Topics

Tek 7T11A issues


n49ex
 

I'm a new member, but have been an avid Tek scope user/collector/fixer ever since my extensive time using them as a former, now retired Bell Labs engineer.
But my ability to fix has recently been challenged with some problems in my 7S11/7T11A pair:

The 7T11A and 7S11 are situated in my 7904A scope, and the problem I am having with it is that I can’t get a stable trace. In fact, (and I think it is all set up correctly) with a square wave input (from my PG503) I get either a splatter of dots on two lines representing the top and bottom of the signal, but not triggering on a transition, or in some settings just a splatter of dots on the screen. And in some settings, from 50uS on down - non real time, you see nothing at all.

Any suggestions on how to proceed with diagnosis/repair would be appreciated.


Dan G
 

One way to see if the 7T11A can produce a non-real-time sweep at all is to try the following setup:

1. Disconnect all signals from the 7S11/sampling head (not needed for these tests)
2. Set 7T11A as follows:
SEQUENTIAL: pushed in
HF SYNC: pushed in
REP: pushed in
TRIG INPUT: no connection

If the 7T11A is working properly and is well calibrated, then you should be able to
get a sweep at all settings of TRIG POS RNG, TIME/DIV SWEEP RANGE,
TRIG LEVEL and STABILITY.

If the test passes, then I would suggest the following next:

3. Set the 7T11A as follows:
50 Ohm 2V MAX: pushed in
TRIG AMP: X1
STABILITY: fully clockwise
TRIG LEVEL: mid-range
TIME POS RNG: 50 us

With SLOPE in either + or - position, there should be a TRIG LEVEL position that
produces a constant sweep (ideally near mid-range). If this does not work, than
something is not quite right with the 7T11A (though it possibly just needs to be
re-calibrated).

If the test passes, then turn STABILITY fully counter-clockwise. Now, *no* position of
TRIG LEVEL should produce a sweep.

If any of these tests fail, then the manner in which they fail should provide useful
troubleshooting hints.


dan


Jim Ford
 

Well, these 7000 series sampling plug-ins are notorious for being temperamental, as far as triggering goes. Does TekWiki mention this? I don't know. I don't have a 7T11 or 7T11A, but I do have a 7S11, a 7S12, and a 7904, and it is very difficult to even get a stable trace on the screen. That's with S-4 sampling heads in the 7S11 and left slot of the 7S12 and either the S-53 trigger recognizer or S-51 trigger countdown head in the right slot of the 7S12.

I'm looking for one of the 11800 series scopes with the very nice SD-24 sampling head/TDR unit. I had that combo at work several decades ago in the mid 1990s when that series was new, and I don't recall any particular triggering issues. Not to mention higher BW/faster rise and fall times and the most accurate pulse response according to Picosecond Pulse Labs (sold to Tek in 2014, IIRC). Don't tell my wife! ;)

Jim Ford

------ Original Message ------
From: "n49ex via groups.io" <n49ex=aol.com@groups.io>
To: TekScopes@groups.io
Sent: 1/27/2021 5:53:47 PM
Subject: [TekScopes] Tek 7T11A issues

I'm a new member, but have been an avid Tek scope user/collector/fixer ever since my extensive time using them as a former, now retired Bell Labs engineer.
But my ability to fix has recently been challenged with some problems in my 7S11/7T11A pair:

The 7T11A and 7S11 are situated in my 7904A scope, and the problem I am having with it is that I can’t get a stable trace. In fact, (and I think it is all set up correctly) with a square wave input (from my PG503) I get either a splatter of dots on two lines representing the top and bottom of the signal, but not triggering on a transition, or in some settings just a splatter of dots on the screen. And in some settings, from 50uS on down - non real time, you see nothing at all.

Any suggestions on how to proceed with diagnosis/repair would be appreciated.





Jim Ford
 

Say, Dan, any advice for using a 7S12 with an S-51 or S-53 in the Pulse Generator(/Trigger) hole? As I mentioned in my recent reply to the OP n49ex, I have a hard time getting a stable trace with those plug-ins in my 7904.

Thanks.

Jim Ford

------ Original Message ------
From: "Dan G" <dgajanovic@sympatico.ca>
To: TekScopes@groups.io
Sent: 1/28/2021 11:16:18 PM
Subject: Re: [TekScopes] Tek 7T11A issues

One way to see if the 7T11A can produce a non-real-time sweep at all is to try the following setup:

1. Disconnect all signals from the 7S11/sampling head (not needed for these tests)
2. Set 7T11A as follows:
SEQUENTIAL: pushed in
HF SYNC: pushed in
REP: pushed in
TRIG INPUT: no connection

If the 7T11A is working properly and is well calibrated, then you should be able to
get a sweep at all settings of TRIG POS RNG, TIME/DIV SWEEP RANGE,
TRIG LEVEL and STABILITY.

If the test passes, then I would suggest the following next:

3. Set the 7T11A as follows:
50 Ohm 2V MAX: pushed in
TRIG AMP: X1
STABILITY: fully clockwise
TRIG LEVEL: mid-range
TIME POS RNG: 50 us

With SLOPE in either + or - position, there should be a TRIG LEVEL position that
produces a constant sweep (ideally near mid-range). If this does not work, than
something is not quite right with the 7T11A (though it possibly just needs to be
re-calibrated).

If the test passes, then turn STABILITY fully counter-clockwise. Now, *no* position of
TRIG LEVEL should produce a sweep.

If any of these tests fail, then the manner in which they fail should provide useful
troubleshooting hints.


dan





Dan G
 

Hi Jim,

I also used to have triggering problems with my 7S12/S-53 setup years ago, but it turned
out that the S-53 just needed to be re-calibrated.

In a 7S11/7T11 setup, the triggering path is rather complicated, and triggering issues
could be in the sampling head trigger pick-off circuits, the routing of the trigger signal
through the 7S11 to the 7T11, or the tunnel diode/Schmitt Trigger circuits within
the 7T11 itself. (This is why my previous post started with tests that deliberately
side-step the sampling head and the 7S11.)

With the 7S12/S-53, triggering problems are most likely due to S-53 not working quite right.
If you suspect that your S-53 is not triggering properly, I would suggest performing step 7
of the Performance Check (Check INPUT Signal and Sensitivity), as specified in the S-53
manual. If the S-53 is malfunctioning or not calibrated properly, step 7 tends to fail.


dan


Jim Ford
 

I will check the S-53 manual for Step 7 of the performance check.
Fortunately I have two S-53's, so that's an extra point on the curve.

Thanks, Dan!

Jim

------ Original Message ------
From: "Dan G" <dgajanovic@sympatico.ca>
To: TekScopes@groups.io
Sent: 1/29/2021 12:22:43 AM
Subject: Re: [TekScopes] Tek 7T11A issues

Hi Jim,

I also used to have triggering problems with my 7S12/S-53 setup years ago, but it turned
out that the S-53 just needed to be re-calibrated.

In a 7S11/7T11 setup, the triggering path is rather complicated, and triggering issues
could be in the sampling head trigger pick-off circuits, the routing of the trigger signal
through the 7S11 to the 7T11, or the tunnel diode/Schmitt Trigger circuits within
the 7T11 itself. (This is why my previous post started with tests that deliberately
side-step the sampling head and the 7S11.)

With the 7S12/S-53, triggering problems are most likely due to S-53 not working quite right.
If you suspect that your S-53 is not triggering properly, I would suggest performing step 7
of the Performance Check (Check INPUT Signal and Sensitivity), as specified in the S-53
manual. If the S-53 is malfunctioning or not calibrated properly, step 7 tends to fail.


dan





Albert Otten
 

Hi n49ex,

Are you using a sampling head with trigger signal output? If not (for instance S-6) then you have to supply external triggering to the 7T11A.

Albert


n49ex
 

Thanks all for the valuable input - I now have a few things to try/look at. Since my original post I decided to try the 7S11 with my 7S12. Since that, the 7S12 has developed its own problem - It no longer will show rise times. Instead, it only brightens a portion of the trace corresponding to the time/division setting, but does not expand the trace anymore to enlarge in on the leading edge. There's always something! I've never had the 7S11 in with the 7S12, always just used the 7S12 stand alone as a TDR to measure cables or look for opens/shorts - Now I'm wondering if the 7S11 could have caused some damage to the 7S12?


Albert Otten
 

" it only brightens a portion of the trace corresponding to the time/division setting "
Isn't that the LOCATE button?

Albert


n49ex
 

No, I don't think it's the locate button. It behaves just like when on a regular analog scope you use the delayed time base in the intensify mode, where a piece of the primary trace is brighter for a segment corresponding to the delayed time based time setting, and can be scrolled across the primary trace. That's what appears on the TDR trace. E.G. with the fastest time settings, say 100pS, only a short piece of trace is intensified, and with the distance/delay knob, it can be walked across the trace, and the trace stays exactly the same for all the settings, instead of focusing in with decreasing time scales.


Dan G
 

On Fri, Jan 29, 2021 at 06:29 PM, n49ex wrote:

No, I don't think it's the locate button. It behaves just like when on a
regular analog scope you use the delayed time base in the intensify mode,
where a piece of the primary trace is brighter for a segment corresponding to
the delayed time based time setting, and can be scrolled across the primary
trace. That's what appears on the TDR trace. E.G. with the fastest time
settings, say 100pS, only a short piece of trace is intensified, and with the
distance/delay knob, it can be walked across the trace, and the trace stays
exactly the same for all the settings, instead of focusing in with decreasing
time scales.
I believe Albert is right. N49ex, what you are describing sounds exactly like
how the 7S12 is supposed to behave when the LOCATE mode is enabled.
On a properly functioning 7S12, the LOCATE button is a mechanical toggle
(every time you press it, it should alternate between being pushed in and
popped out -- there should be a visible yellow ring around the base of the
button when it pops out to let you know that it's in locate mode).

When LOCATE is pushed in (i.e. not in locate mode), the bright portion of
the trace should expand horizontally to fill the whole screen, and the
entire sweep should have uniform brightness.

Perhaps your 7S12 is permanently stuck in locate mode.


dan


n49ex
 

Dang, you are right!!!! Thank you, I guess I didn't pay attention to the state of the button, and never used the locate function before! That was it! Works just fine after pushing the button in, it must have gotten inadvertently pushed somewhere along the way and I didn't notice. Now, if I could find an "operator error" to fix my 7T11A!

Despite many years of using/collecting/fixing my Tek scopes, I finally found and joined this group - absolutely outstanding info and folks here!
Thanks again!