Focus adjustment on a Tek 2235 CRO

[Lionel]

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?

 

[carneyke]

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.

 

[Jim Yanik]

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.

 

[Lionel]

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.

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]

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.

 

[carneyke]

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

 

[David C. Partridge]

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

 

[Jim Yanik]

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.

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.

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.

 

[Hugh Prescott]

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

 

[Jim Yanik]

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

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.

 

[Lionel]

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.

 

[Lionel]

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?

 

[Jim Yanik]

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?

I "express" the preference for carbon comp resistors because that's what
the TEK engineers specified for use in the mod.
I could not say whether MF resistors would be a good sub. They could be
worse,I just don't know.I don't know the specific cause of failure for the
CF resistors.
Are there MF resistors available of the correct value,physical size and
wattage?

I suppose if you could fit 1W CF resistors in there,they would
suffice,too,or do the "teepee" mod another suggested.

 

[Lionel]

I "express" the preference for carbon comp resistors because that's what
the TEK engineers specified for use in the mod.
I could not say whether MF resistors would be a good sub. They could be
worse,I just don't know.I don't know the specific cause of failure for the
CF resistors.
Are there MF resistors available of the correct value,physical size and
wattage?

Yes. They're even in stock! (Unusual for these guys, who're mostly
geared towards hobbyists & rev-heads.

I suppose if you could fit 1W CF resistors in there,they would
suffice,too,or do the "teepee" mod another suggested.

<nod> If I hadn't been able to find 510K/0.5W, I was considering using
pairs of 1M/0.5W in parallel, but fitting the right part will be lot
quicker & neater.

Thanks again for your help, it's much appreciated.

 

[David C. Partridge]

Film parts won't take the pulse current. Look to carbon ceramic parts if
you can't get carbon comp.

Dave

 

[Jim Yanik]

Film parts won't take the pulse current. Look to carbon ceramic
parts if you can't get carbon comp.

Dave

No pulse current on a 2200 focus divider string.

 

[David C. Partridge]

Blush, of course you are right - this one is over voltage drift.

D.

 

[Lionel]

[Top posting to retain the original context for the search engines]

I've finally done the repair on this unit. As suggested, I replaced
the six 510K focus chain resistors, all of which were way out of
tolerance. (In fact, the worst one was 1.7M!) I used six 510K, 1/2W,
metal-film resistors, which seem to be working okay, & have definitely
brought the focus control back into the correct range. Only time will
tell if the metal-film resistors are up to the job over the the long
time, of course.

Thanks again to everyone who chipped in with advice, particularly
Carneyke, Jim Yanik & Dave Partridge, whose comments were particularly
helpful.