New member and owner of a dead 2225


phillip.lyne.barrett
 

Long term electronics hobbyist. I've had a Tek 2225 for about 15 years and used it a lot. The other day I turned it on to test a circuit I'm prototyping and nothing. Dead. Check outlet, fuse, power cord. Nope. The only sign of life is a little blip on the power and leds - maybe half a second. I'm guessing it's the power supply. This scope has been an old friend.


I got the service manual and pulled the chassis out of the case. Found a loose torx screw inside though It didn't appear to be shorting anything and after removing it the symptoms persist. The scope had been sitting on the bench in the same spot for a couple of years so I doubt the screw was the problem. Inspection didn't show anything obvious - burnt components, bulging caps, blue smoke. In fact, it was surprisingly clean for 30 year old scope.


Anyway, I took some test point measurements on the bottom of the main baord and all the voltage levels were around half what they were supposed to be. More confident it's the PS. Looked at the mains input board. rectifiers (CR901-CR904) are ok. Pulled the board and pulled the big @ss 2200 uF cap (C900). It tested out ok (2270 uF).


Next step is to look at the preregulator section after I put the mains input board back.


I'm just following a fairly logical path here but thought there might be smarter people than me here that could help short cut the process. Any suggestions of where to look would be greatly appreciated.


Phil


 

The best one to help on the 22xx scopes is Tom Jobe.

Anyway, just a caution on measuring the 2200 uF cap. I have seen bad caps check ok for capacitance but have a very high ESR. Sans an ESR meter, you can take a look at the ripple across the cap for a better measurement.

Not sure on this scope as to the need for an isolation transformer. Tom would know.

----- Original Message -----
From: phillip.lyne.barrett@gmail.com [TekScopes]
To: TekScopes@yahoogroups.com
Sent: Sunday, November 12, 2017 6:29 PM
Subject: [TekScopes] New member and owner of a dead 2225



Long term electronics hobbyist. I've had a Tek 2225 for about 15 years and used it a lot. The other day I turned it on to test a circuit I'm prototyping and nothing. Dead. Check outlet, fuse, power cord. Nope. The only sign of life is a little blip on the power and leds - maybe half a second. I'm guessing it's the power supply. This scope has been an old friend.


I got the service manual and pulled the chassis out of the case. Found a loose torx screw inside though It didn't appear to be shorting anything and after removing it the symptoms persist. The scope had been sitting on the bench in the same spot for a couple of years so I doubt the screw was the problem. Inspection didn't show anything obvious - burnt components, bulging caps, blue smoke. In fact, it was surprisingly clean for 30 year old scope.


Anyway, I took some test point measurements on the bottom of the main baord and all the voltage levels were around half what they were supposed to be. More confident it's the PS. Looked at the mains input board. rectifiers (CR901-CR904) are ok. Pulled the board and pulled the big @ss 2200 uF cap (C900). It tested out ok (2270 uF).


Next step is to look at the preregulator section after I put the mains input board back.


I'm just following a fairly logical path here but thought there might be smarter people than me here that could help short cut the process. Any suggestions of where to look would be greatly appreciated.


Phil


Glenn Little
 

Check the ESR as well as the capacitance on all of the electrolytics.
These fail with age, especially in a switching supply.
The capacitance may be ok, but, if the ESR is too high, it will not
properly filter the DC generated by the high frequency switching supply.

Glenn

On 11/12/2017 6:29 PM, phillip.lyne.barrett@gmail.com [TekScopes] wrote:

Long term electronics hobbyist. I've had a Tek 2225 for about 15 years
and used it a lot. The other day I turned it on to test a circuit I'm
prototyping and nothing. Dead. Check outlet, fuse, power cord. Nope.
The only sign of life is a little blip on the power and leds - maybe
half a second. I'm guessing it's the power supply. This scope has been
an old friend.


I got the service manual and pulled the chassis out of the case. Found
a loose torx screw inside though It didn't appear to be shorting
anything and after removing it the symptoms persist. The scope had
been sitting on the bench in the same spot for a couple of years so I
doubt the screw was the problem. Inspection didn't show anything
obvious - burnt components, bulging caps, blue smoke. In fact, it was
surprisingly clean for 30 year old scope.


Anyway, I took some test point measurements on the bottom of the main
baord and all the voltage levels were around half what they were
supposed to be. More confident it's the PS. Looked at the mains input
board. rectifiers (CR901-CR904) are ok. Pulled the board and pulled
the big @ss 2200 uF cap (C900). It tested out ok (2270 uF).


Next step is to look at the preregulator section after I put the mains
input board back.


I'm just following a fairly logical path here but thought there might
be smarter people than me here that could help short cut the process.
Any suggestions of where to look would be greatly appreciated.


Phil



--
-----------------------------------------------------------------------
Glenn Little ETCS(SS) USN Ret, ARRL Technical Specialist, SBE ARRL TAPR
Amateur Callsign: WB4UIV wb4uiv@arrl.net AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI, FRA, NRA LM QCWA LM 28417
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
---------------------------------------------------------------------------


phillip.lyne.barrett
 

Thanks. It will take me some time to get an ESR meter. I did find one odd thing - Q913 in the preregulator measures 67.5V on E, B and C. It also show 0 ohms between E, B and C. Kind of looks shorted out to me. Not sure if that's just a symptom. I uploaded the relevant part of the schematic.

https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/134408395?orderBy=mtime&;sortOrder=desc&photoFilter=ALL# https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/134408395?orderBy=mtime&;sortOrder=desc&photoFilter=ALL#


 

That's not good. The Q913 is a TIP30C and is available pretty cheap. The analog pass xistor Q923 is also on ebay. You should also check the driver to Q913, Q912. Probably pull Q913 to double check it and follow up with some other measurements.

The switching regulator is a TL594. They are common. Two are used. probably both are now toast.

You will need to check everything that is supplied the pre-regulated voltage (38 volts) as it would have exceeded many ratings. Besides the power supply schematic #7, check schematics 2 and 5 as this overvoltage goes to those sections.

Good luck,
Tom M

----- Original Message -----
From: phillip.lyne.barrett@gmail.com [TekScopes]
To: TekScopes@yahoogroups.com
Sent: Sunday, November 12, 2017 11:38 PM
Subject: Re: [TekScopes] New member and owner of a dead 2225



Thanks. It will take me some time to get an ESR meter. I did find one odd thing - Q913 in the preregulator measures 67.5V on E, B and C. It also show 0 ohms between E, B and C. Kind of looks shorted out to me. Not sure if that's just a symptom. I uploaded the relevant part of the schematic.

https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/134408395?orderBy=mtime&sortOrder=desc&photoFilter=ALL# https://groups.yahoo.com/neo/groups/TekScopes/photos/photostream/lightbox/134408395?orderBy=mtime&sortOrder=desc&photoFilter=ALL#


phillip.lyne.barrett
 

I'm wondering if I should just replace all the electrolytics and be done with it.


phillip.lyne.barrett
 

yikes. Thanks for taking the time to look at this. Makes sense.

Probably should replace the electrolytics and any obvious damage. I'm now wondering if it's worth it at this point. I'll follow the overvoltage but if the damage is extensive, I may opt to retire this scope. At least it would be a good excuse to get a nice DSO.


phillip.lyne.barrett
 

One other question, is there an authoritative cross reference between the tektronix PN and the actual part number? I see a goodgle search on the tektronix part number typically gives at least one hit that shows it. A lot of places just show the Tek PN and list an amazing high price (one I saw was $8 for $0.50 part). I'll buy from Mouser, if I can.


 

A shorted Q913 should have protected driver Q912. Q911 should have
protected TL594 U910.

The linear series regulator would have tried to control the output
voltages until Q923 overheated and likely failed shorted. Switching
regulator controller U940, another TL594, operates open loop to drive
the inverter but uses one of its error amplifiers to detect the supply
voltage to the inverter and if it rises above 42.5 volts, it shuts
down so the rest of the oscilloscope circuits should be protected.

I am not sure about how the +40 volt supply rising to +67.5 volts is
going to affect the sweep circuit on schematic 5 and the vertical CRT
amplifier on schematic 5. Normally both circuits should be able to
protect themselves until they overheat but when the low voltage
outputs shut off, they may not be able to. I am inclined to believe
Tektronix took this into account but the theory section says nothing
about Q913 failing as a short.

On Mon, 13 Nov 2017 01:01:56 -0500, you wrote:

That's not good. The Q913 is a TIP30C and is available pretty cheap. The analog pass xistor Q923 is also on ebay. You should also check the driver to Q913, Q912. Probably pull Q913 to double check it and follow up with some other measurements.

The switching regulator is a TL594. They are common. Two are used. probably both are now toast.

You will need to check everything that is supplied the pre-regulated voltage (38 volts) as it would have exceeded many ratings. Besides the power supply schematic #7, check schematics 2 and 5 as this overvoltage goes to those sections.

Good luck,
Tom M


 

Absolutely there is; it is called the Tektronix Semiconductors Common
Parts Design Catalog and a search for that will turn up scans.

On 13 Nov 2017 19:20:14 +0000, you wrote:

One other question, is there an authoritative cross reference between the tektronix PN and the actual part number? I see a goodgle search on the tektronix part number typically gives at least one hit that shows it. A lot of places just show the Tek PN and list an amazing high price (one I saw was $8 for $0.50 part). I'll buy from Mouser, if I can.


 

If I had to order any parts, then I would include replacement aluminum
electrolytic capacitors.

On 13 Nov 2017 18:34:01 +0000, you wrote:

I'm wondering if I should just replace all the electrolytics and be done with it.


 

I think U940 would have been overvoltaged. Max is 41 volts. It would have seen 67 - 15 or 52 volts. But those are very cheap and available. C972, 470/50v could have survived a 17 volt overvoltage. But they are cheap and available. I agree Q912 is probably ok as long as the stressor didn't get it before 913 shorted.

Hell, I would sure try to bring it back to life.

Tom

----- Original Message -----
From: David davidwhess@gmail.com [TekScopes]
To: TekScopes@yahoogroups.com
Sent: Monday, November 13, 2017 6:12 PM
Subject: Re: [TekScopes] New member and owner of a dead 2225



A shorted Q913 should have protected driver Q912. Q911 should have
protected TL594 U910.

The linear series regulator would have tried to control the output
voltages until Q923 overheated and likely failed shorted. Switching
regulator controller U940, another TL594, operates open loop to drive
the inverter but uses one of its error amplifiers to detect the supply
voltage to the inverter and if it rises above 42.5 volts, it shuts
down so the rest of the oscilloscope circuits should be protected.

I am not sure about how the +40 volt supply rising to +67.5 volts is
going to affect the sweep circuit on schematic 5 and the vertical CRT
amplifier on schematic 5. Normally both circuits should be able to
protect themselves until they overheat but when the low voltage
outputs shut off, they may not be able to. I am inclined to believe
Tektronix took this into account but the theory section says nothing
about Q913 failing as a short.

On Mon, 13 Nov 2017 01:01:56 -0500, you wrote:

>That's not good. The Q913 is a TIP30C and is available pretty cheap. The analog pass xistor Q923 is also on ebay. You should also check the driver to Q913, Q912. Probably pull Q913 to double check it and follow up with some other measurements.
>
>The switching regulator is a TL594. They are common. Two are used. probably both are now toast.
>
>You will need to check everything that is supplied the pre-regulated voltage (38 volts) as it would have exceeded many ratings. Besides the power supply schematic #7, check schematics 2 and 5 as this overvoltage goes to those sections.
>
>Good luck,
>Tom M


Tom Jobe <tomjobe@...>
 

Hi Phillip.
I see you are getting some very good help on this problem, and when you
combine that with the fact that your voltages are only down by half, I'd
say the 2225 is only lightly wounded and that you will have it working
perfectly very soon. In order for the low voltages to be half there,
most of the power supply must be fundamentally sound.
The 2225 needs all of the aluminum electrolytic capacitors 10 uF and
larger changed anyway to regain that sharp trace you once had, and the
new ones might possibly help with your current problem as well. Get
brand new high quality 105 degree caps from a major electronics parts
supplier if you can.
There are several variations of the architecture of the small 22xx power
supplies but there are not many problems with the 2225 type of power
supply that ever show up on the Tekscopes Group .
When I read your original post about your 2225 problem, I looked on
YouTube to see what kinds of videos were available about the 2225.
Take a look at these two when you have time. This 2225 Dave Jones bought
worked just as it was received, and he gives you quite a nice lesson on
its use and claibration, as well as a tour of much of it's inner
workings (less the power supply unfortunately). He also gives you some
good common sense tips about working around the high voltages present in
the oscilloscope.

EEVBlog #196 operational review of the 2225
https://www.youtube.com/watch?v=8GR_6QH3uZk

EEVBlog #208 teardown and calibration of the 2225
https://www.youtube.com/watch?v=mHODWDBcQDg

Best of luck!
tom jobe...




On 11/12/2017 3:29 PM, phillip.lyne.barrett@gmail.com [TekScopes] wrote:

Long term electronics hobbyist. I've had a Tek 2225 for about 15 years
and used it a lot. The other day I turned it on to test a circuit I'm
prototyping and nothing. Dead. Check outlet, fuse, power cord. Nope.
The only sign of life is a little blip on the power and leds - maybe
half a second. I'm guessing it's the power supply. This scope has been
an old friend.


I got the service manual and pulled the chassis out of the case. Found
a loose torx screw inside though It didn't appear to be shorting
anything and after removing it the symptoms persist. The scope had
been sitting on the bench in the same spot for a couple of years so I
doubt the screw was the problem. Inspection didn't show anything
obvious - burnt components, bulging caps, blue smoke. In fact, it was
surprisingly clean for 30 year old scope.


Anyway, I took some test point measurements on the bottom of the main
baord and all the voltage levels were around half what they were
supposed to be. More confident it's the PS. Looked at the mains input
board. rectifiers (CR901-CR904) are ok. Pulled the board and pulled
the big @ss 2200 uF cap (C900). It tested out ok (2270 uF).


Next step is to look at the preregulator section after I put the mains
input board back.


I'm just following a fairly logical path here but thought there might
be smarter people than me here that could help short cut the process.
Any suggestions of where to look would be greatly appreciated.


Phil

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



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


n4mf_sc
 

I have the 1982 version I think and one from a website that is no longer there called Tek-cross-ref-free. It has a sig in the pdf of www.aa4df.com http://www.aa4df.com which is available for someone to purchase but no longer online. I can't post either because they are way past the limit on upload sizes. Transistors and IC's are in both..

Mitch
N4MF


phillip.lyne.barrett
 

Thanks, David, Tom, Tom and Glen.

I've seen those videos. Dave's tear down is really good though my wife makes me wear headphones for any of his videos - she's a non-fan. I kind of get her point...

After following the circuits on the schematic, I think the 38V/67.5V feeds are probably OK because there doesn't seem to be anything particularly sensitive there. Two 100n 50V ceramics are worth looking at, one on the X-Axis Amp and one in the vertical output amp. Everything else feeds though resistors. Though the lowest is 330 so bears looking at a bit more.


It looks like Q912 is not blown, at least a resistance check across the collector and base sees a cap charging up - probably C909 though the 2.2K resistor on the base. Not sure how to determine if either TL594 is dead. I've got the datasheet but haven't looked yet. Will probably order a few just in case, they're cheap.


On the caps, I've found what look like decent replacements - reasonably low ESR, same or better tolerance and all are 105C. However, not a single lead pitch matches. I can make them fit by bending the leads in an L to make them fit the original pitch. I assume that there is no issue with that. So, not worried except for C900 - the big 2200 uF 80V on the mains input board. The only matches are snap-in caps with fat, stiff, stubby leads and the wrong terminal pitch. (10mm vs a measured 12mm on the one that's there now). I guess I can make a little adapter but it will be a bit ugly. If I was industrious, I'd 3D print a foot for it to keep it from moving around - though it will probably be hot glued instead.


Another question. There are at least 9 10 uF axial electrolytics in various places. I should replace them as well? I'm not wild about removing the attenuator/timebase board, though it's probably easier than it looks.


Once I get my ESR meter (due this Friday), I'll hopefully find a smoking gun.


Anyway, again, thanks for all the help. I really do appreciate all your efforts.


Phil


 

On 14 Nov 2017 07:18:45 +0000, you wrote:

I've seen those videos. Dave's tear down is really good though my wife makes me wear headphones for any of his videos - she's a non-fan. I kind of get her point...
Crikey! Dave's Australian speech makes for entertaining listening but
I can understand how it would get old after a while. The same thing
happens to me if I binge watch Doctor Who episodes.

After following the circuits on the schematic, I think the 38V/67.5V feeds are probably OK because there doesn't seem to be anything particularly sensitive there. Two 100n 50V ceramics are worth looking at, one on the X-Axis Amp and one in the vertical output amp. Everything else feeds though resistors. Though the lowest is 330 so bears looking at a bit more.
Damage to the vertical CRT amplifier transistors is possible.
Depending on the bias conditions which are not real clear with the
power supply malfunctioning, the 2N3866s could have a specified
maximum breakdown voltage between Vceo of 30 volts and Vcbo of 55
volts. Luckily they should be easy to replace if damaged.

I am surprised that the switching preregulator did not include an SCR
crowbar circuit to protect against a shorted switching transistor but
maybe Tektronix determined that it was not necessary.

On the caps, I've found what look like decent replacements - reasonably low ESR, same or better tolerance and all are 105C. However, not a single lead pitch matches. I can make them fit by bending the leads in an L to make them fit the original pitch. I assume that there is no issue with that. So, not worried except for C900 - the big 2200 uF 80V on the mains input board. The only matches are snap-in caps with fat, stiff, stubby leads and the wrong terminal pitch. (10mm vs a measured 12mm on the one that's there now). I guess I can make a little adapter but it will be a bit ugly. If I was industrious, I'd 3D print a foot for it to keep it from moving around - though it will probably be hot glued instead.
Usually capacitors with the correct lead pitch can be found. Higher
voltage parts can be selected if necessary and this may even be
desirable for improved reliability and operating life.

The big bulk input capacitor is one of the aluminum electrolytics
which is likely to last the longest so maybe leaving it in place would
be best unless testing shows that it has failed. The reason for this
is that it was originally selected based on capacitance instead of ESR
or ripple current rating which will be more than enough and it is the
last one which impacts operating life.

Another question. There are at least 9 10 uF axial electrolytics in various places. I should replace them as well? I'm not wild about removing the attenuator/timebase board, though it's probably easier than it looks.
They will not have caused the failure but replacing them may lead to
better performance and perhaps less noise in the displayed trace. On
the other hand, replacing parts unnecessarily risks causing damage.

Once I get my ESR meter (due this Friday), I'll hopefully find a smoking gun.
Unfortunately an ESR meter will consider a shorted capacitor good.
They are useful and convenient for in-circuit measurements but not
always easy to interpret.

Soon your 2225 will be operating with peak efficiency and Bob's your
uncle!

You should hear some of the San Dimas slang I picked up when I worked
there which was not featured in the Bill and Ted movies.


phillip.lyne.barrett
 

Thanks. I went through Mouser looking for the caps. 300 mil pitch doesn't seem to exist anymore. Will try Digi. On higher voltage caps, it's it the case the higher V often means higher ESR?

I am planning on pulling all the switcher electrolytic caps out and am hoping to find what went bad to help with the general diagnosis. Right now, my bet is on either C915 or C971. Hopefully the series regulator and inverter are ok as I don't have a scope to debug my scope with...


As to Dave, it's not his Aussie accent and turn of phrase, rather his high pitched voice and geeky mannerisms that bug my Wife. It sometimes gets a bit wearing to me too. Plus it can be hard to figure out if he thinks something is good or bad. I went to school with a bunch of Aussies - great group of friends.


phillip.lyne.barrett
 

After a marathon session at distributor's web sites, I found a number of perfect fit replacement caps but still missing a few.

What is glaring is the absence of axial cap selection. It looks like these are disappearing fast. Not that I blame the manufacturers - I hate bending leads and would rather use SMDs any day. I was wrong about the axial caps' value - 15 uF, not 10. I wasn't able to find anything at mouser or digikey that matched except for some tant caps that were like $100. Since I need 7 of those, that ain't happening. These aren't in the power supply section so I don't HAVE to replace them. What are people doing in this situation? accepting worse tolerance? Significantly higher voltage (like 100+ for 10V caps), if that's even available?


There is a wonderful selection of SMD caps with tight tolerances for well less than a buck. I could imagine making a tiny little carrier board that would convert the SMD to the right pitch and size to replace an axial cap. Anything wrong with that idea?


Phil


Tom Jobe <tomjobe@...>
 

Another question. There are at least 9 10 uF axial electrolytics in
various places. I should replace them as well? I'm not wild about removing the attenuator/timebase board, though it's probably easier than it looks.

Hi Phil,
Sorry for not being clearer about my suggestion of replacing the 10 uF and up aluminum electrolytic caps, my comments are only about the repairing of the power supply, and not about any other sections of your oscilloscope.
The leads on those 'snap' caps bend easily, maybe spread them to fit and plan on not having the cap sit down on the board tightly? Maybe use two pair of pliers so the joint between the leads and the capacitor body are not disturbed?
In studying the schematic of the 2225 power supply, we see that it has at least twice as many parts as the more common 22xx family which includes the 2235, etc. Maybe this explains why it is fairly rare to hear about 2225 power supply problems?
tom jobe...


 

Just go up to 15uF, or 18uF, or maybe even 20uF. Odds are that the old part
was -10%,+100%, so an 18uF part would likely be spot on, and 20uF would not
harm any.

These are after all just decoupling caps.

Dave

-----Original Message-----
From: TekScopes@yahoogroups.com [mailto:TekScopes@yahoogroups.com]
Sent: 14 November 2017 13:45
To: TekScopes@yahoogroups.com
Subject: [TekScopes] Re: New member and owner of a dead 2225

After a marathon session at distributor's web sites, I found a number of
perfect fit replacement caps but still missing a few.

What is glaring is the absence of axial cap selection. It looks like these
are disappearing fast. Not that I blame the manufacturers - I hate bending
leads and would rather use SMDs any day. I was wrong about the axial caps'
value - 15 uF, not 10. I wasn't able to find anything at mouser or digikey
that matched except for some tant caps that were like $100. Since I need 7
of those, that ain't happening. These aren't in the power supply section so
I don't HAVE to replace them. What are people doing in this situation?
accepting worse tolerance? Significantly higher voltage (like 100+ for 10V
caps), if that's even available?


There is a wonderful selection of SMD caps with tight tolerances for well
less than a buck. I could imagine making a tiny little carrier board that
would convert the SMD to the right pitch and size to replace an axial cap.
Anything wrong with that idea?


Phil






------------------------------------
Posted by: phillip.lyne.barrett@gmail.com
------------------------------------


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

Yahoo Groups Links