Focus adjustment on a Tek 2235 CRO

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?

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Lionel
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Reply to
carneyke

"carneyke" wrote in news: snipped-for-privacy@y43g2000cwc.googlegroups.com:

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.
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Jim Yanik
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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, so I doubt I'll have any trouble reading the colour codes.

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?

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Lionel

Lionel wrote in news: snipped-for-privacy@4ax.com:

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.

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Jim Yanik
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Jim Yanik

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 > "carneyke" wrote in
Reply to
carneyke

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

Reply to
David C. Partridge

"David C. Partridge" wrote in news: snipped-for-privacy@pipex.net:

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

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.

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.

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Jim Yanik
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Jim Yanik

Hugh Prescott wrote in news:449b9f9e snipped-for-privacy@newsfeed.slurp.net:

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.

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Jim Yanik
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Jim Yanik

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.

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Reply to
Lionel

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?

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Lionel

Lionel wrote in news: snipped-for-privacy@4ax.com:

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.

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Jim Yanik
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Jim Yanik

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

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.

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Lionel

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

Dave

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David C. Partridge

"David C. Partridge" wrote in news: snipped-for-privacy@pipex.net:

No pulse current on a 2200 focus divider string.

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Jim Yanik
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Jim Yanik

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

D.

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David C. Partridge
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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. :)

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Lionel

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