Tektronix 2246 Oscilloscope Problem.

[jordankersten]

tektronix 2246 service manual

I just got a Tektronix 2246 Mod A Oscilloscope. Everything works great on it except for one thing: The CRT seems to have some "sparkle" or "noise" in it. The trace looks as if it has dashes or breaks in it. It gets worse the more channels you have on the screen (its a 4 channel scope) and gets worse still if you use Line triggering. I am thinking maybe a power supply filter problem? HV CRT problem? It passes all it's self tests and runs it's calibration fine. Also, the problem doesn't seem to affect the screen readouts or cursors. Any ideas?

 

[XNOX_Rambo]

tektronix 2246 flicker

Are the dashes equidistant?

 

[jordankersten]

tektronix 2246 dots on display

Are the dashes equidistant?

No, and they move through the wave form. When viewing a square wave, it is worse in the vertical deflection.

Added after 9 minutes:

Are the dashes equidistant?

Well, I take that back, they are. Here is a pic of the traces in 10X Mag mode...[/img]

 

[echo47]

tektronix 2246 service

Some folks won't want to download such a big JPEG file. Here's a smaller version.

 

[XNOX_Rambo]

tektronix 2246 oscilloscope

Does switching back and forth between CHOP and ALT do anything?

 

[jordankersten]

tektronix 2246 repair

Does switching back and forth between CHOP and ALT do anything?

The pic above is in ALT mode. If you switch to CHOP, then the traces have smooth parts in them, but there are "dotted" spots (like above) that move along the waveform.

Added after 7 minutes:

Here is a pic in CHOP mode...those "dotted" chuncks move along the traces.

 

[jordankersten]

tek 2246

Here's a smaller file, sorry.[/img]

 

[XNOX_Rambo]

tektronix 2246

Odd. I would have expected the chopped-up traces in CHOP mode.
Has the scope been refurbished/repaired? Maybe some wiring has been crossed.

You can check that ALT and CHOP work properly by setting the time base to 0.1 s/div and enabling CH1 and CH2.
In ALT mode the traces will take turns in sweeping the screen.

The smaller wandering dots might be attributed to the screen readout - the beam has to switch between painting them and the traces.
I see you have set the time base to x10 - that might have something to do with it.
You can try and turn of the readout and see what happens to the dots.

/Ram

 

[jordankersten]

tektronix 2246 problem

If you turn the readout off, the dots smooth out.

 

[XNOX_Rambo]

tektronix 2246 no trace no signal

Well, it seems to me that your scope is working as it should and that the dots you see are just artifacts from the chopping operation.
Either you or the scope have mixed up the CHOP/ALT setting.

On the first chopped-up picture one can see that there is one dash and three "spaces" in a row, which is consistent with displaying four traces.
If you switch to two traces the pattern will be dash-space-dash-space etc, but the dashes will still be of the same length as previously.
From the picture I can calculate the chopping frequency to be about 1 MHz which seems normal.

You should use ALT most of the time and only switch to CHOP at lower sweep frequencies when the display starts to flicker.

/Ram

 

[jordankersten]

tektronix 2246 u602

The problem goes away when the readout is turned off. The readout intensity doesn't affect it at all, but when you turn the readout intensity all the way down (off) the problem disappears.

 

[E-design]

2465 tektronix problems

There seems to be a problem with the Z mod data interfering with the normal trace operation. You will need to get hold of the service manual and scope the control signals responsible for blanking and readout functions. You will need another working scope to do so.

There are service diagnostics to help you. You can enter this mode by pressing the top & bot menu-item buttons at the same time. One item on the service menu is DIAGNOSE which may point to your problem area.

 

[jordankersten]

tektronix 2246 service menu

According to the service manual, there is a Z Response variable capacitor. However, on the main board of the scope, there is a fixed capacitor, no adjustment. Would the Z response have anything to do with it?

 

[E-design]

tektronix2246 two bad trace lines

I very much doubt that. I would suspect a logic problem - faulty gate or something that causes a timing error with the Z blanking timing. This may result in readout data that modulates the beam, hence the dots.

The Z response cap would only affect the frequency response of the Z mod input.

 

[jordankersten]

tek 2246 screen not signal

I can't find anything in the service manual about Z axis blanking. Would it be considered something else?

 

[E-design]

tektronix 2465 readout problem

Read the circuit description in the manual about the Z mod, readout and DAC circuits. There are some multiplexing circuits involved that may be the cause of your problems.

 

[jordankersten]

tektronix 2246 fault ram

Ok, the service manual says that When ROR (Readout Request or something like that) input is low, chop blanking is disabled and the ROB (Read Out Blanking) is inverted and controls the blanking. When ROR goes from low to high, the blank output remains connected to the readout blank signal for an additional four to six TC clock periods to mask vertical source-switching transients. My question is this: on my ROR, high input is 5.06V and low is 4.65. Is 4.65 low enough to be considered "low" and enable the readout blanking? When the readout pot is turned all the way off, I get the 5.06 supply voltage on the ROR pin. When the readout is on (doesn't matter to what intensity, just on) then the ROR drops to 4.65V to enable the readout blanking (ROB). Does that voltage difference sound right?

 

[XNOX_Rambo]

tek-2246

It depends - is the ROR input analogue or digital?
If digital, "low" is way off - it should be below 0.8V.

/Ram

 

[jordankersten]

tektronix 2246 problem clock

Honestly, I don't know. I do no that the Freq on the ROR is 24Khz when the readout is on and 0Hz when it's off. Also, in messing with the 7.5 adjust, the behavior changed, but did not go away. The "dashing" of the traces now scan across the trace is are only noticeable some of the time now. One thing: it doesn't matter where I put the 7.5 adjust, the -7.5 is always slightly higher. Ex. If 7.5 is excactly 7.5V then the -7.5 will be around 7.56 or 7.57. The High logic for the ROR is now right at 5.03 volts, the low logic is 4.69V.

 

[XNOX_Rambo]

tektronix 2246 manual

Some sort of schematics would be nice... :wink:

/Ram