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Tek 2235 display problem

Creede

Apr 22, 2021
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I recently picked up a Tex 2235 scope. The display shows a solir band 5 units high. You can move it up and down with the front panel controls. A signal input to the vertical amp is superimposed on the display. The signals to the vertical and horizontal pins of the CRT are clean and as would be expected.
There is a high level of 20 Khz present. it is surely coming from the HV power supply. I don't know how much filtering the HV supply has.Could this radiation be getting into the scope tube and causing the display problem? I have not been able to find much info on these HV power supplies.
Any help would be greatly appreciated.
Creede
 

bertus

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Nov 8, 2019
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Hello,

The failure you are describing looks like a focus failure.
When the dot is out of focus a broad line may occur.
The focus is also in the high voltage section.

The service manual can be obtained here:
http://www.jabse.com/L1Rla3Ryb25peC8yMjM1

From that manual:
TEK 2235 High voltage section focus.png
Bertus
 

Creede

Apr 22, 2021
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I measured the voltage at the focus pot and it is as specified on the schematic. But is has no effect on the trace.
Any suggestion on trouble shooting this further?
Thanks very much.
Creede
 

Creede

Apr 22, 2021
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I measured the voltage at the focus pot and it is as specified on the schematic. But is has no effect on the trace.
Any suggestion on trouble shooting this further?
Thanks very much.
Creede
My last post was in error. I had measured the astigmatism pot thinking that might be the problem. I will look further into the focus circuit.
Looks like high voltage there.
Thanks a lot for your help.

Creede
 

bertus

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Nov 8, 2019
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Hello,

The voltag will be rather high.
C893 wich is connected to the focus is 1 nF 3000 Volts.

Bertus
 

Creede

Apr 22, 2021
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Hello,

The voltag will be rather high.
C893 wich is connected to the focus is 1 nF 3000 Volts.

Bertus
Hi Bertus
I checked the waveform at test point 45 and it is as described in the manual and does change with intensity (brightness). when looking at the focus pot, high speed pulses. I only have a x10 probe so trace is off scale. I could try to make a voltage divider to be able to look at the wave form if that would be helpful. The drawings that I have found don't have voltages to check in that circuit, Not sure how to proceed.
Very much appreciate your help,
Creede
 

bertus

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Nov 8, 2019
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Hello,

The focus voltage can be upto 2 kV.
See this snippet from the manual:
TEK 2235 focus circuit.png
Strange effects can occur when Q885 or Q804 are not working properly.

Bertus
 

Creede

Apr 22, 2021
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I have checked al the resistors in the focus area and all are within specs. I have checked some of the diagrams and have found:
1, Symbol 48 near TP842 has a +/- 10 volt square wave.
2. Collector and Emitter of Q825 both have a square wave +/- .1 volt
The frequency is about 300Hz.
These are no way near the drawings in the manual.
I may be getting more lost all the time.
It seems like there may be some sort of logic problem?
Really appreciate the help! I hate to give up on this.
Creede
 

bertus

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Nov 8, 2019
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Hello,

Perhaps this thread on EEVBLOG helps a bit:
https://www.eevblog.com/forum/testg...any-potentialserious-problems-for-this-model/

From the page:
I do not know of any specific issues with the 2235A or the 2235 series for that matter. The common issues that come mind include the power supply capacitors wearing out and CRT DC restorer failures although no more often than other CRT oscilloscopes.

And an repair thread over there:
https://eevblog.com/forum/testgear/tektronix-2235-repair-thread/

Bertus
 

Creede

Apr 22, 2021
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Thanks Bertrus
I appreciate you taking the time to try to help me.
I will keep plugging away.
Best Regards
Creede
 

bertus

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Hello,

How are the testpoints before 48?
TEK 2235 testpoints 46 47 48.png
Are those also faulty?

Bertus
 

Creede

Apr 22, 2021
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Good morning

I have decided to start from scratch on this and check all the reference points in the manual with sample wave forms.
I have only tested 3 so far, 1.2.3.
Non of the three have any wave forms, only DC levels.
1, Pin 1 U537 (tp537) 4.2 Volts.
2, Pin 6 U537 .25 Volts
3, Pin6 U540 .3 Volts
I will jump to 47 and 48 and answer your questions, but I don't think I will have time today.
This is tedious work with my old eyes.
Thanks for staying with me on this.
Creede
 

Creede

Apr 22, 2021
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I managed to sneak off and make the measurements you asked about.
There is no structure like in the drawings. Only a pulse. No step.
47. 2.3 Volt to 2.5 Volt pulse. High for 2.7 Msec and Low for .05 Msec.
48. 12v to 28 volt pulse. High for 2.7 Msec and Low for 2.3 Msec.
I will resume going through the reference points tomorrow.

Creede
 

bertus

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Hello,

As the diagram 48 says, the amplitude should change with the intensity.
Does it change with the intensity?

Did you use the settings as in the manual?

tek 2235 ac waveform settings.png

bertus
 

Creede

Apr 22, 2021
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Good morning

The answer is yes. It goes from 0 Volts to a pulse about -10 to +30 Volt. Also as I said before it does not have the intermediate step as shown in the wave form diagram.
I still plan to check all the check points unless you recommend a different approach. I am not very fast at this. It seems to take me a long time to find the points to check.
Thanks again for your help.

Creede
 

Creede

Apr 22, 2021
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Missed a point. Yes I used those settings except for the 1Khz input.
 

Creede

Apr 22, 2021
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Yes the transistor seems okay. The signal on the collector +15 to +48 Volt pulse. I might have had the scope ac coupled before.
Continuing with my testing I found U502 (MC10198), an monostable multivibrator, has no activity I am an old RF designer and not too up on digital stuff.The data sheet I found shows supply voltage's to be Vee -5.2V, Vcc1 +2V and Vcc2 2V. When I measure, I find Vee, 0V, Vcc1 5.05V and Vcc2 4.2V.
The theory of operation for this device might be in the manual, But I have not yet tried to find it.
What do you think?
Creede
 

bertus

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Hello,

Here I have the datasheet of the MC10198.

Bertus
 

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