2236 Focus range.


Dave Peterson
 

Hi All,

I'm running through calibration on a 2236. Everything's going great. But something I noticed is that the focus is at the extreme CCW end of the FOCUS knob/pot.

I'm getting a fairly good trace line. Not as good as my bench 465 (I can just about hide the trace behind a graticule line on that), and not quite as good as my 2236A. But even the 2236A isn't as good as the 465. I can't help but wonder if the focus could be/would be better if the adjustment weren't at the end of its range.

I've gone through the power supply and CRT sections several times making sure voltage levels are staying good, setting a proper grid bias, geometries, and astigmatism. The cathode voltage is within spec at -1933v. It started at -1934v this morning, and the scope's been on all day. So pretty stable. All the other PS values are within mVs from this morning too. When I circled back to the grid bias adjustment this evening it needed a tiny tweak, but hardly anything that suggests a larger problem with the grid circuit.

I got through the vertical section of the calibration today, and all is well. The measured BW is around 130MHz. Even the 2mV/div BW was about 118MHz (spec is 90MHz). The focus is reasonably good - again, I don't know that it would get better with more range on the knob. But I can't help but think that the adjustment range should be closer to center, and that it might be indicative of a problem. Something marginal on its way to failure?

When I look at the schematic nothing really jumps out as me as a likely problem, but the focus circuit appears powered by the HV multiplier output. Could the anode voltage be sagging? How do I check that? I don't mean go buy an HV probe - I've done that. I mean, how do you safely access and measure the anode voltage? It's all sealed up, and I'm not keen on the idea of piercing insulation or jamming sharp things into the anode connector. Do 'yall just unplug it and use a wire or something to measure the socket inside the female side of the connector?

And I haven't seen what it should be. What should the anode voltage be for a 2236? What are it's specs? I have a physical manual, but I'm not completely familiar with it - I don't know where it would tell me what the anode should be, and what its limits are. It's not part of the calibration procedure.

Any help is appreciated. I need to call it a day and get some sleep. Thanks all.
Dave


Alex
 

This is a well known and documented problem in the 22xx line. There is a string of 5 (or 6) 510K resistors that over time change value and produce the focus problem in some cases even going off the potentiometer adjustment range. Just replace all resistors with metaI film types and 1-2W rating, and also make sure that each resistor is rated to handle at least 300V or more. Don't have any specific knowledge of the 2236, but for sure had to replace these resistors in two separate 2213A which basically are the same in this sense. Do a search here and I am sure you will find many other messages referring to this particular problem.


Dave Peterson
 

Got into the PS section today. Visually things look pristine, but:

R888: 472k
R889: 519k
R890: 943k
R891: 1601k
Total: 3535k (3520k measured).

Or a 70% increase over 4 x 510k. Guess it's time to order up some metal film resistors. Existing appears to be 1/2 W carbon composition? The book says "RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave


 

Dave,

The book is telling you that these should be carbon FILM resistors, which makes sense if they are supposed to be high precision (5%). The range of their values is absurd, both high and low by more than 70%!

[dawning a beige trench coat and mumbling through a cigar] there's just one thing that bothers me...

Are these resistors in series? When I add up the values I get only 2094k Ω. Why are you measuring 3535k Ω?

-- Jeff Dutky


Dave Peterson
 

Yes, they are series resistors. You missed the '1' at the end of R891: 1601 k, not 160 k.


 

1601k Ω. YIKES!

It sure sounds like these are carbon comp resistors, no matter what the parts list says, or did carbon film resistors suffer from the same drift problems for which carbon comps are (in)famous?

(also, "donning" not "dawning")

-- Jeff Dutky


Tom Lee
 

Yikes, indeed! Carbon film resistors are generally much more stable than are carbon comps, but I have seen enough off-tolerance carbon film resistors to know that they, too, can drift (and generally upwards). If they run hot (though still within their rated dissipation), that can accelerate their drift. Also, if there is arcing or corona, that can degrade the films as well. HV circuits are hard on components.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 8/25/2021 23:47, Jeff Dutky wrote:
1601k Ω. YIKES!

It sure sounds like these are carbon comp resistors, no matter what the parts list says, or did carbon film resistors suffer from the same drift problems for which carbon comps are (in)famous?

(also, "donning" not "dawning")

-- Jeff Dutky




greenboxmaven
 

One detail often forgotten is that resistors have a maximum voltage rating. Be sure the voltage across each one will not exceed the rating, if need be use more resistors in series  but choose them to keep the same total resistance. Heathkit ignored this in many of their oscilloscopes, and many have been trashed as having a bad CRT because of it. Another resistor to be alert for in any equipment is a 1/2 watt carbon resistor that looks "shriveled", a bit smaller than others and with a "woody" look to the case. They usually increase their resistance by 2-300% of their markings.

     Bruce Gentry, KA2IVY

On 8/26/21 0:32, Dave Peterson via groups.io wrote:
Got into the PS section today. Visually things look pristine, but:

R888: 472k
R889: 519k
R890: 943k
R891: 1601k
Total: 3535k (3520k measured).

Or a 70% increase over 4 x 510k. Guess it's time to order up some metal film resistors. Existing appears to be 1/2 W carbon composition? The book says "RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave





Alex
 

On Wed, Aug 25, 2021 at 09:32 PM, Dave Peterson wrote:

Guess it's time to order up some metal film
resistors. Existing appears to be 1/2 W carbon composition? The book says
"RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave
As I said before, just replace all resistors with metaI film types and 1-2W rating (they will be about the same size of a 1/2w carbon), and also make sure that each resistor is individually rated to handle at least 300V or more. Use the Mouser parametric search to find a suitable candidate.


Dave Peterson
 

Ah, thanks for the clarification. I misread your reply as saying 1/2W, not 1-to-2W. RTFM Dave.

I've also wondered about the installation: Is it better to A) snug the resistor to the board, B) stand the resistor off a couple/few mm, C) it doesn't matter?


Dave

On Thursday, August 26, 2021, 06:25:25 AM PDT, <tekscopegroup@...> wrote:

On Wed, Aug 25, 2021 at 09:32 PM, Dave Peterson wrote:

Guess it's time to order up some metal film
resistors. Existing appears to be 1/2 W carbon composition? The book says
"RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave
As I said before, just replace all resistors with metaI film types and 1-2W rating (they will be about the same size of a 1/2w carbon), and also make sure that each resistor is individually rated to handle at least 300V or more. Use the Mouser parametric search to find a suitable candidate.


n4buq
 

I recently had a 1/2W film resistor flame-out on the LV Supply board in my 2445 and it burned a significant divot in the board. I mounted the replacement about 1/8" or more off the board to give it some breathing room. Probably unnecessary as the cap that caused the resistor to burn out is now replaced but I did it just for good measure.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Dave Peterson via groups.io" <davidpinsf@...>
To: "TekScopes@groups.io" <tekscopes@groups.io>
Sent: Thursday, August 26, 2021 9:14:33 AM
Subject: Re: [TekScopes] 2236 Focus range.

Ah, thanks for the clarification. I misread your reply as saying 1/2W, not
1-to-2W. RTFM Dave.

I've also wondered about the installation: Is it better to A) snug the
resistor to the board, B) stand the resistor off a couple/few mm, C) it
doesn't matter?


Dave


On Thursday, August 26, 2021, 06:25:25 AM PDT, <tekscopegroup@...>
wrote:

On Wed, Aug 25, 2021 at 09:32 PM, Dave Peterson wrote:

Guess it's time to order up some metal film
resistors. Existing appears to be 1/2 W carbon composition? The book says
"RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave
As I said before, just replace all resistors with metaI film types and 1-2W
rating (they will be about the same size of a 1/2w carbon), and also make
sure that each resistor is individually rated to handle at least 300V or
more. Use the Mouser parametric search to find a suitable candidate.












greenboxmaven
 

In power supply or other low frequency circuits, I always stand resistors off  circuit boards. The resistor gets room to cool, and won't roast the circuit board.

     Bruce Gentry, KA2IVY

On 8/26/21 10:19, n4buq wrote:
I recently had a 1/2W film resistor flame-out on the LV Supply board in my 2445 and it burned a significant divot in the board. I mounted the replacement about 1/8" or more off the board to give it some breathing room. Probably unnecessary as the cap that caused the resistor to burn out is now replaced but I did it just for good measure.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Dave Peterson via groups.io" <davidpinsf@...>
To: "TekScopes@groups.io" <tekscopes@groups.io>
Sent: Thursday, August 26, 2021 9:14:33 AM
Subject: Re: [TekScopes] 2236 Focus range.

Ah, thanks for the clarification. I misread your reply as saying 1/2W, not
1-to-2W. RTFM Dave.

I've also wondered about the installation: Is it better to A) snug the
resistor to the board, B) stand the resistor off a couple/few mm, C) it
doesn't matter?


Dave


On Thursday, August 26, 2021, 06:25:25 AM PDT, <tekscopegroup@...>
wrote:
On Wed, Aug 25, 2021 at 09:32 PM, Dave Peterson wrote:

Guess it's time to order up some metal film
resistors. Existing appears to be 1/2 W carbon composition? The book says
"RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave
As I said before, just replace all resistors with metaI film types and 1-2W
rating (they will be about the same size of a 1/2w carbon), and also make
sure that each resistor is individually rated to handle at least 300V or
more. Use the Mouser parametric search to find a suitable candidate.












kim.herron@sbcglobal.net
 

Tek had a service bulletin on this and they suggested
replacing these resistors with 1 or even 2 watt resistors
and mounting them up off the board for air circulation.
I' ve done this on any 22XX sacope I've been in even if
the focus circuit seems to be working.



On 25 Aug 2021 at 22:18, Jeff Dutky wrote:

Dave,

The book is telling you that these should be carbon FILM resistors,
which makes sense if they are supposed to be high precision (5%).
The range of their values is absurd, both high and low by more than
70%!

[dawning a beige trench coat and mumbling through a cigar] there's
just one thing that bothers me...

Are these resistors in series? When I add up the values I get only
2094k . Why are you measuring 3535k ?

-- Jeff Dutky




Kim Herron W8ZV
kim.herron@...
1-616-677-3706



--
Kim Herron
W8ZV
kim dot herron at sbcglobal dot net


Mark Vincent
 

Dave,

As others have said, raise the wattage and stand off the board for air flow. Always look at the physical size of new resistors! New ones made are smaller in physical size than older types. The material for the body can take higher temperatures. The heat negates the resistance coefficient and will get as hot or hotter than original. Buying ones from China on fleabay are the easiest ones to find that are 1% and keep the original physical size. Some of the ones Mouser stocks are 3,68 x 8,74mm and are rated 2W. Those are physically smaller than a standard 1/2W. It was suggested to use 2W. An excellent suggestion. These will be able to handle the voltage across them and the heat generated will be over a larger surface making the body warm instead of hot. The resistor you asked about, I would not use. It is too small physically and the voltage rating is not high enough. Typical 1/2W types are 350V, 1W types are 500V and 2W types are 750V. Smaller physical sizes will have a lower voltage rating.

Getting the 1% types from China, these are 100 ppm/C. Many 5% types can be 1300-2200 ppm/C while carbons are -1300 ppm/C. There are 1% types that are below 100 ppm/C that Mouser stocks. These are usually 1W and below. The 22meg 1/2W 1% I use as the bleeder in the G1 circuit of the dc restorer circuit is 200 ppm/C. The original carbons in the circuit are well out of tolerance (30-50meg). The 100meg and 39meg in the 2400 series (look to see if that model uses both) are also way out of tolerance. Resistors in the high voltage strings I replace with 1W 1% types unless it is an older type that uses 2W, then a 2W 1% is used. The original carbons are operated at or above the voltage rating, 453/4, 7000 series, etc.. I do not have a 2200 series, yet. The precision types generate low/very low noise. I know in a focus circuit resistor noise is not a problem.

Mark


Dave Peterson
 

This is my latest candidate:

https://www.mouser.com/ProductDetail/Vishay-BC-Components/PR02000205103JR500?qs=sGAEpiMZZMtlubZbdhIBIDqBBcd6vlUvC8QRhSjO%252B9c%3D

Meets all the specs, has a good temperature coefficient, and the 4mm diameter is close to the 3.7mm I just measured on the existing parts.

I now have a little backlog of parts to order, so I think I'll go for it. Just need to decide between 10 and 100.


Alex
 

On Thu, Aug 26, 2021 at 07:14 AM, Dave Peterson wrote:
Ah, thanks for the clarification. I misread your reply as saying 1/2W, not
1-to-2W. RTFM Dave.

I've also wondered about the installation: Is it better to A) snug the
resistor to the board, B) stand the resistor off a couple/few mm, C) it
doesn't matter?


Dave

How you mount them does not really matter much as these resistors do not really get hot, and will definitively not smoke the board as there is not enough energy stored in that circuit to be able to burn up these resistors in any way. The value drift is mainly due to the fact that common carbon comp resistors where used and someone in the design department seems to have overlooked the amount of voltage each resistor will be subjected to in operation, well above their rating, hence the slow drift over time (years). You will also not find any heat related stress on them or discoloration, even tough the original resistors are only 1/2W rated. That is why it is highly recommended to use resistors that individually can handle 300-350V, which is the main selling point of any substitute candidate, and the additional wattage 1-2W is just because modern MF/Silicon resistors tend to be physically smaller so will still fit in the same space, and will be mechanically sturdier but there is absolutely no real need to be able to dissipate additional power over the originals.


Alex
 

On Thu, Aug 26, 2021 at 02:19 PM, Dave Peterson wrote:
This is my latest candidate:

https://www.mouser.com/ProductDetail/Vishay-BC-Components/PR02000205103JR500?qs=sGAEpiMZZMtlubZbdhIBIDqBBcd6vlUvC8QRhSjO%252B9c%3D
Seems like a perfect candidate to me.


kim.herron@sbcglobal.net
 

Actually the service bulletin said to mount them above the
board as they DO get hot and I've seen the boards
burnt black due to heat. Follow the SB directive and
mount them 1/2" above the board.

On 27 Aug 2021 at 6:45, tekscopegroup@...
wrote:

On Thu, Aug 26, 2021 at 07:14 AM, Dave Peterson wrote:
Ah, thanks for the clarification. I misread your reply as saying
1/2W, not
1-to-2W. RTFM Dave.

I've also wondered about the installation: Is it better to A) snug
the
resistor to the board, B) stand the resistor off a couple/few mm,
C) it
doesn't matter?


Dave

How you mount them does not really matter much as these resistors do
not really get hot, and will definitively not smoke the board as
there is not enough energy stored in that circuit to be able to burn
up these resistors in any way. The value drift is mainly due to the
fact that common carbon comp resistors where used and someone in the
design department seems to have overlooked the amount of voltage
each resistor will be subjected to in operation, well above their
rating, hence the slow drift over time (years). You will also not
find any heat related stress on them or discoloration, even tough
the original resistors are only 1/2W rated. That is why it is highly
recommended to use resistors that individually can handle 300-350V,
which is the main selling point of any substitute candidate, and the
additional wattage 1-2W is just because modern MF/Silicon resistors
tend to be physically smaller so will still fit in the same space,
and will be mechanically sturdier but there is absolutely no real
need to be able to dissipate additional power over the originals.

On Thursday, August 26, 2021, 06:25:25 AM PDT,
<tekscopegroup@...>
wrote:

On Wed, Aug 25, 2021 at 09:32 PM, Dave Peterson wrote:

Guess it's time to order up some metal film
resistors. Existing appears to be 1/2 W carbon composition? The
book says
"RES, FXD, FILM: 510K OHM, 5%, 0.5W.

Is this a sufficient replacement?
https://www.mouser.com/ProductDetail/Vishay-BC-Components/SFR25H0005
103JR500?qs=sGAEpiMZZMtlubZbdhIBIDRM2xq4Qtv%252BVka%2FZ0bowUU%3D

Dave
As I said before, just replace all resistors with metaI film types
and 1-2W
rating (they will be about the same size of a 1/2w carbon), and
also make sure
that each resistor is individually rated to handle at least 300V
or more. Use
the Mouser parametric search to find a suitable candidate.









Kim Herron W8ZV
kim.herron@...
1-616-677-3706



--
Kim Herron
W8ZV
kim dot herron at sbcglobal dot net


Alex
 

On Fri, Aug 27, 2021 at 12:29 PM, kim.herron@... wrote:

Actually the service bulletin said to mount them above the
board as they DO get hot and I've seen the boards
burnt black due to heat. Follow the SB directive and
mount them 1/2" above the board.
Yes agree, still a good idea, specially as if you say they get hot under certain circumstances. I do not pretend to be an expert on this issue but did not recall seeing any sign of heat stress on any of the resistors in the two 2213A where I replaced them. Might be a different case in other models, or perhaps overheating happens if the whole string drifts low instead of high, and then the current through them increases as well as the power dissipation. I'd be interesting to check the new resistors once installed (or even the old ones if still present in circuit) with an IR thermometer if available to verify any heat related issues for future references. I would not recommend to touch them with a finger during operation to feel the temperature, a nasty "HV" bite will surely get you good.


Dave Peterson
 

Seeing the feedback, and dwelling on the subject, my take is: duh. Why not? No harm I can think of in giving them a little breathing room. I suspect 1/2" is a bit much. So sorry, kind of dumb question.

I'm itching to get an IR tool. Not only am I curious, for example there's a large amount of heat coming off the 465 main board in the vicinity of the sweep generators. Why? An IR meter would help me narrow down the culprit(s). I can certainly use a finger, but that's a rather crude IR instrument. Not that I can do anything about it, but I'm a curious cat.

I also had a recent experience with a PS that was misbehaving, and a quick look with an IR meter would have shown me the problem right away. Such a tool may well be a good thing to have when restarting a repair. Sometimes the magic smoke comes out right away, other times the thing just doesn't work, and excess heat is always a good indicator of a problem.

R's are on the way. I'll let 'yall know how it turns out.