On Thu, 14 Mar 2013 15:31:53 +0000, Chris Jones <chris@...>
wrote:

Question 3: The traces above show what appears to be noise and notably
Channel 2 seems to be riding on a ragged sine wave. Is there a way to
determine if this is actual signal or some kind of
digitization/quantization noise? If I reduce bandwidth of the scope
to 20MHz and use smoothing, the signals look a lot cleaner, but I am
then concerned that I am missing actual data.

That looks like good old-fashioned noise in your wiring, circuit or
sensors, and nothing to do with the digitization/quantization process.

I hope this helps
Chris

I agree with Chris.

Here is what my 2440 displays under similar circumstances with good
coaxial probe connections using peak detect mode:



Contrast that with my 2232 displaying the same signals. The 2232 has
a funky noise reduction algorithm making the lower trace look cleaner
than it actually is. If I disabled the noise reduction, then it would
look the same as the 2440:



Here is a similar 2440 display of almost the same signal but with
bandwidth limiting. The noise looks the same in this case:

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?

[snip]

> Question 3: The traces above show what appears to be noise and notably
> Channel 2 seems to be riding on a ragged sine wave. Is there a way to
> determine if this is actual signal or some kind of
> digitization/quantization noise? If I reduce bandwidth of the scope
> to 20MHz and use smoothing, the signals look a lot cleaner, but I am
> then concerned that I am missing actual data.
>
That looks like good old-fashioned noise in your wiring, circuit or
sensors, and nothing to do with the digitization/quantization process.

How are your probes grounded?

I hope this helps
Chris

--
Chris Jones - chris@...

On Thu, 14 Mar 2013 10:53:53 -0400, Mark Richards
<mark.richards@...> wrote:

Question 1: If I reduce the sweep frequency, the amplitude of both the
transmitted and received signals drops. I assume that by changing the
sweep rate the bandwidth is not touched, and therefore attenuation would
not be present. Does this attenuation suggest a scope issue?

That should not happen unless aliasing is occurring which is
definitely a possibility. The 2440 has a pretty short record length
which will limit the sample rate at low horizontal speeds. Switch
your 2440 to peak detect mode (envelope mode over 1 acquisition) and
see if that works better.

Question 2: I can't find a way to more closely examine the return
signal. I'd like to see more detail in the return signal on Channel 2.
A digital capture only provides about 100us of detail. Ideally would
like to trigger such that Channel 2 is captured after a selectable delay.

The 2440 supports delay by time acquisitions which will do exactly
this. Select A intensified by B or B, select delay by time, and
adjust the acquisition to see what you want. This is equivalent to
delayed sweep in an analog oscilloscope. You may want to study the
manual on this subject.

Question 3: The traces above show what appears to be noise and notably
Channel 2 seems to be riding on a ragged sine wave. Is there a way to
determine if this is actual signal or some kind of
digitization/quantization noise? If I reduce bandwidth of the scope to
20MHz and use smoothing, the signals look a lot cleaner, but I am then
concerned that I am missing actual data.

Using peak detect mode (above) will reveal what is going on.

It seems a DSO brings more troubles, but seeing these signals with an
analog scope (I have an old 100MHz Philips scope) is near impossible.

I never have any problems with my 2440 but it does require some care
and understanding to avoid misleading displays caused by aliasing.
Peak detect is your friend.

On 14/03/2013 14:53, Mark Richards wrote:

Question 1: If I reduce the sweep frequency, the amplitude of both the transmitted and received signals drops. I assume that by changing the sweep rate the bandwidth is not touched, and therefore attenuation would not be present. Does this attenuation suggest a scope issue?

That's a sampling effect, and is to be expected. If the scope's sampling frequency (I don't know what it is for a 2440, but it'll be a few tens of samples per division, I expect) drops below twice the signal frequency, you'll get all sorts of odd effects, called aliasing, which will sometimes look like a reduced amplited. Search for 'Nyquist limit' to find out more.

My 2430A does exactly the same, and maths predicts it, so it's not a faulty scope. The only way to avoid it is to use an analogue scope, a digital one with a faster sample rate (and hence larger memory) or a digital one with a clever peak detection function.

Question 2: I can't find a way to more closely examine the return signal. I'd like to see more detail in the return signal on Channel 2. A digital capture only provides about 100us of detail. Ideally would like to trigger such that Channel 2 is captured after a selectable delay.

The 2440 should be able to do that. My 2430A can. I have it running normally with timebase A, then, to select a portion of the trace to examine in more detail, press:
'A INTEN' mode (above the SEC/DIV knob)
Press 'TIME' in the DELAY section at the top. Now turn SEC/DEV clockwise a few stops and you should see a highlighted section of trace (you might have to turn the intensity down a bit) which you can move with the knob which normally drives the cursors. The setting of SEC/DIV now controls the speed of timebase B, and hence the length of the highlighted part. When you've got what you want, press MODE 'B' (above the SEC/DIV) knob and you will see your highlighted area expanded to fill the screen.

Note: this only works with repetitive waveforms, but you can set up timebase B and the delay for a one-shot event too, it's just that you don't get the immediate feedback.

This is all based on the 2430A on my bench which I've just tried it on. Your 2440 may be different!

Question 3: The traces above show what appears to be noise and notably Channel 2 seems to be riding on a ragged sine wave. Is there a way to determine if this is actual signal or some kind of digitization/quantization noise? If I reduce bandwidth of the scope to 20MHz and use smoothing, the signals look a lot cleaner, but I am then concerned that I am missing actual data.

That looks like good old-fashioned noise in your wiring, circuit or sensors, and nothing to do with the digitization/quantization process.

I hope this helps
Chris

--
Chris Jones - chris@...

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Hopeful that operation questions will be appropriate here.? The Tektronix 2440 user manual only goes so far.? If not appropriate, admonish me and send me packing.? Just gently, please.


I have a 2440 DSO.? Bought used a few years ago and has been a very useful tool.? I had the benefit of this group to help me do some purchasing research and was very helpful indeed.


Lately puzzling on an issue measuring ultrasonic signaling.? Transmit signal on Channel 1 consists of 20 pulses at 1MHz rate with amplitude of about 2.5V.? Receive signal on Channel 2 occurs about 70us after transmit signal and amplitude as expected is less.?

Hopefully this scope capture will come through.? If it does not, the image shows Ch1 and 2 displayed, with Ch1 a set of pulses superimposed on a DC shift of +1V with the pulses between +1 and +2.5V.? The return signal is AC of about 250mv p/p.

Question 1: If I reduce the sweep frequency, the amplitude of both the transmitted and received signals drops.? I assume that by changing the sweep rate the bandwidth is not touched, and therefore attenuation would not be present.? Does this attenuation suggest a scope issue?

Question 2: I can't find a way to more closely examine the return signal.? I'd like to see more detail in the return signal on Channel 2.? A digital capture only provides about 100us of detail.? Ideally would like to trigger such that Channel 2 is captured after a selectable delay.?

Question 3: The traces above show what appears to be noise and notably Channel 2 seems to be riding on a ragged sine wave.? Is there a way to determine if this is actual signal or some kind of digitization/quantization noise?? If I reduce bandwidth of the scope to 20MHz and use smoothing, the signals look a lot cleaner, but I am then concerned that I am missing actual data.

It seems a DSO brings more troubles, but seeing these signals with an analog scope (I have an old 100MHz Philips scope) is near impossible.

I don't see much mention of these on the group. Are they unpopular, rare, or just so reliable that no one ever needs to fix them?

My 475A is ill, and a couple extra channels & digital output would be very useful. Fixing the 475A would probably be much easier with a second scope as well.

Doug White

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On 03/13/2013 06:35 PM, Kenneth G. Gordon wrote:

?

Might anyone have a schematic, or even a users or maintenance manual for the B&K Precision model 3025 SweepFunction Generator?

Mine crapped out a few years ago, and I would really like to get it working.

B&K has so far been no help whatever.

Ken W7EKB

One thing that you might check is the ttl sync output.
If this is OK the problem is probably in the output amps,
fairly common.
if not, check out the power supplies.
I have had a 3020 for many years.
Don't know if it's different.

.

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Looks good. I am also building several extenders from John's kits. And I have a dysfunctional 7L14 to get working at point. I will be interested to see how you make out with your repair effort.

DaveD

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Cheap probes have their place. I use them because of price AND if they break it does not cause me the angst that breaking a decent probe would.
These probes are used with my 7854 & I know there are issues with them but when I feel the need for a better probe I use the HP or Tek probes that are in the box aabove my bench.

Being "financially challenged" (the word in the UK is skint) means that I have to look after the nice kit I have scrounged up over the years for when I need the peformance advantage it gives. Otherwise the cheap stuff works adequately for most of the time when I am not working with high speed / high slew rate signals.

Just my thoughts.

Phil

Hi to All,

Anybody who has this document (2 pages) and is willing to send me scans of it?

Greetings,

Egge Siert

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A line and the variable control behavior makes me think that there is
an open or short involving a bad part somewhere in the input buffer.
The collectors of Q30 and Q38 and relays K44 and K48 on schematic 1
would be a good place to start.

Q192 is not strictly necessary depending on the output drive
capability of U192. It just acts as a current amplifier and protects
the output of U192 which given that it was originally a 709 type of
operational amplifier, is almost necessary. I am not surprised if
Tektronix removed Q192 and replaced U192 with something better as soon
as they could. What is the actual 155 or 156 Tektronix part number on
yours?

In the service manual I have, U192 is a 709 type (LM709, uA709, RC709,
whatever) operational amplifier:

"The first op-amp to get real exposure in the UK was the Fairchild
uA709, designed by the renowned Bob Widlar and introduced in 1965. It
was a rather awkward item that required quite complicated external
compensation and was devoid of output short-circuit protection. One
slip of the probe and an expensive IC was gone. It was prone to
latch-up with high common-mode voltages and did not like capacitive
loads. I for one found all this most discouraging, and gave up on the
709 pretty quickly. If you're going to quit, do it early, I say."



Needless to say, there are lots of suitable and improved replacements
if it is bad. The only problem will be Tektronix used a TO-99 metal
can package but it is possible to make an 8 pin DIP fit with some
cleverness.

On Thu, 14 Mar 2013 05:42:41 -0000, "woooey" <woooey@...>
wrote:

Thanks, David.

When I set up the 7B70 in 'amplifier' mode, instead of a dot
I get a horizontal trace about 4 units wide. Turning up the
gain causes the trace to disappear off to the left of the
screen.

Since the other trigger functions work, this seems to me to
indicate the 'external horizontal amplifier' (U192/Q192 in
the manual). The circuitry in my 7B70 is somewhat different
in this area from the manual (there is no Q192 on my PCB).

U192 is a Tek-numbered opamp. I imagine I will need to find
a salvage 7B70 to fix this.

--ian

--- In TekScopes@..., David <davidwhess@...> wrote:

I took a quick look at the 7B70 schematics.

What you are describing with the variable control causing the external
X input to shift seems like a problem with the DC balance. Page 5-6
of the service manual has a procedure that involves adjusting R186
(Schematic 2 external horizontal amplifier balance) and then R37
(Schematic 1 external input amplifier balance) while rotating the
variable control until the X position does not change.

I would start there as it will likely provide further information
about the problem.

On Wed, 13 Mar 2013 19:38:21 -0000, "woooey" <woooey@...>
wrote:

I found the manual on BAMA.

Relay K780 has an o/c coil. Replacing this with one borrowed
from a 7B71 fixed the 'magnifier' problem but had no effect
on the problem with external input. Still, this is encouraging,
so I will peer at the schematic some more.

--ian

--- In TekScopes@..., "woooey" <woooey@> wrote:

In 'amplifier' (external input) mode, the variable control
shifts the trace off to the left when turned clockwise. There
is some low-level signal on the X-axis but this seems to have
no relation to a signal applied to the external input.

The Magnifier no longer operates. The numeric indicator on
the display changes correctly, but the timebase itself doesn't
change.

Is it worth tapping on relays, etc, or should I just look for
a "new" 7B70?

Thanks,

--ian

Thanks, David.

When I set up the 7B70 in 'amplifier' mode, instead of a dot
I get a horizontal trace about 4 units wide. Turning up the
gain causes the trace to disappear off to the left of the
screen.

Since the other trigger functions work, this seems to me to
indicate the 'external horizontal amplifier' (U192/Q192 in
the manual). The circuitry in my 7B70 is somewhat different
in this area from the manual (there is no Q192 on my PCB).

U192 is a Tek-numbered opamp. I imagine I will need to find
a salvage 7B70 to fix this.

--ian

I recently built the three kits kindly supplied by John Griessen and am
now using them to work on my 7L14 spectrum analyzer, including doing
open-first-mixer surgery.

Here is a photo of extender cables (built to the two foot length of
John's pre-cut wires) coming out of the right side of a 7306:



The cables are long enough to come out of the front of the scope, but I
did it this way to give me more length for turning the 7L14 on its side
and so the 7L14 would be beside the scope, rather than in front of it.

Thanks John!

- Robin

To make these two messages from the thread "Re: Isolation transformers
are not a panacea [was: Variak talk]" easier to find in the archives,
here is a summary of them, with links to the messages, under a new
Subject heading.

Steve <ditter2@...> wrote an interesting piece on differential
probes, including:



1 - Why ordinary probes are resonant at certain frequencies if the
ground connection to the device under test is not connected.

2 - The value and potential limits of differential probes.

3 - Difficulties measuring power transistor drive voltages in the
few hundred millivolt range when they are both at 900V or more.

4 - Achieving differential measurements with a pair of equal length
cable P6009 probes. Apparently the DC gain can be finely adjusted
on these 100x attenuation 120MHz 1500V probes:



David <davidwhess@...> continued the interesting discussion:



with mention of success using this dual probe technique, keeping the
cables close together, adding a ferrite core around the two cables,
modifying other probes to achieve the required close match of
attenuations, and: "As Glenn mentioned in the concurrent Cheap Probes
thread, if you run your hand along the probe cable and the waveform
changes, then you have problems."

- Robin

On 03/13/2013 07:04 AM, jerry massengale wrote:

Hi,

Really good soldering work.

If you mean the photos on Chris hayes's email, that's one I assembled. Thanks!

The edges of the edge connector need to be trimmed down a bit. As it is you cannot daisy chain the

extender as the edge is wider than the black connector can take.

That is true. I just found it out after the last order of PCBs for Tek_7K_flex.
I plan to fix it so they can daisy chain if possible while making a profit, but
for the first runs I was more worried about having minimal connector tolerance
slop so no wrong electrical connections could be made.

John Griessen
(back from Louisiana)

On 03/13/2013 07:04 AM, jerry massengale wrote:

Hi,

Really good soldering work.

If you mean the photos on Dave D's original email, that's one I assembled. Thanks!

The edges of the edge connector need to be trimmed down a bit. As it is you cannot daisy chain the

extender as the edge is wider than the black connector can take.

That is true. I just found it out after the last order of PCBs for Tek_7K_flex.
I plan to fix it so they can daisy chain if possible while making a profit, but
for the first runs I was more worried about having minimal connector tolerance
slop so no wrong electrical connections could be made.

John Griessen
(back from Louisiana)

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