Topics

Another scope 7854

Joe Laffey
 

On Fri, 5 May 2017, Joe Laffey joe@... [TekScopes] wrote:

On Fri, 5 May 2017, David @DWH [TekScopes] wrote:

I did a lot of poking around this evening.

I put a scope on the output of both the trigger and signal followers
Q280-Q380 Q480-Q580. I could not see any glitches in phase on these
signals. I was triggering off the sync out from the siggen. I did notice a
fair amount of noise on the signals, but that could just be the probes and
the ground leads, etc. ** Shouldn't I see the glitching on these waveforms
if the problem is in the vertical plugin?

Sure enough the problem is in the TimeBase 7B53A.

I can see the input to the timebase as claen and working fine.

However, there is something up with the HoldOff circuit! When I scope the output of the Sweep Disable (pin 17) of U520 (by connecting to R360 on Schematic 2, which is accessible without an extender) I can see Sweep Disable signal shift around in synchronization with the glitch. Basically, the Sweep Disable is going high when it should not be, essentially causing a hold off condition.

I am not sure WHY this is happening yet, but I am just starting to look at the 7B53A docs.

Still digging through the docs.

--
73
Joe Laffey
The Stable
Visual Effects
http://TheStable.tv/?e40773M/

 

On Fri, 5 May 2017 21:39:24 -0500 (CDT), you wrote:

On Fri, 5 May 2017, David @DWH [TekScopes] wrote:

I did a lot of poking around this evening.

I put a scope on the output of both the trigger and signal followers
Q280-Q380 Q480-Q580. I could not see any glitches in phase on these
signals. I was triggering off the sync out from the siggen. I did notice a
fair amount of noise on the signals, but that could just be the probes and
the ground leads, etc. ** Shouldn't I see the glitching on these waveforms
if the problem is in the vertical plugin?
If the problem is with the vertical plug-in, then it should show up
here. Besides noise that the probe picks up via its ground lead, the
noise level is high because the signal is differential so if only one
side is probed, the common mode noise is there also.

I was able to get the glitches to occur with both Ch 1 and Ch 2
triggering. They are however much more often (3-5x more often) on the
channel 2 trigger.

The glitches seem to only occur when the trigger level is negative and
the slope is positive, or the trigger level is positive and the slope is
downward on a normal centered signal (think AC coupled, or 0 V baseline).

It is very difficult to get the glitches to occur with a square wave. It
really wants to have a slower rise time to cause the problem. Triangle and
sine waves do it fine.
Oh, so it is not caused by a glitch on the edge of the square wave.

** Or perhaps the noise is what is causing the triggering. However the
waveforms displayed on the 7854 are not particularly noisy. They look
pretty thin and normal for a ~100MHz scope (with this plugin).

I am beginning to wonder if the problem might lie in the horizontal
amplifier. I am pretty confused at this point.
If the glitch, then I would expect it in the 7B53A trigger circuit or
because of noise from the mainframe's power supply outputs.

Changing the trigger source form Ch 1 to Ch 2 DOES make a difference, even
when displaying just Ch 1 (or Ch2). I have a T at the siggen and two wires
come down to the scope. Both wires have terminator on them (same problem
happens without termination). I tried another siggen and had the same
problem. I tried putting this siggen into another scope and it triggered
and looked fine. I have tried the internal calibrator, and I have seen the
issue, but it is much less common on a square wave, especially a higher
frequency square wave.
Some DDS function generators produce enough glitch energy in their
stairstep output to falsely trigger sensitive oscilloscopes.

I swapped the two switching ICs to no avail. The DC voltages on the
switching ICs read like the reference.

I should receive another Vertical amplifier plugin next week, and I guess
I can use that to rule out or in the timebase.

The timebase is a 7B53A.
I have seen this behavior before in all of my 7B53A timebases and I
suspect you may be seeing noise falsely trigger it. But if that is
the case, it should not matter which vertical channel is used or how
they are configured except for bandwidth which you cannot control on
the 7A18.

On Sat, 6 May 2017 01:09:00 -0500 (CDT), you wrote:

Sure enough the problem is in the TimeBase 7B53A.

I can see the input to the timebase as claen and working fine.

However, there is something up with the HoldOff circuit! When I scope the
output of the Sweep Disable (pin 17) of U520 (by connecting to R360 on
Schematic 2, which is accessible without an extender) I can see Sweep
Disable signal shift around in synchronization with the glitch. Basically,
the Sweep Disable is going high when it should not be, essentially causing
a hold off condition.

I am not sure WHY this is happening yet, but I am just starting to look at
the 7B53A docs.

Still digging through the docs.
I have never studied the timebase logic in enough detail to make
directed suggestions.

I would take a really close look however at noise on the power supply
outputs at the plug-in compartment. The internal trigger path inside
the mainframe is differential however it is single ended inside the
7B53A making it susceptible to power supply noise.

EB4APL
 

One standard tool used in NASA Deep Space Network soldering courses in
the 70's was a kind of stainless steel stencil which has holes for the
common solder pads (round, square, droplet, etc.) that was used to
remove the gold from the PCBs before soldering, using a common ink
eraser. It was a standard item but unfortunately I lost mine.

Regards,
Ignacio, B4APL


El 05/05/2017 a las 14:49, David @DWH [TekScopes] escribió:

On Fri, 5 May 2017 07:11:08 +0100, you wrote:

I originally thought that but when I ran across the problem,
removing the solder revealed that the
entire solder to gold interface was a purple-brown color. I suspect
this was a long term effect
of
whatever flux Tektronix used during assembly.
Using tin-lead solder on gold plating is not a good thing. In space
application, component leads are
gold plated to prevent oxidation of the underlying wire and maintain
solderability. But the leads
are "degolded" prior to assembly using a solder bath (the gold
plating dissolves in the solder). If
you don't degold, you get contact embrittlement because of the
gold/tin/lead intermetallics.

Do a google search on degolding - there is a bunch of stuff on there
including you tube videos, and
automatic machines to bulk degold.

Craig
One of the NASA papers on the subject recommended removing the gold by
dissolving it in tin which is what I ended up doing. It took a few
passes of applying solder and removing it. The gold dissolves in the
tin better at higher temperatures but I had better results at a low
temperature applied for a longer time to avoid damaging the
polysulfone board substrate.


------------------------------------------------------------------------
Posted by: David <@DWH>
------------------------------------------------------------------------


[Non-text portions of this message have been removed]

Joe Laffey
 

On May 6, 2017, at 10:21 AM, David @DWH [TekScopes] <TekScopes@...> wrote:

Still digging through the docs.
I have never studied the timebase logic in enough detail to make
directed suggestions.

I would take a really close look however at noise on the power supply
outputs at the plug-in compartment. The internal trigger path inside
the mainframe is differential however it is single ended inside the
7B53A making it susceptible to power supply noise.
Sounds reasonable. I haven't examined the power supply at all yet, though I didn't see any ripple on the power rails of the vertical amplifier.

Will measure power supply itself. It may have original caps.


--
Joe Laffey

Joe Laffey
 

At the timebase plugging, measuring the rails on the trigger board I see

-15V rail. 0.32miliVAC RMS
+15V rail 0.73miliVAC RMS
+5V rail 2.07milVAC RMS

When the plugin in NOT triggering the ripple on the +5V goes to 0V

I will see about removing the power supply for the scope and testing it. This whole thing may go on hold for a week due to work.


--
Joe Laffey
The Stable
Visual Effects

On May 6, 2017, at 12:40 PM, Joe Laffey joe@... [TekScopes] <TekScopes@...> wrote:



On May 6, 2017, at 10:21 AM, David @DWH [TekScopes] <TekScopes@...> wrote:
Still digging through the docs.
I have never studied the timebase logic in enough detail to make
directed suggestions.

I would take a really close look however at noise on the power supply
outputs at the plug-in compartment. The internal trigger path inside
the mainframe is differential however it is single ended inside the
7B53A making it susceptible to power supply noise.
Sounds reasonable. I haven't examined the power supply at all yet, though I didn't see any ripple on the power rails of the vertical amplifier.

Will measure power supply itself. It may have original caps.

--
Joe Laffey




[Non-text portions of this message have been removed]

Joe Laffey
 

Well, it was a good thing that my schematic no vertical amplifier came today. Because the same timebase works just fine with the second amplifier 7A26

So now I am really confused as to why the signals coming out of the two switching ICs on the 7A18 looked fine, but still had the issue.

Kinda confused at this point. Good thing I am taking a break this week for work. Perhaps things will be more clear when I get back into it.
--
Joe Laffey
The Stable
Visual Effects

On May 6, 2017, at 2:45 PM, Joe Laffey <joe@...> wrote:

At the timebase plugging, measuring the rails on the trigger board I see

-15V rail. 0.32miliVAC RMS
+15V rail 0.73miliVAC RMS
+5V rail 2.07milVAC RMS

When the plugin in NOT triggering the ripple on the +5V goes to 0V

I will see about removing the power supply for the scope and testing it. This whole thing may go on hold for a week due to work.


--
Joe Laffey
The Stable
Visual Effects

On May 6, 2017, at 12:40 PM, Joe Laffey joe@... [TekScopes] <TekScopes@...> wrote:



On May 6, 2017, at 10:21 AM, David @DWH [TekScopes] <TekScopes@...> wrote:
Still digging through the docs.
I have never studied the timebase logic in enough detail to make
directed suggestions.

I would take a really close look however at noise on the power supply
outputs at the plug-in compartment. The internal trigger path inside
the mainframe is differential however it is single ended inside the
7B53A making it susceptible to power supply noise.
Sounds reasonable. I haven't examined the power supply at all yet, though I didn't see any ripple on the power rails of the vertical amplifier.

Will measure power supply itself. It may have original caps.

--
Joe Laffey




[Non-text portions of this message have been removed]

 

That sounds like an eraser shield used for drafting.

<https://www.amazon.com/Alvin-Stainless-Steel-Erasing-Shield/dp/B000HF6VK6>

I always thought one of those rotary electric erasers used for
drafting with one of the more abrasive eraser elements for ink would
work for this.

On Sat, 6 May 2017 19:08:31 +0200, you wrote:

One standard tool used in NASA Deep Space Network soldering courses in
the 70's was a kind of stainless steel stencil which has holes for the
common solder pads (round, square, droplet, etc.) that was used to
remove the gold from the PCBs before soldering, using a common ink
eraser. It was a standard item but unfortunately I lost mine.

Regards,
Ignacio, B4APL

 

On Sat, 6 May 2017 14:45:56 -0500, you wrote:

At the timebase plugging, measuring the rails on the trigger board I see

-15V rail. 0.32miliVAC RMS
+15V rail 0.73miliVAC RMS
+5V rail 2.07milVAC RMS

When the plugin in NOT triggering the ripple on the +5V goes to 0V

I will see about removing the power supply for the scope and testing it. This whole thing may go on hold for a week due to work.
The peak to peak high frequency noise matter more than the low
frequency RMS noise in this case.

On Sat, 6 May 2017 15:45:48 -0500, you wrote:

Well, it was a good thing that my schematic no vertical amplifier came today. Because the same timebase works just fine with the second amplifier 7A26

So now I am really confused as to why the signals coming out of the two switching ICs on the 7A18 looked fine, but still had the issue.

Kinda confused at this point. Good thing I am taking a break this week for work. Perhaps things will be more clear when I get back into it.
I do not think the common mode rejection of the trigger circuit path
in the 7B53A is all that good so it is possible that your 7A18 is
producing a lot of common mode noise which would not be immediately
apparent. Comparing the trigger signal waveforms at the output of the
7A18 to the 7A26 might reveal something.

It may not be worth spending too much time on this. The 7B53A is not
a very good timebase to use with the 7854; the timebase itself
triggers too slowly to show the leading edge with the 400MHz 7854's
delay line which is only 65 nanoseconds versus 120 nanoseconds of the
100MHz 76xx mainframes. And like I said earlier, all of my 7B53As so
this to one extent or another; adjusting the trigger level and
coupling is usually enough to solve it.

Of course some of us have a lot of fun tracking down obscure problems
like this. :)

Joe Laffey
 

On May 6, 2017, at 4:19 PM, David @DWH [TekScopes] <TekScopes@...> wrote:

On Sat, 6 May 2017 14:45:56 -0500, you wrote:

At the timebase plugging, measuring the rails on the trigger board I see

-15V rail. 0.32miliVAC RMS
+15V rail 0.73miliVAC RMS
+5V rail 2.07milVAC RMS

When the plugin in NOT triggering the ripple on the +5V goes to 0V

I will see about removing the power supply for the scope and testing it. This whole thing may go on hold for a week due to work.
The peak to peak high frequency noise matter more than the low
frequency RMS noise in this case.

Ah... well I did put a 1X probe on the 5V rail and Peak detect mode on the 2232 I saw edge peaks of around 75-90mV P-P. For grins and experimentation/learning tried clipping in various sized capacitors on the 5V rail and it did not alleviate the issue.

I do not think the common mode rejection of the trigger circuit path
in the 7B53A is all that good so it is possible that your 7A18 is
producing a lot of common mode noise which would not be immediately
apparent. Comparing the trigger signal waveforms at the output of the
7A18 to the 7A26 might reveal something.

I may compare these just to see what I see.
It may not be worth spending too much time on this. The 7B53A is not
a very good timebase to use with the 7854; the timebase itself
triggers too slowly to show the leading edge with the 400MHz 7854's
delay line which is only 65 nanoseconds versus 120 nanoseconds of the
100MHz 76xx mainframes. And like I said earlier, all of my 7B53As so
this to one extent or another; adjusting the trigger level and
coupling is usually enough to solve it.

Of course some of us have a lot of fun tracking down obscure problems
like this. :)
I am learning stuff in the process-- so that is good. In the real world I would just enable HF reject in the trigger filter and it is stable.

I will seek out a better timebase.

Thanks for all your help. I really appreciate it.

--
Joe Laffey

bobh@joba.com
 

From: Joe Laffey joe@... [TekScopes]
Sent: Saturday, May 06, 2017 2:59 PM
To: TekScopes@...
Subject: Re: [TekScopes] Another scope 7854


I will seek out a better timebase.
Joe Laffey

I use a 7B92A and a 7B87 in mine. I think the 7B87 was basically built for the 7854 scope.
Bob.

Dave Casey
 

Correct, and I believe it is necessary if you want to capture single shot
events before the trigger (pre-trigger acquisition). When you cut your
teeth on modern DSOs, you take features like this for granted.

Dave Casey

On Sat, May 6, 2017 at 9:21 PM, 'bobh' bobh@... [TekScopes] <
TekScopes@...> wrote:





From: Joe Laffey joe@... [TekScopes]
Sent: Saturday, May 06, 2017 2:59 PM
To: TekScopes@...
Subject: Re: [TekScopes] Another scope 7854

I will seek out a better timebase.
Joe Laffey
I use a 7B92A and a 7B87 in mine. I think the 7B87 was basically built for
the 7854 scope.
Bob.

[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]

 

One of the more specialized 7000 series plug-in extenders could swap
the signal out and trigger out signals allowing the trigger output to
be viewed on the CRT.

Another test which might show something is the vertical signal output
on the back of the 7854. It is a copy of the currently selected A
trigger signal.

Joe Laffey
 

I'd love a 7B87, but they seem tough to find. If anyone has one they want to get rid of give me a shout off list.


--
Joe Laffey

On May 6, 2017, at 10:08 PM, Dave Casey polara413@... [TekScopes] <TekScopes@...> wrote:

Correct, and I believe it is necessary if you want to capture single shot
events before the trigger (pre-trigger acquisition). When you cut your
teeth on modern DSOs, you take features like this for granted.

Dave Casey

On Sat, May 6, 2017 at 9:21 PM, 'bobh' bobh@... [TekScopes] <
TekScopes@...> wrote:





From: Joe Laffey joe@... [TekScopes]
Sent: Saturday, May 06, 2017 2:59 PM
To: TekScopes@...
Subject: Re: [TekScopes] Another scope 7854

I will seek out a better timebase.
Joe Laffey
I use a 7B92A and a 7B87 in mine. I think the 7B87 was basically built for
the 7854 scope.
Bob.

[Non-text portions of this message have been removed]


[Non-text portions of this message have been removed]


[Non-text portions of this message have been removed]