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Tektronix 2246 Oscilloscope Problem.

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adjusting oscilloscope display problems

Here are schematics of the Display/Trigger logic and the Readout System. U600 Slow Logic IC is the display Logic.
 

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By the way, it is most noticeable at slower times. At faster times, the problem goes away.
 

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Maybe it's normal behavior for that model. About 20 years ago I briefly saw an unfamiliar model Tek scope, and was surprised by the annoying chopping artifacts. I forget which model it was.

I rarely see that effect in my 2465/2467 scopes. These scopes have a "refresh prioritizer" that does a pretty good job of hiding the chopping artifacts. Maybe your scope doesn't have that feature.
 

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Well, I had one of these 3 years ago (can't remember if it was a 2246 or 2246 Mod A) and it did not have this problem.
 

tektronix 2246 does not appears voltage on screen

I don't have the service manual available to look at but there are often loop routines that you can activate through the service menu to make scope measurements. These measurements can be compared to the waveform signatures given in the manual. It should be fairly easy to compare for timing and logic level issues that will be the cause of your problem.
 

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U2410 generates RO_BLANK (pin 16) and RO_REQ (pin 14), and those should be either high>2.0V or low<0.8V.
In the receiving end sits U600.

If either one of these non-standard ICs are faulty you are pretty much screwed - but if you have good levels
from U2410 you can check if they disappear along the way.
Check R2401, R2419 and R2420 and all the solder joint and the connectors in the path.

/Ram
 

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Pin 14 has 5 volts when when the readout is off and 4.7 V when it is on.

Added after 37 seconds:

And those resistors and connections check OK.
 

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By the way, are you measuring the signals with another oscilloscope?

/Ram
 

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No, I am using a DMM with a volt meter and frequency counter.
 

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OK, does the frequency counter register anything on pin 14?
I guess the pulse is low very briefly and that the DMM measures the average voltage.
Something seesm to be happening on the pin, though.

/Ram
 

tektronix u602

Pin 14 is 25Khz when the readout is on and 0 Hz when its off. Pin 16 is 13 KHz when readout is on and 0 when its off.
 

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Would a problem with horiz and vert buffers U2416's have anything to do with this issue?
 

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If I understand the operation correctly the output of those opamps give the X-Y position for readout dots
and as they apparently are in the right space on the screen they are OK.

Another thing springs to mind - do the dots appear on every trace?
In your picture I can see them in the bottom trace. Is that CH4?

/Ram
 

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They appear on all traces and it becomes more noticeable the more traces that are on the screen. It doesn't get any worse if you turn on the cursors or volt meter. And the readout intensity has no affect either. If one, or all four, traces are on the screen and the readout is off, they are perfect. When you turn the readout on, you see the "dashes" go through the traces across the screen and then settle in. As soon as the readout is turned off, the traces look smooth.

Added after 2 hours 44 minutes:

On pin 1 of U600 (TC input) I only get 2.4 KHz. Schematic says 10 MHz should be there. On Blank (pin 18 of U600 and pin 2 of U602) there is 26 MHz with readout on and nothing when its off. Also, in my scope, there is a capacitor across R666. There is no cap in the schematic.
 

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If you aren't getting 10 MHz at U600 pin 1, is the crystal oscillator is dead?
 

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Would this cause the readout to malfunction? Does the TC input on U600 feed the readout blanking?
 

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It is highly unlikely that you would get 26 MHz out of U600 if TC is 2.4 kHz - or even 10 MHz for that matter.
You really need an oscilloscope to measure those signals properly.

You could try and remove the extra capacitor across R666 to see what happens.

/Ram
 
tektronix 2246 no correct trace

Woaps! My mistake, I've made a typing error. It's 26 KHz, not MHz.
 

tektronix 2246 not screen voltage

Does that oscillator circuit going into the TC input of U600 have anything to do with the readout?
 

2246 oscilloscope

U600 has a lot to do with the display. It will probably work with 2.4 kHz -
but all timing will be way off.
It sounds farfetched though...
Nevertheless you should see to it that it gets 10 MHz.

/Ram
 

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