Some peculiar SMPS designs sometimes employ special rating components, so
there is a slight chance that some of those "small, less significant"
capacitors is somehow special. More experienced members of this group might
help here, but anyways it is always a good practice to examine each part
you remove to check whether its ratings/specs are standard.

I'm no expert for 2xxx series, but you should also check the high voltage
power supply (if it is separate from the main power supply). Stresses in
high voltage low current converters are usually inferior compared to their
counterparts, but still any SMPS is suspect for eating out its own
(electrolytic) capacitors.

You should also be able to find good advice in the archives of this group,
capacitor replacements are very common, I'm sure more experienced members
have already covered this topic many times, much more precisely than me.

Best Regards,
Nenad FIlipovic

On Thu, Oct 5, 2017 at 8:21 AM, Michael Pickwell mikepickwell@...
[TekScopes] <TekScopes@...> wrote:



Hello folks,

This is my first post so please be gentle!

I have a 2235 which I occasionally use for amateur hobby use repairing
audio equipment.

Just recently it has started making a high pitched whining sound which
appears to be coming from the power supply section. Although it still
*works* I find the noise worrying and I think the trace is also getting
more fuzzy too.

My first thought was to check for tired electrolytic capacitors in the
power supply. I had a look yesterday and none of them show any external
signs of failure (i.e. bulging or leaking electrolyte).

My soldering skills are good and I have a decent DMM a steady hand and a
keen eye but I'm no electronics engineer.

Any advice will be gratefully received.

Best regards, Mike

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








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

Nenad Filipovic
 

Hi Mike,

You're welcome, I also recommend you wait until experts (from the US time
zone) join the discussion.
In my opinion you should not touch the voltage adjustments after the
replacement, just check if voltages are within ranges specified in the
manual. Readjustment of power supply voltages would require you to
re-calibrate the rest of the scope, which is no small effort.

Best Regards,
Nenad Filipovic


On Thu, Oct 5, 2017 at 10:49 AM, Michael Pickwell mikepickwell@...
[TekScopes] <TekScopes@...> wrote:

Thank you for the very helpful reply Nenad.

If someone familiar with this model could tell me which capacitors are the
most likely culprits I'll get those changed first.

Will I need to do any recalibration after changing caps in the PSU?

Best regards, Mike



Sent from my Samsung device


-------- Original message --------
From: "Nenad Filipovic ilmuerte@... [TekScopes]" <
TekScopes@...>
Date: 05/10/2017 09:27 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise



Hi Mike,

In most well designed equipment, electrolytic capacitors do not tend to
bulge and leak that easily like in cheap Chinese stuff. Your 2235 is surely
old enough to exceed the lifespan of capacitors in areas where ripple
currents are high (and have a high slew rate, typical for SMPS). My advice
would be to stop using the scope to prevent any potential damage, and
straightforwardly replace all electrolytic capacitors in the power supply,
with (perhaps) the exception of the mains bulk capacitor (the big one which
filters the rectified mains supply). The cost of full replacement is
usually less than the effort you would spend to find exactly the one
responsible for the symptoms, not to mention future proof repair. The
replacements should all be of the 105°C temperature rating and low ESR.

Filtering capacitors on the mains transformer secondary side are usually
the most important (these usually fail first due to stress), there you
should use some good reputable brand like Nippon Chemi-con, Rubycon,
Panasonic..., you should consult the catalogs to see which series are
recommended for SMPS, there exist some 2-3 letter markings for the series.
Also note the size, low ESR radial capacitors are usually tall and thin,
and sometimes it can be difficult to find a replacement that exactly fits.
You should not have a problem to use a slightly larger capacity rating than
the original.

Some peculiar SMPS designs sometimes employ special rating components, so
there is a slight chance that some of those "small, less significant"
capacitors is somehow special. More experienced members of this group might
help here, but anyways it is always a good practice to examine each part
you remove to check whether its ratings/specs are standard.

I'm no expert for 2xxx series, but you should also check the high voltage
power supply (if it is separate from the main power supply). Stresses in
high voltage low current converters are usually inferior compared to their
counterparts, but still any SMPS is suspect for eating out its own
(electrolytic) capacitors.

You should also be able to find good advice in the archives of this group,
capacitor replacements are very common, I'm sure more experienced members
have already covered this topic many times, much more precisely than me.

Best Regards,
Nenad FIlipovic

On Thu, Oct 5, 2017 at 8:21 AM, Michael Pickwell mikepickwell@...
[TekScopes] <TekScopes@...> wrote:



Hello folks,

This is my first post so please be gentle!

I have a 2235 which I occasionally use for amateur hobby use repairing
audio equipment.

Just recently it has started making a high pitched whining sound which
appears to be coming from the power supply section. Although it still
*works* I find the noise worrying and I think the trace is also getting
more fuzzy too.

My first thought was to check for tired electrolytic capacitors in the
power supply. I had a look yesterday and none of them show any external
signs of failure (i.e. bulging or leaking electrolyte).

My soldering skills are good and I have a decent DMM a steady hand and a
keen eye but I'm no electronics engineer.

Any advice will be gratefully received.

Best regards, Mike

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


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





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



------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links




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

Bert Haskins
 

On 10/5/2017 2:21 AM, Michael Pickwell mikepickwell@...
[TekScopes] wrote:

Hello folks,

This is my first post so please be gentle!

I have a 2235 which I occasionally use for amateur hobby use repairing
audio equipment.

Just recently it has started making a high pitched whining sound which
appears to be coming from the power supply section. Although it still
*works* I find the noise worrying and I think the trace is also
getting more fuzzy too.

My first thought was to check for tired electrolytic capacitors in the
power supply. I had a look yesterday and none of them show any
external signs of failure (i.e. bulging or leaking electrolyte).

My soldering skills are good and I have a decent DMM a steady hand and
a keen eye but I'm no electronics engineer.

Any advice will be gratefully received.

Best regards, Mike
If you can hear this it means that one of the three SMPS has drifted is
now running at too low of a frequency.
The last time that I ran into this the cause was just a less than good
ground...... or more than one.
I'll never know because I studied/worked on every one shown on the power
schematic.
The other cause can be a shorted turn in on of the transformers and of
course, one or more degraded ecaps.
This is one of the places that you might be able to use the scope to
find it's own problems but be very, very careful and go back and read
the many! posts about working around a "hot" chassis.

Bert

Bert Haskins
 

On 10/5/2017 2:21 AM, Michael Pickwell mikepickwell@...
[TekScopes] wrote:

Hello folks,

This is my first post so please be gentle!

I have a 2235 which I occasionally use for amateur hobby use repairing
audio equipment.

Just recently it has started making a high pitched whining sound which
appears to be coming from the power supply section. Although it still
*works* I find the noise worrying and I think the trace is also
getting more fuzzy too.

My first thought was to check for tired electrolytic capacitors in the
power supply. I had a look yesterday and none of them show any
external signs of failure (i.e. bulging or leaking electrolyte).

My soldering skills are good and I have a decent DMM a steady hand and
a keen eye but I'm no electronics engineer.

Any advice will be gratefully received.

Best regards, Mike
I got interrupted and set the last one too soon.
On just about all of the 22xx scopes, anytime you have them opened up
!!carefully!! check out the focus resistor string.
Lots of material in the archives on this.
I had notes on the resistors that I used to replace the original parts
but they were on my raid nas which somehow has managed to eat three hard
drives in a four month period.!

 

On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors are the most likely culprits I'll get those changed first.
The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.

Tom Jobe <tomjobe@...>
 

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...





On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:

On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.



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

Michael Pickwell <mikepickwell@...>
 

Good morning, David, Tom, Bert, and anyone else I've forgotten.

Thank you all for your excellent advice.

Bert, that's a good call on the focus resistor string, I had to replace those last year.

Tom, thanks for the tip about using a snap-in cap for C906!

I have some updates for you. Yesterday I ordered replacements for:

C961, 962, 963, 960, 970 and 968
C954
C940
C956
C942, 943
C925

I also ordered all of the parts necessary to install a small PC cooling fan as found in the 2236:

CR965
C965
R965

It was only after placing the order for all these parts that I got around to removing the old capacitors (schoolboy error). I then tested them all with my little Peak Atlas capacitor tester and they all tested as healthy!

As a result I'm now worried that the capacitors aren't the problem after all. Apparently a common fault on the 2235 is an internal short on the windings of inductor T906.

Replacements for T906 are apparently unobtanium so I may well gave an irreparable scope now after wasting money on lots of new parts. Doh!

The new parts should be delivered later today and I'm trying to decide if I should bother fitting them.

I suppose I should at least remove and test the big capacitor C906.







Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]" <TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...





On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:

On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.







------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links

Michael Pickwell <mikepickwell@...>
 

I just pulled the big capacitor C906 and it measures 84uF with 1.00 Ohm ESR.





Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]" <TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...





On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:

On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.







------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links

Tom Jobe <tomjobe@...>
 

Hi Mike,
On that C906 replacement, grab the leads of the 'snap-in' capacitor
right as they come out of the capacitor with a pointed pair of pliers,
and then do your bending with another pair of pliers so you don't
disturb the connection and seal of the lead where it goes into the
capacitor. It only takes a little tweak of the leads to make it fit into
the PCB.
Your 2235 really needs all of the aluminum electrolytic capacitors
replaced to freshen it up whether it solves your current problem or not.
Put all of your new caps in and see where you stand at that point.
T906 is a very rare failure in 22xx scopes as far as I know, but there
are plenty of them available. For example, there's some on eBay right
now for $15 US and I don't think it matters which dash number T906 part
you get (-00, -01, etc).
http://www.ebay.com/itm/Tektronix-120-1439-00-01-SwitchmodeTransformer-for-22xx-Oscilloscope-/282648416169?hash=item41cf288fa9
Walter at 'Sphere Research' in Canada has some in stock too.
Parts availability should not be a problem for anything you might need
for your 2235.
It seems to me that you either need to take a serious diagnostic
approach to solving the problem you have, or throw some simple parts at
it that you know it needs anyway and just see what happens.
tom jobe...



On 10/6/2017 12:08 AM, Michael Pickwell mikepickwell@...
[TekScopes] wrote:

Good morning, David, Tom, Bert, and anyone else I've forgotten.

Thank you all for your excellent advice.

Bert, that's a good call on the focus resistor string, I had to
replace those last year.

Tom, thanks for the tip about using a snap-in cap for C906!

I have some updates for you. Yesterday I ordered replacements for:

C961, 962, 963, 960, 970 and 968
C954
C940
C956
C942, 943
C925

I also ordered all of the parts necessary to install a small PC
cooling fan as found in the 2236:

CR965
C965
R965

It was only after placing the order for all these parts that I got
around to removing the old capacitors (schoolboy error). I then tested
them all with my little Peak Atlas capacitor tester and they all
tested as healthy!

As a result I'm now worried that the capacitors aren't the problem
after all. Apparently a common fault on the 2235 is an internal short
on the windings of inductor T906.

Replacements for T906 are apparently unobtanium so I may well gave an
irreparable scope now after wasting money on lots of new parts. Doh!

The new parts should be delivered later today and I'm trying to decide
if I should bother fitting them.

I suppose I should at least remove and test the big capacitor C906.

Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]" <TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the
leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...

On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:

On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.



------------------------------------

------------------------------------

------------------------------------

Yahoo Groups Links





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

Michael Pickwell <mikepickwell@...>
 

I've just installed the last of the replacement capacitors (the big cap C906) and unfortunately the whining/squeal sound from the PSU is as bad as ever.


So what now? Do I need to try and find a replacement for T906?


I've noticed that the PCB under diode CR907 is slightly discoloured suggesting that it has been getting hot. Is this normal or something I should be concerned about?


Thanks is advance, Mike


________________________________
From: TekScopes@... <TekScopes@...> on behalf of Michael Pickwell mikepickwell@... [TekScopes] <TekScopes@...>
Sent: 06 October 2017 08:08
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise



Good morning, David, Tom, Bert, and anyone else I've forgotten.

Thank you all for your excellent advice.

Bert, that's a good call on the focus resistor string, I had to replace those last year.

Tom, thanks for the tip about using a snap-in cap for C906!

I have some updates for you. Yesterday I ordered replacements for:

C961, 962, 963, 960, 970 and 968
C954
C940
C956
C942, 943
C925

I also ordered all of the parts necessary to install a small PC cooling fan as found in the 2236:

CR965
C965
R965

It was only after placing the order for all these parts that I got around to removing the old capacitors (schoolboy error). I then tested them all with my little Peak Atlas capacitor tester and they all tested as healthy!

As a result I'm now worried that the capacitors aren't the problem after all. Apparently a common fault on the 2235 is an internal short on the windings of inductor T906.

Replacements for T906 are apparently unobtanium so I may well gave an irreparable scope now after wasting money on lots of new parts. Doh!

The new parts should be delivered later today and I'm trying to decide if I should bother fitting them.

I suppose I should at least remove and test the big capacitor C906.

Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]" <TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...

On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:

On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.

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

------------------------------------

------------------------------------

------------------------------------

Yahoo Groups Links







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

Tom Jobe <tomjobe@...>
 

A look at some similar 22xx's does not find any hint of temperature around CR907 on the bottom of the mainboards, so my guess is that this is a reflection of the problem you have.
My knowledge of the circuit theory involved is minimal but I printed out the relevant pages of the service manual theory section to study today, not that it will help my understanding very much.
On the schematic you can see that much of the power coming to the scope is going through one of the windings of T906, so there is plenty of what it takes to cook things.
There aren't many components directly involved around T906, so I would consider changing them all (one by one) if no one comes forward with a better plan.

Yesterday I was working on a 2215A which is the same scope with less bandwidth, and I thought about the problem of your 2235 making the high pitched squealing sound.
Because of my age and former working life occupation I can't even hear those kinds of sounds, so I could have the same problem and not even know it.
I wonder if there is a way for us old folks to "see" a noise like that? Maybe a meter of some kind?
tom jobe...

On 10/9/2017 6:18 AM, Michael Pickwell mikepickwell@... [TekScopes] wrote:
I've just installed the last of the replacement capacitors (the big cap C906) and unfortunately the whining/squeal sound from the PSU is as bad as ever.


So what now? Do I need to try and find a replacement for T906?


I've noticed that the PCB under diode CR907 is slightly discoloured suggesting that it has been getting hot. Is this normal or something I should be concerned about?


Thanks is advance, Mike


________________________________
From: TekScopes@... <TekScopes@...> on behalf of Michael Pickwell mikepickwell@... [TekScopes] <TekScopes@...>
Sent: 06 October 2017 08:08
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise



Good morning, David, Tom, Bert, and anyone else I've forgotten.

Thank you all for your excellent advice.

Bert, that's a good call on the focus resistor string, I had to replace those last year.

Tom, thanks for the tip about using a snap-in cap for C906!

I have some updates for you. Yesterday I ordered replacements for:

C961, 962, 963, 960, 970 and 968
C954
C940
C956
C942, 943
C925

I also ordered all of the parts necessary to install a small PC cooling fan as found in the 2236:

CR965
C965
R965

It was only after placing the order for all these parts that I got around to removing the old capacitors (schoolboy error). I then tested them all with my little Peak Atlas capacitor tester and they all tested as healthy!

As a result I'm now worried that the capacitors aren't the problem after all. Apparently a common fault on the 2235 is an internal short on the windings of inductor T906.

Replacements for T906 are apparently unobtanium so I may well gave an irreparable scope now after wasting money on lots of new parts. Doh!

The new parts should be delivered later today and I'm trying to decide if I should bother fitting them.

I suppose I should at least remove and test the big capacitor C906.

Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]" <TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...

On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:
On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.

------------------------------------

------------------------------------

------------------------------------

Yahoo Groups Links









------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links



Jon Batters
 

Hi Tom Jobe, See Noise ? You’ll love this; I have a Smart Phone, download from the Web : Spectrum Analyze. Basic version is free and pay $1 ( somehow ) for more features. Two- finger swipe will open/reduce bandwidth. I’ve used it to show a tone I was hearing and wanted to know the exact freq to aid troubleshooting and just because it was cool.

I have 5 cheap plastic stethoscopes I recycled from my hospital ( you may get from drugstores ) that I’ve used to track down little buzzing/tone noise from a ckt pcb. Around a hot ( powered on ) pcb, the insulated plastic is good thing. Stole a small funnel from my kitchen stock to replace the diaphragm for a sound funnel. When I get close to the tone source I pull the funnel off and just use the tube end to localize the source.

I’ve used a small microphone ( electret ? ) hooked up to my scope probe and w/knob twiddling display the tone. It will never be a sine tone, but a buzzing/spiking sound pulse train. Fascinating to get two noise pulses displayed and see the actual waveshape; when you get to the xfmr/part that is buzzing, pressing on the part often changes waveshape and tone sound ! I’ve stopped one sounding xfmr with 3 small 3” ty-raps strapped down hard around the part ! Remember, sound is created by something moving, so you usually stop it by clamping, RTV, etc.

The weirdest problem I worked on was a device, X_Y position driven by stepper motors. One day it started singing out when driving. With a stethoscope I tracked down that the sound was coming not from the motors, but from the large TO-3 mounted drive transistors. Right out of the cases ! I cannot explain that phenom’. It was not a “ problem “ but I was looking forward to track the issue down in my spare time, when Damn , the unit got swapped-out for a new imaging device……….

Jon Batters, Grants Pass, Or.

Sent from Mail for Windows 10

From: Tom Jobe tomjobe@... [TekScopes]
Sent: Monday, October 9, 2017 1:19 PM
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

A look at some similar 22xx's does not find any hint of temperature
around CR907 on the bottom of the mainboards, so my guess is that this
is a reflection of the problem you have.
My knowledge of the circuit theory involved is minimal but I printed out
the relevant pages of the service manual theory section to study today,
not that it will help my understanding very much.
On the schematic you can see that much of the power coming to the scope
is going through one of the windings of T906, so there is plenty of what
it takes to cook things.
There aren't many components directly involved around T906, so I would
consider changing them all (one by one) if no one comes forward with a
better plan.

Yesterday I was working on a 2215A which is the same scope with less
bandwidth, and I thought about the problem of your 2235 making the high
pitched squealing sound.
Because of my age and former working life occupation I can't even hear
those kinds of sounds, so I could have the same problem and not even
know it.
I wonder if there is a way for us old folks to "see" a noise like that?
Maybe a meter of some kind?
tom jobe...




On 10/9/2017 6:18 AM, Michael Pickwell mikepickwell@...
[TekScopes] wrote:
I've just installed the last of the replacement capacitors (the big cap C906) and unfortunately the whining/squeal sound from the PSU is as bad as ever.


So what now? Do I need to try and find a replacement for T906?


I've noticed that the PCB under diode CR907 is slightly discoloured suggesting that it has been getting hot. Is this normal or something I should be concerned about?


Thanks is advance, Mike


________________________________
From: TekScopes@... <TekScopes@...> on behalf of Michael Pickwell mikepickwell@... [TekScopes] <TekScopes@...>
Sent: 06 October 2017 08:08
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise



Good morning, David, Tom, Bert, and anyone else I've forgotten.

Thank you all for your excellent advice.

Bert, that's a good call on the focus resistor string, I had to replace those last year.

Tom, thanks for the tip about using a snap-in cap for C906!

I have some updates for you. Yesterday I ordered replacements for:

C961, 962, 963, 960, 970 and 968
C954
C940
C956
C942, 943
C925

I also ordered all of the parts necessary to install a small PC cooling fan as found in the 2236:

CR965
C965
R965

It was only after placing the order for all these parts that I got around to removing the old capacitors (schoolboy error). I then tested them all with my little Peak Atlas capacitor tester and they all tested as healthy!

As a result I'm now worried that the capacitors aren't the problem after all. Apparently a common fault on the 2235 is an internal short on the windings of inductor T906.

Replacements for T906 are apparently unobtanium so I may well gave an irreparable scope now after wasting money on lots of new parts. Doh!

The new parts should be delivered later today and I'm trying to decide if I should bother fitting them.

I suppose I should at least remove and test the big capacitor C906.

Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]" <TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...

On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:
On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.


------------------------------------

------------------------------------

------------------------------------

Yahoo Groups Links

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





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



------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links





------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links






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

mikepickwell@...
 

Some good news; I've managed to source a replacement used T906 transformer (part number 120-1439-00) from Sphere Research Cooperation in Canada.


I'll let you know when it arrives and I've fitted it.


Does this look like a suitable replacement for diode CR907 which has been getting hot?


http://uk.farnell.com/taiwan-semiconductor/her155/rectifier-single-400v-1-5a-do/dp/2677292?ost=2677292&iscrfnonsku=false&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch http://uk.farnell.com/taiwan-semiconductor/her155/rectifier-single-400v-1-5a-do/dp/2677292?ost=2677292&iscrfnonsku=false&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch


Many thanks, Mike

Tom Jobe <tomjobe@...>
 

Thanks Jon,
That's great stuff about how to 'see' the noise for us old folks!
I'm on it!
tom jobe...




On 10/9/2017 11:45 PM, Jon Batters jonbatters32@... [TekScopes] wrote:

Hi Tom Jobe, See Noise ? You’ll love this; I have a Smart Phone,
download from the Web : Spectrum Analyze. Basic version is free and
pay $1 ( somehow ) for more features. Two- finger swipe will
open/reduce bandwidth. I’ve used it to show a tone I was hearing and
wanted to know the exact freq to aid troubleshooting and just because
it was cool.

I have 5 cheap plastic stethoscopes I recycled from my hospital ( you
may get from drugstores ) that I’ve used to track down little
buzzing/tone noise from a ckt pcb. Around a hot ( powered on ) pcb,
the insulated plastic is good thing. Stole a small funnel from my
kitchen stock to replace the diaphragm for a sound funnel. When I get
close to the tone source I pull the funnel off and just use the tube
end to localize the source.

I’ve used a small microphone ( electret ? ) hooked up to my scope
probe and w/knob twiddling display the tone. It will never be a sine
tone, but a buzzing/spiking sound pulse train. Fascinating to get two
noise pulses displayed and see the actual waveshape; when you get to
the xfmr/part that is buzzing, pressing on the part often changes
waveshape and tone sound ! I’ve stopped one sounding xfmr with 3 small
3” ty-raps strapped down hard around the part ! Remember, sound is
created by something moving, so you usually stop it by clamping, RTV,
etc.

The weirdest problem I worked on was a device, X_Y position driven by
stepper motors. One day it started singing out when driving. With a
stethoscope I tracked down that the sound was coming not from the
motors, but from the large TO-3 mounted drive transistors. Right out
of the cases ! I cannot explain that phenom’. It was not a “ problem “
but I was looking forward to track the issue down in my spare time,
when Damn , the unit got swapped-out for a new imaging device……….

Jon Batters, Grants Pass, Or.

Sent from Mail for Windows 10

From: Tom Jobe tomjobe@... [TekScopes]
Sent: Monday, October 9, 2017 1:19 PM
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

A look at some similar 22xx's does not find any hint of temperature
around CR907 on the bottom of the mainboards, so my guess is that this
is a reflection of the problem you have.
My knowledge of the circuit theory involved is minimal but I printed out
the relevant pages of the service manual theory section to study today,
not that it will help my understanding very much.
On the schematic you can see that much of the power coming to the scope
is going through one of the windings of T906, so there is plenty of what
it takes to cook things.
There aren't many components directly involved around T906, so I would
consider changing them all (one by one) if no one comes forward with a
better plan.

Yesterday I was working on a 2215A which is the same scope with less
bandwidth, and I thought about the problem of your 2235 making the high
pitched squealing sound.
Because of my age and former working life occupation I can't even hear
those kinds of sounds, so I could have the same problem and not even
know it.
I wonder if there is a way for us old folks to "see" a noise like that?
Maybe a meter of some kind?
tom jobe...

On 10/9/2017 6:18 AM, Michael Pickwell mikepickwell@...
[TekScopes] wrote:
I've just installed the last of the replacement capacitors (the big
cap C906) and unfortunately the whining/squeal sound from the PSU is
as bad as ever.


So what now? Do I need to try and find a replacement for T906?


I've noticed that the PCB under diode CR907 is slightly discoloured
suggesting that it has been getting hot. Is this normal or something I
should be concerned about?


Thanks is advance, Mike


________________________________
From: TekScopes@... <TekScopes@...> on
behalf of Michael Pickwell mikepickwell@... [TekScopes]
<TekScopes@...>
Sent: 06 October 2017 08:08
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise



Good morning, David, Tom, Bert, and anyone else I've forgotten.

Thank you all for your excellent advice.

Bert, that's a good call on the focus resistor string, I had to
replace those last year.

Tom, thanks for the tip about using a snap-in cap for C906!

I have some updates for you. Yesterday I ordered replacements for:

C961, 962, 963, 960, 970 and 968
C954
C940
C956
C942, 943
C925

I also ordered all of the parts necessary to install a small PC
cooling fan as found in the 2236:

CR965
C965
R965

It was only after placing the order for all these parts that I got
around to removing the old capacitors (schoolboy error). I then tested
them all with my little Peak Atlas capacitor tester and they all
tested as healthy!

As a result I'm now worried that the capacitors aren't the problem
after all. Apparently a common fault on the 2235 is an internal short
on the windings of inductor T906.

Replacements for T906 are apparently unobtanium so I may well gave
an irreparable scope now after wasting money on lots of new parts. Doh!

The new parts should be delivered later today and I'm trying to
decide if I should bother fitting them.

I suppose I should at least remove and test the big capacitor C906.

Sent from my Samsung device

-------- Original message --------
From: "Tom Jobe tomjobe@... [TekScopes]"
<TekScopes@...>
Date: 06/10/2017 02:34 (GMT+00:00)
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

Hi Mike,
I see you have gotten some very good advice on your 2235.
Replace all of the aluminum electrolytic capacitors, there's not that
many of them and they are all worn out.
In general, go up on the voltage rating a bit (but not down!) and get
fresh new 105 degree C caps if possible.
Look at the lead spacing and size of your new caps to make sure they fit
nicely. There is plenty of room on the mainboard so there is no problem
finding new replacements for any of them.
There is nothing special about any of the caps as far as I know and I
have done quite a few 22xx recaps.
C906 is a higher voltage cap, and one of those common 'snap lock' caps
is available and will fit in perfectly with just a slight bend of
the leads.
Some of those caps David told you about are in the back right hand
corner of the mainboard, and a couple of them are mounted under the
metal edge of the frame of the scope.
With good soldering equipment you can reach under the metal edge and get
the old ones out and the new ones in with no problems.
Best of luck with your recap project!
tom jobe...

On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:
On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:

If someone familiar with this model could tell me which capacitors
are the most likely culprits I'll get those changed first.

The preregulator's input capacitor C906 and output capacitor C940 are
under the most stress and should be changed first. Usually I would
agree with Nenad about input capacitors but not in this case because
the input capacitor has an unusually low value.

C925, C943, C942, and C944 are not under a lot of stress but are
usually worn out anyway. And that applies to the inverter's output
capacitors as well.

Will I need to do any recalibration after changing caps in the PSU?
No recalibration should be necessary.

If you adjust the -8.6 volt output as described in the manual, then
this will change the CRT's cathode voltage altering the horizontal and
vertical deflection. If the horizontal and vertical deflection are
off by the same amount, then it may be feasible to adjust the -8.6
volt output to bring them into spec as long as the -8.6 volt output is
within the range specified in the service manual.

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

------------------------------------

------------------------------------

------------------------------------

Yahoo Groups Links

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





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



------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links



------------------------------------

------------------------------------

------------------------------------

Yahoo Groups Links

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



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

Tom Jobe <tomjobe@...>
 

I just checked this working 2215A and CR907 has a square wave across it
of about 165 volts at 14 uS.
I have no idea what happens during scope start up or other transition.
I'm in a 120 volt mains part of the planet, and my guess is that you are
in a 220 or 240 volt mains part of the planet, so I think you will see
about twice the voltage and have your CR907 see a square wave of
something like 330 volts at 14 uS.
Hopefully one of the people who actually know something will comment on
your CR907 diode question.
tom jobe...




On 10/10/2017 12:55 AM, mikepickwell@... [TekScopes] wrote:

Some good news; I've managed to source a replacement used T906
transformer (part number 120-1439-00) from Sphere Research Cooperation
in Canada.


I'll let you know when it arrives and I've fitted it.


Does this look like a suitable replacement for diode CR907 which has
been getting hot?


http://uk.farnell.com/taiwan-semiconductor/her155/rectifier-single-400v-1-5a-do/dp/2677292?ost=2677292&iscrfnonsku=false&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch
http://uk.farnell.com/taiwan-semiconductor/her155/rectifier-single-400v-1-5a-do/dp/2677292?ost=2677292&iscrfnonsku=false&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch



Many thanks, Mike



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



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

Kevin Clark
 

Hi Jon

This is similar to what you can do on mechanical items (i'm mechanical engineer
by trade) and we would quite often use a similar technique to find problems in
mechanical items, bearing noise etc.

Would love to see a u tube video of this on electronics.

Kevin

On 10 October 2017 at 07:45 "Jon Batters jonbatters32@... [TekScopes]"
<TekScopes@...> wrote:



Hi Tom Jobe, See Noise ? You’ll love this; I have a Smart Phone, download
from the Web : Spectrum Analyze. Basic version is free and pay $1 ( somehow )
for more features. Two- finger swipe will open/reduce bandwidth. I’ve used it
to show a tone I was hearing and wanted to know the exact freq to aid
troubleshooting and just because it was cool.

I have 5 cheap plastic stethoscopes I recycled from my hospital ( you may
get from drugstores ) that I’ve used to track down little buzzing/tone noise
from a ckt pcb. Around a hot ( powered on ) pcb, the insulated plastic is good
thing. Stole a small funnel from my kitchen stock to replace the diaphragm for
a sound funnel. When I get close to the tone source I pull the funnel off and
just use the tube end to localize the source.

I’ve used a small microphone ( electret ? ) hooked up to my scope probe
and w/knob twiddling display the tone. It will never be a sine tone, but a
buzzing/spiking sound pulse train. Fascinating to get two noise pulses
displayed and see the actual waveshape; when you get to the xfmr/part that is
buzzing, pressing on the part often changes waveshape and tone sound ! I’ve
stopped one sounding xfmr with 3 small 3” ty-raps strapped down hard around
the part ! Remember, sound is created by something moving, so you usually stop
it by clamping, RTV, etc.

The weirdest problem I worked on was a device, X_Y position driven by
stepper motors. One day it started singing out when driving. With a
stethoscope I tracked down that the sound was coming not from the motors, but
from the large TO-3 mounted drive transistors. Right out of the cases ! I
cannot explain that phenom’. It was not a “ problem “ but I was looking
forward to track the issue down in my spare time, when Damn , the unit got
swapped-out for a new imaging device……….

Jon Batters, Grants Pass, Or.

Sent from Mail for Windows 10

From: Tom Jobe tomjobe@... [TekScopes]
Sent: Monday, October 9, 2017 1:19 PM
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

A look at some similar 22xx's does not find any hint of temperature
around CR907 on the bottom of the mainboards, so my guess is that this
is a reflection of the problem you have.
My knowledge of the circuit theory involved is minimal but I printed out
the relevant pages of the service manual theory section to study today,
not that it will help my understanding very much.
On the schematic you can see that much of the power coming to the scope
is going through one of the windings of T906, so there is plenty of what
it takes to cook things.
There aren't many components directly involved around T906, so I would
consider changing them all (one by one) if no one comes forward with a
better plan.

Yesterday I was working on a 2215A which is the same scope with less
bandwidth, and I thought about the problem of your 2235 making the high
pitched squealing sound.
Because of my age and former working life occupation I can't even hear
those kinds of sounds, so I could have the same problem and not even
know it.
I wonder if there is a way for us old folks to "see" a noise like that?
Maybe a meter of some kind?
tom jobe...

On 10/9/2017 6:18 AM, Michael Pickwell mikepickwell@...
[TekScopes] wrote:
> I've just installed the last of the replacement capacitors (the big cap
> C906) and unfortunately the whining/squeal sound from the PSU is as bad
> as ever.
>
>
> So what now? Do I need to try and find a replacement for T906?
>
>
> I've noticed that the PCB under diode CR907 is slightly discoloured
> suggesting that it has been getting hot. Is this normal or something I
> should be concerned about?
>
>
> Thanks is advance, Mike
>
>
> ________________________________
> From: TekScopes@... <TekScopes@...> on behalf of
> Michael Pickwell mikepickwell@... [TekScopes]
> <TekScopes@...>
> Sent: 06 October 2017 08:08
> To: TekScopes@...
> Subject: Re: [TekScopes] 2235 power supply noise
>
>
>
> Good morning, David, Tom, Bert, and anyone else I've forgotten.
>
> Thank you all for your excellent advice.
>
> Bert, that's a good call on the focus resistor string, I had to replace
> those last year.
>
> Tom, thanks for the tip about using a snap-in cap for C906!
>
> I have some updates for you. Yesterday I ordered replacements for:
>
> C961, 962, 963, 960, 970 and 968
> C954
> C940
> C956
> C942, 943
> C925
>
> I also ordered all of the parts necessary to install a small PC cooling
> fan as found in the 2236:
>
> CR965
> C965
> R965
>
> It was only after placing the order for all these parts that I got
> around to removing the old capacitors (schoolboy error). I then tested
> them all with my little Peak Atlas capacitor tester and they all tested
> as healthy!
>
> As a result I'm now worried that the capacitors aren't the problem after
> all. Apparently a common fault on the 2235 is an internal short on the
> windings of inductor T906.
>
> Replacements for T906 are apparently unobtanium so I may well gave an
> irreparable scope now after wasting money on lots of new parts. Doh!
>
> The new parts should be delivered later today and I'm trying to decide
> if I should bother fitting them.
>
> I suppose I should at least remove and test the big capacitor C906.
>
> Sent from my Samsung device
>
> -------- Original message --------
> From: "Tom Jobe tomjobe@... [TekScopes]"
> <TekScopes@...>
> Date: 06/10/2017 02:34 (GMT+00:00)
> To: TekScopes@...
> Subject: Re: [TekScopes] 2235 power supply noise
>
> Hi Mike,
> I see you have gotten some very good advice on your 2235.
> Replace all of the aluminum electrolytic capacitors, there's not that
> many of them and they are all worn out.
> In general, go up on the voltage rating a bit (but not down!) and get
> fresh new 105 degree C caps if possible.
> Look at the lead spacing and size of your new caps to make sure they fit
> nicely. There is plenty of room on the mainboard so there is no problem
> finding new replacements for any of them.
> There is nothing special about any of the caps as far as I know and I
> have done quite a few 22xx recaps.
> C906 is a higher voltage cap, and one of those common 'snap lock' caps
> is available and will fit in perfectly with just a slight bend of the
> leads.
> Some of those caps David told you about are in the back right hand
> corner of the mainboard, and a couple of them are mounted under the
> metal edge of the frame of the scope.
> With good soldering equipment you can reach under the metal edge and get
> the old ones out and the new ones in with no problems.
> Best of luck with your recap project!
> tom jobe...
>
> On 10/5/2017 12:27 PM, David @DWH [TekScopes] wrote:
>> On Thu, 5 Oct 2017 08:49:50 +0000, you wrote:
>>
>>> If someone familiar with this model could tell me which capacitors
>> are the most likely culprits I'll get those changed first.
>>
>> The preregulator's input capacitor C906 and output capacitor C940 are
>> under the most stress and should be changed first. Usually I would
>> agree with Nenad about input capacitors but not in this case because
>> the input capacitor has an unusually low value.
>>
>> C925, C943, C942, and C944 are not under a lot of stress but are
>> usually worn out anyway. And that applies to the inverter's output
>> capacitors as well.
>>
>>> Will I need to do any recalibration after changing caps in the PSU?
>> No recalibration should be necessary.
>>
>> If you adjust the -8.6 volt output as described in the manual, then
>> this will change the CRT's cathode voltage altering the horizontal and
>> vertical deflection. If the horizontal and vertical deflection are
>> off by the same amount, then it may be feasible to adjust the -8.6
>> volt output to bring them into spec as long as the -8.6 volt output is
>> within the range specified in the service manual.
>>
>>
> [Non-text portions of this message have been removed]
>
> ------------------------------------
>
> ------------------------------------
>
> ------------------------------------
>
> Yahoo Groups Links
>
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
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Kevin Clark
 

Hi Mike

Re CR907, I will check my 2235 in the morning ref the burn marks, which is still
on the bench at work following extensive repairs which Tom Jobe very kindly held
my hand through repair (thanks Tom). I still have to do a write up for the group
on its repair but I have been premeditated with potential redundancy from
work..... I still have a problem with the channel 1 trigger, which is why its
not been closed up yet.

Anyway, your question with regard to CR907, I used RS 700-3964 Vishay BYV26C-TAP
High speed 600V 1A 30nS. I had to order minimum number when I got mine so have
some spare, you in the UK?

Cheers

Kevin


On 10 October 2017 at 16:31 "Tom Jobe tomjobe@... [TekScopes]"
<TekScopes@...> wrote:


I just checked this working 2215A and CR907 has a square wave across it
of about 165 volts at 14 uS.
I have no idea what happens during scope start up or other transition.
I'm in a 120 volt mains part of the planet, and my guess is that you are
in a 220 or 240 volt mains part of the planet, so I think you will see
about twice the voltage and have your CR907 see a square wave of
something like 330 volts at 14 uS.
Hopefully one of the people who actually know something will comment on
your CR907 diode question.
tom jobe...




On 10/10/2017 12:55 AM, mikepickwell@... [TekScopes] wrote:
>
> Some good news; I've managed to source a replacement used T906
> transformer (part number 120-1439-00) from Sphere Research Cooperation
> in Canada.
>
>
> I'll let you know when it arrives and I've fitted it.
>
>
> Does this look like a suitable replacement for diode CR907 which has
> been getting hot?
>
>
> http://uk.farnell.com/taiwan-semiconductor/her155/rectifier-single-400v-1-5a-do/dp/2677292?ost=2677292&iscrfnonsku=false&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch
> http://uk.farnell.com/taiwan-semiconductor/her155/rectifier-single-400v-1-5a-do/dp/2677292?ost=2677292&iscrfnonsku=false&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fsearch
>
>
>
> Many thanks, Mike
>
>
>
>
>
>







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mikepickwell@...
 

Hi Tom,


Unfortunately I don't have a second oscilloscope to look at the waveform across CR907 with but I'll try with my DMM which has a frequency counter to see what I can find.


I've had confirmation that my replacement T906 is in the post so fingers crossed!


Many thanks, Mike