Dead Tek 2235

[isw]

A friend came across one of these, a bit dirty but otherwise looking OK.
Green pilot lights up, but the screen does not, even in a darkened room.

Are there well-known failure points in these? Where to find a schematic
and/or service manual?

Isaac

 

[tom]

"isw" <isw@witzend.com> wrote in message
news:isw-A34D3E.21452125082016@news-roam.garlic.com...

A friend came across one of these, a bit dirty but otherwise looking OK.
Green pilot lights up, but the screen does not, even in a darkened room.

Are there well-known failure points in these? Where to find a schematic
and/or service manual?

Isaac

Service manual and Operators manual here:

http://w140.com/tekwiki/wiki/2235

Also you might join the tek yahoo group and describe your findings there.

https://groups.yahoo.com/neo/groups/TekScopes/info


Good luck.

--

 

[isw]

In article <npojvq$dnb$1@dont-email.me>,
"tom" <tmiller11147@verizon.net> wrote:

"isw" <isw@witzend.com> wrote in message
news:isw-A34D3E.21452125082016@news-roam.garlic.com...
A friend came across one of these, a bit dirty but otherwise looking OK.
Green pilot lights up, but the screen does not, even in a darkened room.

Are there well-known failure points in these? Where to find a schematic
and/or service manual?

Isaac

Service manual and Operators manual here:

http://w140.com/tekwiki/wiki/2235

Also you might join the tek yahoo group and describe your findings there.

https://groups.yahoo.com/neo/groups/TekScopes/info

Thanks.

Isaac

 

[Dave Platt]

A friend came across one of these, a bit dirty but otherwise looking OK.
Green pilot lights up, but the screen does not, even in a darkened room.

Are there well-known failure points in these? Where to find a schematic
and/or service manual?

One known weak point in this family is a chain of high-value
carbon-composition resistors which divide down the high voltage to
provide the focus voltage. They run fairly hot, and tend to drift
over time, resulting in a fuzzy image which can't be focused properly.

If one of these were to fail "open" I suspect it might disrupt the
operation of the CRT badly enough that you wouldn't see a trace.

Repair costs a few dollars in parts (either some old-stock carbon
comp resistors selected for the right value, or more modern leaded
resistors of a type rated for high voltage) and an hour or two of
work (a few of the resistors in the chain are tucked into slightly
tricky places on the board). It's not a bad job, though: I picked up
a 2235 at a flea market for $60, found it had this problem, and had it
repaired by the end of the afternoon.

Failure-to-light-up might mean a more serious problem in the high
voltage supply - if I recall correctly it's a switcher. Dried-out
(high-ESR) or leaky caps would be another common suspect.

 

This is where an old VTVM and 1090 meg HV probe come in handy. The self contained ones most people used for TV work could not measure negative voltages and it is a hell of alot easier to get to the cathode than the anode on a scope.

Those voltages are usually on the print, so you can tell if it is in regulation or not. If I am not mistaken, the dividing resistors for the focus are part of the HV regulation feedback loop, so a fairly accurate measurement of the cathode voltage can be quite useful. Also, turn it off and let it all discharge, then turn it on with the probe connected and see if you get HV for a split second and then goes away. If so it is going into shutdown, dead giveaway.

Most of the scopes we bought/buy we get a chance to plug them in and tend to get the ones that at least produce a trace. We are sorta in the market for one of those 1090 ohm jobs but the ones I see are a little to high for something that might get used once a year. Also that the market for them is so limited. So that means I get to wrestle with the fun ones, like B sweep not starting on a 465B. You REALLY want B sweep to work on a 465B because it has the split function where you can see the whole trace and the expanded trace at the same time. Even though it is not something you would use everyday, it is still a feature and it should work.

And don't overlook the possibility of a failure in the main power supply. Check everywhere it says 15V, 5V, 8V -5V, -15V, 130V, 50V, -50V and so forth.. Those seem to be the supply voltages Tek used to like to use.

 

[Ian Field]

"isw" <isw@witzend.com> wrote in message
news:isw-2D3B4E.09442026082016@news-roam.garlic.com...

In article <npojvq$dnb$1@dont-email.me>,
"tom" <tmiller11147@verizon.net> wrote:

"isw" <isw@witzend.com> wrote in message
news:isw-A34D3E.21452125082016@news-roam.garlic.com...
A friend came across one of these, a bit dirty but otherwise looking OK.
Green pilot lights up, but the screen does not, even in a darkened
room.

Are there well-known failure points in these? Where to find a schematic
and/or service manual?

Isaac

Service manual and Operators manual here:

http://w140.com/tekwiki/wiki/2235

Also you might join the tek yahoo group and describe your findings there.

https://groups.yahoo.com/neo/groups/TekScopes/info

Thanks.

The simplest and easiest thing to do is check the CRT heater lights - a dry
joint somewhere would probably be more likely than an OC heater.

 

"The self contain HV probes are easy to change polarity. Either
reverse the meter leads, or connect a small bridge rectifier between >the
meter and the probe and ground lead. "

Not so fast. These things had a current detect function for the horizontal output TUBE (vale for those overseas). Now if you put a DPDT switch there and reverse the meter coil polarity that would work.

However, a bridge rectifier would introduce error, and that error would be greater at lower voltages. Many meter movement barely need a volt but a standard diode drops .6 of that. At scope cathode voltages which might only be like 2 KV or less, this is significant.

We now have a Keithley HV probe but acn't really use it. Dude found a good deal on it, but it has a 350 meg output resistance which means you need an electrometer to use it. A regular VTVM will not work without alot of math, and even then... We do have one meter is would work on and it is seriously aged. (two syllables there, really) Actually it agrees with the best of the Flukes we got, but nobody is going to do the NIST routine on it and certify it.

I have seriously considered building one, but not with 1090 megs. The meter is 10 megohm, so 90 megs gives it enough range to measure scope CRT voltages. We are sort of committeds to repairing and restoring CROs. I do not like DSOs, let alone LCD based scopes. I believe that every electronics class in highschool should have a CRO for the kids to play with, once they know enough to not fuck it up of course. I also think they should not graduate without certain other knowledge, like the different types of firearms ad how to at least unload them and make sure there is not one in the chamber. If we are going to have guns here, dammit, make sure people know how to be safe with them.

They should know how to change a tire on the care, some at least simple mechanics. How many times has someone called on a neighbor or whatever to do one of the simplest of tasks claiming that are "not mechanically inclined" ? We put a Man on the moon and now we are down to this ? No wonder conspiracy theorists think it was fake.

But I can tell you the moonshot was real. Not that I was there but it is not hard to figure out. This was a milestone in human history, a worldwide event. There were Russian kids with telescopes all over the place and it was during the cold war. If ANY ONE of them caught even one little piece of evidence that it was faked they would be rich and the USSR would have never let us live it down. And if you really think about it, and I mean really, it would have cost more to fake it than to actually do it. Plus they really did want to do it.

But I have digressed quite a bit. As I look at my old 561A Tek that was built before I was born with dual trace and dual time base, sometimes I wonder just how far we have come. They have minuturized things and packed more transistor and other devices into one case, but what have they really done ?

When I was young my Uncle gave me the basic books from the air force. Whatever branch of the US military you serve(d) in, if it was the air force you usually had the least risk and got the best jobs when you got out. At least back then.

But those books said there are only three electronic circuits, rectifier, amplifier and oscillator. And every circuit to this day you see can be described as one of those. It might be gated or controlled, fancied up in all kinds of ways but it is still the same thing. Consider the function, a computer gate can be considered rectifiers feeding an amplifier.

But anyway, I am probably boring the OP (et al) so it is time to stop.

To the OP, do you got 15 volts ? Do you got 130 volts ? Before tearing into the HV shit that doesn't like to be touched, make sure you got everything else.

 

[Michael A. Terrell]

jurb6006@gmail.com wrote:

Most of the scopes we bought/buy we get a chance to plug them in
and tend to get the ones that at least produce a trace. We are
sorta in the market for one of those 1090 ohm jobs but the ones
I see are a little to high for something that might get used once
a year.

The self contain HV probes are easy to change polarity. Either
reverse the meter leads, or connect a small bridge rectifier between the
meter and the probe and ground lead.


--
Subject: Spelling Lesson

The last four letters in American.........I Can
The last four letters in Republican.......I Can
The last four letters in Democrats.........Rats

End of lesson. Test to follow in November, 2016

Remember, November is to be set aside as rodent extermination month.

 

[Michael A. Terrell]

jurb6006@gmail.com wrote:

"The self contain HV probes are easy to change polarity. Either
reverse the meter leads, or connect a small bridge rectifier between >the
meter and the probe and ground lead. "

Not so fast. These things had a current detect function for the horizontal output TUBE (valve for those overseas). Now if you put a DPDT switch there and reverse the meter coil polarity that would work.

However, a bridge rectifier would introduce error, and that error would be greater at lower voltages. Many meter movement barely need a volt but a standard diode drops .6 of that. At scope cathode voltages which might only be like 2 KV or less, this is significant.

Not all of them have a current function. The bridge rectifier would
introduce a little over 1 volt error. There is a shunt in the ones that
monitor cathode current for the horizontal output. You have no need for
hat in a scope repair. The reason I wouldn't use the switch is that
there isn't enough wetting current, so after a few years it can start
arcing. Even at low current, you don't want to create plasma. Calculate
the current, through the resistor for the full scale range. It is quite
low to keep from loading the HV supply and affecting the reading.


--
Subject: Spelling Lesson

The last four letters in American.........I Can
The last four letters in Republican.......I Can
The last four letters in Democrats.........Rats

End of lesson. Test to follow in November, 2016

Remember, November is to be set aside as rodent extermination month.

 

[Ian Field]

<jurb6006@gmail.com> wrote in message
news:b4c546df-5b9b-413d-b800-6fab1310ef66@googlegroups.com...

"The self contain HV probes are easy to change polarity. Either
reverse the meter leads, or connect a small bridge rectifier between >the
meter and the probe and ground lead. "

Not so fast. These things had a current detect function for the horizontal
output TUBE (vale for those overseas). Now if you put a DPDT switch there
and reverse the meter coil polarity that would work.

However, a bridge rectifier would introduce error, and that error would be
greater at lower voltages. Many meter movement barely need a volt

I remember meter shunts being available in 2 different types - one was 100mV
and the other might have been something like 70mV.

It was usually printed in a corner of the dial plate so you could order the
right shunt.

 

"I remember meter shunts being available in 2 different types - one >was 100mV
and the other might have been something like 70mV.

It was usually printed in a corner of the dial plate so you could >order the
right shunt. "

What Terrell say about the bridge rectifier only introducing about a volt of error only applies if there is no shunt. I am not sure. Now this Keithley we got has a shunt but will ot rui into 10 megs, so that leaves regular VTVMs out.

These totally passive HV probes must load more than a 1090 meg one, and certainly do for the Keithley probe which is way up in the gigohms. I forget the specs but the resistance is very high.

I am not so sure we need it that high. A CRT pulls current, some even do a bit on the focus electrode though I think i a perfect one it doesn't. But there is current from the cathode to anode so I think 1100 megs will be high enough to determine if it is i regulation and whatever. Most of the scope print I have seen seem to indicate it would be alright for the control grid or the blanking grid as well. That is what we need. We are not measuring the output of a Van De Graff generator here.

Still waiting to hear if all the low voltage supplies are there. Many of the older Tek CROs used the supply that fed the 15V regulator, unregulated and just regulated the HV directly.There could be fusibles to that supply. Sorry, but I am just about doe studying prints for people, let them download them and have a look, ad maybe learn something in the process. I am tired, half blind and my computer is quite full. I am tired of going to BAMA, electrotanya, hifiengie and that 401 whatever for nothing, I don't even do that for work anymore since my computer there disappeared. I couldn't stand the thing anyway with Win 10 on it. Can't up the font so I can see the damn thing, can't change the homepage to Google, fukit. I get paid by the hour and if they slow me down it costs me NOTHING. It costs them. I am tired of fucking around. I got a stack of 53 amplifiers that need a modification, go ahead and drag your feet. Take some more tools away, I don't give a fuck. I used to but now I will sit on the head, do my nails or who fucking knows.

I said what I said, check the low voltage supplies first. Not having a trace can be caused by 100 different things, not just lack of HV.

 

[Michael A. Terrell]

jurb6006@gmail.com wrote:

"I remember meter shunts being available in 2 different types - one >was 100mV
and the other might have been something like 70mV.

It was usually printed in a corner of the dial plate so you could >order the
right shunt. "

What Terrell say about the bridge rectifier only introducing about a volt of error only applies if there is no shunt. I am not sure. Now this Keithley we got has a shunt but will ot rui into 10 megs, so that leaves regular VTVMs out.

These totally passive HV probes must load more than a 1090 meg one, and certainly do for the Keithley probe which is way up in the gigohms. I forget the specs but the resistance is very high.

What is the model of that probe? You should be able to find out what
Keithley meters it was made to work with.

The 1600A probe looks to be made for a 1 Meg input impedance meter.

The manual says the resistor is ~ 1000Meg, and it it a 1000:1 probe, so
it would be used on the 50V range.


<http://www.libertytest.com/assetmanager/uploaded/pdf-2011721-141719-kei_1600a-ug.pdf>