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jordankersten
Joined: 07 Jun 2008
Posts: 38
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 07 Jun 2008 20:13 Tektronix 2246 Oscilloscope Problem. |
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| 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?
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XNOX_Rambo
Joined: 13 Jul 2002
Posts: 430
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Location: Far out, man!
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| 07 Jun 2008 20:29 Re: Tektronix 2246 Oscilloscope Problem. |
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| Are the dashes equidistant?
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 07 Jun 2008 21:03 Re: Tektronix 2246 Oscilloscope Problem. |
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| XNOX_Rambo wrote: |
| 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:
| XNOX_Rambo wrote: |
| Are the dashes equidistant? |
Well, I take that back, they are. Here is a pic of the traces in 10X Mag mode...[/img]
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echo47
Joined: 07 Apr 2002
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| 07 Jun 2008 21:50 Re: Tektronix 2246 Oscilloscope Problem. |
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Some folks won't want to download such a big JPEG file. Here's a smaller version.
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XNOX_Rambo
Joined: 13 Jul 2002
Posts: 430
Helped: 86
Location: Far out, man!
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| 07 Jun 2008 22:04 Re: Tektronix 2246 Oscilloscope Problem. |
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| Does switching back and forth between CHOP and ALT do anything?
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 07 Jun 2008 22:16 Re: Tektronix 2246 Oscilloscope Problem. |
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| XNOX_Rambo wrote: |
| 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.
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 07 Jun 2008 22:36 Re: Tektronix 2246 Oscilloscope Problem. |
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Here's a smaller file, sorry.[/img]
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XNOX_Rambo
Joined: 13 Jul 2002
Posts: 430
Helped: 86
Location: Far out, man!
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| 07 Jun 2008 23:04 Re: Tektronix 2246 Oscilloscope Problem. |
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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
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 08 Jun 2008 0:13 Tektronix 2246 Oscilloscope Problem. |
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| If you turn the readout off, the dots smooth out.
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XNOX_Rambo
Joined: 13 Jul 2002
Posts: 430
Helped: 86
Location: Far out, man!
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| 08 Jun 2008 11:26 Re: Tektronix 2246 Oscilloscope Problem. |
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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
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 08 Jun 2008 18:21 Re: Tektronix 2246 Oscilloscope Problem. |
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| 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.
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E-design
Joined: 01 Jun 2002
Posts: 768
Helped: 57
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| 08 Jun 2008 19:28 Re: Tektronix 2246 Oscilloscope Problem. |
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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.
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 08 Jun 2008 22:18 Tektronix 2246 Oscilloscope Problem. |
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| 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?
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E-design
Joined: 01 Jun 2002
Posts: 768
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| 08 Jun 2008 22:56 Re: Tektronix 2246 Oscilloscope Problem. |
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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.
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 08 Jun 2008 23:17 Re: Tektronix 2246 Oscilloscope Problem. |
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| I can't find anything in the service manual about Z axis blanking. Would it be considered something else?
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E-design
Joined: 01 Jun 2002
Posts: 768
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| 08 Jun 2008 23:29 Re: Tektronix 2246 Oscilloscope Problem. |
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| 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.
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 10 Jun 2008 8:18 Re: Tektronix 2246 Oscilloscope Problem. |
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| 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?
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XNOX_Rambo
Joined: 13 Jul 2002
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Location: Far out, man!
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| 10 Jun 2008 11:40 Tektronix 2246 Oscilloscope Problem. |
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It depends - is the ROR input analogue or digital?
If digital, "low" is way off - it should be below 0.8V.
/Ram
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 10 Jun 2008 16:22 Re: Tektronix 2246 Oscilloscope Problem. |
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| 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.
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XNOX_Rambo
Joined: 13 Jul 2002
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| 10 Jun 2008 16:47 Tektronix 2246 Oscilloscope Problem. |
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Some sort of schematics would be nice... 
/Ram
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 11 Jun 2008 2:58 Re: Tektronix 2246 Oscilloscope Problem. |
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Here are schematics of the Display/Trigger logic and the Readout System. U600 Slow Logic IC is the display Logic.
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 11 Jun 2008 4:14 Tektronix 2246 Oscilloscope Problem. |
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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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echo47
Joined: 07 Apr 2002
Posts: 4205
Helped: 564
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| 11 Jun 2008 5:17 Tektronix 2246 Oscilloscope Problem. |
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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.
Last edited by echo47 on 11 Jun 2008 5:42; edited 1 time in total |
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 11 Jun 2008 5:41 Re: Tektronix 2246 Oscilloscope Problem. |
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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.
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E-design
Joined: 01 Jun 2002
Posts: 768
Helped: 57
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| 11 Jun 2008 6:03 Re: Tektronix 2246 Oscilloscope Problem. |
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| 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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XNOX_Rambo
Joined: 13 Jul 2002
Posts: 430
Helped: 86
Location: Far out, man!
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| 11 Jun 2008 11:27 Re: Tektronix 2246 Oscilloscope 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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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 11 Jun 2008 17:18 Tektronix 2246 Oscilloscope Problem. |
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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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XNOX_Rambo
Joined: 13 Jul 2002
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Helped: 86
Location: Far out, man!
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| 11 Jun 2008 22:40 Tektronix 2246 Oscilloscope Problem. |
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By the way, are you measuring the signals with another oscilloscope?
/Ram
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jordankersten
Joined: 07 Jun 2008
Posts: 38
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| 12 Jun 2008 0:19 Re: Tektronix 2246 Oscilloscope Problem. |
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| No, I am using a DMM with a volt meter and frequency counter.
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XNOX_Rambo
Joined: 13 Jul 2002
Posts: 430
Helped: 86
Location: Far out, man!
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| 12 Jun 2008 0:48 Re: Tektronix 2246 Oscilloscope Problem. |
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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
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