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Focus adjustment on a Tek 2235 CRO
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Lionel
electronics forum addict


Joined: 16 Apr 2006
Posts: 54

PostPosted: Fri Jul 21, 2006 7:35 am    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

On 23 Jun 2006 15:35:09 GMT, Jim Yanik <jyanik@abuse.gov> opined:

Quote:
It was NOT a "chronic" problem(a -common- problem,yes);once the 1/2W carbon
FILM resistors were replaced with 1/2W carbon COMP resistors of the same
original value,the scopes were not seen again for the same problem.

I've finally gotten some time to work on this CRO.

Jim; you've expressed a strong preference for carbon comp resistors
for this repair, but they'll be a pain to obtain locally. Would metal
film (1/2W, 1% tol.) resistors be a reasonable substitute?
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
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Lionel
electronics forum addict


Joined: 16 Apr 2006
Posts: 54

PostPosted: Thu Jun 29, 2006 3:41 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

On Fri, 23 Jun 2006 02:59:05 -0500, Hugh Prescott <hugh345@adams.net>
opined:

Quote:
There was a recent discussion on Yahoo TekScopes group about this problem.

IIRC a retired Tek engineer has looked at the problem as came up with a
new resistor and value (12 used) that should fix this chronic problem
forever.

Thanks for the tip. I'll take a look in that group. :)

I haven't had time to work on the scope since I first posted about it
here, so I'm still interested in peoples' suggestions.
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
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Jim Yanik
electronics forum Guru


Joined: 01 May 2005
Posts: 635

PostPosted: Fri Jun 23, 2006 3:35 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

Hugh Prescott <hugh345@adams.net> wrote in
news:449b9f9e_2@newsfeed.slurp.net:

Quote:
There was a recent discussion on Yahoo TekScopes group about this
problem.

IIRC a retired Tek engineer has looked at the problem as came up with
a new resistor and value (12 used) that should fix this chronic
problem forever.

Hugh


carneyke wrote:
There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will
find which resistors and their value. I think it's about 5 resistors
all 510 Kohm.
Lionel wrote:

I'm finding that my elderly Tek 2235 CRO (a very nice 100MHz
Dual-trace) can't focus the spot any smaller than about a millimetre
in diameter, & that is at the the extreme anti-clockwise setting of
the front panel focus control. Turning the control clockwise can
defocus the spot to nearly size of the whole screen.
I assume this is due to component aging, rather than an actual fault.

Does anyone know if there's an internal focus control or setting (an
HV adjustment?) I can to adjust to re-centre the main focus control?
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda
est
---^----^-------------------------------------------------------------
--




It was NOT a "chronic" problem(a -common- problem,yes);once the 1/2W carbon
FILM resistors were replaced with 1/2W carbon COMP resistors of the same
original value,the scopes were not seen again for the same problem.

--
Jim Yanik
jyanik
at
kua.net
Back to top
Hugh Prescott
electronics forum beginner


Joined: 21 Jun 2005
Posts: 17

PostPosted: Fri Jun 23, 2006 7:59 am    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

There was a recent discussion on Yahoo TekScopes group about this problem.

IIRC a retired Tek engineer has looked at the problem as came up with a
new resistor and value (12 used) that should fix this chronic problem
forever.

Hugh


carneyke wrote:
Quote:
There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will find
which resistors and their value. I think it's about 5 resistors all 510
Kohm.
Lionel wrote:

I'm finding that my elderly Tek 2235 CRO (a very nice 100MHz
Dual-trace) can't focus the spot any smaller than about a millimetre
in diameter, & that is at the the extreme anti-clockwise setting of
the front panel focus control. Turning the control clockwise can
defocus the spot to nearly size of the whole screen.
I assume this is due to component aging, rather than an actual fault.

Does anyone know if there's an internal focus control or setting (an
HV adjustment?) I can to adjust to re-centre the main focus control?
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

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Jim Yanik
electronics forum Guru


Joined: 01 May 2005
Posts: 635

PostPosted: Fri Jun 09, 2006 4:05 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

"David C. Partridge" <mimx64xwy@dsl.pipex.com> wrote in
news:evednWm4MIzJ1xTZRVnyiw@pipex.net:

Quote:
First port of call is to check all the power supply voltages,
including the cathode (using a suitable HT probe). Make sure these
are on specification before assuming the focus resistor chain is off.

standard good advice,but..
the resistor chain problem is a very common focus problem.


Quote:
IIRC, if you calculate the voltage across these resistors, you'll
find that they are working on the ragged edge of the voltage rating
for 1/2W Carbon comps, so it is likely they have drifted in value, and
if you replace like for like, then you may see the same problem again
a year or so down the road.

the problem was with carbon FILM types,replaced BY comp types for better
longevity.We never had any problems with the 2200s after that mod.

Quote:

So check them and if they have drifted (as I suspect), then I suggest
that you use two resistors in series of 1/2 the value of the originals
to replace each one. It wont look pretty as you will have a join in
the air above the board, but will probably last much better.

Note that you should try to minimise pointy bits at the junction
between each of the two replacements to reduce any likelihood of
corona discharge (though the voltage here is probably a bit low for
that problem.

Dave


If you do choose to do this mod,use HV dope to coat the teepee leads.
IMO,it's unnecessary if you use carbon comp resistors.



--
Jim Yanik
jyanik
at
kua.net
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David C. Partridge
electronics forum addict


Joined: 27 Jul 2005
Posts: 75

PostPosted: Fri Jun 09, 2006 10:15 am    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

First port of call is to check all the power supply voltages, including the
cathode (using a suitable HT probe). Make sure these are on specification
before assuming the focus resistor chain is off. IIRC, if you calculate
the voltage across these resistors, you'll find that they are working on the
ragged edge of the voltage rating for 1/2W Carbon comps, so it is likely
they have drifted in value, and if you replace like for like, then you may
see the same problem again a year or so down the road.

So check them and if they have drifted (as I suspect), then I suggest that
you use two resistors in series of 1/2 the value of the originals to replace
each one. It wont look pretty as you will have a join in the air above the
board, but will probably last much better.

Note that you should try to minimise pointy bits at the junction between
each of the two replacements to reduce any likelihood of corona discharge
(though the voltage here is probably a bit low for that problem.

Dave
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carneyke
electronics forum addict


Joined: 07 Sep 2005
Posts: 74

PostPosted: Thu Jun 08, 2006 11:01 am    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

I have the manual in front of me. They (R888,889,890,891,892 & 894) are
510Kohm. R893 is the focus pot ! So that makes 6 resistors that need to
be changed ! I also have R885 (5.1Kohm) highlightled in the manual,
maybe it changes value too. Take Care

Jim Yanik wrote:
Quote:
"carneyke" <carneyke@localnet.com> wrote in
news:1149687730.822228.80680@y43g2000cwc.googlegroups.com:

There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will find
which resistors and their value. I think it's about 5 resistors all 510
Kohm.


The focus string resistors will all be located right around the focus
pot,on the main board.
1/2W carbon composition types are preferred for replacements,not carbon
film resistors.

--
Jim Yanik
jyanik
at
kua.net
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Jim Yanik
electronics forum Guru


Joined: 01 May 2005
Posts: 635

PostPosted: Wed Jun 07, 2006 11:21 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

Lionel <usenet@imagenoir.com> wrote in
news:h02e829hpg5fdlams02u0b9g8r043qvs83@4ax.com:

Quote:
On 7 Jun 2006 16:20:35 GMT, Jim Yanik <jyanik@abuse.gov> opined:

"carneyke" <carneyke@localnet.com> wrote in
news:1149687730.822228.80680@y43g2000cwc.googlegroups.com:

There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will find
which resistors and their value. I think it's about 5 resistors all 510
Kohm.

I couldn't find any schematics or component details on Google for the
2235, unfortunately. Still, with your description & Jim's, I shouldn't
have any trouble locating them on the PCB, & nothing looks cooked,

They don't get "Cooked",they discolor( a few shades darker,mostly in the
middle),and their value increases until they eventually open.
Carbon-film types,that is.


Quote:
so
I doubt I'll have any trouble reading the colour codes.

The focus string resistors will all be located right around the focus
pot,on the main board.
1/2W carbon composition types are preferred for replacements,not carbon
film resistors.

Thanks guys, those explanations make a lot of sense.

While I'm on the subject, does anyone have any ideas on whether it's
possible to download a copy of the schematic, calibration manual, or
service manuals without handing over cash to some dodgy website?



--
Jim Yanik
jyanik
at
kua.net
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Lionel
electronics forum addict


Joined: 16 Apr 2006
Posts: 54

PostPosted: Wed Jun 07, 2006 5:15 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

On 7 Jun 2006 16:20:35 GMT, Jim Yanik <jyanik@abuse.gov> opined:

Quote:
"carneyke" <carneyke@localnet.com> wrote in
news:1149687730.822228.80680@y43g2000cwc.googlegroups.com:

There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will find
which resistors and their value. I think it's about 5 resistors all 510
Kohm.

I couldn't find any schematics or component details on Google for the
2235, unfortunately. Still, with your description & Jim's, I shouldn't
have any trouble locating them on the PCB, & nothing looks cooked, so
I doubt I'll have any trouble reading the colour codes.

Quote:
The focus string resistors will all be located right around the focus
pot,on the main board.
1/2W carbon composition types are preferred for replacements,not carbon
film resistors.

Thanks guys, those explanations make a lot of sense.

While I'm on the subject, does anyone have any ideas on whether it's
possible to download a copy of the schematic, calibration manual, or
service manuals without handing over cash to some dodgy website?
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
Back to top
Jim Yanik
electronics forum Guru


Joined: 01 May 2005
Posts: 635

PostPosted: Wed Jun 07, 2006 4:20 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

"carneyke" <carneyke@localnet.com> wrote in
news:1149687730.822228.80680@y43g2000cwc.googlegroups.com:

Quote:
There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will find
which resistors and their value. I think it's about 5 resistors all 510
Kohm.


The focus string resistors will all be located right around the focus
pot,on the main board.
1/2W carbon composition types are preferred for replacements,not carbon
film resistors.

--
Jim Yanik
jyanik
at
kua.net
Back to top
carneyke
electronics forum addict


Joined: 07 Sep 2005
Posts: 74

PostPosted: Wed Jun 07, 2006 1:42 pm    Post subject: Re: Focus adjustment on a Tek 2235 CRO Reply with quote

There is a string of resistors that have turned brown & changed value
in the focus string. Do a google search on this model and you will find
which resistors and their value. I think it's about 5 resistors all 510
Kohm.
Lionel wrote:
Quote:
I'm finding that my elderly Tek 2235 CRO (a very nice 100MHz
Dual-trace) can't focus the spot any smaller than about a millimetre
in diameter, & that is at the the extreme anti-clockwise setting of
the front panel focus control. Turning the control clockwise can
defocus the spot to nearly size of the whole screen.
I assume this is due to component aging, rather than an actual fault.

Does anyone know if there's an internal focus control or setting (an
HV adjustment?) I can to adjust to re-centre the main focus control?
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
Back to top
Lionel
electronics forum addict


Joined: 16 Apr 2006
Posts: 54

PostPosted: Wed Jun 07, 2006 9:36 am    Post subject: Focus adjustment on a Tek 2235 CRO Reply with quote

I'm finding that my elderly Tek 2235 CRO (a very nice 100MHz
Dual-trace) can't focus the spot any smaller than about a millimetre
in diameter, & that is at the the extreme anti-clockwise setting of
the front panel focus control. Turning the control clockwise can
defocus the spot to nearly size of the whole screen.
I assume this is due to component aging, rather than an actual fault.

Does anyone know if there's an internal focus control or setting (an
HV adjustment?) I can to adjust to re-centre the main focus control?
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
Back to top
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